Download 1. Overview of Gene Expression Overview of Gene Expression Chapter 10B:

Chapter 10B:
Gene Expression
1. Overview of Gene Expression
2. Transcription
3. The Genetic Code
4. Translation
1. Overview of Gene Expression
Overview of
Gene Expression
DNA (genetic info)
transcription
(in nucleus)
RNA (copy)
translation
(via ribosomes
in cytoplasm)
Protein
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Gene Expression
The expression of a gene into an actual
protein occurs by 2 basic processes:
1) Transcription of a gene into RNA
• RNA is a nucleic acid very similar to DNA
(RNA uses “U” instead of “T”)
• this is essentially creating a “photocopy” of the gene
• occurs in the nucleus
2) Translation of the RNA transcript into protein
• accomplished by ribosomes, in the cytoplasm
Comparison of DNA & RNA
RNA
DNA
• sugar = Ribose
• sugar = Deoxyribose
• single-stranded
• double-stranded
• A, C, G & U (uracil)
• A, C, G & T (thymine)
2. Transcription
2
Transcription
Transcription of gene to make an RNA copy is much
like DNA replication except for the following:
RNA nucleotides
RNA
polymerase
A
T
C C A A
U
T
G
A
G G T
Direction of
transcription
Newly made RNA
G
C A U C C A
G
U
T
• catalyzed by RNA
polymerase
T
C
T
A
• only 1 strand is used
as a template
C
• produces
complementary RNA
C
T
A
A
• involves only 1 gene
Template
Strand of DNA
• RNA is released, DNA
“zips” back up!
*results in a copy of one DNA strand*
The Process of Transcription
3
Functions of
RNA made by
Transcription
*
1) mRNA*
• copy of gene
2) tRNA
• delivers AAs
to ribosomes
3) rRNA
*provides genetic
info for translation
• part of
ribosomes
3. The Genetic Code
How are Genes related to DNA?
Genes are segments of DNA that code for a
particular protein (or RNA molecule)
• the human genome contains ~3 billion base
pairs (bps) and ~25,000 genes
• almost all genes encode proteins
• when we talk about “genes” we will focus on those
that express proteins
( the “end products” for a small percentage of genes are
special types of RNA molecules)
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What does DNA actually code for?
In other words, “How do genes encode
proteins”?
• recall that proteins are linear polymers made of
the 20 different amino acids
***genes need simply to encode the identity of
each amino acid in a given protein***
• i.e., genes must be capable of encoding 20 different
amino acids and their order in a protein
• although DNA contains only 4 “letters” (i.e.,
nucleotides), this is more than sufficient…
The Genetic Code
Each amino acid in a protein is specified by
3 nucleotide sequences called codons
• each of the 20 amino acids is coded for by a
unique set of codons:
e.g.
ATG = methionine (start codon)
GGN = glycine
CAA or CAG = glutamine
• there are 64 possible “codon” triplets (4 x 4 x 4)
• more than enough to encode 20 amino acids and the
signal to “stop” or end the protein (TGA, TAA or TAG)
The Genetic Code
Second base
If the DNA sequence is:
CATGCCTGGGCAATAG
Third base
First base
(transcription)
The mRNA copy is:
CAUGCCUGGGCAAUAG
(translation)
The polypeptide is:
*Met-Pro-Gly-Gln-(stop)
all proteins begin w/Met
5
From DNA to RNA to Protein
DNA strand
Transcription
RNA
Codon
Translation
Polypeptide
Amino acid
The Effects of Mutation
MUTATION: any change in DNA sequence
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
• deletions,
insertions
cause a
change in
reading frame
(frameshift)
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Lys
Met
Leu
Ala
His
Sickle Cell Anemia is due to a
Base Substitution
Normal hemoglobin DNA
C
T
T
G
A
A
mRNA
Mutant hemoglobin DNA
C
A
T
G
U
A
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
1 nucleotide changes 1 amino acid resulting in a misfolded
hemoglobin protein (clump together in RBCs > sickle shape)
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4. Translation
Overview of Translation
Translation occurs in the cell cytoplasm and
involves:
Messenger RNA (mRNA)
• copy of the gene to be expressed into protein
Ribosome
• structure made of rRNA and protein that catalyzes
the polymerization of amino acids into a polypeptide
Transfer RNAs (tRNAs)
• deliver amino acids to ribosomes during translation
Messenger RNA (mRNA)
Start of genetic message
AUG…
5’
End
3’
UGA or
UAA or
UAG
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Ribosome Structure
tRNA-binding sites
Large
subunit
mRNA
binding
site
Small
subunit
tRNA Structure & Function
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
mRNA codons will
“base pair” with
a complementary
anticodon in a tRNA…
Anticodon
Ribosomes translate mRNA to Protein
Next amino acid
to be added to
polypeptide
in the
cytoplasm
Growing
polypeptide
tRNA
mRNA
5’
3’
Codons
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Translation (“protein synthesis”)
The building of a polypeptide, 1 amino acid at
a time, by ribosomes using info in mRNA:
• ribosomes bind directly to mRNA, “read” codon by
codon
• ribosomes always start at AUG (methionine)
• translation also involves tRNAs, each of which is
attached to 1 of the 20 amino acids (AAs)
• ribosomes match the right tRNA (via the anticodon)
with the right codon in the mRNA, then add its AA to
the growing protein
Initiation of Translation
M et
M et
Large
ribosomal
subunit
Initiator tRNA
P site
1
Start
codon
mRNA
Small ribosomal
subunit
A site
2
• small ribosomal subunit, mRNA, and intiator tRNA
(tRNAMet) assemble first (at P site)
• large ribosomal subunit then joins the complex
after which a 2nd tRNA enters the A site…
• tRNA with complementary anticodon
Amino
acid
Polypeptide
A site
P site
Anticodon
mRNA
Elongation of
Translation
Codons
1 Codon recognition
1) tRNA with
complementary
anticodon enters
A site
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
peptide
bond
3 Translocation
2) peptide bond
forms between
amino acids from
each tRNA
3) ribosome shifts
(translocates) to
next codon,
process repeats…
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Termination of
Translation
New peptide
bond forming
Growing
polypeptide
Elongation
A succession of tRNAs
add their amino acids
to the polypeptide chain
as the mRNA is moved
through the ribosome,
one codon at a time.
Codons
mRNA
Polypeptide
Termination
The ribosome
recognizes a stop
codon. The polypeptide
is terminated and
released.
Stop codon
General Steps of Translation
1) ribosome begins translation at AUG of mRNA
2) ribosome binds 2 tRNA-AAs, 2 codons at a time
• i.e., tRNAs with anti-codons complementary to the mRNA
codons
3) ribosome then catalyzes peptide bond formation
between the amino acids attached to each tRNA
4) ribosome shifts 3 nucleotides (1 codon) on mRNA
and repeats the process
5) this continues until the ribosome reaches a
“stop” codon which causes translation to end
Summary
of Gene
Expression
Transcription
DNA
1
mRNA
RNA
polymerase
mRNA is transcribed
from a DNA template.
Translation
Amino acid
2
Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and
ATP.
Enzyme
ATP
tRNA
Anticodon
Large
ribosomal
subunit
Initiator
tRNA
3
Initiation of
Translation
The mRNA, the first
tRNA, and the ribosomal
sub-units come together.
Start Codon
mRNA
Small
ribosomal
subunit
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Key Terms for Chapter 10B
• mRNA, tRNA, rRNA
• transcription, RNA polymerase
• codon, anti-codon
• genetic code
• mutation: substitution, deletion, insertion
• reading frame, frameshift
• translation, ribosome
Relevant Review Questions:
1, 3-5, 7, 9
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