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From DNA to Proteins
Chapter 14
Marvelous Mussel Adhesive

Mussel binds itself to rocks with threads
coated with the protein bysuss

Gene for bysuss has been put into yeast

Yeast synthesize the protein based on the
instructions in the mussel DNA
Steps from DNA to Proteins
Same two steps produce all proteins:
1) DNA is transcribed to form RNA


Occurs in the nucleus
RNA moves into cytoplasm
2) RNA is translated to form polypeptide
chains, which fold to form proteins
Three Classes of RNAs

Messenger RNA


Ribosomal RNA


Carries protein-building instruction
Major component of ribosomes
Transfer RNA

Delivers amino acids to ribosomes
A Nucleotide Subunit of RNA
uracil (base)
phosphate
group
sugar
(ribose)
Figure 14.2
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Base Pairing during
Transcription
DNA
G
C
A
T
RNA
G
C
A U
DNA
C
G
T
A
DNA
C
G
T A
base pairing in DNA replication
base pairing in transcription
Transcription & DNA
Replication

Like DNA replication


Nucleotides added in 5’ to 3’ direction
Unlike DNA replication

Only small stretch is template

RNA polymerase catalyzes nucleotide addition

Product is a single strand of RNA
Promoter

A base sequence in the DNA that
signals the start of a gene

For transcription to occur, RNA
polymerase must first bind to a
promoter
Gene Transcription
transcribed DNA
winds up again
DNA to be
transcribed unwinds
mRNA
transcript
RNA polymerase
Figure 14.4c
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Adding Nucleotides
5’
growing RNA transcript
3’
5’
3’
direction of transcription
Figure 14.4d
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Genetic Code

Set of 64 base
triplets

Codons

61 specify amino
acids

3 stop translation
Figure 14.7
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tRNA Structure
codon in mRNA
anticodon
amino-acid
attachment site
amino
acid
OH
Figure 14.8
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Ribosomes
tunnel
small ribosomal subunit large ribosomal subunit
intact ribosome
Figure 14.9b,c
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Three Stages of Translation
Initiation
Elongation
Termination
Initiation



Initiator tRNA binds to
small ribosomal subunit
Small subunit/tRNA
complex attaches to
mRNA and moves along it
to an AUG “start” codon
Large ribosomal subunit
joins complex
Fig. 14.10a-c
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Binding Sites
binding site for mRNA
P (first
binding site
for tRNA)
A (second
binding site
for tRNA)
Figure 14.10d
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Elongation

mRNA passes through ribosomal subunits

tRNAs deliver amino acids to the ribosomal
binding site in the order specified by the
mRNA

Peptide bonds form between the amino acids
and the polypeptide chain grows
Elongation
Fig. 14.10e-g
Page 233
Termination




Stop codon into place
No tRNA with anticodon
Release factors bind to the
ribosome
mRNA and polypeptide
are released
mRNA
new
polypeptide
chain
Fig. 14.10j-k
Page 233
What Happens to the
New Polypeptides?

Some just enter the cytoplasm

Many enter the endoplasmic reticulum and
move through the cytomembrane system
where they are modified
Transcription
Overview
mRNA
Mature mRNA
transcripts
Translation
rRNA
ribosomal
subunits
tRNA
mature
tRNA
Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
Base-Pair Substitution
a base substitution
within the triplet (red)
original base triplet
in a DNA strand
During replication, proofreading
enzymes make a substitution
possible outcomes:
or
original, unmutated
sequence
a gene mutation
Figure 14.11
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Frameshift Mutations

Insertion


Extra base added into gene region
Deletion

Base removed from gene region

Both shift the reading frame

Result in many wrong amino acids
Frameshift Mutation
mRNA
parental DNA
arginine
glycine
tyrosine
tryptophan asparagine amino acids
altered mRNA
arginine
glycine
leucine
leucine
glutamate
DNA with
base insertion
altered aminoacid sequence
Figure 14.12
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Mutation Rates



Each gene has a characteristic mutation rate
Average mutation rate is approximately 1 in
every 100,000 genes
Only mutations that arise in germ cells can be
passed on to next generation
Mutagens

Ionizing radiation (X rays)

Nonionizing radiation (UV)

Natural and synthetic chemicals