Protein synthesis Protein synthesis Protein synthesis

Protein synthesis

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‘s

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Lesson Objectives
•explain in outline how DNA encodes for
the amino acid sequence of proteins

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Review
_____________ are nitrogen-containing organic substances
that form the basis of the ________ ______ DNA and RNA.
All nucleotides contain the following _______ groups:
In DNA the sugar is _________, whereas in RNA the sugar
is _______.
a _______
_________
a _______
_________
a _________
___________

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Answers
Nucleotides are nitrogen-containing organic substances
that form the basis of the nucleic acids DNA and RNA.
All nucleotides contain the following three groups:
In DNA the sugar is deoxyribose, whereas in RNA the
sugar is ribose.
a phosphate
group
a pentose
sugar
a nitrogen-
containing base

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More questions from previous lessons
1.What is a mutation?
2.Name four times of mutations
3.Provide some examples of diseases
cause by these mutations.

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Bases Review
There are ______ bases, split into two types:
______ (A) and
_______ (G) are
purine bases.
A G
T U
_________ (T),
cytosine (C) and
________ (U) are
pyrimidine bases.
_____ contains A, G, T and C, whereas ______
contains A, G, U and C.
C

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Bases Answers
There are five bases, split into two types:
adenine (A) and
guanine (G) are
purine bases.
A G
T U
thymine (T),
cytosine (C) and
uracil (U) are
pyrimidine bases.
DNA contains A, G, T and C, whereas RNA contains
A, G, U and C.
C

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Protein Synthesis
New terms:
•The genetic code
•Replication
•Transcription
•Translation
•Codons

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Determining the structure of DNA
The structure of DNA was
determined in 1953 by
James Watson and
Francis Crick.
Previous X-ray studies
by Rosalind Franklin and
Maurice Wilkins had
suggested that DNA was
a double helix.
Erwin Chargraff had found evidence that DNA bases
occurred in pairs.
These discoveries were not fully accepted until other
scientists had performed their own tests and experiments.

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Protein Synthesis follows three steps

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DNA Replication
•DNA replication 
is
the process by
which
 
DNA 
makes a
copy of itself during
cell division. The
first step in
 
DNA
replication 
is to
'unzip' the double
helix structure of
the
 
DNA
?
 
molecule.
•Each new strand
formed is identical to
the parent DNA.

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DNA Transcription
•Transcription is the process by which the information in a strand of
DNA is copied into a new molecule of messenger RNA (mRNA). ...
The newly formed mRNA copies of the gene then serve as blueprints
for protein synthesis during the process of translation.

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Transcription and codons
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
mRNA
T A C

G C A G A T T A C
A U GC G U C U A A U G
The genetic code is non-overlapping: each base is only
part of one triplet/codon, and each triplet/codon codes just
one amino acid.

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What 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.
Each gene codes for a specific polypeptide.
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.
one gene
thousands more bases
in gene (not shown)

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The triplet code
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.

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Codons

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What is translation?
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.
How are the correct amino acids transported to the ribosome,
and how are they linked together in the correct order?
mRNA
strand
ribosome

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What is tRNA?
nucleotides
amino acid
attachment site
anticodon
In the cytoplasm, amino acids become attached to transfer
RNA (tRNA) molecules. Each tRNA is specific for one amino
acid.
Each tRNA molecule
has a sequence of
three bases called an
anticodon. These
are complementary
to codons on the
mRNA molecule.
3’ end
5’ end
hydrogen bond
What is the anticodon
for the codon A U G?
U A C

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What happens during translation?
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.

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Stages of protein synthesis