Transcript GENE EXPRESSION CH 17

GENE EXPRESSION
CH 17
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I. Basic principles of gene expression
• A. General characteristics
• process by which genetic info in DNA is converted to
protein
– DNA → RNA is transcription
– RNA → protein is translation
• RNA is the bridge between proteins and genes that
code for them
• The concept of gene is universal to all domains of life
• The general process of gene expression is also
universal
• The genetic code is also universal
B. The genetic code
• Language of DNA and RNA are nucleotides
• Language of proteins are amino acids
• The nucleotide sequence must be translated
into amino acid sequence
• Nucleotide sequence is read in groups of 3
nucleotides called codons
• The genetic code is redundant
II. Transcription
• Process by which the genetic info in DNA is
copied into RNA
• Occurs at specific regions of DNA called genes
• the basic structure of genes are the same in
all domains
Promotor: where transcription starts.
Coding sequence: what is transcribed to RNA
Terminator: where transcription stops
A. The process of transcription
• Only one strand of the
DNA is transcribed into
RNA, the template
strand
• RNA strand is
complementary to DNA
strand copied
• Enzyme is RNA
polymerase
Three stages: initiation, elongation,
termination
Initiation: RNA polymerase
binds to promoter with the
help of various transcription
factors and unzips DNA.
Elongation: RNA
polymerase reads
template stand of
DNA making RNA
Termination: transcription stops
B. Post-transcriptional processing of
mRNA in eukaryotes
Before mRNA is usable it must be processed
• 5’ CAP and 3’ poly A tail put on
• Purpose of CAP and tail: help RNA leave
nucleus, prevent its degradation, and
help ribosome bind to 5’end
• Splicing out of introns
• Introns: segments of gene that are transcribed into
mRNA but don’t code for protein
• Must be cut out
• snRNPs binds to
intron/exon junction
• snRNPs attract each
other looping out the
introns
• introns are cut out and
exons are glued together
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III. Translation
• Process by which genetic
information carried in the
mRNA is converted into
protein
• Requires the help of tRNA
which transfers amino
acids to growing protein in
the ribosome
A. The structure of a tRNA
• Anticodon: a group of 3
nucleotides
complementary to a
codon in mRNA
• CCA site: place where
amino acid is attached
• Accurate translation requires 2 steps:
– There must be a correct match between tRNA
and amino acid which is done by aminoacyl tRNA
synthase
– There must be a correct match between
anticodon and codon
B. Ribosomes
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Where translation occurs
Facilitates interaction of tRNA and mRNA
Made of 2 subunits of rRNA
Overall structure of bacterial and eukaryotic
ribosomes are similar but many antibiotics
target bacterial ribosomes without affecting
eukaryotic ribosomes
Ribosomes have 3 binding sites for tRNA
P site: holds the tRNA that carries growing
polypeptide chain
A site: holds the tRNA carrying the next amino acid to
be added to growing chain
E site: exit site where free tRNAs leave
C. The Process of Translation
• Occurs at the ribosome and is fundamentally
the same in prokaryotes and eukaryotes but
eukaryotic ribosomes are larger
• occurs in 3 stages: initiation, elongation,
termination
1. Initiation
• mRNA interacts with a ribosome such that 1st
AUG sits in the P site
• initiator tRNA binds to the FIRST AUG codon in
mRNA
2. Elongation and translocation
• 2nd tRNA with the
correct anticodon binds
to the 2nd codon in
mRNA in the A site
• The 2 adjacent amino
acids are linked via
dehydration reaction
• Ribosome moves down
3 nucleotides
• 1st tRNA leaves thru the
E site
• This process continues
3. Termination
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• At the stop codon:
Release factor binds to stop codon in the A
site
Translation stops and protein leaves
If a protein is destined for another location like
the cell membrane or is to be secreted, it has a
signal sequence that brings the ribosome to
the RER
Many ribosomes can translate mRNA at the
same time forming polyribosome. Can make a
lot of protein quickly
• Protein can be modified after translation to
make the functional protein:
– 2 or more protein chains interact to form the
functional protein (quaternary structure)
– Small carbohydrate chains can be added to some
proteins
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IV. Mutations
• Changes in the DNA sequence
• Can be a product of mistakes made during
replication, transcription, or DNA repair
• Most are caused by mutagens: agents that
damage DNA
• Most change the way the protein folds,
affecting its function
• Two types of small scale mutations:
substitution mutations and frameshift
mutation
•
Base substitution mutation: a single nucleotide is
changed. Can be silent, missense, nonsense
Silent: no effect on protein due to
redundancy in genetic code
Missense: mutation results in a different
amino acid
Nonsense: amino acid changed to stop codon
• Insertion /deletion mutations: loss or
addition of nucleotides and are most often
disastrous
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V. Evolutionary significance of
Mutations
• Mutation rate is relatively low. Keeps genome
constant from generation to generation
– DNA repair mechanisms
– DNA polymerase proofreads
– Double strandedness and coiling of DNA protect it
• However, mutations do occur. Mutations
provide genetic variation for evolution to act on.