Download 3.2.2 DNA Structure Lesson 3

3.2.2 DNA Structure Lesson 3
Genetic Code
Protein Synthesis
Genes-What do they code for?
Genes-What do they code for?
• Genes code for proteins
• How?
Protein
• What is a protein?
• Your genes control the production of a
protein. Your sequence of DNA codes
for a specific sequence of amino
acids. This agrees with Beadle and
Tatum’s ONE GENE ONE ENZYME
hypothesis
The Genetic Code
A
T
C
G
T
C
A
A
T
C
C
G
The Genetic Code
• All proteins are synthesised by just 20
amino acids. However the sequence
and number of amino acids can
produce a limitless range of proteins.
• The code of bases in a gene relates to
a specific code for a protein-this is the
genetic code.
The genetic code of an organism is the sequence of bases
along its DNA. It contains thousands of sections called genes
or cistrons. Each gene codes for a specific polypeptide.
one gene/cistron
thousands more bases
in gene (not shown)
All polypeptides are made from amino acids, so the
sequence of bases in a gene must code for amino acids.
The genetic code is almost universal – the same sequence
of bases codes for the same amino acids in all organisms.
HOW DOES IT WORK?
• How many bases do you think code for a
particular amino acid?
• One?
• Two?
• Three?
• Four?
Given that there are four bases in DNA, and these code for
20 amino acids, what is the basis for the genetic code?

If one base = one amino acid,
possible amino acids = 4

If two bases = one amino acid,
possible amino acids = 16 (4×4)

If three bases = one amino acid,
possible amino acids = 64 (4×4×4)
The existence of a three-base (triplet) code was confirmed
by experiments by Francis Crick and his colleagues in 1961.
The triplet code is degenerate, which means that each
amino acid is coded for by more than one triplet.
DEGENERATE, NONOVERLAPPING CODE
• A triplet of bases codes for an amino
acid
• How many possible amino acids can
be produced?
• But there are only 20 amino acids so
what do all the other …….. Codes
represent
2ND T
C
A
G
3RD
1ST
T
C
A
G
PHENYLALANINE
SERINE
TYROSINE
CYSTEINE
T
PHENYLALANINE
SERINE
TYROSINE
CYSTEINE
C
LEUCINE
SERINE
STOP
STOP
A
LEUCINE
SERINE
STOP
TRYPTOPHAN
G
LEUCINE
PROLINE
HISTIDINE
ARGININE
T
LEUCINE
PROLINE
HISTIDINE
ARGININE
C
LEUCINE
PROLINE
GLUTAMINE
ARGININE
A
LEUCINE
PROLINE
GLUTAMINE
ARGININE
G
ISOLEUCINE
THREONINE
ASPARAGINE
SERINE
T
ISOLEUCINE
THREONINE
ASPARAGINE
SERINE
C
ISOLEUCINE
THREONINE
LYSINE
ARGININE
A
METHIONINE
THREONINE
LYSINE
ARGININE
G
VALINE
ALANINE
ASPARTIC ACID
GLYCINE
T
VALINE
ALANINE
ASPARTIC ACID
GLYCINE
C
VALINE
ALANINE
GLUTAMIC ACID
GLYCINE
A
VALINE
ALANINE
GLUTAMIC ACID
GLYCINE
G
DEGENERATE, NONOVERLAPPING CODE
DNA Double Strand- Coding
Strand and template Strand
Questions
1. A section of DNA has the following
sequence TACGCTCCGCTGTAC.
What is the template strand?
2. How many amino acids does this
code for?
3. What are the amino acids?
4. What is the advantage of the
degenerate code?
PROTEIN SYNTHESIS
• Where does it happen?
• How can the TRIPLET CODE get here?
• How do the Amino Acids know that
they are going to be synthesised?
Transcription
The copying of DNA on to messenger RNA
(mRNA)
The messenger RNA takes the sequence of
bases of a gene from the nucleus to the
ribosome
Coding
/Sense
Strand
G
A
T
A
C
T
G
G
C
C
C
T
A
T
G
A
C
C
G
G
Template
Antisense
Strand
Transcription
• You want a copy of the DNA Coding
Sense strand
• So what strand do you copy and why?
Transcription
• Transcribe a particular sequence of DNA to
produce a sequence of amino acids that
corresponds to a protein.
• A sequence of DNA, called a Cistron, is
unwound by a HELICASE enzyme
• DNA dependent RNA Polymerase copies
the Template Antisense strand, producing
an exact copy of the Coding sense strand.
mRNA
• The sequence of bases on the mRNA
molecule codes for a sequence of
bases.
• The code is split into triplet codes
• On the mRNA, this triplet code that
corresponds to an amino acid is
known as a CODON
mRNA
• What’s not on the messenger RNA
molecule that is on the DNA molecule
that was copied?
When a polypeptide is required, the triplet code of its gene is
converted into a molecule of messenger RNA (mRNA).
This process is called transcription and is the first stage of
protein synthesis.
Like DNA, mRNA is a nucleic
acid, but it differs in that:

it is single stranded,
not double stranded

it contains ribose instead of
deoxyribose

it contains uracil instead of thymine.
mRNA strand
during
transcription
During transcription, the mRNA is built up by complementary
base pairing, using the DNA as a template. The DNA’s base
triplets are converted into mRNA codons.
What are the codons in the mRNA transcribed from this
sequence of DNA base triplets?
DNA
TAC GCA GAT
TAC
mRNA
AUG CGU CUA AUG
The genetic code is non-overlapping: each base is only
part of one triplet/codon, and each triplet/codon codes just
one amino acid.
Translation
• Transfer RNA
In the cytoplasm, amino acids become attached to transfer
RNA (tRNA) molecules. Each tRNA is specific for one amino
acid.
amino acid
3’ end
attachment site
Each tRNA molecule
5’ end
has a sequence of
three bases called an
hydrogen bond
anticodon. These
are complementary
to codons on the
mRNA molecule.
What is the anticodon
for the codon A U G?
UAC
nucleotides
anticodon
Translation
• The mRNA arrives at the ribosome and
is recognised. The first two codons
enter the ribosomal structure.
• tRNA units, complementary to the
codon travel from the cytoplasm to
the ribosome.
• The tRNA molecule binds to the mRNA
molecule in the ribosome
Translation
• A second tRNA unit complementary to
the second codon then enters the
ribosome and binds to the mRNA
molecule.
• A peptide bond then forms between
the two amino acids
• The mRNA then moves through the
ribosome allowing more mRNA codons
to attach to tRNA units.
Translation
• The tRNA molecules that have already
connected to a mRNA molecule and have
had their amino acids attached to a
peptide chain are released back in to the
cytoplasm.
• This process continues until a STOP codon is
reached
• More than one ribosome may be attached
to a mRNA molecule at any one time. This
creates a POLYSOME unit.
• Up to 50 a second
TRANSLATION
• Peptide bonds form between the
amino acids.
• Once the entire mRNA molecule has
been translated to a sequence of
amino acids, the created protein can
be used.
Once a molecule of mRNA has been transcribed, it moves
out of the nucleus via a nuclear pore.
In the cytoplasm, the
mRNA combines with a
ribosome – the cellular
structure on which the
polypeptide chain will
be built in a process
called translation.
ribosome
mRNA
strand
How are the correct amino acids transported to the ribosome,
and how are they linked together in the correct order?
tRNA molecules attach to the ribosome,
and their anticodons pair up with the
appropriate codons on the mRNA.
The amino acids transported by the
tRNA link together, and the tRNA
molecules then return to the cytoplasm.
The ribosome moves along
the mRNA, and amino acids
continue to join together until
all the codons have been
translated and the
polypeptide is complete.