DNA

STRUCTURE OF NUCLEIC ACIDS

● 2 types of nucleic acids in humans:

1. DNA (deoxyribonucleic acid)

2. RNA (ribonucleic acid)

● consists of monomers called nucleotides

● each nucleic acid consists of a phosphate (P), sugar molecule (S) & a nitrogenous base

(NB)

BRIEF HISTORY OF THE DISCOVERY OF DNA

● 1952 - Rosalind Franklin & assistant Maurice Wilkins researched the structure of DNA

using x-ray diffraction images

● 1953 - Watson & Crick did independent research on DNA & upon seeing Franklin’s

images, they proposed a 3-D double helix model for DNA

● 1962 - Watson & Crick received a nobel prize for discovering DNA’s structure, Wilkins

received an award for his x-ray photography & Franklin died of cancer
LOCATION OF DNA
● DNA is found in a cell in 2 locations:

1. in the nucleus

2. outside the nucleus

1. in the nucleus:

○ referred to as nuclear DNA

2. outside the nucleus:

○ referred to as extra-nuclear DNA

○ 2 types of extra-nuclear DNA

- chloroplastic DNA, which is found in the chloroplasts of plant cells

- mitochondrial DNA, which is found in the mitochondria & is useful for tracing

ancestory

THE STRUCTURE OF DNA

● has a double helix structure that consists of monomers called nucleotides

● the nucleotides link to form long chains called polymers

● sugar is called deoxyribose sugar & is attached to a nitrogenous base

● phosphate & sugar molecules are attached by strong bonds to form polymers
● 4 types of nitrogenous bases:

1. adenine (A)

2. thymine (T)

3. cytosine (C)

4. guanine (G)

● nitrogenous bases are complementary & join speci cally:

- adenine & thymine

- cytosine & guanine

● pairing of bases means that 2 strands of DNA are joined together, which forms a long

ladder-like structure

● nitrogenous bases are held together by weak hydrogen bonds

● the ladder-like structure becomes coiled & is known as a double helix structure

● DNA strands wind around proteins called histones

double helix struxture

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 simple structure of DNA

ROLE OF DNA
● carries hereditary information in the form of genes

● genes are short sections of DNA which code for a speci c trait & determine the physical

characteristics & behaviour of an organism

● most DNA strands don’t code for anything & are known as non-coding DNA

● the importance of non-coding DNA is still being researched

● 3 main functions of DNA:

1. controls the functioning of cells

2. regulates the functioning of genes

3. passes on hereditary characteristics

RNA
● 3 types of RNA:

1. messenger RNA (mRNA)

2. ribosomal RNA (rRNA)

3. transfer RNA (tRNA)

● the 3 types of RNA are formed in the nucleus by DNA

● they perform different functions in different places in a cell

LOCATION OF RNA
● messenger RNA (mRNA) is formed in the nucleus & then enters the cytoplasm where it

attaches to ribosomes

● ribosomal RNA (rRNA) is found in ribosomes in the cytoplasm of the cell

● transfer RNA (tRNA) is found freely in the cytoplasm of the cell

THE STRUCTURE OF RNA

● RNA also consists of monomers (nucleotides), which link to form long chains (polymers)

● RNA is a single-stranded structure

● it’s not coiled

● sugar is called ribose sugar & is attached to a nitrogenous base

● phosphate & sugar molecules are attached to each other to form the long chains

(polymers)
● 4 types of nitrogenous bases:
adenine & uracil join
1. adenine (A) together

cytosine & guanine join

2. uracil (U) together

3. cytosine (C) they join together when

DNA is transcribed into
RNA
4. guanine (G)

simple structure of RNA

ROLE OF RNA
● the 3 types of RNA play an important role to protein synthesis

● each type plays a unique role to protein synthesis

COMPARISON BETWEEN DNA & RNA
● similarities:

1. contains sugar with a phosphate

2. contains adenine, cytosine & guanine

3. plays a role in protein synthesis

● differences:

DNA RNA

1. deoxyribose sugar 1. ribose sugar

2. double helix structure 2. single-stranded structure

3. nitrogenous base - thymine (T) 3. nitrogenous base - uracil (U)

4. only found in nucleus 4. found in nucleus, ribosomes &

cytoplasm

DNA REPLICATION
● DNA double helix structure unwinds
● the weak hydrogen bonds between nitrogenous bases are broken

● DNA strands unzip (separate)

● each original DNA strand serves as a template on which its complement is built

● free oating nucleotides build a DNA strand onto each of the original DNA strands by

attaching their complementary nitrogenous bases

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● this results in 2 identical DNA molecules

● each molecule consists of 1 original strand & 1 new strand

IMPORTANCE OF DNA REPLICATION

● it’s important for cell division (mitosis)

● it allows each chromosome to be copied, so that each new identical daughter cell

produced, contains the same no. & type of chromosomes

ERRORS THAT OCCUR DURING DNA

REPLICATION
● errors that occur during DNA replication may lead to mutations, such as a change in the

nitrogenous base sequence

● if the incorrect nitrogenous base attaches to the original strand & a nitrogenous base is

added or deleted, then the sequence of the nitrogenous bases changes on the new DNA

molecule, which results in a change in the gene structure

DNA PROFILING
● a pattern produced on x-ray lm

● consists of lines that are different lengths, thicknesses & are in different positions

● all individuals, except identical twins, have a unique DNA pro le

● DNA pro les are used to:

- identify crime suspects in forensic investigations

- prove paternity (father) & maternity (mother)

- determine the causes of genetic defects

- establish the compatibility of tissue types for organ transplants

● DNA pro ling is generally extremely reliable

● the interpretation & comparison of pro les should be done with caution bc:

- humans interpret the results, which means mistakes could be made

- the method of pro ling may be different in different labs, which produces inconsistencies

- only a small piece of DNA is used, so the pro le may not be 100% unique to a particular

individual

- it’s expensive & not easily accessible to those who can’t afford it

- it may reveal information about a person, which could be used against them in a

prejudicial way

a DNA profile for 3 different

individuals
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PROTEIN SYNTHESIS
● the process in which proteins are made is called protein synthesis

● proteins are made by linking different amino acids that are present in the cytoplasm of

cells

● there are 20 different amino acids & they combine in a large variety of combinations

● the no. & sequence of amino acids determines the type of protein formed

● the bond between amino acids is called a peptide bond

● the genes found in DNA contain the code which determines which type of protein will be

formed

● the smallest protein contains 50 amino acids linked together

● proteins generally contain 300+ amino acids

● 3 consecutive nitrogenous bases on the DNA strand is called the base triplet

● base triplets determine which amino acid will be placed into the protein & the sequence in

which the amino acids will be joined

● protein synthesis occurs in 2 stages:

1. transcription (stage 1)

2. translation (stage 2)
1. transcription:

○ occurs in the nucleus

○ DNA double helix unwinds

○ weak hydrogen bonds between the nitrogenous bases of DNA break

○ the DNA strand unzips

○ 1 strand acts as a template

○ this DNA template forms a complementary strand of messenger RNA (mRNA) by

using free RNA nucleotides in the nucleoplasm

○ the mRNA now contains the code for the protein that will be formed

○ 3 adjacent nitrogenous bases on mRNA are called codons

○ codons code for a speci c amino acid

○ mRNA moves out of the nucleus through a nuclear pore & into the cytoplasm, where

is attaches onto a ribosome

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2. translation:

○ occurs in the cytoplasm

○ transfer RNA (tRNA) in the cytoplasm has 3 adjacent nitrogenous bases called an

anti-codon

○ a mRNA’s codon is complementary to a tRNA’s anti-codon

○ each tRNA will carry a speci c amino acid

○ according to the codons on mRNA, tRNA will bring the required amino acid to the

ribosome

○ the amino acids are linked by a peptide bond to form the required protein

DNA = base triplets

mRNA = codons

tRNA = anti-codons
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● the diagram below shows the process protein synthesis in a cell:

THE EFFECT OF MUTATION ON DNA

SEQUENCE
● a mutation is a change in the nitrogenous base sequence of a DNA molecule or gene

● mRNA is copied from the DNA molecule during transcription, which results in a change in

fhe codons

● this means that different tRNA molecules carrying different amino acids are required

● the sequence of amino acids changed & forms a different protein

● if the same amino acid is coded for, then there will be no change in the protein structure