Transcript Gene, Protein Synthesis & Gene Regulation

Structure & concept of gene,
One gene one enzyme hypothesis,
Genetic Code
PROTEIN SYNTHESIS,
Regulation of gene expression
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany Deptt.
P.G.G.C.G. -11,Chandigarh
Definitions of the gene
• The gene is the unit of genetic
information that controls a specific
aspect of the phenotype.
• The gene is the unit of genetic
information that specifies the synthesis
of one polypeptide.
1942: George Beadle and Edward Tatum

Studied relationships between genes and enzymes in the haploid
fungus Neurospora crassa (orange bread mold).

Discovered that genes act by regulating definite chemical events.

One Gene-One Enzyme Hypothesis
Each gene controls synthesis/activity of a single enzyme.
“one gene-one polypeptide”
1958: George Beadle (Cal Tech) & Edward Tatum (Rockefeller Institute)
Beadle and Tatum (1942)--One
Gene, One Enzyme
•
•
•
Bread mold Neurospora can
normally grow on minimal
media, because it can
synthesize most essential
metabolites.
If this biosynthesis is under
genetic control, then mutants in
those genes would require
additional metabolites in their
media.
This was tested by irradiating
Neurospora spores and
screening the cells they
produced for additional
nutritional requirements
(auxotrophs).
Beadle and Tatum proposed: “One Gene-One Enzyme Hypothesis”
However, it quickly became apparent that…
1.
More than one gene can control each step in a pathway
(enzymes can be composed of two or more polypeptide chains,
each coded by a separate gene).
2.
Many biochemical pathways are branched.
“One Gene-One Enzyme Hypothesis”
“One Gene-One Polypeptide Hypothesis”
Modern Concept of Gene
• Until 1940, the gene was considered as
the basic unit of genetic information as
defined by three criteria.
- Cistron:The unit of function, controlling
the inheritance of one “character” or
phenotypic attribute.
– Recon : The unit of recombination
– Muton:The unit of mutation.
Genetic code:
Def. Genetic code is the nucleotide base sequence on DNA ( and
subsequently on mRNA by transcription) which will be translated into a
sequence of amino acids of the protein to be synthesized.
The code is composed of codons
Codon is composed of 3 bases ( e.g. ACG or UAG). Each codon is
translated into one amino acid.
The 4 nucleotide bases (A,G,C and U) in mRNA are used to produce the
three base codons. There are therefore, 64 codons code for the 20 amino
acids, and since each codon code for only one amino acids this means
that, there are more than one cone for the same amino acid.
How to translate a codon (see table):
This table or dictionary can be used to translate any codon sequence.
Each triplet is read from 5′ → 3′ direction so the first base is 5′ base,
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followed by the middle base then the last base which is 3′ base.
Examples: 5′- A UG- 3′ codes for methionine
5′- UCU- 3′ codes for serine
5′ - CCA- 3′ codes for proline
Termination (stop or nonsense) codons:
Three of the 64 codons; UAA, UAG, UGA do not code for any amino
acid. They are termination codes which when one of them appear in
mRNA sequence, it indicates finishing of protein synthesis.
Characters of the genetic code:
1- Specificity: the genetic code is specific, that is a specific codon
always code for the same amino acid.
2- Universality: the genetic code is universal, that is, the same codon is
used in all living organisms, procaryotics and eucaryotics.
3- Degeneracy: the genetic code is degenerate i.e. although each codon
corresponds to a single amino acid,one amino acid may have more than
one codons. e.g arginine has 6 different codons
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Gene mutation (altering the nucleotide sequence):
1- Point mutation: changing in a single nucleotide base on the mRNA
can lead to any of the following 3 results:
i- Silent mutation: i.e. the codon containg the changed base may code
for the same amino acid. For example, in serine codon UCA, if A is
changed to U giving the codon UCU, it still code for serine. See table.
ii- Missense mutation: the codon containing the changed base may code
for a different amino acid. For example, if the serine codon UCA is
changed to be CCA ( U is replaced by C), it will code for proline not
serine leading to insertion of incorrect amino acid into polypeptide chain.
iii- Non sense mutation: the codon containing the changed base may
become a termination codon. For example, serine codon UCA becomes
UAA if C is changed to A. UAA is a stop codon leading to termination
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of translation at that point.
How your cell makes very
important proteins
• The production (synthesis) of proteins.
• 3 phases:
1. Transcription
2. RNA processing
3. Translation
• DNA  RNA  Protein
DNA  RNA  Protein
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
Before making proteins, Your
cell must first make RNA
• How does RNA (ribonucleic acid) differ
from DNA (deoxyribonucleic acid)?
RNA differs from DNA
1. RNA has a sugar ribose
DNA has a sugar deoxyribose
2. RNA contains uracil (U)
DNA has thymine (T)
3. RNA molecule is single-stranded
DNA is double-stranded
1. Transcription
• Then moves along one of the DNA strands
and links RNA nucleotides together.
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
1. Transcription OR
RNA production
• RNA molecules are produced by copying
part of DNA into a complementary
sequence of RNA
• This process is started and controlled by
an enzyme called RNA polymerase.
1. Transcription
DNA
RNA Polymerase
pre-mRNA
Types of RNA
• Three types of RNA:
A. messenger RNA (mRNA)
B. transfer RNA (tRNA)
C. ribosome RNA (rRNA)
• All types of RNAproduced in the nucleus!
mRNA
• Carries instructions from DNA to the
ribosome.
• Tells the ribosome what kind of
protein to make
A. Messenger RNA (mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein methionine
codon 2
codon 3
glycine
serine
codon 4
isoleucine
codon 5
codon 6
glycine
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
rRNA
• Part of the structure of a ribosome
• Helps in protein production
tRNA
• Bring right amino acid to make the right protein
according to mRNA instructions
B. Transfer RNA (tRNA)
amino acid
attachment site
methionine
U A C
anticodon
amino acid
RNA Processing
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
RNA Processing
(Post Transcriptional Changes)
• Introns are pulled out and exons
come together.
• End product is a mature RNA
molecule that leaves the nucleus
& move to the cytoplasm.
RNA Splicing
pre-RNA molecule
exon
intron
exon
intron
exon
intron
intron
exon
splicesome
exon
exon
splicesome
exon
exon
exo
n
Mature RNA molecule
Ribosomes
Large
subunit
P
Site
A
Site
mRNA
A U G
Small subunit
C U A C U U C G
3. Translation - making
proteins
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
3. Translation
• Three parts:
1. initiation: start codon (AUG)
2. elongation:
3. termination: stop codon (UAG)
3. Translation
Large
subunit
P
Site
A
Site
mRNA
A U G
Small subunit
C U A C U U C G
Initiation
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
Elongation
peptide bond
aa3
aa1
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
aa1
peptide bonds
aa4
aa2
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
peptide bonds
aa1
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
mRNA
terminator
or stop
codon
C A U G U U U A G
End Product
• The end products of protein synthesis is
a primary structure of a protein.
• A sequence of amino acid bonded
together by peptide bonds.
aa2
aa1
aa3
aa4
aa5
aa199
aa200
Regulation of Gene
Expression
The control of gene
expression
• Each cell in the human contains all the genetic
material for the growth and development of a human
• Some of these genes will be need to be expressed all
the time called Constitutive genes
• These are the genes that are involved in of vital
biochemical processes such as respiration
• Other genes are not expressed all the time
• They are switched on an off at need called Nonconstitutive genes
Operons
• An operon is a
group of genes that
are transcribed at
the same time.
• They usually control
an important
biochemical
process.
Jacob, Monod & Lwoff
Inducible Genes - Operon Model
• Definition: Genes whose expression is
turned on by the presence of some
substance
– Lactose induces expression of the lac
genes
• Catabolic pathways
Lactose Operon
• Structural genes
– lac z, lac y, & lac a
– P-Promoter
– Polycistronic
mRNA
• R-Regulatory
gene
– Repressor
• Operator
• Inducer - lactose
Regulatory
Gene
i
Operon
p
o
z
y
a
DNA
m-RNA
Protein
-Galactosidase
Transacetylase
Permease
Lactose Operon
• Inducer -lactose
Absence of lactose
i
p
y
a
No lac mRNA
• Active repressor
• No expression
• Inactivation of
repressor
• Expression
z
Active
– Absence
– Presence
o
Presence of lactose
i
p
o
z
y
a
Inactive
-GalactosidasePermease Transacetylase
1. When lactose is absent
• A repressor protein is continuously synthesised. It sits on
a sequence of DNA just in front of the lac operon, the
Operator site
• The repressor protein blocks the Promoter site where
the RNA polymerase settles before it starts transcribing
Repressor
protein
DNA
I
O
Regulator
gene
Operator
site
© 2007 Paul Billiet ODWS
RNA
polymerase
Blocked
z
y
lac operon
a
2. When lactose is present
• A small amount of a sugar allolactose is formed within
the bacterial cell. This fits onto the repressor protein at
another active site (allosteric site)
• This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the
operator site. RNA polymerase can now reach its
promoter site
DNA
I
O
z
y
Promotor site
a
Repressible Genes - Operon Model
• Definition: Genes whose expression is
turned off by the presence of some
substance (co-repressor)
– Tryptophan represses the trp genes
• Co-repressor is typically the end
product of the pathway
Tryptophan Operon
• Structural genes
– trp E, trpD, trpC
trpB & trpA
– Common promoter
• Regulatory Gene
– Apo-Repressor
Regulatory
Gene
R
Operon
P
O
L
E
D
C
• Inactive
• Operator
• Co-repressor
– Tryptophan
Inactive repressor
(apo-repressor)
5 Proteins
B
A
Tryptophan Operon
Absence of Tryptophan
• Co-repressor -tryptophan
– Absence of
tryptophan
• Gene expression
R
O
L
E
Inactive repressor
(apo-repressor)
C
B
A
B
A
5 Proteins
R
P
O
L
E
• Activates repressor
– No gene expression
D
Presence of Tryptophan
• Negative control
– Presence of
tryptophan
P
D
C
No trp mRNA
Inactive repressor
(apo-repressor)
Trp
(co-repressor)