TE

Molecular genetics
27
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bioccfc27.e

Computer artwork of a polypeptide chain (chain of yellow spheres)

being synthesized by a ribosome.

Links to prior knowledge Chapter preview

In the previous chapter, we learned that genes
can determine our body characteristics by 27.1 From DNA to proteins
controlling the types of proteins our cells make. 27.2 Mutations
Now let us look at how genes work at the
molecular level. Your knowledge of
complementary base pairing will help you
understand how proteins are synthesized using
the instructions carried in DNA. You will also find
out what happens when errors occur in the DNA.
 27 Molecular genetics

Inborn errors of metabolism

In Hong Kong, about one in 4000 newborn babies # suffers from inborn errors of
metabolism (IEMs). IEMs are genetic disorders caused by mutations of genes. The
patients cannot produce some metabolically important proteins (e.g. enzymes). If left
untreated, IEMs can lead to serious health problems and even death.

IEM Health problems

Mental retardation, delayed development,
Phenylketonuria
hyperactivity, seizures

Severe brain dysfunction, weakened and enlarged

Primary carnitine deficiency
heart, muscle weakness, low blood glucose levels

Enlarged adrenal glands, hormone imbalance,

Congenital adrenal hyperplasia
abnormal development of sex organs

 Examples of IEMs and associated health problems

Early detection and treatment of IEMs can

minimize their adverse impact on the child.
The Hong Kong Government launched the
Pilot Study on Newborn Screening for Inborn
Errors of Metabolism in 2015 and is expanding
the programme to provide early detection of
a wider range of IEMs.
 Blood samples are collected from an infant
for screening of IEMs.

(# Source: The Department of Obstetrics and Gynaecology of The Chinese University of Hong Kong)

Think about …
1. How are proteins produced in our body?
2. What is a mutation? What causes mutations? Answer

(Refer to p.A2 for answers.)

inborn errors of metabolism congenital adrenal hyperplasia

27- 2 phenylketonuria
primary carnitine deficiency
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27 Molecular genetics

Learning objective 27.1 From DNA to proteins

• Understand the nature of the
genetic code
• Recognize the role of DNA and
A. The genetic code
RNA in gene expression
The genetic code is the set of rules that determines the sequence of
• Be able to outline the process
of protein synthesis amino acids in a protein. The code is stored in the sequence of
nucleotide bases in the DNA of a gene. It has the following features:

Connection to Maths The code is a triplet code. A sequence of three consecutive

bases (i.e. a triplet) on a DNA strand codes for one amino acid.
Why a triplet code?
If one base coded for one The code is a degenerate code. There are four types of bases
amino acid, four types of
in DNA. Different combinations of the four bases produce a
bases could only code for 41
= 4 different amino acids. If a total of 64 triplet codes to specify the 20 types of amino acids
combination of two bases used to make proteins. The excessive number of triplet codes
coded for an amino acid, means that certain triplet codes code for the same amino acid.
then 42 = 16 amino acids
could be specified. A triplet Some triplet codes serve as ‘start signals’ or ‘stop signals’ for
code composed of three
bases has 43 = 64
protein synthesis.
combinations, which is more
than enough to specify the The code is read in a continuous manner. There is no break
20 amino acids. between codes.

The code is non-overlapping. This means that adjacent triplets

do not share a base.

The code is universal. The same triplet code codes for the same
amino acid in all species.

triplet codes on
DNA strand

specify

amino acids /
start stop
signal
same amino acids

Figure 27.1 The genetic code is a triplet code.

triplet code
degenerate code 27- 3
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27 Molecular genetics

Flipped classroom B. Gene expression and protein synthesis

Protein synthesis
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bioccflip2701.e
Genes are located on chromosomes in the nucleus of a cell but
proteins are made in the cytoplasm, at ribosomes. Since DNA
cannot leave the nucleus, a molecule called messenger RNA
(mRNA) carries the genetic code to the cytoplasm to direct protein
synthesis.

Protein synthesis occurs in two stages:

 Transcription—the genetic code of a gene is copied to an mRNA

molecule in the nucleus. The mRNA molecule is small enough
to move out of the nucleus through a pore in the nuclear
membrane.

 Translation—amino acids are assembled into a polypeptide by

a ribosome in the cytoplasm, based on the genetic code carried
by the mRNA.

nucleus
DNA

 Transcription
mRNA

cytoplasm polypeptide

 Translation

ribosome

Figure 27.2 Protein synthesis: transcription and translation

Link it
All body cells of an organism have the same DNA and thus the
In a multicellular organism, cells same genes. However, in any one type of cell, only some genes are
have specific functions because
they make different proteins. They ‘turned on’ (or expressed)—the rest are inactive. Once a gene is
make different proteins because expressed, the protein product of that gene is usually made.
different genes are expressed in Therefore, gene expression and protein synthesis are often
different cell types.
considered the same process.

messenger RNA RNA gene expression

27- 4 transcription
translation
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27 Molecular genetics

1. Transcription
Transcription is the synthesis of mRNA using part of the DNA as a
template. The process occurs in the nucleus through the following
steps (Figure 27.3):

 An enzyme called RNA polymerase binds to a region of the

DNA near the beginning of the gene to be transcribed. The
hydrogen bonds holding the two DNA strands of the double
helix break. A section of the DNA double helix unwinds, exposing
the bases of each DNA strand.

Only one of the two DNA strands is transcribed. This strand is

called the template strand because it acts as the template for
synthesizing mRNA. The other strand that is not transcribed is
called the coding strand.
Remember this
 As the RNA polymerase moves along the template strand, free
In DNA, A pairs with T, and G with RNA nucleotides in the nucleus pair with the exposed bases
C. In RNA, T is replaced by U, and
so A pairs with U, and G with C. following the rules of complementary base pairing. RNA
polymerase catalyses the joining of adjacent RNA nucleotides
to form an mRNA strand.

The base sequence of the mRNA is complementary to the

template strand, and is therefore identical to the coding strand
except for the fact that the mRNA has the base uracil (U)
instead of thymine (T).

 When the RNA polymerase reaches the end of the gene, it

separates from the DNA and releases the mRNA. The DNA
strands rewind to reform the double helix.

RNA polymerase

coding strand
 Free RNA nucleotides pair
with their complementary
bases on the template strand.

 The DNA strands rewind.

 The DNA double helix
unwinds.

mRNA template strand

direction of transcription
Figure 27.3 The process of transcription
RNA polymerase RNA
template strand 27- 5
coding strand
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27 Molecular genetics

Through transcription, every triplet code on the DNA template

strand is transcribed into a complementary sequence of three bases
on the mRNA, called a codon.

The mRNA produced by transcription then leaves the nucleus

through a pore in the nuclear membrane and enters the cytoplasm.

2. Translation
Link it
Translation leads to the synthesis of a polypeptide, with an amino
Ribosomes are made up of acid sequence determined by the series of codons on the mRNA.
ribosomal RNA (rRNA) and
proteins. They may be found free
The process occurs at ribosomes in the cytoplasm.
in the cytoplasm or bound to the
endoplasmic reticulum (ER) to a. The role of transfer RNA
form rough ER (see Chapter 3).
In addition to mRNA molecules and ribosomes, translation requires
another type of RNA, called transfer RNA (tRNA). tRNA molecules
act as carriers of amino acids, bringing specific amino acids to the
ribosomes.

There are different tRNA molecules in the cytoplasm. All have a

similar structure (Figure 27.4):

At one end of the tRNA molecule is a binding site for a specific
amino acid.

At the other end of the tRNA molecule is a sequence of three

unpaired bases, called an anticodon. It can pair with a
complementary mRNA codon.

binding site for a

specific amino acid

Remember this
tRNA is a small single-stranded
molecule twisted into the shape of
a clover leaf. Like mRNA, tRNA is
made in the nucleus through
transcription and then moves out
to the cytoplasm.

anticodon
Figure 27.4 Structure of a tRNA molecule

codon
27- 6 transfer RNA RNA
anticodon
 27 Molecular genetics

b. Synthesizing a polypeptide
first amino acid

 In the cytoplasm, a molecule of mRNA ribosome

binds to a ribosome. A tRNA carrying the
first amino acid binds to the start codon tRNA
(AUG) on the mRNA. Notice that the
tRNA has the complementary anticodon
UAC. The start codon signals the start of
translation. mRNA

start codon
peptide bond

 A second tRNA carrying another amino

acid binds to the second codon with its another tRNA binds
complementary anticodon. The two to mRNA

amino acids are joined together by a

peptide bond.

growing polypeptide
 The ribosome moves one codon further
along the mRNA. The first tRNA is first tRNA is
released to pick up another amino acid released

molecule and be used again. Another

tRNA binds to the third codon on the
mRNA, bringing in the third amino acid.
A peptide bond forms between the
second and third amino acids. The
process repeats, adding one amino acid
at a time to the growing polypeptide. direction of translation

 The ribosome reaches a stop codon on

the mRNA. There is no corresponding stop
tRNA for the stop codon (see the chart in codon
Figure 27.7 on the next page). Translation
is now complete. The mRNA and the
polypeptide will be released from the
ribosome.
Figure 27.5 The synthesis of a polypeptide during
translation

27- 7
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27 Molecular genetics

Usually, many ribosomes move simultaneously along the same

mRNA molecule. Therefore, many identical polypeptides can be
made within a short time.
polypeptide
released
growing polypeptide

ribosomes reading same

mRNA at the same time

mRNA
stop codon
start
ribosome
codon
x48,000 released

Figure 27.6 Ribosomes moving along the same mRNA molecule

Remember this
The chart below lists all of the 64 possible codons on mRNA. Each
You do not need to memorize the
codons for amino acids. You will codon corresponds to a specific amino acid or a stop signal.
always be given this information in
an exam question.

Second base Key:

U C A G Ala = Alanine

UUU UCU UAU UGU U Arg = Arginine

Phe Tyr Cys Asn = Asparagine
UUC UCC UAC UGC C
U Ser Asp = Aspartic acid
UUA UCA UAA (Stop) UGA (Stop) A
Leu Cys = Cysteine
UUG UCG UAG (Stop) UGG Trp G Gln = Glutamine

CUU CCU CAU CGU U Glu = Glutamic acid

His Gly = Glycine
CUC CCC CAC CGC C
C Leu Pro Arg His = Histidine
CUA CCA CAA CGA A
Gln Ile = Isoleucine
First base

Third base

CUG CCG CAG CGG G Leu = Leucine

AUU ACU AAU AGU U Lys = Lysine
Asn Ser
AUC Ile ACC AAC AGC C Met = Methionine
A Thr
Phe = Phenylalanine
AUA ACA AAA AGA A
Lys Arg Pro = Proline
AUG Met (Start) ACG AAG AGG G
Ser = Serine
GUU GCU GAU GGU U Thr = Threonine
Asp
GUC GCC GAC GGC C Trp = Tryptophan
G Val Ala Gly
Tyr = Tyrosine
GUA GCA GAA GGA A
Glu Val = Valine
GUG GCG GAG GGG G

Figure 27.7 The mRNA codons and the corresponding amino acids or signals they specify

27- 8
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27 Molecular genetics

c. Assembling a protein
The polypeptide produced by translation then coils and folds to
form a protein. Some proteins consist of more than one polypeptide.
The amino acid sequence of each polypeptide determines the
three-dimensional structure (conformation) of the protein
molecule.

coiling and combination of multiple

folding polypeptides

polypeptide

Figure 27.8 Formation of proteins folded polypeptide protein consisting of

from polypeptides (protein) four polypeptides

Different proteins have different fates:

Proteins made on free ribosomes in the cytoplasm stay in the

cell and will be used within it. They may be enzymes that
catalyse chemical reactions in the cell.

Proteins made on ribosomes attached to the rough ER are either

embedded in the cell membrane (e.g. channel proteins, carrier
proteins and receptor proteins) or exported out of the cell (e.g.
digestive enzymes, hormones and antibodies).

Key point
1. The base sequence of DNA in a gene serves as the genetic code that
determines the sequence of amino acids in a protein.
2. Protein synthesis involves two stages:
• Transcription—the genetic code of a gene is copied to an mRNA
molecule in the nucleus.
• Translation—amino acids are assembled into a polypeptide by a
ribosome in the cytoplasm, based on the genetic code carried by the
mRNA.

Clear your concepts

In our body, only cells that can produce a particular protein have the gene
coding for that protein.
Body cells are produced by the repeated mitotic cell divisions of the
fertilized egg, and so they all have the same set of genes. However, in any
one cell, only some genes are expressed to make proteins. For example, all
body cells contain the gene coding for the hormone insulin, but it is only
expressed in the insulin-secreting cells of the pancreas.

27- 9
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27 Molecular genetics

Checkpoint
1. Which of the following descriptions about transcription is correct?
A. tRNA pairs with ribosome to produce amino acids.
B. mRNA pairs with ribosome to produce proteins.
C. Free DNA nucleotides pair with DNA template strand to
produce DNA.
D. Free RNA nucleotides pair with DNA template strand to
produce mRNA.

HKDSEE Biology 2012 Paper 1 Section A Q18

2. If the base sequence on the coding strand of the DNA is AAC, which
of the following combinations correctly shows the mRNA codon and
the tRNA anticodon?
mRNA codon tRNA anticodon
A. AAC UUG
B. AAC TTG
C. UUG AAC
D. TTG AAC

HKDSEE Biology 2017 Paper 1 Section A Q9

Learning objective 27.2 Mutations

• Distinguish between gene
mutations and chromosome
mutations A. What is a mutation?
• Understand the causes of
mutations A mutation is a sudden and permanent change in the DNA of an
organism. Mutations can be broadly classified into two types: gene
mutations and chromosome mutations.

Flipped classroom 1. Gene mutations

Gene mutations
e-aristo.hk/r/ A gene mutation is a change in the base sequence of DNA in a
bioccflip2702.e
gene. It can cause a change in the amino acid sequence of a protein,
and subsequently may affect the phenotype of an organism.

mutation
27- 10 gene mutation
chromosome mutation
 27 Molecular genetics

Gene mutations can occur by insertion, deletion, substitution or

inversion of bases (Table 27.1).

DNA template strand mRNA Amino acid sequence

Normal TAC TAG AAA CCG GTA AUG AUC UUU GGC CAU Met Ile Phe Gly His

Insertion TAC TAG AAC ACC GGT A AUG AUC UUG UGG CCA U Met Ile Leu Trp Pro

Deletion TAC TAG AAC CGG TA AUG AUC UUG GCC AU Met Ile Leu Ala

Substitution TAC TAG AAA TCG GTA AUG AUC UUU AGC CAU Met Ile Phe Ser His

Inversion TAC TGA AAA CCG GTA AUG ACU UUU GGC CAU Met Thr Phe Gly His

Table 27.1 Types of gene mutations and their effects on the amino acid sequence

Because the genetic code is read as a triplet, an insertion or a

deletion of bases by a number other than a multiple of three will
change the grouping of bases (i.e. the reading frame is shifted).
Every codon after the point of mutation will be affected. This results
in a different sequence of amino acids, and thus a different protein.
The protein usually cannot function.

A substitution or an inversion of bases does not cause a reading

frame shift, but changes one codon for another. The effect on the
amino acid sequence varies:

If the new codon codes for the same amino acid, the amino acid
sequence will remain the same. This is called a silent mutation.

If the new codon codes for a different amino acid in the

polypeptide, the polypeptide may fold into a protein with a
different conformation. This protein may not function properly.

If the mutation changes the codon for an amino acid to a stop

codon, translation will come to a stop. The polypeptide produced
will be shorter than normal and probably cannot function.

insertion inversion silent mutation

deletion reading frame 27- 11
substitution
 27 Molecular genetics

Sickle-cell anaemia—a disorder caused by a single base

substitution
Remember this
Sickle-cell anaemia is a genetic disorder of the blood. It is caused
A haemoglobin molecule consists by a substitution of one base in the gene coding for a polypeptide
of four polypeptide chains: two
alpha (α) chains and two beta (β)
chain in haemoglobin. As shown in Figure 27.9, base T is replaced
chains. by base A at one position on the DNA template strand. This causes
a change in the mRNA codon (from GAG to GUG), resulting in a
different amino acid (valine instead of glutamic acid) in the
polypeptide.

Normal DNA template strand

mRNA transcribed

Polypeptide of haemoglobin Thr Pro Glu

Mutated DNA template strand

mRNA transcribed

Polypeptide of haemoglobin Thr Pro Val

Figure 27.9 Gene mutation in sickle-cell anaemia

The change in a single amino acid in the polypeptide significantly

alters the conformation and properties of haemoglobin. In conditions
of low oxygen concentrations, the abnormal haemoglobin molecules
form long fibres within red blood cells. This causes the red blood
cells to become sickle-shaped (Figure 27.10).

normal sickle-shaped

x5000
Figure 27.10 SEM showing normal and sickle-shaped red blood cells

sickle-cell anaemia
27- 12
 27 Molecular genetics

sickle-shaped cells Sickle-shaped cells do not carry oxygen around the body as
sticking together
efficiently as normal red blood cells. They tend to stick together,
blocking blood vessels (Figure 27.11). This prevents normal blood
flow and can cause organ damage.

Moreover, sickle-shaped cells are more likely to be damaged and

have a shorter lifespan than normal red blood cells. The patients do
Figure 27.11 Sickle-shaped
red blood cells can block blood not produce red blood cells fast enough to replace dead red blood
vessels cells. This leads to anaemia.

Worked example 27.1

The sequence of bases on a segment of mRNA is shown below.

CGGAUGUACCGA
(a) Give the sequence of bases on the template DNA strand from which the mRNA was transcribed.
(1 mark)
(b) The table below shows the mRNA codons for three amino acids and the stop signal.
Amino acid / stop signal mRNA codon
Met AUG
Tyr UAU UAC
Arg CGU CGC CGA CGG
Stop signal UAA UAG

(i) Give the sequence of amino acids in the polypeptide coded for by this segment of mRNA.
(1 mark)
(ii) The effects of two different gene mutations on the base sequence of the mRNA are shown
below.
Mutation 1 CGGAUGUACCGC
Mutation 2 CGGAUGUAACGA
Use the information in the table to explain the effect of each mutation on the resulting
polypeptide. (4 marks)

Solution
Reminder
(a) GCCTACATGGCT ................................................................. (1)
The base sequence of the template
(b) (i) Arg – Met – Tyr – Arg ...................................................... (1) strand is complementary to that of
the mRNA molecule.
(ii) Mutation 1 causes no change to the polypeptide ............. (1)
because both the codes before and after the mutation code
Reminder
for the same amino acid (Arg). ...................................... (1)
The genetic code is degenerate, i.e.
Mutation 2 changes a codon for an amino acid to a stop some codons code for the same
codon. ........................................................................... (1) amino acid.

The polypeptide formed will be shorter than normal. .... (1)

27- 13
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27 Molecular genetics

2. Chromosome mutations
A chromosome mutation is a change in the structure or number of
chromosomes. Unlike a gene mutation which affects one gene only,
chromosome mutations affect large segments of DNA containing
many genes.

a. Changes in the structure of chromosomes

There are four main ways that the structure of a chromosome may
be changed: deletion, duplication, inversion and translocation
(Figure 27.12). These changes most often arise from errors during
the crossing over of chromosomes in meiosis.

a Deletion—a segment of a chromosome

(along with its genes) is lost.

break

break

b Duplication—a segment of a
chromosome is inserted to the
homologous chromosome,
resulting in duplication of genes.

break

break
c Inversion—a segment of a chromosome
breaks off and rotates through 180°
before rejoining. The order of genes is
reversed.

break
d Translocation—a segment of a
chromosome breaks off and becomes
attached to a non-homologous
chromosome. Some genes are
exchanged between the chromosomes.

break

Figure 27.12 Deletion, duplication, inversion and translocation in chromosomes

deletion translocation
27- 14 duplication
inversion
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27 Molecular genetics

b. Changes in the number of chromosomes

Sometimes, the members of a pair of homologous chromosomes
fail to separate during meiosis I and they end up in the same
gamete. This phenomenon is called non-disjunction. This results in
gametes with an abnormal number of chromosomes—some have
both members of a homologous chromosome and some have
neither.

Alternatively, sister chromatids may fail to separate during meiosis

II. The results are similar.

diploid cell
Members of a pair (2n)
of homologous
chromosomes fail
to separate Meiosis I

Sister
chromatids
Meiosis II fail to
separate

abnormal abnormal
abnormal gametes with abnormal gametes with normal gametes gamete gamete
an extra chromosome a chromosome missing (n) (n + 1) (n - 1)
(n + 1) (n - 1)

Figure 27.13 Non-disjunction during gamete formation (only a pair of homologous chromosomes is shown)

If a gamete having an abnormal number of chromosomes fuses

with a normal gamete during fertilization, the resulting zygote will
also have an abnormal number of chromosomes. Through mitosis,
the mutation will be passed on to all the cells derived from the
zygote.

Zygotes that lack an entire chromosome rarely survive. Zygotes

that have an extra chromosome may survive and develop, but the
individuals usually develop a syndrome of disorders caused by the
abnormal number of genes.

non-disjunction
27- 15
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27 Molecular genetics

Down syndrome
Down syndrome is caused by non-disjunction of chromosomes 21
during gamete formation in a parent. If a gamete containing an
extra copy of chromosome 21 fuses with a normal gamete during
fertilization, the resulting zygote will have three copies of
chromosome 21 (Figure 27.14). The zygote will develop into an
individual with Down syndrome. There will be an extra copy of
chromosome 21 in each of the body cells (Figure 27.15). This
condition is called trisomy 21.

abnormal ovum
(n + 1)

1 2 3 4 5
fertilization

6 7 8 9 10 11 12

zygyote
13 14 15 16 17 18
(2n + 1)

normal sperm 19 20 21 22 X Y
(n)
Figure 27.14 Formation of a zygote with an extra copy Figure 27.15 Karyotype of a male with Down
of chromosome 21 (only chromosomes 21 are shown) syndrome

In most cases, the extra copy of chromosome 21 comes from the

mother’s ovum. The cause of the non-disjunction is still unknown,
but the risk increases with the mother’s age.

People with Down syndrome usually have a lifespan shorter than

normal. They also have varying degrees of physical and mental
disabilities. However, with proper care, they can grow up healthily
and live a productive life in society.

Surf the net

Visit the website below to learn
more about Down syndrome.
e-aristo.hk/r/
bioccstn2701.e

Figure 27.16 People with Down syndrome have distinctive facial

features, such as a round face and a flattened nose bridge.
Down syndrome
27- 16 trisomy
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27 Molecular genetics

Taking it further
Sex chromosome mutations
Chromosome mutations can also occur in sex chromosomes. An abnormal number of sex chromosomes does not
usually affect survival, but can affect sexual development. Below are two examples.

Turner syndrome Klinefelter syndrome

Turner syndrome affects females. It results when there is Klinefelter syndrome affects males. It results when there
only one X chromosome (i.e. the genotype is XO). The is an extra X chromosome (i.e. the genotype is XXY).
ovaries do not develop properly, and so these females Insufficient production of the sex hormone testosterone
do not produce enough sex hormones to become interferes with sexual development and can result in
sexually mature or develop secondary sexual enlarged breasts, small testes and infertility.
characteristics. They are typically infertile.

1 2 3 4 5 1 2 3 4 5

6 7 8 9 10 11 12 6 7 8 9 10 11 12

13 14 15 16 17 18 13 14 15 16 17 18

19 20 21 22 x y 19 20 21 22 x y

 Karyotype of a female with Turner syndrome  Karyotype of a male with Klinefelter syndrome

B. Causes of mutations
Spontaneous mutations are mutations that occur naturally and
randomly with no outside causes. They may arise from errors in
DNA replication. The rate of spontaneous mutations varies among
genes and organisms, but is generally low.

Induced mutations are mutations caused by exposure to mutagens

(i.e. agents that can increase the rate of mutations). Certain
chemicals and radiations are mutagens (Table 27.2 on the next
page).

spontaneous mutation Turner syndrome

induced mutation Klinefelter syndrome 27- 17
mutagen
 27 Molecular genetics

Example Possible source

Chemical Nitrous acid Food preservatives

mutagen
Tar Cigarette smoke

Dioxin Industrial waste

Radiation Ultraviolet radiation Sunlight

X-rays Medical imaging

Gamma rays Radiotherapy

Table 27.2 Examples of mutagens and their sources

Some chemicals affect DNA by changing the structure of the bases

and causing them to pair with the wrong base. Some other chemical
mutagens have structures that resemble nucleotides, so they can
be incorporated into the DNA molecule and cause errors in DNA
thymine
replication.

Ultraviolet (UV) radiation can change the structure of DNA and

interfere with DNA replication (Figure 27.17). Ionizing radiations
(e.g. X-rays and gamma rays) can damage DNA molecules directly,
or they can ionize other molecules (e.g. water) to form free radicals.
Free radicals are highly reactive. They can cause DNA damage and
Figure 27.17 UV radiation can give rise to mutations.
cause adjacent thymine bases
to bind to each other,
preventing normal base
pairing. Extras: Do you know ...
Some viruses and bacteria are sources of mutations
Some viruses can induce mutations in host cells. For example, human
papillomavirus (HPV) infections increase the rate of mutations that give rise to
cancer (e.g. cervical cancer).
Some bacteria can cause DNA damage. For example, Helicobacter pylori
attacks the cells lining the stomach, damaging DNA molecules and increasing
the rate of mutations.

x2500
 HPV vaccines reduce the risk of  Helicobacter pylori
cervical cancer associated with HPV
infections.

nitrous acid Gamma ray

27- 18 asbestos free radical
dioxin
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27 Molecular genetics

C. Effects of mutations
Mutations can occur in any cells:

Mutations that occur in body cells (somatic cells) of an organism

are not passed on to the offspring. Only daughter cells derived
from the mutated cells are affected.

Mutations that occur in gametes or gamete-producing cells can

be passed on to the offspring. The mutated DNA will occur in
every cell of the offspring.

Mutations can be harmful to an organism:

A gene mutation can result in a change in the conformation

and, subsequently, the function of the protein. Many diseases
are caused by proteins that do not function properly.

Mutations that result in a significant loss of genetic material

(e.g. deletion of a segment of a chromosome) are most likely
lethal.

However, the majority of mutations are neutral. A gene mutation

may cause no change in the phenotype of an organism if:

it occurs in a non-coding region of the DNA. Such a region does

not code for proteins.

it is a silent mutation, usually a substitution or inversion.

Although the triplet code has changed, it still codes for the same
amino acid, and so the amino acid sequence of the protein
remains unchanged.
Link it the changes of amino acids do not change the shape of the
If a gene is altered by a change in protein sufficiently, so that the protein can still function properly.
its base sequence, it becomes
another version of the same gene. the mutated allele is recessive to the normal allele and
It is an allele of the gene (Chapter
26).
therefore is not expressed in the heterozygous condition.

If a mutation does cause a change in the phenotype, but the change

gives no particular advantage or disadvantage to the organisms,
then the effect is also regarded as neutral.

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27 Molecular genetics

Sometimes mutations are beneficial to organisms. Mutations

provide a source of genetic variations, and may give rise to new
characteristics that help organisms become better adapted to a
particular environment. The well-adapted organisms have a better
Link it
chance to survive and reproduce—they pass on their genes to the
Natural selection and evolution next generation. This is natural selection, which is the mechanism
will be discussed in Chapter 31.
of evolution.

Extras: Health issue

Mutations and cancer
Mutations can cause cells to grow and divide out of control, giving rise to
cancer. However, cancer does not result from a mutation in a single gene.
Instead, the development of a cancer involves mutations accumulated in
several genes. The process of accumulating mutations can take many years.
This explains why cancer is more common in older people.
Many mutagens are carcinogens (i.e. cancer-causing). For example, exposure
to UV radiation can cause mutations that lead to skin cancer. Avoiding
mutagens can help reduce the risk of developing cancer.

 Use sunscreen to reduce the amount  Avoiding tobacco can reduce the
of UV radiation reaching the skin. risk of lung cancer.

STSE connections
Uses of induced mutations
Induced mutations may be useful in science and agriculture. Scientists induce
mutations to study how genes normally function or to create new strains of
organisms. For example, mutated strains of Penicillium that can produce a
higher concentration of penicillin have been created. Plant breeders also
induce mutations to create new varieties of crops with improved characteristics.

 Rio Red grapefruits (left) and several type of rice (right) have been created by
using radiation to induce mutations.

natural selection
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All answers TE
27 Molecular genetics

Key point
1. A mutation is a sudden and permanent change in the DNA of an
organism.
2. A gene mutation is a change in the base sequence of DNA in a gene.
3. A chromosome mutation is a change in the structure or number of
chromosomes.
4. Exposure to mutagens can increase the rate of mutations.

Checkpoint
The following events may occur in a cell after exposure to radiations.
I. A change in protein conformation
II. A change in amino acid sequence
III. A change in cellular activity
IV. A mutation
What is the correct sequence of the events?
A. III  IV  II  I
B. III  II  IV  I
C. IV  II  I  III
D. IV  I  II  III

STEM activity 27.1

Using DNA as a data storage
DNA is an ideal genetic material because it is stable, can store large amounts
of genetic information and replicate. Scientists are exploring the use of DNA
to store digital data—pictures, videos, texts, etc. In this activity, you will
design DNA base sequences to encode information for your classmates to
decode.
e-aristo.hk/r/bioccstem.e

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27 Molecular genetics

Article reading

The Fukushima nuclear

accident and mutations
In 2011, a nuclear accident took place at a nuclear
plant in Fukushima of Japan. The release of radioactive
materials had a negative impact on the wildlife there.

Some butterflies in Fukushima were found to have

smaller wings and irregularly developed eyes. By
breeding these butterflies, scientists noticed that the
offspring showed abnormalities, such as malformed
antennae, that had not been observed in the parents.
It is believed that these abnormalities result from
mutations induced by high levels of radioactive
▲ Studies found that mutation rates were
materials. much higher among the pale grass blue
butterfly in Fukushima.

Questions

1. How do mutations cause changes in the phenotype of an organism? (3 marks)

2. What type of mutations can be passed on to the offspring? (1 mark)

3. Why are some abnormalities observed in the butterfly offspring but not in their parents?
(2 marks)

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 27 Molecular genetics

e-dictionary
Key terms e-aristo.hk/r/
bioccedict.e

anticodon 反密碼子 p.6 messenger RNA (mRNA) 信使 RNA p.4

chromosome mutation 染色體突變 p.10 mutagen 誘變劑 p.17

codon 密碼子 p.6 mutation 突變 p.10

degenerate code 簡併密碼 p.3 non-disjunction 不分離現象 p.15

deletion 缺失 p.11,14 sickle-cell anaemia 鎌狀細胞性貧血 p.12

Down syndrome 唐氏綜合症 p.16 spontaneous mutation 自發突變 p.17

duplication 複製 p.14 substitution 取代 p.11

gene expression 基因表達 p.4 transcription 轉錄 p.4

gene mutation 基因突變 p.10 transfer RNA (tRNA) 轉移 RNA p.6

induced mutation 誘發突變 p.17 translation 轉譯 p.4

insertion 插入 p.11 translocation 易位 p.14

inversion 倒位 p.11,14 triplet code 三聯體密碼 p.3

Summary

From DNA to proteins

27.1

1. The base sequence of DNA in a gene serves as the genetic code that determines the sequence of amino
acids in a protein. The genetic code has the following features:

• It is a triplet code. Three consecutive bases on a DNA strand code for one amino acid.
• It is a degenerate code. More than one triplet code may code for the same amino acid.
• It is read in a continuous manner.
• It is non-overlapping.
• It is universal. The same triplet code codes for the same amino acid in all species.

2. Protein synthesis involves two stages: transcription and translation.

Transcription Translation

Site of occurrence Nucleus Cytoplasm (ribosome)

Template used One of the DNA strands of a gene mRNA

Base pairing Bases on the DNA template strand pair Codons on mRNA pair with anticodons
with bases on free RNA nucleotides on tRNA

Raw material RNA nucleotides Amino acids

Product mRNA Polypeptides

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 27 Molecular genetics

Mutations
27.2

3. A mutation is a sudden and permanent change in the DNA of an organism.

4. A gene mutation is a change in the base sequence of DNA in a gene.

Gene mutation Description Results

Insertion One or more bases are added to the Every codon after the point of mutation
sequence. is changed (i.e. there is a frame shift).
The amino acid sequence after the point
Deletion One or more bases are lost from of mutation is also changed. The
the sequence. resulting protein is usually non-
functional.

Substitution One or more bases are replaced by The amino acid sequence may or may
others. not change because more than one
codon can code for the same amino
Inversion A sequence of bases is reversed. acid.

5. Sickle-cell anaemia is caused by a substitution of a base in the gene coding for a polypeptide chain in
haemoglobin.

6. A chromosome mutation is a change in the structure or number of chromosomes.

Chromosome mutation Description Results

Deletion A segment of a chromosome is lost. Some genes are lost.

Duplication A segment of a chromosome is Some genes are duplicated.

inserted to the homologous
chromosome.

Inversion A segment of a chromosome breaks The order of the genes is reversed.

off and rotates through 180° before
rejoining.

Translocation A segment of a chromosome breaks Some genes are exchanged between the
off and becomes attached to a two non-homologous chromosomes.
non-homologous chromosome.

7. Down syndrome is a condition in which a person has an extra copy of chromosome 21 in the body cells. It
is caused by non-disjunction of chromosomes 21 during gamete formation in a parent.

8. Spontaneous mutations are mutations that occur naturally and randomly with no outside causes.

9. Induced mutations are mutations caused by exposure to mutagens.

10. Mutagens are agents that can increase the rate of mutations. These include certain chemicals and
radiations.

11. The effects of mutations can be harmful, neutral or beneficial.

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27 Molecular genetics

Concept map
Complete the following concept map to review the key points of this chapter.

affects
Protein synthesis

involves two stages

transcription
induce
Mutation

occurs in produces occurs on produces occurs at

two levels

nucleus polypeptides
in the
coil and fold
cytoplasm to form

gene chromosome

Time allowed: 40 minutes

Self quiz Total score: 30 marks

Level 1: Understanding basic concepts (8 marks, 1 mark for each multiple-choice question)
Answer
1. The diagram below shows an overview of protein (a) Identify A to D. (4 marks)
synthesis in a eukaryotic cell.
(b) The following describes the events occurring
during protein synthesis. Arrange them in the
DNA correct order by placing numbers 1 to 6 in the
boxes provided. (1 mark)
The mRNA molecule binds to a ribosome.
nucleus cytoplasm The DNA double helix unwinds.
C Free nucleotides bind with complementary
bases of the template strand to form an
A mRNA molecule.
tRNA molecules carrying specific amino
amino acids bind to the mRNA molecule.
acid
B The mRNA molecule leaves the nucleus
and enters the cytoplasm.
A peptide bond is formed between
D adjacent amino acids.

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27 Molecular genetics

2. The tRNA anticodon for the sequence CAT on the 4. A person with Down syndrome has 47 instead of
coding strand of DNA is 46 chromosomes in each body cell. What is the
cause of this?
A. CAT. B. GUA.
A. Random alignment of chromosomes in the
C. CAU. D. GTA.
formation of gametes
3. What base sequence could be formed by the B. Gene mutation in the formation of gametes
deletion and substitution of bases from the
sequence GGACTCCTC? C. Non-disjunction in the formation of gametes
D. Fertilization of an ovum by two sperms
Deletion of a triplet Substitution of a base
A. GGACTCC GGACUCUCC
B. GGACTC GGTCTCCTC
C. GACTCCC GCGATCCTC
D. CTCCTC GGACTCCTCC

Level 2: Applying concepts (18 marks)

5. (a) Complete the table below concerning three types of nucleic acid. (4 marks)

Nucleic acid
DNA mRNA
Feature
Molecular size 50–250 million base pairs 75–3000 nucleotides 70–80 nucleotides

Shape double helix clover shape

Five-carbon sugar ribose ribose

Nitrogenous bases A, G, C, U A, G, C, U

Answer
(b) Explain the difference in length between DNA and mRNA molecules. (2 marks)

Answer
6. (a) Describe how cells use the information in a gene to make a protein. (6 marks)

(b) CFTR is a transmembrane protein consisting of 1480 amino acids.

Answer
(i) What is the minimum number of base pairs in the CFTR gene? Explain your answer. (2 marks)
(ii) People with cystic fibrosis have a mutation in the CFTR gene, which results in a defective CFTR protein
Answer
with one amino acid missing. Name this type of gene mutation. (1 mark)
Answer
(iii) Name one factor which may increase the frequency of gene mutations. (1 mark)
(iv) The diagrams below show the structure of a normal CFTR protein and that of a defective CFTR protein.
The normal CFTR protein has a sugar molecule attached to it which makes it functional.

Normal CFTR protein Defective CFTR protein

sugar molecule
With reference to the diagram, suggest why the defective CFTR protein, with one amino acid missing, is
Answer
not functional. (2 marks)

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 27 Molecular genetics

Level 3: Building a better answer (4 marks)

7. Read the following question and student A’s answer. Re-write and improve the answer based on the teacher’s
comment.

Question
Explain, in terms of protein synthesis, why some mutations are harmful while others are harmless. (4 marks) Answer

Student A’s answer

A gene mutation such as a substitution may result in only one different amino acid in the
polypeptide. ✔
If the different amino acid does not affect the structure of the protein, it will not affect the
functioning of the protein. ✔

Teacher's comment
Although the student has the right idea about the effect of mutation on protein structure, there is no reference
to codons. The answer makes no mention of the different effects of addition or deletion of bases. Only two
marks are awarded here.

Answers are available on p. A2. If you miss any of the questions, review the relevant section(s) again.

Question 1 2 3 4 5 6(a) 6(b) 7

Section(s) 27.1 27.1 27.2 27.2 27.1 27.1 27.2 27.2

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 TE All answers
27 Molecular genetics

Exam practice

Multiple-choice questions
Section 27.1 Section 27.2
1. What substances are involved in the process of 4. A DNA mutation is shown below.
transcription?
AAGCGCAAT
A. DNA and mRNA
B. DNA and ribosome
AAGCCGAAT
C. mRNA and tRNA
D. mRNA, tRNA and ribosome This type of mutation is called

A. a deletion.
2. Which of the following statements about codons is B. an insertion.
correct?
C. a substitution.
A. A codon may consist of bases A, C, G or T.
D. an inversion.
B. A codon is a triplet of bases on transfer RNA.
C. Most amino acids are coded by more than one 5. One type of gene mutation involves a base
codon. substitution.
D. All codons code for amino acids.
Original DNA sequence:
HKDSEE Biology 2013 Paper 1 Section A Q11 GAC TGA GGA CTT CTC TTC AGA
Mutated sequence 1:
3. The following table shows six codons and the GAC TGA GGA CAT CTC TTC AGA
corresponding amino acids translated from these
codons: Mutated sequence 2:
GAC TGA GGA CTC CTC TTC AGA
mRNA codon AAG CUA CCU
mRNA codons for valine GUU GUC GUA GUG
amino acid lysine leucine proline
mRNA codons for glutamic acid GAA GAG

mRNA codon GUA GAU CAU

amino acid valine aspartic histidine What are the consequences of the base
acid substitutions in the two new sequences of DNA?

A. Both are mutations that would result in different

The sequence of amino acids in a certain part of a polypeptides.
polypeptide is shown below:
B. Sequence 2 would result in a changed
– lysine – proline – histidine – aspartic acid – polypeptide but sequence 1 would not.

Which of the following is the correct sequence of C. All three DNA sequences would translate into
the nucleotides on the non-template DNA (coding the same polypeptide.
DNA) for this part of the polypeptide? D. Only the original DNA and sequence 2 would
translate into the same polypeptide.
A. AAGCCTCATGAT
B. TTCGGAGTACTA IBDP Higher Level Biology Paper 1 (TZ2)
May 2013 Q11
C. AAGCCUCAUGAU
D. UUCGGAGUACUA

HKDSEE Practice Paper Biology Paper 1

Section A Q9

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27 Molecular genetics

6. What is the cause of sickle-cell anaemia? 8. Males affected by Klinefelter syndrome may have
two X chromosomes and a Y chromosome (XXY).
A. A base in the haemoglobin gene is substituted. This condition arises from
B. Errors occur in the translation of mRNA.
A. independent assortment.
C. Valine is replaced by glutamic acid in the
B. sex-linkage.
polypeptide chain of haemoglobin.
C. crossing over.
D. Homologous chromosomes fail to separate
during meiosis. D. non-disjunction.

7. During pregnancy, amniotic fluid containing foetal 9. Some fruit flies are subjected to radiation in a
cells can be obtained for karyotyping. This helps to laboratory and are allowed to reproduce. The
determine whether the foetus offspring produced show new characteristics, such
as white eyes. This is caused by
(1) is male or female.
A. continuous variation.
(2) has Down syndrome or not.
B. discontinuous variation.
(3) is a carrier of Sickle-cell anaemia.
C. spontaneous mutation.
A. (1) and (2) only
D. induced mutation.
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)

HKDSEE Biology 2015 Paper 1 Section A Q26

Short questions

Section 27.1
Answer
10. Give four ways in which transcription differs from translation. (4 marks)

11. Read the following passage.

A method called in vitro translation is often used by scientists to produce proteins in the laboratory. The method
uses extracts from animal cells, plant cells or bacteria. These are chosen because they have high levels of protein
synthesis. The cells are treated so that the cell walls, if present, and cell membranes are broken down and then
treated so that any of the cell’s own DNA and mRNA are destroyed. When mRNA from any source is added to these
extracts, it will be translated into the corresponding protein.

(a) Explain why:

Answer
(i) the cells are chosen on the basis of their high level of protein synthesis. (2 marks)
Answer
(ii) the cell walls (if present) and cell membranes need to be broken down. (1 mark)
Answer
(iii) the cell’s own mRNA needs to be destroyed. (1 mark)
Answer
(iv) mRNA from any source can be translated in any type of extract. (2 marks)
(b) Scientists usually find that the method of in vitro translation is less efficient than in vivo translation,
Answer
which occurs in cells. Suggest a reason for this. (1 mark)

Cambridge International AS & A Level Biology 9700 Paper 23 Q2(a),(c) June 2013

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 All answers
27 Molecular genetics

Section 27.2
12. The table below shows the sequence of bases on a segment of a DNA strand and the effects of four different gene
mutations.

Base sequence of the DNA strand

Original AGCTTACG
Mutation 1 AGCTTAGC
Mutation 2 AGCATACG
Mutation 3 ACGCTTACG
Mutation 4 AGCTACG

Answer
(a) (i) Identify gene mutations 1 and 2. (2 marks)
(ii) Mutations 3 and 4 would probably cause the greatest changes in the structure of the protein coded for by
Answer
the original DNA strand. Explain why. (2 marks)
Answer
(b) Certain chemicals can induce mutations. Give one example of such a chemical. (1 mark)

13. Table 1 gives the mRNA codons for six different amino acids.

Table 1

Amino acid mRNA codon

Ala GCU GCC GCA GCG
Gly GGU GGC GGA GGG
Phe UUU UUC
Pro CCU CCC CCA CCG
Ser UCU UCC UCA UCG AGU AGC
Thr ACU ACC ACA ACG

A section of DNA contained the sequence of bases given in Table 2.

Table 2

DNA bases C C A C G T A A G T G G C C G
mRNA bases
Amino acids

(a) (i) Complete Table 2 to show the sequence of bases in the mRNA transcribed from this section of DNA.
(1 mark)
(ii) Use information from Table 1 to complete the sequence of amino acids coded for by this section of DNA in
Table 2. (1 mark)
(b) (i) A substitution mutation occurred in this section of DNA. The base T, in the grey box, was replaced by the
Answer
base C. Explain why this had no effect on the sequence of amino acids. (2 marks)
(ii) A deletion mutation occurred in which the base T, in the grey box, was lost from the DNA. What effect
Answer
would this deletion have on the sequence of amino acids? (1 mark)

AQA GCE AL Human Biology Unit 4 Jun 2014 Q4

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27 Molecular genetics

14. In humans, three copies of chromosome number 21 result in the medical condition Down syndrome. The condition
arises when a chromosome mutation causes two copies of chromosome number 21 to occur in an egg. When this
egg is fertilized with a normal sperm, a zygote is produced with the 47 chromosomes characteristic of Down
syndrome.
Answer
(a) Name the type of chromosome mutation involved in Down syndrome. (1 mark)
(b) There is a close positive correlation between the incidence of Down syndrome and the age of the mother at the
time of birth. The graph below shows the relationship between the age of the mother and the risk of having a
baby with Down syndrome.
3.5

3.0

2.5

2.0
Risk of Down syndrome
in live births (%) 1.5

1.0

0.5

0
20 25 30 35 40 45
Maternal age (years)

(i) Determine the risk of having a Down syndrome baby at age

30 _____________% live births
40 _____________% live births (1 mark)
Amniocentesis is used to diagnose whether a pregnancy is likely to produce a child with Down syndrome. This
is an invasive procedure which involves removal of fluid containing foetal cells from the womb. If this shows that
the developing foetus has Down syndrome, the parents are offered the option to terminate the pregnancy.
However, amniocentesis carries a 1% risk of miscarriage (loss of foetus). Only mothers over the age of 35 years
are routinely offered amniocentesis for Down syndrome.
(ii) Using the information provided, explain fully why only pregnant mothers over 35 years of age are normally
Answer
offered amniocentesis screening. (2 marks)
Answer
(iii) Most Down syndrome children are born to mothers under the age of 35 years. Suggest why. (1 mark)

CCEA GCE AL Biology Assessment Unit A2 2 Jun 2013 Q6

Structured questions

Section 27.1
15. The electron micrograph on the next page shows some structures of a human cell.
12
27 (a) Label A and B. (2 marks)
(b) Which stage of the cell cycle is shown in this photomicrograph? Give a reason to support your answer.
(2 marks)
(c) The cell was obtained from the pancreas. How do A and B work together such that this cell can perfrom its
function? (4 marks)

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 27 Molecular genetics

HKDSEE Biology 2017 Paper 1 Section B Q4

16. The hormone insulin is a relatively small protein. Researchers studying the production of insulin in the cells of the
pancreas noted that one of the early steps in this process was the formation of a polypeptide called preproinsulin.

Researchers noted that the formation of this polypeptide required repeated use of different types of molecule X,
shown below.

molecule X

anticodon

Answer
(a) (i) What is the name of molecule X? (1 mark)
Answer
(ii) How does molecule X play a role in the production of preproinsulin? (3 marks)
(b) The coding information in the DNA molecule for preproinsulin is initially transferred to another molecule
(molecule W). However, molecule W has a different nucleotide sequence from the coding section of the DNA
molecule.
Describe how molecule W is synthesized. (3 marks)Answer

VCAA VCE Biology Written Examination 2016 Section B Q6

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27 Molecular genetics

Section 27.2
17. In 2014, parts of West Africa were hit by an epidemic of Ebola Fever. Most people who caught the disease died.
The diagram below shows the virus which causes the disease.

RNA

secreted
glycoprotein

capsule
glycoprotein

(a) (i) This virus reproduces in the cytoplasm of the host cell. Using information from the diagram, suggest why
Answer
this virus does not have to enter the nucleus of cells in order to produce proteins. (2 marks)
(ii) Suggest the organelle in the infected cell which would complete the production of glycoprotein. (1 mark)Answer
(iii) Describe the process by which the genetic information of the virus would be translated into amino acid
Answer
sequences in its proteins. (4 marks)
(b) When Ebola viruses enter human cells one of their genes controls the production of a glycoprotein that is
immediately released.
The gene which codes for this glycoprotein is changed after infecting cells. An extra adenine is inserted at
position 1016.
The changed gene then controls the production of the second glycoprotein which forms part of the viral capsule.
Diagrams of the changed and original base sequences for the glycoproteins are shown below.

Changed base sequence

1010

1015

1020

1025

Nucleotide position number

Changed nucleotide sequence CUU GUU AUA AAA AAA AUA C

Changed amino acid sequence Leu Val

Original base sequence

1010

1015

1020

1025

Nucleotide position number

Original nucleotide sequence CUU GUU

Original amino acid sequence Leu Val

(i) Use the table on the next page to complete the changed amino acid sequence in the diagram above.
(1 mark)

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 27 Molecular genetics

Second

U C A G
Phe Ser Tyr Cys U
Phe Ser Tyr Cys C
U
Leu Ser STOP STOP A
Leu Ser STOP Trp G
Leu Pro His Arg U
Leu Pro His Arg C
C
Leu Pro Gln Arg A
Leu Pro Gln Arg G

Third
First

Ile Thr Asn Ser U

Ile Thr Asn Ser C
A
Ile Thr Lys Arg A
Met Thr Lys Arg G
Val Ala Asp Gly U
Val Ala Asp Gly C
G
Val Ala Glu Gly A
Val Ala Glu Gly G

(ii) Complete the original nucleotide sequence in the diagram. (1 mark)

(iii) Use the table again to complete the original amino acid sequence. (1 mark)
(iv) Explain why the original glycoprotein is smaller than the changed glycoprotein. (2 marks)Answer

WJEC GCE A Level Biology/Human Biology Module BY5 Jun 2016 Q4(a)–(b)

Answer
18. (a) (i) Explain the term chromosome mutation. (2 marks)
(ii) Chromosome mutations usually have much more serious consequences than single gene mutations.
Answer
Explain why. (1 mark)
(b) Turner syndrome is the most common chromosome mutation in females.
(i) A female affected by Turner syndrome may have an entire X chromosome missing. Explain how this
Answer
condition can arise. (3 marks)
In some forms of Turner syndrome, one of the pair of X chromosomes is damaged so that part of its structure is
missing. The diagram below shows a normal X chromosome and two forms of mutation in the X chromosome.
Normal X chromosome Mutation 1 Mutation 2
This section of X
chromosome is
missing; the affected
female may have a
short stature
This section of X
chromosome is missing;
the affected female may
have poor development
of the ovaries

Answer
(ii) Identify the type of these two mutations of the X chromosome. (1 mark)
Answer
(iii) Explain why mutations 1 and 2 result in different phenotypes. (2 marks)

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 27 Molecular genetics

Essays

Section 27.1
19. DNA is found in the nucleus but RNA is found in both the nucleus and cytoplasm. Account for this observation by
explaining the functions of the different types of nucleic acids found in cells. (10 marks)Answer

WJEC GCE A level Biology/Human Biology Module BY5 Jun 2015 Q7(a)

Section 27.2
Answer
20. Give an account of gene mutations and their effects on an organism. (10 marks)

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