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Chapter 12
Gene Expression
From DNA to Protein
 Things to remember:
 Proteins can be structural (muscles) or
functional (enzymes).
 Proteins are polymers of amino acids.
 Each protein has a specific sequence of
amino acids.
 The shape of the protein determines the
function of the protein.
 The DNA code holds the key for the
sequence of amino acids for each protein.
From DNA to Protein…
DNA
transcription
RNA
translation
protein
So, we need RNA, right?
Single stranded – although it can fold back on itself
for short double-stranded segments
 Ribose (rather than deoxyribose) – this sugar has a
hydroxyl group (OH) in the 2’ position rather than
just H
 Uracil (rather than thymine) – this is a pyrimidine
that can form 2 hydrogen bonds
 Three types:
1. messenger RNA (mRNA) – carries the code
2. transfer RNA (tRNA) – 20 different ones; folds back
on itself into a particular shape which allows it to
carry a specific amino acid
3. ribosomal RNA (rRNA) – globular; has catalytic
properties during protein synthesis
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Transcription means to copy
 Transcription comes 1st:(in the alphabet too!)
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mRNA is going to copy the DNA code in the gene
DNA is split – only one strand is read – the template strand
The DNA strand that is not read is the nontemplate strand
Three DNA nucleotides are a triplet. There are 64 possible
triplets that code for the 20 different amino acids.
RNA polymerase makes the mRNA by following the rules of
base pairing from the sense strand of the DNA, going from 5’
of the RNA to 3’.
Each group of 3 mRNA bases are called a codon.
When RNA polymerase reaches the termination signal,
transcription stops and the mRNA can leave the nucleus.
Translation converts the code to a
protein
 mRNA needs the help of tRNA and rRNA
 mRNA binds to a ribosome (Initiation)
 3 tRNA anticodons – complementary to the mRNA
codons - bring the specified amino acid into position
 A condensation reaction occurs to link the amino acids
together (Elongation)
 The bond between the amino acids is a peptide bond –
the chain of amino acids is sometimes called a
polypeptide
 Translation continues until the end of the mRNA is
reached – voila! A protein is born! (Termination)
Nontemplate strand
Transcription
DNA
Template strand
mRNA
(complementary
copy of template
DNA strand)
Codon 1 Codon 2 Codon 3 Codon 4 Codon 5 Codon 6
Polypeptide
Translation
Second letter
C
A
G
U
U
C
A
G
C
U
C
A
G
A
U
C
A
G
G
U
C
A
G
= Stop Codon
= Start Codon
Third letter (3’ end)
First letter (5’ end)
U
Three representations of a tRNA molecule
Ribosome
structure
Credit: © Kiseleva and Donald Fawcett/Visuals Unlimited
Ribosomes along two long mRNA molecules. Upper: Emerging nascent polypeptides that
emerge from each ribosome during translation. X150,000.
27005
Bacterial vs. Eukaryotic Cells
 In bacterial cells, transcription and
translation is coupled – translation of
the mRNA often begins before the 3’
end of the transcription is complete
 mRNA in eukaryotic cells is modified
in two ways – adding a 5’ cap and
polyadenylation at the 3’ end.
These probably help stabilize the
mRNA
Inactive DNA
segment
RNA
polymerase
Active
DNA
segment
Direction of
RNA synthesis
Active DNA
segment
Ribosomes
Polyribosome
Direction of
protein
synthesis
(a)
0.5 µm
mRNA
(b)
Interrupted coding sequences
 Eukaryotic DNA has sections of genes
that do not code for a protein –
introns.
 The coding sections are exons
 After the mRNA is transcribed, the
introns must be removed and the
exons spliced together before
translation begins
Retroviruses
 Reverse transcriptase makes a
DNA molecule from RNA
 Found in retroviruses
 Example: HIV – 1: the virus that
causes AIDS
Mutations
 A change in the nucleotide sequence
 Once it is in the DNA, replication will cause it to be
copied over and over
 Base substitution:
 Simplest type
 May cause a different amino acid
 Missense – altered function
 Nonsense – stops function
 Frameshift mutation:
 Caused by additions or deletions
 Alters the reading frame for all downstream
nucleotides
 Mutagens – radiation, certain chemicals, that cause
mutations
Mutations