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Chapter 10
Gene Action and Expression
Central Dogma
DNA is the genetic material
within the nucleus.
Replication
The process of replication
creates new copies of DNA.
The process of transcription
creates an RNA using
DNA information.
DNA
Transcription
RNA
Nucleus
The process of translation
creates a protein using
RNA information.
Translation
Protein
Cytoplasm
Two types of nucleic acids
# of strands
kind of sugar
bases used
Two types of nucleic acids
RNA
DNA
• Carries proteinencoding
information
• Carries RNAencoding
information
• Can be catalytic
• Not catalytic
Types of RNA
mRNA Messenger RNA
encodes protein
rRNA
Ribosomal RNA part of ribosome,used to
translate mRNA into protein
tRNA
Transfer RNA
couples the region which
binds the mRNA codon and its
amino acid
rRNA is part of ribosome, used
to translate mRNA into protein
tRNA is a connection between
anticodon and amino acid
The genetic code
• There is a 3 to 1 correspondence between RNA
nucleotides and amino acids.
• The three nucleotides used to encode one amino
acid are called a codon.
• The genetic code refers to which codons encode
which amino acids.
The genetic code
A codon of three nucleotides
determines choice of amino acid
The genetic code is nonoverlapping
The genetic code is universal
• All known organisms use the same genetic code.
•
(Rare organisms use one codon for an additional amino acid.)
The genetic code is degenerate
Some codons encode the same amino acid.
e.g. GGU, GGC, GGA, and GGG all encode glycine
Degeneracy is mostly at the third base of the codon.
Some codons have additional functions
AUG encodes methionine.
Methionine can be used within a protein sequence
and is often the first amino acid cueing the beginning
of translation.
UAA, UAG, and UGA do not encode an amino acid
These codons signal termination of the protein.
Transcription
• Three step process:
Initiation
Control of transcription
is regulated by
transcription factors.
Elongation
RNA polymerase adds
nucleotides to growing
RNA.
Termination
Sequences in the DNA
prompt the RNA
polymerase to fall off
ending the transcript.
Initiation of transcription
Transcription begins at regions called the promoters.
The promoter recruits TATA protein,
a DNA binding protein, which in turn recruits
other proteins.
TATA binding protein
Promoter
DNA
GG
Transcription factor
Gene sequence
to be transcribed
TATA CCC
TATA box
Transcription begins
When a complete transcription complex is formed
RNA polymerase binds and transcription begins.
•
Transcription
DNA is used as a template for creation of RNA using
• the enzyme RNA polymerase.
DNA
5
’
G T C A T T C G G
3’
3’
C A G T A A G C C
5’
Transcription
• RNA polymerase reads the nucleotides on the template
strand from 3’ to 5’ and creates an RNA molecule
that looks like the coding strand.
DNA
DNA coding strand
5
’
G T C A T T C G G
3’
3’
C A G T A A G C C
DNA template strand
5’
Transcription
DNA coding strand
5
’
DNA
G T C A T T C G G
3’
3’
G U C A U U C G G
3’
C A G T A A G C C
5’
DNA template strand
5
’
RNA
mRNA processing
• mRNA transcripts are modified before use as a
• template for translation:
•Addition of capping nucleotide at the 5’ end
•Addition of polyA tail to 3’ end


Important for moving transcript out of nucleus
And for regulating when translation occurs
•Splicing occurs removing internal sequences
introns are sequences removed
exons are sequences remaining
mRNA processing
•
Gene Expression
• Together transcription and translation are called gene
expression.
• The genetic information encoded in the DNA of an embryo
includes all of the genes needed to develop and maintain
the organism.
• Different cell types express different subsets of genes.
• Differential gene expression during development
establishes the role of a cell within the body.
Translation
• The process of reading the RNA sequence of an
mRNA and creating the amino acid sequence of a
protein is called translation.
DNA
Transcription
T
T
C
A
G
T
C
A
G
A
A
G
U
C
A
G
U
C
DNA
template
strand
Messenger
RNA
mRNA
Codon
Codon
Codon
Translation
Protein
Lysine
Serine
Valine
Polypeptide
(amino acid
sequence)
Translation is composed of three steps
Initiation
translation begins at
start codon (AUG=methionine)
Elongation
the ribosome uses the tRNA
anticodon to match codons to
amino acids and adds those amino
acids to the growing peptide chain
Termination
translation ends at the stop codon
UAA, UAG or UGA
Translation initiation
Initiation Complex
Small ribosomal subunit
5’
3’
Leader
mRNA
sequence
mRNA
U U C G U C A U G G G A U G U A A G C G A A
U A C
Initiator tRNA
Assembling to
begin translation
Met
tRNA of methione hydrogen bonds with the
anticodons UAC at the
start codon AUG on the mRNA
small rRNA subunit
Translation Elongation
Ribosome
5’
3’
mRNA
A U G G G A U G U A A G C G A
U A C C C U
tRNA
Amino acid
Met
Gly
Large ribosomal subunit
Translation Elongation
• Large rRNA subunit attaches to the small
rRNA subunit
• tRNA of the second amino acid
complementary to the second triplet code
attaches by H bonds
• Enzyme catalyzes the dehydration synthesis
reaction between the two amino acids
• MettRNA is released and leaves the complex to
go back and get recharged with more met.
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
U A C C C U
Met
Gly
Translation Elongation
• The rRNA complex moves one triplet space
• The third charged tRNA attaches by the
anticodons to the third triplet codon on mRNA
• These steps are repeated and amino acids are
bonded to make the protein sequence
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
C C U A C A
Gly
Cys
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
C C U A C A
Gly
Cys
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
A C A U U C
Cys
Lengthening
polypeptide
(amino acid chain)
Lys
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
A C A U U C
Cys
Lys
Translation Elongation
5’
3’
mRNA
A U G G G A U G U A A G C G A
A C A U U C
Cys
Lys
Translation Elongation
Stop codon
5’
mRNA
A U G G G A U G U A A G C G A U A A
U U C
Lys
Release
factor
Translation Termination
Stop codon
Ribosome reaches stop codon
5’
mRNA
A U G G G A U G U A A G C G A U A A
Release
factor
Translation Termination
Once stop codon is reached,
elements disassemble.
Release
factor
Translation Termination
• Elongation continues till the stop codon is reached
• A release factor attaches to the stop codon
• The rRNAs and mRNA complex is broken
• Protein released
Translation: multiple copies of
a protein are made
simultaneously
Levels of protein
structure
sequence of amino acids
Primary structure
Secondary structure
shapes formed with
regions of the protein
(helices, coil, sheets)
Tertiary structure
shape of entire folded
protein due to interactions
between particular peptides
Quaternary structure
structures formed by
interaction
of several proteins together
e.g. Functional hemoglobin is two alpha-hemoglobin proteins
and two beta-hemoglobin proteins form a heterotetramer
Levels of protein structure
Misfolding of protein impairs
function
•Misfolded prion protein disrupts functions of other
normally folded prion proteins.
•Aberrant conformation can passed on propagating like
an “infectious” agent.
Human Genome
• 3.2 million DNA base pairs
• 1.5% encode proteins < = > 98.5% not protein encoding
• ~ 31,000 genes encoding 100,000 - 200,000 proteins
• How are 100,000 to 200,000 proteins produced from 31,000 genes?
• What is the 98.5% of the human genome that does not encode
proteins?
Alternative splicing of exons
forms distinct proteins:
one gene, many proteins
Exon shuffling forms distinct
proteins:
several genes, multiple proteins
Noncoding portion of the human
genome
Type of sequence
Function or characteristic
Noncoding RNAs
Translation (tRNA,rRNA)
Pseudogenes
RNA processing
Introns
Removed with RNA processing
Promoters and other
regulatory regions
Determine when and where
transcription occurs
Repeats:
Transposons
DNA that moves around genome
Telomeres
Chromosome tips
Centromeres
Important for attachment to spindle
Duplications
Unknown
Simple short repeats
unknown
Transposons
•
Mobile DNA elements
•
First identified in corn by Barbara McClintock in 1940s
•
“selfish DNA”?
•
Grouped by size and sequence similiarity:
•
LINES
•
•
long interspersed elements of 6,000 bases
transcribed and trimmed to 900 bases
SINES
short interspersed elements of 100-500 bases
•
Alu element most common SINE in humans
•
300,000 to 500,000 copies => 2-3% of genome!
•
Gene expression changes
during development