Key area 3: Control of gene
expression
Unit 1: DNA and the Genome
This topic cover:
• Structure and function of proteins
• Structure and function of RNA
• Transcription
• Translation
• One gene…many proteins
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DNA and
the
Genome
Proteins
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DNA and
the
Genome
What are proteins made of?
Think back to National 5…
What elements are in proteins?
What are the sub-units in proteins
called?
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DNA and
the
Genome
Proteins are made of long chains of amino
acids.
There are 20 different amino acids.
They are joined together by peptide
bonds.
The proteins fold into different shapes.
The shape of the protein dictates the
role it will have in the cell.
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DNA and
the
Genome
Protein structure
Proteins fall into two distinct groups:
1. Fibrous proteins
2. Globular proteins
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DNA and
the
Genome
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DNA and
the
Genome
Fibrous
Globular
Keratin
Enzymes –
Elastin
Messengers –
Collagen
Transporters
Regulatory roles
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DNA and
the
Genome
Investigating structure and
function
Investigating a variety of proteins using
RasMol modelling software.
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DNA and
the
Genome
Protein function
• You will have come across many proteins
before and covered some of the many
roles they undertake in living organisms.
• Choose one of the proteins listed below
to research and complete its identity
card, which will be used to create a
classroom display.
helicase
keratin
kinase
integrins
phosphorylase
tubulin
insulin
catalase
oxytocin
antibody
actin
elastin
polymerase
collagen
myosin DNA and
haemoglobin
pepsin
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the
porin
amylase
cytochromes
Genome
Structure: _____________________________
Location: _____________________________
Function: _____________________________
_____________________________________
_____________________________________
_____________________________________
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DNA and
the
Genome
Protein folding
Once the polypeptide has
formed, hydrogen bonds can
form between amino acids –
creating secondary
structures.
These can fold again to
form – tertiary structures
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Several different polypeptides can then
join together forming the final protein.
DNA and
the
Genome
Structure and function of
RNA
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DNA and
the
Genome
RNA
There is a second type of nucleic acid in
the cell, called RNA.
RNA plays a vital role in the production of
protein from the code in the DNA.
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DNA and
the
Genome
Differences between DNA and RNA
RNA nucleotides are similar in
structure to DNA, except Ribose
sugar replaces Deoxyribose sugar…
5’ end
Phosphate
3’ end
… and Uracil
replaces thymine
Ribose
Sugar
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5’ pronounced
“5 prime”
DNA and
the
Genome
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DNA and
the
Genome
DNA
RNA
Double stranded
Single stranded
Deoxyribose sugar
Ribose sugar
Bases: A, T, C and G
Bases: A, U*, C and G
*Uracil replaces
thymine
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DNA and
the
Genome
Types of RNA
There are three types of RNA:
1. Messenger RNA (mRNA)
2. Transfer RNA (tRNA)
3. Ribosomal RNA (rRNA)
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DNA and
the
Genome
mRNA
mRNA is formed in the nucleus from free
nucleotides and carries a copy of the
DNA code from the nucleus to the
ribosomes (where protein synthesis
occurs).
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DNA and
the
Genome
tRNA
tRNA molecules collect amino acids and
bring them to the ribosome to build
proteins.
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DNA and
the
Genome
rRNA
rRNA molecules combine with proteins to
create the ribosome – the organelle
responsible for assembling proteins
following the DNA code.
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DNA and
the
Genome
Transcription
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DNA and
the
Genome
Protein synthesis
What is a gene?
A gene is a section of DNA which
carries the code for the production of
one protein.
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DNA and
the
Genome
Nucleus
DNA
Overview
of gene
expression
AGAGGTTGACGAA
T CT CCAACTGCTT
Transcription
mRNA
U CU CCAACUGCUU
codon
Ribosome
ser
pro
thr
Translation
Protein
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DNA and
the
Genome
ala
Transcription
Transcription is the synthesis of mRNA
from a section of DNA.
Transcription of a gene starts from a
region of DNA known as the promoter.
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DNA and
the
Genome
Promoter: Start
of a gene
Terminator: End
of a gene
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DNA and
the
Genome
RNA polymerase
This enzyme is responsible for
transcription.
RNA polymerase binds at the promoter
and unwinds the DNA.
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DNA and
the
Genome
RNA polymerase adds nucleotides onto
the 3’ end of the growing mRNA molecule.
Due to the base-pairing rules the mRNA
produced will be complementary to the
DNA.
The molecule elongates until it reaches
the terminator sequence.
The molecule produced is called the
primary transcript.
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DNA and
the
Genome
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DNA and
the
Genome
Modification of the primary
transcript
Not all the regions in a eukaryotic gene
are required to produce the final protein.
These non-coding regions are called
introns.
The coding regions are called exons.
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DNA and
the
Genome
Primary
Primary
transcript
transcript
Mature
transcript
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DNA and
the
Genome
RNA splicing
After the mRNA has been transcribed
the introns are removed.
The remaining exons are spliced together
to form a continuous sequence.
This is called the mature transcript.
The mature transcript then leaves the
nucleus to travel to the cytoplasm.
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DNA and
the
Genome
Translation
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DNA and
the
Genome
Genetic code
Translation is the synthesis of protein
following the code with in the mature mRNA
transcript.
The mRNA is made of sequences of three
nucleotides (a triplet of bases) called
codons.
Each codon is code for one amino acid.
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DNA and
the
Genome
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DNA and
the
Genome
tRNA
A further type of RNA is found in the
cell’s cytoplasm.
This is called tRNA (transfer RNA) and is
made of a single chain of nucleotides.
It is folded into a 3D structure, held
together by hydrogen bonds.
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DNA and
the
Genome
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DNA and
the
Genome
Each tRNA has an attachment site for a
specific amino acid and a triplet of bases
known as an anticodon.
Many different types of tRNA are
present in cell, one or more for each type
of amino acid.
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DNA and
the
Genome
The tRNA picks up its appropriate amino
acid and takes it to the ribosome to be
matched with the mRNA.
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DNA and
the
Genome
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DNA and
the
Genome
Ribosomes
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DNA and
the
Genome
Ribosomes are small, roughly spherical
structures found in all cells.
They contain the enzymes essential for
protein synthesis.
The ribosome’s function is to bring the
tRNA molecules bearing amino acids in
contact with the mRNA.
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DNA and
the
Genome
Site P – holds the
tRNA carrying the
growing
polypeptide chain.
Site A – holds the
tRNA carrying the
next amino acid to
be joined to the
chain.
Site E – releases the
empty tRNA once it
has dropped off its
amino acid.
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DNA and
the
Genome
The translation process
1. The ribosome binds to the 5’ end of the
mRNA so that the start codon (AUG) is
in site P.
2. Next a tRNA carrying the amino acid
methionine becomes attached to site P.
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DNA and
the
Genome
3. The mRNA codon at site A bonds
complementary anticodon on the
appropriate tRNA bearing the correct
amino acid.
4. A peptide bond then forms between
these two amino acids.
5. The ribosome then moves along one codon.
6. The tRNA from Site P is move to Site E
and released.
7. Steps 3-6 then repeat until it reaches a
stop codon.
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DNA and
the
Genome
One gene…many proteins
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DNA and
the
Genome
One gene…many proteins
The same gene can be used to make
several different proteins by:
1. Alternative RNA splicing
2. Post-translational modification
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DNA and
the
Genome
1. Alternative RNA Splicing
Previously we learned that the primary
transcript is separated into exons and
introns and the exons are spliced
together to make the mature transcript.
Under certain conditions alternative
segments of RNA may be treated as
exons and introns.
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DNA and
the
Genome
In other words one gene can produce
several different mature mRNA
transcripts.
And therefore, several different
proteins.
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DNA and
the
Genome
Primary
transcript
Alternatively
spliced
mature
mRNA
transcripts
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Different
DNA and
proteins
the are
Genome
formed
2. Post-translational modification
Once translation is completed, proteins
can be modified by:
(a) Cleavage
(b) Addition of other molecules
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DNA and
the
Genome
(a) Cleavage
A single poly-peptide chain can be cleaved
(cut) by enzymes to make it active.
Insulin is an example of the protein
modified in this way.
A central section of the “pro-insulin”
molecule is removed to make insulin.
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DNA and
the
Genome
Inactive “proinsulin”
Active
insulin
Redundant
central section
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DNA and
the
Genome
(b) Addition of other molecules
Carbohydrates and phosphate groups can
be added to proteins.
Mucus is a glycoprotein made by the
addition of a protein and a
carbohydrate.
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DNA and
the
Genome
Some regulatory proteins need phosphate
groups added to them to make them
active.
e.g. p53 is a regulator involved in DNA
repair. Normally it is inactive. When
phosphate is added it becomes active.
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DNA and
the
Genome