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DNA and proteins
The genetic code
• Polypeptides are chains of amino acid residues
joined by peptide bonds.
• Proteins are large polypeptides.
• There are 20 different aa and their sequence
determines the structure and function of the
protein.
• The sequence of bases in a DNA molecule
determines the sequence of aa.
• A gene is a length of DNA that codes for one
(or more) polypeptides.
• A genome is the entire sequence of DNA of an
organism (about 25000 genes in the human
genome).
• Each gene occupies a specific locus (position) on a
chromosome and each chromosome consists of one
molecule of DNA.
• The DNA is wrapped around basic histone proteins
(Chromatin)
• In between genes is
non coding DNA
• Several genes that function
together are called a cistron
Genes code for polypeptides such as:
Structural proteins (keratin, collagen,
actin, myosin)
Enzymes
electron carriers
Haemoglobin
Antigens, immunoglobulins
Cell surface receptors
Channel proteins
The genetic code
• The sequence of bases in a gene
determines that protein that it will code
for
• The bases are in groups of 3 called
codons
• Each codon codes for an amino acid
The genetic
Code translated
according to the
codes on mRNA
Features of the Genetic Code
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It is a triplet code
It is degenerate
It has punctuation
It is widespread but not universal
It is a non overlapping code
Principles of Protein synthesis
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DNA stays in the nucleus
Proteins are made on ribosomes in the cytoplasm
SO
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The genetic code must be copied to messenger
RNA (mRNA)
mRNA leaves the nucleus via the nuclear pores
taking the code to the ribosomes
• So the process of using the genetic
code held in DNA to make a protein
occurs in 2 stages:
1. Transcription – the code on part of a
DNA molecule is copied to a mRNA
molecule.
2.Translation – the mRNA takes the code
to a ribosome where it is used to make a
polypeptide.
Transcription
• The process by which the base sequence of a
gene is converted into a complimentary base
sequence of mRNA
• One strand of the DNA (template strand) is
used to make mRNA the other stand is the
coding strand.
• Only part of a DNA molecule is transcribed
at one time.
• This part of the DNA molecule ‘unzips’ –
DNA helicase breaks the hydrogen bonds
between the bases.
Transcription
Transcription
• The part of the DNA molecule to be transcribed
enters the nucleolus to unwind and ‘unzip’
• DNA helicase breaks the hydrogen bonds between
the bases
• RNA polymerase catalyses the binding of activated
free nucleotides to the template
• Uracil binds to adenine NOT thymine
• The nucleotides condense together forming
phospho diester bonds
• The mRNA is complimentary to the template strand
and a copy of the coding strand
mRNA and
the Genetic
Code
Translation
• The second stage of gene
expression/protein synthesis
• It is the assembly of polypeptides
at the ribosome based on the
sequence of codons
• This means it is the translation of
the sequence of bases into a
sequence of amino acids
Several
ribosomes attach
to mRNA forming
a polysome
Ribosomal
groove
mRNA
attaches
to small
ribosomal
subunit
Made in the
nucleolus
from rRNA
and protein
Transfer RNA (tRNA)
• Made in the nucleus and
passes into the cytoplasm.
• It is a length of RNA that
folds into hairpin shapes
to form a clover leaf
• It can attach to a specific
aa in the cytoplasm
• It can recognise the
specific mRNA codon for
the aa it carries
tRNA structure
Smallest RNA
Three exposed bases at one
end where an amino acid can
bind
Over 20
different tRNA’s
Recognises
a specific
activation
enzyme
Three unpaired
nucleotide bases
at the other end
Translation
mRNA used to make polypeptide chain
(protein)
1.
1. mRNA attaches itself to the small subunit of a ribosome so 6 bases of
the mRNA are exposed to the large subunit.
2. The first exposed mRNA codon is always AUG (start codon)
3. A tRNA molecule (with its aa -met) with an anticodon complimentary to
the 1st codon lines up in position P
4. Complimentary base pairs form H bonds between the codon and
anticodon (UAC with the AUG)
5. Another tRNA (Pro) complimentary base pairs with the next codon in
the ribosome at position A.
6. The enzyme peptidyl transferase forms a peptide bond between the two
aa (met and Pro)
7. The first tRNA is released and leaves the ribosome without its aa.
Translation 2
8. The ribosome moves along the mRNA 3 bases to the next
codon.
9. The second tRNA molecule (Pro) is now position P.
10.A new tRNA molecule pairs with the mRNA in position A
bringing its aa (Tyr).
11.A growing polypeptide is formed in this way until a stop
codon is reached.
End of Translation
12.A stop codon (UAA, UAC or UAG) on the mRNA signals
the ribosome to leave the mRNA.
13.A newly synthesised protein is now complete and can
take up its tertiary structure.
14.Some proteins need to be activated by cAMP to take up
a 3D shape that is the best fit to their complimentary
molecule e.g by binding to an allosteric site on an enzyme
Translation - outline
A polysome
Overview of Translation