Lecture 10
DNA Translation and Control
Translation
 Translation converts the order of the nucleotides of a gene
into the order of amino acids in a protein
 The rules that govern translation are called the genetic code
 mRNAs are the “blueprint” copies of nuclear genes
 mRNAs are “read” by a ribosome in three-nucleotide
units, termed codons
 Each three-nucleotide sequence codes for an amino
acid or stop signal
The Genetic Code
 The genetic code is (almost)
universal
 Only a few exceptions have
been found
Ribosomes
 The protein-making factories of cells
They use mRNA to direct the assembly of a protein
A ribosome is made up of
two subunits
Each of which is
composed of proteins
and rRNA
Sites play key
roles in
translation
Transfer RNA
Hydrogen
bonding causes
hairpin loops
 tRNAs bring amino acids to
the ribosome
 They have two business ends
 Anticodon which is
complementary to the
codon on mRNA
 3’–OH end to which the
amino acid attaches
3-D shape
Making the Protein
 mRNA binds to the
small ribosomal subunit
 The large subunit joins
the complex, forming
the complete ribosome
 mRNA threads through
the ribosome producing
the polypeptide
How translation works
 The process continues until a stop codon enters the A site
 The ribosome complex falls apart and the protein is released
Play
Protein Synthesis
Architecture of the Gene
 In eukaryotes, genes are fragmented
 They are composed of
 Exons – Sequences that code for amino acids
 Introns – Sequences that don’t
 Eukaryotic cells transcribe the entire gene, producing a
primary RNA transcript
 This transcript is then heavily processed to produce the
mature mRNA transcript
 This leaves the nucleus for the cytoplasm
Processing eukaryotic mRNA
Protect from
degradation
and facilitate
translation
 Different combinations of exons can generate different polypeptides via
alternative splicing
Play
How Spliceosomes Process RNA
How protein synthesis works in eukaryotes
6. The polypeptide
chain grows until the
protetin is
completed.
7. Phosphorylation or other
chemical modifications can
alter the activity of a protein
after it is translated.
Amin
o
acid
Complete
d
polypepti
de
tRNA
5’
Ribosome
moves toward
3’ end
Cytoplas
m
5. tRNAs bring their
amino acids in at the A
site of the ribosome.
Peptide bonds form
between amino acids at
the P site, and tRNAs
exit the ribosome from
the E site.
Ribosom
e
4. tRNA molecules
become attached
to specific amino
acids with the
help of activating
enzymes. Amino
acids are brought
to the ribosome in
the order dictated
by the mRNA.
Nuclear
membran
e
DNA
3
’
RNA
polymera
1. In the cellsenucleus,
RNA polymerase
transcribes RNA from
DNA
Play
Control of Gene Expression
3
Poly-A
’
tail
Intron
s
3
’
3’
5
’
3
Primary
RNA ’
transcript
Exon
s
5’
5
’
Small
ribosomal
subunit
Nuclea
r pore
5
Ca’
p
mRNA
2. Introns are excised from the
RNA transcript, and the
remaining exons are spliced
together, producing mRNA
Poly-A
tail
mRN
A
Ca
p
Large
ribosomal
subunit
3. mRNA is transported out of
the nucleus. In the cytoplasm,
ribosomal subunits bind to the
mRNA
Architecture of the Gene
 Most eukaryotic genes exist in multiple copies
 Clusters of almost identical sequences called multigene families
 As few as three and as many as several hundred genes
 Transposable sequences or transposons are DNA sequences that can
move about in the genome
 They are repeated thousands of times, scattered randomly about the
chromosomes
Turning Genes Off and On
 Genes are typically controlled at the level of transcription
 In prokaryotes, proteins either block or allow the RNA
polymerase access to the promoter
 Repressors block the promoter
 Activators make the promoter more accessible
 Most genes are turned off except when needed
The lac Operon
 An operon is a segment of DNA that contains a cluster of
genes that are transcribed as a unit
 The lac operon contains
 Three structural genes
 Encode enzymes involved in lactose metabolism
 Two adjacent DNA elements
 Promoter
 Site where RNA polymerase binds
 Operator
 Site where the lac repressor binds
The lac Operon
 In the absence of lactose, the lac repressor binds to the
operator
 RNA polymerase cannot access the promoter
 Therefore, the lac operon is shut down
The lac Operon
 In the presence of lactose, a metabolite of lactose called
allolactose binds to the repressor
 This induces a change in the shape of the repressor
which makes it fall off the operator
 RNA polymerase can now bind to the promoter
 Transcription of the lac operon is ON
The lac Operon
The lac Operon
 What if the cell encounters lactose, and it already has glucose?
 The bacterial cell actually prefers glucose!
 The lac operon is also regulated by an activator
 The activator is a protein called CAP
 It binds to the CAP-binding site and gives the RNA polymerase
more access to the promoter
 However, a “low glucose” signal molecule has to bind to CAP before
CAP can bind to the DNA
Play
Combination of Switches
Activators and repressors of the lac operon
Enhancers
 DNA sequences that make the promoters of genes more accessible to
many regulatory proteins at the same time
Usually located far away
from the gene they
regulate
Common in eukaryotes;
rare in prokaryotes
Mutation
 The genetic material can be
altered in two ways
 Recombination
 Change in the
positioning of the
genetic material
 Mutation
 Change in the content
of the genetic material
Bithorax mutant
Mutation
 Mutation and recombination provide the raw material for evolution
 Evolution can be viewed as the selection of particular combinations of
alleles from a pool of alternatives
 The rate of evolution is ultimately limited by the rate at which these
alternatives are generated
 Mutations in germ-line tissues can be inherited
 Mutations in somatic tissues are not inherited
 They can be passed from one cell to all its descendants
Kinds of Mutation
 Mutations are caused in one of two ways
 Errors in DNA replication
 Mispairing of bases by DNA polymerase
 Mutagens
 Agents that damage DNA
Kinds of Mutation
 The sequence of DNA can be altered in one of two main
ways
 Point mutations
 Alteration of one or a few bases
 Base substitutions, insertion or deletion
 Frame-shift mutations
 Insertions or deletions that throw off the reading frame
Kinds of Mutation
Kinds of Mutation
 The position of genes can be altered in one of two main
ways
 Transposition
 Movement of genes from one part of the genome to
another
 Occurs in both eukaryotes and prokaryotes
 Chromosomal rearrangements
 Changes in position and/or number of large segments
of chromosomes in eukaryotes
Kinds of Mutation
Kinds of Mutation
Mutation, Smoking and Lung Cancer
 Agents that cause cancer are called carcinogens
 These are typically mutagens
 The hypothesis that chemicals cause cancer was first advanced in the
18th century
 Many investigations since then have determined that chemicals can
cause cancer in both animals and humans
 For example, tars and other chemicals in cigarette smoke can
cause cancer of the lung