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Exam Question Facts
• DNA and RNA can be worth 5% if asked
as a short question in Section A.
DNA
and it’s
replication
• DNA and RNA can be worth 7.5% if asked
as an experiment in Section B.
• DNA and RNA can be worth 15% if asked
as a long question in Section C.
Note: DNA and RNA question has been asked every year
since 2004 except for 2009.
DNA: The Watson and Crick Hypothesis
The Structure of DNA
Learning Objective:
Be able to describe the structure of
DNA.
• Rosalind Franklin in the early
1950’s showed that
phosphate groups of DNA
must lie on the outside of
the molecule.
• Watson and Crick put
forward the possible
structure of DNA in 1953.
They were awarded a Nobel
prize in 1962.
Chromosomes
What is DNA?
• Eukaryotes chromosomes are linear DNA
molecules associated with histone
proteins.
• Heredity material responsible for
passing genetic information from cell to
cell.
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DNA: The monomers
DNA Structure
• Deoxyribo Nucleic Acid
• One molecule of DNA, consists of 2
strands of repeating units called
nucleotides
• The 2 strands are twisted into a double
helix
• Deoxyribonucleic acid
• Made up of lots of nucleotides joined
together.
Phosphate
A nitrogenous base
Ribose sugar (RNA)
Deoxyribose sugar (DNA)
DNA: The bases
There are 4 different bases
▪ Guanine
▪ Cytosine
▪ Adenine
▪ Thymine
DNA Structure
• DNA is made up of many nucleotides joined
together.
Really strong bond
• Adenine and Guanine are double ringed Purine
bases.
• Cytosine, Thymine and Uracil are single ringed
Pyramidine bases.
Nucleotide
DNA Structure
The sugar and phosphate units
make up the “backbone” of the
nucleic acid.
one nucleotide
A base is attached to each sugar
molecule.
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DNA
DNA: Base pairing
• The two strands of the double helix are held
together by hydrogen bonds between the bases.
• Hydrogen bonds are singularly very weak but
collectively can be very strong.
DNA: Complementary base
pairing
• In DNA Adenine always pairs with Thymine
(A-T)
• In DNA Guanine always pairs with Cytosine (CG)
DNA: Complementary base pairing
A
T
Adenine pairs with Thymine
G
C
Guanine pairs with Cytosine
T
A
A and T are complimentary base pairs.
• However in RNA... Adenine pairs with the base
Uracil....
C
C and G are complimentary base pairs.
T
Structural Differences
DNA
•
•
•
•
Contains deoxyribose sugar
Contains the bases A,T,G,C
Double stranded
Remains in the nucleus
G
A
DNA vs RNA
RNA
•
•
•
•
Contains ribose sugar.
Contains the base A,U,C,G
Singled stranded.
Can leave the nucleus.
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What is the function of DNA
How is it adapted to do its job?
• Stable
• 2 strands can separate so it can selfreplicate.
• Large molecule that carries LOTS of
information
• Base pairing prevents corruption from outside
chemicals or physical forces.
1.
2.
3.
4.
5.
6.
Questions
What elements is DNA made of?
What are the monomer units called in DNA?
Name the 3 parts of this monomer unit?
Which bases form complimentary base pairs?
What holds the 2 strands of DNA together?
If the base sequence on 1 strand of DNA is
GTTACCGTA what would the sequence be on
the other strand
7. If 19.9% of the base pairs in DNA are
Guanine, what percentage is Thymine
When does a DNA molecule
need to replicate?
DNA Replication
Learning Objective:
• Describe how DNA replicates.
• Explain the importance of DNA replication
to living organisms
https://www.hhmi.org/biointeractive/dna-replicationbasic-detail
• Before a cell divides DNA is replicated.
• This occurs during interphase of Mitosis
and Meiosis.
• This is so when a cell divides the two
daughter cells will contain the correct
amount of genetic material.
• Replication takes place in the nucleus of
the cell
DNA Replication
Replication is the production of an
identical copy of the DNA in a cell
A
C
G
G
C
• Replication requires
1. Enzymes
2. Free nucleotides
3. Energy
T
C
G
A
C
T
G
A
T
1. Enzymes
unwind the
double helix
and ‘unzip’ the
two stands of
DNA
T
A
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2. Free
nucleotides
found in the
cytoplasm of
the cell attach
to the old
strands
according to
complementary
base pairing (AT, C-G)
C
3. The result of
this is two
identical
strands of DNA
produced from
the original
strand.
5. One side of
the DNA
contains the
old stand and
one contains
the new
4. The
strand ‘rewinds to
form 2
identical
helices.
…it is called
the semiconservative
method of
replication.
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Steps in DNA Replication
1.
The double helix unwinds
2.
Enzymes breaks the hydrogen bonds between the two
base pairs - the 2 strands of DNA are now separate.
3.
Free nucleotides found in the cytoplasm of the cell
attach to the old strands according to complementary
base pairing (A-T, C-G).
4.
. The result of this is two identical strands of DNA
produced from the original strand.
The two DNA strands will coil up and become
identical chromatids held together by a centromere.
The quantity of genetic material in the cell doubles
but the chromosome number remains the same.
Centromere
Chromatid
4 Chromosomes
4 Chromosomes
Exam Questions
(i) Describe the structure of
DNA (6)
(ii) Outline DNA replication (3)
and its importance.(1)
Marking Scheme
DNA structure
1.double helix (not one strand coiled into double helix)
2.two chains/strands of nucleotides
3.deoxyribose sugar, phosphate and base make up a
nucleotide
4.nucleotides joined together by sugar - phosphate
bonds OR sugar and phosphate joined to form
backbones/chains/strands (of nucleotides)
5.base names (all four)
6.complementary bases pair (to join strands) OR
description – letters acceptable A-T and G-C
7.(weak) hydrogen bonding between bases
Maximum 6
Replication
1. The molecule unwinds/uncoils/untwists AND
unzips/H bonds (between bases) break
2. base pairing of (free) DNA nucleotides with
complementary partners
3. sugar-phosphate bonds/backbones form
4. rewinds into two double helices OR two
double helices form
5. requires enzymes/named enzyme(s) OR ATP
(not energy)
Max 3 Marks
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Importance
• Identical OR exactly the same
DNA/molecules/copies produced.
• Ensures daughter/new cells have
complete/correct/all/the same genetic
information/genes/chromosomes
Protein synthesis:
Transcription
Learning Objective:
• Be able to describe the process of
Transcription
Max 1 Mark
What is the function of DNA?
Coding and Non-Coding DNA
• The function of
DNA is to code for
proteins.
• Not all of the DNA
molecule codes for
proteins.
• The functional
part of DNA that
can is known as a
Gene
RNA
Genes are instructions to build a protein.
• Unfortunately the DNA that contains
the genes can’t leave the nucleus.
• So a copy of the gene must be made.
• This copy is known as Messenger RNA
or mRNA.
• RNA is another type of nucleic acid
RNA
• Ribo nucleic acid
• Structurally
different in 3
ways from DNA.
• What are the
differences?
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Transcription
Protein Synthesis
Step 1: Transcription
Step 2: Translation
The DNA in the nucleus is first unwound….
ATTTTATCGCCCTGCTAGCTGGGCAT
TAAAATAGCGGGACGATCGACCCGTA
This causes the two strands of DNA to separate.
ATTTTATCGCCCTGCTAGCTGGGCAT
TAAAATAGCGGGACGATCGACCCGTA
• Transcription is the copying of a section
DNA (known as a gene) into mRNA.
• This process occurs in the nucleus of
the cell.
The weak hydrogen bonds between the bases are broken…..
ATTTTATCGCCCTGCTAGCTGGGCAT
TAAAATAGCGGGACGATCGACCCGTA
Free RNA nucleotides in the nucleoplasm find their
complementary base pair on the DNA…….
ATTTTATCGCCCTGCTAGCTGGGCAT
U
TAAAATAGCGGGACGATCGACCCGTA
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A hydrogen bond forms between the RNA nuclotide base
and the complementary base on the DNA.
ATTTTATCGCCCTGCTAGCTGGGCAT
U AAA
ATTTTATCGCCCTGCTAGCTGGGCAT
UAAAAUAGCGGGACGAUCGACCCGUA
TAAAATAGCGGGACGATCGACCCGTA
TAAAATAGCGGGACGATCGACCCGTA
The messenger RNA molecule is formed
ATTTTATCGCCCTGCTAGCTGGGCAT
UAAAAUAGCGGGACGAUCGACCCGUA
TAAAATAGCGGGACGATCGACCCGTA
Hydrogen bonds between the bases on mRNA and the
bases on DNA break….
ATTTTATCGCCCTGCTAGCTGGGCAT
UAAAA
TAAAATAGCGGGACGATCGACCCGTA
…….The mRNA is ready to make it’s journey out of the
nucleus and into the cytoplasm.
Steps in Transcription
1. The DNA double helix is ‘unzipped’.
ATTTTATCGCCCTGCTAGCTGGGCAT
TAAAATAGCGGGACGATCGACCCGTA
2. Just one side of the DNA is used as a template to make
mRNA.
3. Free RNA nucleotides found in the nucleoplasm attach to
the DNA strand according to complementary base pairing.
The hydrogen bonds between the complementary bases
on DNA re-form and the DNA is wound back into a helix
again.
4. Thymine is replaced by Uracil on the mRNA strand however.
5. The enzyme RNA polymerase copies the DNA into mRNA
6. The new mRNA strand leaves the nucleus via the nuclear
pores and makes its way to a ribosome.
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Translation
Protein Synthesis:
Translation
Learning Objective:
Describe how mRNA gets
translated into a protein.
• Translation is the manufacture of
protein based on the sequence of bases
on the mRNA.
• Translation occurs on the ribosomes
• The process involves another type of RNA molecule
known as Transfer RNA or tRNA.
• The nitrogenous bases are read in groups of three
and are called codons.
The mRNA leaves the
nucleus through a nuclear
pore and enters the
cytoplasm
UUUCGAUGCAUCGCAACUCGC
UUCGAUGCAUCGCAACUCGC
A ribosome becomes
attached to the mRNA
codon
UUUCGAUGCAUCGCAACUCGC
the genetic code on the
mRNA is read in groups
of 3 letters called
codons
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
the genetic code on the
mRNA is read in groups
of 3 letters called
codons
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codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAA
AAA
aa1
aa1
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAA
tRNA molecules
have specific
anticodons for
each of the 20
amino acids
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAA
GCU
the complementary
anticodon is attracted to
the first codon on the
mRNA and forms a weak
hydrogen bond.
aa1
aa1
aa2
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAA
GCU
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
the second codon on
mRNA also attracts its
complementary anticodon
on tRNA
aa1
aa2
aa1
aa2
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codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
The first tRNA molecule
become detached from it’s
amino acid
ACG
aa1
aa1
aa2
aa2
a peptide bond is formed between the
adjacent amino acids
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
The ribosome moves along
the mRNA molecule
ACG
ACG
aa1
aa2
aa1
aa2
aa3
aa
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU
The ribosome moves along
ACG
the mRNA molecule
aa1
aa2
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU ACG
The third tRNA moves in
place the anticodon linking
with it’s complementary
codon.
aa1
aa2
aa3
aa3
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codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
AAAGCU ACG
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
A A A G C U A C GU A G C G U U G A
A peptide bond forms
between the two amino
acids
aa1
aa2
aa3
aa1
aa2
aa3
aa4
aa5
aa6
A stop codon is reached
codon codon codon codon codon codon codon
UUUCGAUGCAUCGCAACUCGC
A A A G C U A C GU A G C G U U G A
UUUCGAUGCAUCGCAACUCGC
AAA GCU
A polypeptide chain has been formed and is
released into the cytoplasm
aa1
aa2
aa3
aa4
aa5
aa6
aa1
aa2
End Product
UUUCGAUGCAUCGCAACUCGC
AAA GCU
several ribosomes may
pass along the mRNA
behind the first - each
producing an identical
polypeptide
aa1
aa2
The end products of protein synthesis is a primary
structure of a protein.
A sequence of amino acid bonded together by peptide
bonds.
aa2
aa1
aa3
aa4
aa5
aa199
aa200
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Steps involved in Translation
1. The mRNA attaches to the ribosome.
2. The ribosome is made up of another nucleic
acid called ribosomal RNA (rRNA).
3. The rRNA binds weakly to mRNA.
•
Transfer RNA (tRNA) is also found in the ribosome.
4. tRNA uses the mRNA strand as a code to
decide which amino acids to connect together.
•
The tRNA contains an amino acid at one end and an anticodon (made up of 3 bases) on the other end.
5. tRNA brings the anti-codons to line up with its
complementary bases on the mRNA strand.
Translation
• This entire process of transcription
followed by translation is controlled by
enzymes.
• It is in this way that the nucleus (made of
DNA) controls the cell by determining the
proteins present in the cell.
Steps involved in Translation
•
The area where it binds is called the ‘binding site’.
6. The mRNA also carries a start and stop codon
which indicates where the protein begins and
ends.
7. Two tRNA molecules bind to the mRNA at the
one time.
8. The end result is long chains of amino acids
bonded together with peptide bonds to form a
polypeptide chain (i.e a protein!).
Type of RNA
Where it functions
Functions
mRNA
Produced in the nucleus.
Used at ribosome to
make protein
Copy of the genetic message.
Used as a template to make
protein
rRNA
A component of the
ribosome
‘Reads’ the message on the
mRNA.
Used to attach tRNA to the
mRNA
tRNA
Attaches to mRNA at the
ribosome
Places amino acids in the
correct sequence to make a
protein.
The Genetic code
• The sequence of bases on DNA is a ‘codeword’
• The bases (A,T,C,G) are ‘read’ in triplets.
• The combination of these letters in groups of
3 gives 64 base triplets.
• These 64 triplets code for the 20 amino
acids.
• There is therefore more that one triplet that
will code for each amino acid.
• Combinations of these 20 amino acids give
rise to the huge variety of proteins found.
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