gene-expression-text
Download
Report
Transcript gene-expression-text
CHMI 2227E
Biochemistry I
Gene expression
CHMI 2227 - E.R. Gauthier, Ph.D.
1
Nature of the gene
Genes are discrete regions of a DNA
molecule
Genes are permanent fixtures of your
DNA;
Every single cell in your body (every 100
trillion [i.e. 10-15 cells] of them) has the
exact, same DNA, the exact, same set of
genes.
Genes have « encoding » property:
The order of the nucleotide bases in a
given gene (their sequence) has meaning;
For example: the order of nucleotide bases
in the myoglobin gene has all the required
information to make the myoglobin
protein.
CHMI 2227 - E.R. Gauthier, Ph.D.
2
Species
Humans
Drosophila (Drosophila
melanogaster)
Yeast
(Saccharomyces cerevisiae)
Nematode (Caenorhabditis
elegans)
E. coli
Arabidopsis (Arabidopsis
thaliana)
Genome
size
Number
of genes
2.9 billion
base pairs
20,00025,000
120 million
base pairs
13,601
12 million
base pairs
6, 275
97 million
base pairs
19,000
4.1 million
base pairs
4,800
125 million
base pairs
25,000
CHMI 2227 - E.R. Gauthier, Ph.D.
3
Nature of the gene
But: chimps,
humans and even
worms have the
SAME NUMBER OF
GENES…
Moreover, most of
these genes are
SHARED between
these three
species…
What the %$#@?
CHMI 2227 - E.R. Gauthier, Ph.D.
4
Nature of the gene
Genes can be turned on or
off (i.e. regulated):
Temporally: e.g. during
development;
Spatially: e.g. turned on in
the brain/ turned off in the
liver.
Timing and location (i.e.
Different homeotic (Hox) genes
regulation) of gene
(colors) are turned on (expressed)
expression is more
at specific time during development
important than the actual
and lead to the formation of specific
number of genes.
body structures.
CHMI 2227 - E.R. Gauthier, Ph.D.
5
What happens when gene
expression is screwed-up?
Homeotic genes are responsible for the
development of discrete regions of the body:
Antp (antennapedia) specifies the formation
of legs wings/thorax (flies) / thorax (humans)
Dfd (deformed) specifies the formation of the
head (flies)/neck (humans)
If the Antp gene is expressed in the same
cells as Dfd, legs will grow on the head of
the fly. That’s what happens in flies bearing
the Antennapedia mutation.
CHMI 2227 - E.R. Gauthier, Ph.D.
Normal
fly
Antennapedia
mutation
6
Central dogma of molecular biology
CHMI 2227 - E.R. Gauthier, Ph.D.
7
Transcription: turning on a gene
Each gene is preceded by a stretch of DNA called the promoter;
The promoter has DNA sequences that allow the recruitment of a distinct
set of proteins called transcription factors;
These transcription factors help to recruit RNA polymerase, the enzyme
which catalyses the synthesis of an RNA molecule complementary and
antiparallel to one of the DNA strands of the gene (the template strand).
Transcription
factors
Promoter
CHMI 2227 - E.R. Gauthier, Ph.D.
8
Promoters
Two major sequences are found in bacterial promoters:
TATA box (aka Pribnow box):
Centered 10 bp away from the transcription initiation site (denoted
+1 below);
Directly binds RNA pol
Tells the RNA pol where to start transcription
-35 box:
Centered 35 bp away from the transcription initiation site
Directly binds RNA pol
Helps to stabilize the binding of RNA pol to the promoter
CHMI 2227 - E.R. Gauthier, Ph.D.
9
Promoters
Variations in the nucleotide sequence of
the promoter are responsible for changes
in the timing of gene expression;
CHMI 2227 - E.R. Gauthier, Ph.D.
10
Transcription
CHMI 2227 - E.R. Gauthier, Ph.D.
11
Transcription
Messenger RNA
(mRNA) is antiparallel to
template strand
(5’ 3’)
CHMI 2227 - E.R. Gauthier, Ph.D.
Complementarity
12
mRNAs are modified in eukaryotes
CHMI 2227 - E.R. Gauthier, Ph.D.
13
mRNAs are modified in eukaryotes
In bacteria, the mRNA is nice and ready to
make proteins (i.e. being translated) as
soon as it is produced;
In eukaryotes, the mRNA needs to be
further processed before it can be
translated:
Removal of introns splicing
Addition of a non-coded GTP nucleotide at
the 5’ end of the mRNA capping
Addition of 50-200 non-coded adenosine
residues at the 3’end of the mRNA
polyadenylation
Export of the mRNA out of the nucleus and
into the cytoplasm (because translation
ONLY occurs in the cytoplasm).
Introns: part of the gene which is transcribed but NOT found in the mRNA.
CHMI
2227 up
- E.R.
Ph.D. after transcription.
Exons: part of the gene which
ends
inGauthier,
the mRNA
14
mRNAs are translated into
proteins
During translation, the nucleotide
sequence of the mRNA is read and
decoded into an amino acid sequence.
CHMI 2227 - E.R. Gauthier, Ph.D.
15
Translation
To translate an mRNA into a protein, the
following ingredients are needed:
mRNA template
Amino acids
Transfer RNA (tRNA):
adaptor between amino acid and mRNA
In charge of converting the nucleotide sequence code into
an amino acid sequence.
Ribosomes:
organelles directing the translation process.
CHMI 2227 - E.R. Gauthier, Ph.D.
16
Transfer RNA (tRNA)
Short RNA molecule (between 73 and 95
nucleotides long);
Has two major features:
The acceptor arm: where a specific amino
acid is covalently coupled;
The anticodon arm:
has a 3-nucleotide sequence: the anticodon
The anticodon base pairs with a
complementary and antiparallel, 3
nucleotide sequence on the mRNA: the
codon;
The genetic code is the relationship
between the sequence of a codon with a
specific amino acid.
CHMI 2227 - E.R. Gauthier, Ph.D.
17
The genetic code
Each tRNA is bound to a unique, specific amino
acid;
The nature of this amino acid depends on the
sequence of the anticodon;
Since the anticodon on the tRNA base pairs with
the codon on the mRNA, there is a relationship
between the sequence of the codon on the mRNA
and the amino acid bound to the tRNA: this
relationship is the GENETIC CODE.
Thus, in this example, the mRNA codon 5’UGU3’
codes for the amino acid Cys.
The genetic code is (almost) universal: the
meaning of each codon is (pretty much) the same
whatever organism is studied.
CHMI 2227 - E.R. Gauthier, Ph.D.
3’
5’
A
U
5’
3’
mRNA
18
The genetic code
CHMI 2227 - E.R. Gauthier, Ph.D.
19
Translation
Translation is the phenomenon whereby the mRNA code (i.e. the
sequence of codons) is decoded into an amino acid sequence;
Translation involves three major steps:
Initiation: recognition of the translation starting point by the ribosome
and a special tRNA;
Elongation:
successive binding of tRNAs to the ribosome;
formation of a peptide bond between an incoming amino acid and the
growing polypeptide chain;
translocation of the ribosome to decode the next codon;
Termination:
One of three « stop » codons reaches the ribosome
The ribosome is dissociated, freeing the mRNA and the completed
polypeptide;
CHMI 2227 - E.R. Gauthier, Ph.D.
20
Translation - Initiation
fMET = N-formyl methionine
The initiation codon is ALWAYS AUG Methionine
http://biology.unm.edu/ccouncil/Biology_124/Images/RNAtranslation.jpeg
CHMI 2227 - E.R. Gauthier, Ph.D.
21
Translation - Elongation
Peptide bond: synthesized by the « peptidyl transferase »
enzyme on the large ribosomal subunit
Translocation
http://biology.unm.edu/ccouncil/Biology_124/Images/RNAtranslation.jpeg
CHMI 2227 - E.R. Gauthier, Ph.D.
22
Translation - Termination
Three « Stop » codons exist: UGA; UAA, UAG
http://biology.unm.edu/ccouncil/Biology_124/Images/RNAtranslation.jpeg
CHMI 2227 - E.R. Gauthier, Ph.D.
23
Cue from environment (hormones, temperature, etc)
Protein function
Change in cell behavior
Change in organism
http://www.contexo.info/DNA_Basics/images/gene_expression.gif
CHMI 2227 - E.R. Gauthier, Ph.D.
Adaptation to
environment
24