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Chapter 11.4 – 11. 6
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
• Usually single-stranded
• Has uracil as a base
• Ribose as the sugar
• Carries protein-encoding
information
• Can be catalytic
• DNA
• Usually double-stranded
• Has thymine as a base
• Deoxyribose as the sugar
• Carries RNA-encoding
information
• Not catalytic
Types of RNA
function
mRNA Messenger RNA
carries DNA’s info to ribosomes
rRNA
Ribosomal RNA
part of ribosome,
used to translate
mRNA into protein
tRNA
Transfer RNA
brings amino acid to ribosome
rRNA is part of ribosome,
used to translate mRNA into
protein
tRNA is a connection between
anticodon and amino acid
The genetic code
The genetic code
• 3 RNA nucleotides for every one amino acid
• 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 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
• Occurs in three steps:
Initiation
RNA Polymerase finds
the Promotor site on
DNA and begins
transcription
Elongation
RNA polymerase adds
nucleotides to growing
RNA.
Termination
Sequences in the DNA
prompt the RNA
polymerase to fall off
ending the transcript.
Transcription
• Only one DNA strand is used as a template for
making RNA
• Ribonucleotides make H bonds using complementary
DNA
base pair rules.
5’ • A pairs with U
G T C A T T C G G
• G pairs with C
3’• Enzyme RNA polymerase.
3’
C A G T A A G C C
5’
Transcription
DNA coding strand
5’
DNA
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
• The new RNA molecule is formed by incorporating
• nucleotides that are complementary to the template
• strand.
DNA coding strand
5’
3’
DNA
G T C A T T C G G
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
RNA 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
RNA processing
•
RNA Processing
• tRNA
• Folds to the clover
shape
• Activates by
attaching to a
specific amino acid
on the attachment
site
• rRNA
• Joins with proteins
to make a large
subunit and a small
subunit
• Both subunits will
join to make the
working “unit” of
ribosome
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
template
DNA
Transcription
T
T
C
A
G
T
C
A
G
A
A
G
U
C
A
G
U
C
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
Initiation
Leader
sequence
Small ribosomal subunit
5’
3’
mRNA
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
Assembling to
begin translation
Initiator tRNA
Met
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
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
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: multiple copies of
a protein are made
simultaneously
Levels
of
protein
structure
Primary structure
sequence of amino acids
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
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?