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Essential idea: Genetic
information in DNA can be
accurately copied and can be
translated to make the
proteins needed by the cell.
Topic 2: Molecular Biology
2.7 DNA Replication, Transcription and
Translation
Nature of Science
Obtaining evidence for scientific
theories—Meselson and Stahl obtained
evidence for the semi-conservative
replication of DNA. (1.8)
Understandings
2.7.U1 The replication of DNA is semi-conservative
and depends on complementary base pairing.
2.7.U2 Helicase unwinds the double helix and
separates the two strands by breaking hydrogen
bonds.
2.7.U3 DNA polymerase links nucleotides together to
form a new strand, using the pre-existing strand as a
template.
2.7.U4 Transcription is the synthesis of mRNA
copied from the DNA base sequences by RNA
polymerase.
Understandings
2.7.U5 Translation is the synthesis of
polypeptides on ribosomes.
2.7.U6 The amino acid sequence of polypeptides
is determined by mRNA according to the genetic
code.
2.7.U7 Codons of three bases on mRNA
correspond to one amino acid in a polypeptide.
2.7.U8 Translation depends on complementary
base pairing between codons on mRNA and
anticodons on tRNA
Applications and Skills
2.7.A1 Use of Taq DNA polymerase to produce
multiple copies of DNA rapidly by the
polymerase chain reaction (PCR).
2.7.A2 Production of human insulin in bacteria as
an example of the universality of the genetic
code allowing gene transfer between species.
2.7.S1 Use a table of the genetic code to deduce
which codon(s) corresponds to which amino acid.
Key Terms
DNA Replication
 When cells divide, DNA replications needs to occur to
ensure each cell has a full set of chromosomes.
 DNA replication occurs along each chromosome in a
5’3’ direction.
 DNA replication in eukaryotic chromosomes is initiated
at many points.
 In eukaryotes, DNA replication involves many enzymes:
 Helicase
 DNA polymerase III
 RNA primase
 DNA polymerase I
 DNA Ligase
DNA Replication
 Since DNA replication occurs only in a 5’ 3’
direction, a problem arise due to the fact that the
two DNA strands run in opposite directions.
 The two strands are referred to as the leading
strand and the lagging strand.
 Replication on the leading strand is straight
forward, with DNA polymerase III adding
nucleotides in the 5’ 3’ direction.
 On the lagging strand replication also occurs in the
5’3’direction. This results in the formation of
fragments, between 1000 and 2000 nucleotide
long. These fragments are called Okazaki
Fragments and are later joined together by DNA
Ligase.
Ref: Biology for the IB Diploma, Allott
Ref: IB Biology, Oxford Study Courses
Role of the Enzymes
 Helicase
 Unwinds the DNA at the replications fork
 Breaks the hydrogen bonds between the bases
 DNA Polymerase III
 Adds deoxynucleoside triphosphates to the 3’ end
 RNA Primase
 Adds nucleoside triphosphates on the lagging strand to for
an RNA primer
 DNA Polymerase I
 Removes the RNA primer
 Replaces it using deoxynucleoside triphosphates
 DNA Ligase
 Joins the Okazaki Fragments together.
Deoxynucleoside Triphosphates
Ref: IB Biology, Oxford Study Courses
Deoxynucleoside Triphosphates
The Most Beautiful Experiment in Biology
 The Meselson and Stahl Experiment
 Proof of the SEMICONSERVATIVE NATURE of
DNA replication.
 A type of experiment called a PULSE-CHASE
PRIMER.
 PULSE- a ‘tag’ molecule is used that will be incorporated into
a larger molecule.
 CHASE – this tag is then followed through the biochemical
process to see where it ends up.
 In this experiment 15N (a rare isotope of nitrogen
also called Heavy Nitrogen) is used as a tag that will
be incorporated into the DNA
Instead of normal nitrogen (light nitrogen)
14N
Learn More…………….
Look at the Meselson and Stahl
Worksheet to examine this famous
experiment in more detail.
DNA Replication
 DNA Replication is semi-conservative.
 This means that when DNA replicates, each strand of the
DNA molecule is used as a template for a new DNA
molecule.
 Thus, after DNA replication, each new DNA double helix
molecule contains:
 One old strand.
 One new strand.
 It also means that each strand is exactly the same.
Complementary Base Pairing
 DNA carries the genetic code for the organism.
 When cells divide, each new cell must get an exact copy of
the DNA.
 Thus DNA replications must be accurate.
 When DNA replicates, it does so using
 Complementary Base Pairing.
 i.e. A is paired with T

C is paired with G
Ref: IB Biology, OSC
Enzymes Involved in DNA Replication
 When DNA replicates, two enzymes are involved in the
process:
 Helicase:
 Helicase unwinds the DNA double helix molecule.
 Helicase then unzips the DNA double helix molecule
by breaking the hydrogen bonds between the
complementary bases.
 DNA Polymerase:
 DNA polymerase attaches new nucleotides to the old
single DNA strands.
Transcription
 A process is needed to convert this DNA code
into useful proteins and peptides.
 This is a two stage process:
 mRNA (messenger RNA) strand is made from the
DNA code. (transcription)
 The mRNA is then decoded to produce a
polypeptide. (translation)
RNA:
•Made up of
only one
strand
•Contains the
sugar ribose
•Bases C, G,
U, A
DNA:
•Made up of
two strands
•Contains the
sugar
deoxyribose
•Bases C, G,
T, A
DNA
RNA
1. Is a double helix shape
1. Is a single helix shape
2. Is made of deoxyribose
sugar
2. Is made of ribose sugar
3. Large molecule
3. Small molecule
4. Bound in nucleus
4. In nucleus & cytoplasm
5. Only one type of DNA
5. Three types of RNA
(mRNA, tRNA, rRNA)
*refer to pg 42
6. Has A, U, G, C bases
6. Has A, T, G, C bases
Transcription
 Transcription is the formation of a molecule of
messenger RNA (mRNA) from a DNA template in the
nucleus.
 Transcription is carried out in a 5’  3’ direction.
 DNA acts as a template but only one side of the DNA
double helix is a gene. This is called the antisense strand.
 The antisense strand is transcribed into mRNA by RNA
polymerase.
 The other side is complementary to the antisense strand
and is called the sense strand.
 The sense strand has the same sequence of bases as the
mRNA except with T not U.
Transcribe the following……….
1. DNA: A T A C G A A A T C G C G A T C G C G G C G A T T C G G
mRNA:
2. DNA: T T T A C G G C C A T C A G G C A A T A C T G G
mRNA:
3. DNA: T A C G G G C C T A T A C G C T A C T A C T CA T G G A T C G G
mRNA:
4. DNA: G T A C G C G T A T A C C G A C A T T C
mRNA:
Transcription
Ref: Biology for the IB Diploma, Allott
Transcription
Ref: IB Biology, Oxford Study Courses
This mRNA strand has the same sequence of
nucleotides as the DNA (except U instead of T).
The mRNA then leaves the nucleus through the
pores and enters the cytoplasm.
Transcription
 The Promoter Region
 A specific sequence of DNA bases at the start of the gene
to which RNA polymerase binds.
 It is found on the sense strand.
 RNA Polymerase
 The enzyme adds nucleoside triphosphates (NTPs) using
base pairing to the DNA antisense strand.
 Terminator Region
 A specific sequence of DNA bases marking the end of the
transcription process. RNA polymerase breaks free and the
mRNA strand is released. It is found on the sense strand.
Role of the Promoter and
Terminator in Transcription
Ref: Biology for the IB Diploma, Allott
Gene Expression
 The process by which a gene has an effect on a cell is called gene
expression.
 Every cell in a multicellular organism contain all of the organisms
genes.
 However, only some of them will be expressed. This is the basis of
cell differentiation.
 Gene expression involves several steps:
 Transcription of the gene
 Processing of the mRNA to remove introns
 post transcriptional modification
 Translation of the mRNA into a protein
 Modification of the protein
 post translational modification
Introns and Exons
 Many genes in Eukaryotes contain Introns.
 These are non-coding sequences that are
transcribed but not translated.
 The are found in newly transcribed mRNA but
removed to form mature mRNA.
 The sequences that are not removed are
called Exons.
 Prokaryotes do not usually have introns.
 The removal of introns is called posttranscriptional modification of RNA.
Introns and Exons
Ref: IB Biology, Oxford Study Courses
Extension: An Example of Transcription
Groups of genes that are regulated together
are called operons.
Operons are only located in prokaryotes.
In E.coli three genes produce enzymes to
absorb and digest lactose sugar.
These three genes are adjacent in the DNA
and are regulated as a single unit, the lac
operon.
The lac operon in OFF mode
Refer to notes
below for
explanations…
Refer to diagram above…
The promoter is the site where RNA Polymerase
attaches and hence initiates transcription of the
3 genes.
Between the promoter and the enzyme genes is a
DNA segment called an operator
The operator acts as a switch that is turned on
or off, depending on whether a specific protein
(made by a regulatory gene) is bound there.
continued…
The operator and protein together determine
whether RNA Polymerase can attach to the
promoter and start transcribing the genes.
When the operator switch is turned on, all the
enzymes needed to metabolise lactose are made
at once.
The lac operon in ON mode
Translation: The Structure of tRNA
 Translation involves reading the mRNA in sets
of 3 nucleotides called codons.
 There are 61 codons (excluding 3 stop codons)
for the 20 amino acids.
 This also means there are 61 anticodons and
hence 61 different types of tRNA
 This is called the triplet code.
 Since there are only 20 amino acids, the
triplet code allows for degeneracy. (more than
one tRNA per amino acid.
The Structure of tRNA
 Transfer RNA (tRNA) has a
vital role in translating the
genetic code:
 All tRNA molecules have:
 A triplet of bases called the
anticodon,in a loop of 7
nucleotides.
 Two side loops.
 The base sequence CCA at the
3’ terminal, which forms a site
for attaching an amino acid.
 Sections that become double
stranded by complementary
base pairing.
These features allow all tRNA
molecules to bind to sites on the
ribosome and mRNA.
Ref: Biology for the IB Diploma, Allott
tRNA Activating Enzymes
 The variable features of each tRNA molecule give them a
distinctive 3 dimensional shape.
 This allows the correct amino acid to be attached to the 3’
terminal by an enzyme called the tRNA activating enzyme.
 There are 20 different tRNA activating enzymes (one for each
of the 20 amino acids).
 Each enzyme attaches one particular amino acids to all of the
tRNA molecules that have an anticodon corresponding to that
amino acid.
 Energy from ATP is needed for the attachment of amino acids.
 The reaction of joining an amino acid to the tRNA is a
condensation reaction, producing water.
tRNA Activating Enzymes
Ref: IB Biology HL, OSC
Ribosome Structure
 Ribosomes have a complex
structure;
 Proteins and ribosomal RNA
molecules both form part of the
structure.
 There are 2 subunits, one large
and one small.
 There are binding sites for tRNA
on the surface of the ribosome
allow 2 tRNA molecules to bind at
the same time.
 There is a binding site for mRNA
on the surface of the ribosome
Ref: Biology for the IB Diploma, Allott
Translation
 Messenger RNA carries the information needed for
making polypeptides.
 The information is in a code form, which is decoded
during the the process of translation.
 Ribosomes, tRNA molecules and tRNA activating enzymes
are needed to carry out this decoding.
 There are 3 main stages in Translation:
 Initiation
 Elongation
 Termination
 Like DNA replication, translation occurs in a 5’  3’
direction.
Translation
Initiation
Translation Elongation
Translation
Termination
A Dipeptide
Polysomes
 Many polypeptides are needed in
large quantities
 eg: enzymes, antibodies, hormones.
 It would be energetically
inefficient for one mRNA to
synthesise a single polypeptide.
 Thus as a ribosome moves along
the mRNA another one can join
on behind it and so on like beads
on a string.
 Multiple copies of the
polypeptide can be synthesised
rapidly.
Ribosomes
 The distribution of ribosomes within the cell
depends upon the function of the protein they
make.
 Some ribosomes are found bound to the
endoplasmic reticulum while other float free
within the cytoplasm.
 Bound ribosomes produce proteins which are to be
secreted out of the cell or for use in lysosomes.
 Free ribosomes synthesise proteins for use
primarily within the cell.
Simultaneous
transcription and
translation in
prokaryotic cells