2.

5 Changing the code through mutations

LEARNING INTENTION
At the end of this subtopic you will be able to describe the relationship between DNA, chromosomes and
mutations, and outline the factors that contribute to causing mutations.

2.5.1 What are mutations?

Errors or changes in DNA, genes or
FIGURE 2.42 Polydactly is often caused by a DNA mutation
chromosomes can have a variety of
consequences. These genetic mistakes are
called mutations.
Polydactyly (meaning many digits and related
to having more than ten fingers and toes) is
usually due to a DNA mutation.

Resources

Video eLesson Polydactyl cat (eles-2698)

2.5.2 DNA replication

DNA is very stable and can be replicated into exact copies of itself. This process is called DNA replication
and enables genetic material to be passed on unchanged from one generation to the next. DNA replication
begins with the ‘unzipping’ of the paired strands. A new complementary strand is made for each original DNA
strand. This results in the formation of two new double-stranded DNA
FIGURE 2.43 DNA replication molecules, each containing one new DNA strand and one original
is semi-conservative — a DNA strand. This process is vital in cell division, occurring during
combination of original (blue) interphase both at the start of mitosis and meiosis.
and new (purple) strands.
The process of DNA replication has a number of checkpoints to test
CG
for any mistakes that may be made, so that they can be corrected or
AT destroyed. Sometimes, however, the mistakes get through this screening
GC
TA process. When this happens, we say that a mutation has occurred.
AT
A T
C G
A 2.5.3 Mutagenic agents
GT C mutations    changes to DNA
A T Mutations can happen by chance or have a sequence, at the gene or
T A chromosomal level
CG
AT C particular cause.
TA DNA replication    process that
TA
CG • When the cause of the mutation cannot results in DNA making a precise
GC
AT
GC be identified it is called a spontaneous copy of itself
GC mutation. spontaneous mutation    a
AT
CG GC •  mutation of DNA that cannot be
TA AT When the cause can be identified it is
CG explained or identified
TA referred to as an induced mutation. induced mutation    a mutation of
TA
CG DNA in which the cause can be
AT TA A factor that triggers mutations in cells is called
GC CG identified
AT a mutagen or mutagenic agent.
GC mutagen    agent or factor that
can induce or increase the rate of
mutations
Arrows denote direction of synthesis.

94 Jacaranda Science Quest 10 Victorian Curriculum Second Edition

Examples of mutagenic agents include:
FIGURE 2.44 Mutations may result
• radiation, such as ultraviolet radiation, nuclear radiation and
in cancerous growths (tumours) within
X-rays your body, shown as foggy areas here.
• chemical substances, such as formalin, asbestos, tobacco and
benzene (which used to be common in pesticides)
• infectious agents, such as human papillomavirus (HPV).

As a result of the thinning of the ozone layer in the atmosphere, we are

exposed to increasing amounts of UVB radiation that can damage (or
mutate) our DNA. This can lead to the development of skin cancers.
Protective clothing and sunscreens can help reduce our exposure to this
dangerous, potentially mutagenic environmental radiation.

Resources

Video eLesson DNA and Hiroshima (eles-1781)

2.5.4 Errors in the code

Changes in the genetic code due to mutations may result in a particular protein not being made or a faulty version
being produced.
In some cases, the production of an essential enzyme may be impaired, disrupting chemical reactions and
resulting in the deficiencies or accumulation of other substances. This may cause the death of the cell and,
eventually, the organism.

FIGURE 2.45 Different types of mutations

Point mutation Inversion

TGCATTGCGTAGGC

TGCATTCCGTAGGC

Insertion Chromosome Fusion

TGCATTTAGGC
TGCATTCCGTAGGC

CCG

Genome duplication
Deletion

TGCATTCCGTAGGC

TGCATTTAGGC

Gene duplication

TOPIC 2 Getting into genes 95

Point mutations
Occasionally, errors can occur during DNA replication as DNA is being copied. This means that the
instructions carried by the code are not followed exactly.
This may be the result of:
• an incorrect pairing of bases
• the substitution of a different nucleotide
• the deletion or insertion of a nucleotide.

The mutation, where a single nucleotide is affected, is known as a point mutation. Such a point mutation can
change the genetic message by coding for a different amino acid, leading to the production of a different or
non-functional protein through the process of protein synthesis (as explored in subtopic 2.4). The deletion or
insertion mutations are often more serious because they can cause a frameshift, altering the groups of three
in which the sequence is usually read. This can have severe consequences for not just the phenotype of the
organism, but also its survival.
Just like changing letters in a word can change its meaning (as seen in table 2.2), changes in the DNA
sequence can change the meaning of the genetic code.

TABLE 2.2 Analogy of how mutations lead to a change in meaning from an initial message. The initial message
of ‘post’ is changed with the insertion, deletion, inversion or substitution of letters (shown in bold).

Original Insertion Deletion Inversion Substitution

post poster pot pots pest

Types of point mutations

Substitution mutations
One of the most common point mutations is that of a substitution mutation, in which one base is replaced
with another.
Substitution mutations can be one of three types:
• Missense mutations: where one amino acid is swapped for another (as seen in
sickle-cell anaemia, outlined in the case study). point mutation a mutation at
• Silent mutations: no change occurs in the amino acid. one particular point in the DNA
sequence, such as a substitution
• Nonsense: where an amino acid is changed to a STOP codon (causing a
or single base deletion or insertion
shortened protein).

CASE STUDY: Sickle-cell anaemia

Sickle-cell anaemia is a disease that is usually associated with a mutation in the gene that codes for one of
the polypeptides that make up haemoglobin in red blood cells. In this mutation, an adenine base is substituted
by a thymine base. The result is a phenotype of misshapen red blood cells that can clump together and block
blood vessels.

96 Jacaranda Science Quest 10 Victorian Curriculum Second Edition

 TABLE 2.3 Exploring mutations in sickle-cell anaemia

Normal red blood cell Sickle-cell red blood cell

Coding DNA sequence CTG ACT CCT GAG CTG ACT CCT GTG
(complementary)
Template DNA sequence GAC TGA GGA CTC GAC TGA GGA CAC
Complementary RNA CUG ACU CCU GAG CUG ACU CCU GUG
sequence
Amino acid sequence leu — thr — pro — glu leu — thr — pro — val
Phenotype of red blood Normal doughnut-shaped blood cell Sickle-shaped blood cell
cell

Frameshift mutations
Another type of mutation that may result from additions or deletions is a frameshift mutation. In this case,
every amino acid from that point is altered, leading to a completely different amino acid code.
Imagine the following sentence:
THE FAT CAT SAT
The insertion of one ‘letter’ completely changes the groups of three letters, so the sentence now reads:
THT EFA TCA TSA T
These mutations change the amino acid sequence drastically, because they alter all the codons after the
mutation. Examples of these mutations can be seen in table 2.4.

TABLE 2.4 Examples of frameshift mutations

Normal Insertion Deletion

Coding DNA sequence CTG ACT CCT CTA GAC TCC T CGA CTC CT
(complementary)
Template DNA sequence GAC TGA GGA GAT CTG AGG A GCT GAG GA
Complementary RNA sequence CUG ACU CCU CUA GAC UCC U CGA CUC CU
Amino acid sequence leu — thr — pro leu — asp— ser arg — leu

Chromosomal mutations
Point mutations relate to changes in the DNA sequence in genes; however, mutations can also involve
chromosomes. These changes may involve the addition or deletion of entire chromosomes, or the deletion,
addition or mixing of genetic information from segments of chromosomes. Some examples of disorders that
result from chromosome mutations are shown in table 2.5.

TOPIC 2 Getting into genes 97

 TABLE 2.5 Examples of human chromosome abnormalities (mutations). The risk of these mutations increases
with maternal age.

Chromosome abnormality Resulting disorder Incidence (per live births)

Extra chromosome number 21 Down syndrome 1 in 700

Missing sex chromosome (XO) Turner syndrome 1 in 5000

Extra sex chromosome (XXY) Klinefelter syndrome 1 in 1000

FIGURE 2.46 Two examples of karyotypes with chromosomal abnormalities

Down syndrome karyotype Klinefelter syndrome karyotype

2.5.5 Mutants unite!

Not all mutations are harmful. Some mutations can increase the
FIGURE 2.47 Heterochromaia
survival chances of individuals within a population, and hence
(different eye colours) often occurs in
the survival of their species. Other mutations (such as that seen in somatic cells and is not inherited.
figure 2.47) are neither harmful or beneficial to individuals who
possess them.

Spray resistance
Pesticides kill the majority of insects sprayed. Some insects within
the population, however, may survive because they possess slight
variations or mutations in their genes that give them resistance to the
pesticide. The mutated gene in the surviving insects is passed on to
their offspring, who gain that resistance too. While the insects without
the resistance die out, those with resistance increase in numbers.

Good for you, but not for me

When we look at natural selection as a mechanism for evolution in topic 3, we see how mutations can be a
very important source of new genetic material. While such mutations can be beneficial for the survival of the
species under threat, they are not necessarily beneficial to humans. The resistance of bacteria to antibiotics,
for example, has resulted in selection for antibiotic-resistant bacteria. This means we are unable to use these
antibiotics to treat diseases caused by these resistant bacteria, because the drugs are no longer effective.

Malaria and sickle-cell mutation

Malaria is a disease that is very common in many parts of Africa, Asia and South America. It is caused by a
parasite that is transmitted by a species of mosquito. When the mosquito bites an individual, the parasite then
grows in red blood cells of its human host. This disease is one of the main global causes of human disease-
related deaths.

98 Jacaranda Science Quest 10 Victorian Curriculum Second Edition

The mutation that results in sickle-cell anaemia can increase your resistance to malaria. If you are a carrier for
this trait, the parasite cannot grow as effectively in your red blood cells, which means you are less likely to die
from malaria than people in the population without the allele.

2.5.6 Not all mutations are inherited

Only mutations that have occurred in the germline cells such as the sex cells or gametes (sperm and ova) are
inherited. In sexually reproducing organisms, mutations that occur in somatic cells are not passed on to the
next generation.

Resources

eWorkbook Mutations (ewbk-5358)

Video eLesson Types of mutations (eles-4214)

Weblink Scientists warn against vitamins

Additional automatically marked question sets

2.5 Exercise
To answer questions online and to receive immediate feedback and sample responses for every question, go to
your learnON title at www.jacplus.com.au.

Select your pathway

LEVEL 1 LEVEL 2 LEVEL 3

Questions Questions Questions
1, 2, 6, 7 3, 4, 8, 10, 13 5, 9, 11, 12, 14

Remember and understand

1. State whether the following statements are true or false. Rewrite any false statements to make them true.
a. Mutations in both germline and somatic cells are passed on to the next generation.
b. Errors or changes in DNA, genes or chromosomes are called mutations.
c. Radiation (for example, ultraviolet radiation, nuclear radiation and X-rays) is not considered to be a mutagenic
agent.
d. Changes in the genetic code due to mutations may result in a particular protein not being made or a faulty
version being produced.
e. If you are a carrier for sickle-cell anaemia, the malaria parasite cannot grow as effectively in your red blood
cells, which means you are less likely to die from malaria than people in the population without the allele.
f. All mutations are harmful.
2. a. Name the process by which DNA makes copies of itself.
b. Explain why the model used to describe the process identified in (a) is called semi-conservative. Include a
diagram in your response.
c. Explain why it is important for DNA replication to produce exact copies of the original DNA.
3. a. Describe what is meant by the term mutagenic agent. Provide an example.
b. Distinguish between the terms spontaneous mutation and induced mutation.
c. Identify two disorders associated with chromosome mutations.
d. Describe two examples of mutations that can increase chances of survival.
4. Outline the relationship between sickle-cell anaemia and mutated DNA.

TOPIC 2 Getting into genes 99

 Apply and analyse
5. a. Explain the difference between a missense, nonsense and silent mutation.
b. An initial DNA template strand is as follows: AAA GCG TAC.
If the second adenine (A) is changed to a guanine (G), what type of mutation has occurred (refer back to the
genetic code on figure 2.38).
c. How does this type of mutation differ from a frameshift mutation? Which would be more likely have
significant effects?
6. Are mutations always detrimental? Provide an example to justify your response.
7. SIS Suggest why radiographers wear special protective clothing and use remote controls for taking X-rays.
8. SIS The karyotype shows an individual with Turner syndrome.

Turner syndrome karyotype

a. Describe how this karyotype differs from that of a normal human karyotype.
b. Suggest how this mutation may have resulted.
9. The karyotype in figure 2.46 shows an individual with Down syndrome.
a. Describe how this karyotype differs from that of a normal human karyotype.
b. Suggest how this mutation may have resulted.
Search online for Down syndrome research. Use your own knowledge and information found to answer the
following questions.
c. Suggest why the DSCR1 gene is of importance.
d. On which chromosome is the DSCR1 gene located?
e. Outline the advantage suggested by the research of possessing an extra copy of the DSCR1 gene.
10. SIS Research and report on one of the following topics.
• Antibiotic resistance and bacteria
• Pesticide resistance and insects
• Sickle cell anaemia, malaria and heterozygote advantage
• Antioxidant vitamins, brightly coloured vegetables and mutations
• Breast cancer and the BRCA gene

Evaluate and create

11. O
 ver the years, considerable debate has occurred over the use of taking vitamin supplements. James
Watson, one of the scientists who proposed the double helical structure of DNA, is of the opinion that taking
high doses of some vitamins can interfere with cancer treatment.
a. Access the Scientists warn against vitamins weblink and read the article.
b. Summarise the key points.
c. Use the internet to find evidence for or against James Watson’s claims.
d. Outline and justify your own opinion.
12.  SIS Some people claim that eating brightly coloured fruits and vegetables that are high in antioxidants can
reduce the occurrence of mutations within your body.
a. Research this claim and summarise your findings.
b. Formulate a relevant hypothesis that could be investigated scientifically, including identification of
independent, dependent and controlled variables.
c. To enable collection of reliable data to test your hypothesis, design an investigation that also addesses any
safety or ethical issues.
d. Describe results from your investigation that would support your hypothesis.

100 Jacaranda Science Quest 10 Victorian Curriculum Second Edition

 13.  SIS The following graph shows the frequency of 0.03
children being born with Down syndrome based on

Frequency of DS per live births

maternal age. 1
Carefully examine the graph. DS is abbreviated for 46
Down syndrome. 0.02
a. State the axis label of the
i. x-axis
ii. y-axis. 1
b. Suggest how you could improve the axis labels. 100
0.01
c. Suggest a title for the graph. 1
d. What do you think the fractions in the graph 1 1
290
2300 880
represent?
e. Formulate a hypothesis that would be relevant to
the graph. 0.00
f. Describe the pattern or trend. 20 25 30 35 40 45
g. Suggest an interpretation of the data. Age of mother
14.  SIS Carefully examine the provided graph on the rate
of sex-linked mutations in differing levels of radiation.  he rate of sex-linked mutations
T
a. State the axis label (including the units) of the in differing levels of radiation
i. x-axis ii. y-axis.
b. In the graph, identify the

Sex-linked mutations (rate per 104 gametes)

i. independent variable
ii. dependent variable.
1500 •
c. Suggest a title for the graph.
d. Explain what is meant by a sex-linked mutation. •
e. Use the graph to estimate the rate of sex-linked mutations at 2000
Roentgen units of radiation. 1000 •
f. Suggest a reason for the use of error bars on the graph. •
g. Formulate a hypothesis that would be relevant to the graph. •
h. Describe the pattern or trend. Incorporate the axis labels in your 500
description. • •
i. Suggest an interpretation of the data.
•
j. Propose a question that could be used to focus further relevant
research. 0
2000 4000 6000
Fully worked solutions and sample problems are available in your Radiation (Roentgen units)
digital formats.

2.6 Exploring patterns in the genome and genetic

sequences
LEARNING INTENTION
At the end of this subtopic you will be able provide examples of how advances in technology have enabled us to
sequence the human genome and explore patterns in our genetic sequences.

2.6.1 Who do you think you are?

You are incredibly special. Much of who and what you are is determined by genes. Genes determine many of
the traits and characteristics that make you, you.
As a result of rapid advances in technology, the time and cost of the sequencing of your genes has
dramatically reduced. As a consequence, huge amounts of genetic information is being generated.

TOPIC 2 Getting into genes 101