Gene expression

1. GENE EXPRESSION IN
EUKARYOTES-I

2. INTRODUCTION
• The human genome is made up of 23 pairs of chromosome
containing 6 billion base pairs of DNA, with an estimated 30,000-
40,000 genes.
• But not all genes are expressed.
• Gene expression refers to the multistep process that ultimately
results in the production of a functional gene product, either
ribonucleic acid (RNA) or protein.
• Many of the mechanisms that control gene expression are used to
respond to developmental cues, growth factors, hormones,
environmental agents and therapeutic drugs.

3. • The living cells possess a
remarkable property to adapt to
changes in the environment by
regulating gene expression.
e.g. alcohol metabolism by
hepatocytes and synthesis of
insulin by pancreas.
• There are only two types of gene
regulation:
Positive regulation
Negative regulation
Negative
Regulation
Positive
Regulation
Regulator
present
Decreased Increased
Regulator
absent
Increased Decreased
Rate of Gene Expression
Effect of Positive and Negative Regulation
on Gene Expression :-

4. • Biologic systems exhibit three
types of temporal responses to a
regulatory signal-
Type A response
Type B response
Type C response

5. • In eukaryotes, gene expression
involves access to particular
regions of the DNA,
transcription, post-
transcriptional modification,
translation and post-
translational modification.
(Post-
transcriptional
modification)
Post-translational modifications

7. GENE EXPRESSION IN DNA LEVEL
• Gene expression in eukaryotes is also influenced by the accessibility
of DNA to the transcriptional apparatus, the amount of DNA and the
arrangement of DNA.
• DNA is found in chromatin form in eukaryotes.
Euchromatin- They are the decondensed chromatin.
- They are transcriptionally active.
Heterochromatin- More condensed and inactive form.
Interconversion between these two forms is known as chromatin
remodelling.

8. • DNA associates with histones to
form nucleosomes.
• Histones undergo various covalent
modifications known as histone
code, to determine gene activity.
Acetylation- by histone
acetyltransferase (HAT)
- addition of acetyl group
at the amino terminus
of histones.
- eliminates the positive
charge on lysine.
- transcription factors
have easier access to
specicific regions on DNA.
Deacetylation- by histone
deacetylases (HDAC)
- restores the positive
charge.

9. • Different proteins with specific
acetylase and deacetylase
activities are associated with
various components of the
transcription apparatus.
Code writers-
Proteins that catalyse the
histones PTMs.
Code readers-
Proteins that recognise, bind
and interprete these histone PTMs.
Code erasers-
Enzymes that remove histone
PTMs.

10. Methylation of DNA-
- by methyl transferases using S-
adenosylmethionine as the methyl donor.
- Cytosine in the sequence CG of DNA gets
methylated to form 5'- methylcytosine.
-Transcriptionally active genes are less
methylated than their inactive counterparts
- There is evidence that methylation leads to
the inactivation of the genes.
- Methylation of DNA correlates with
deacetylation and provides means for
repression of genes.

11. • Change in the number of copies of a gene can affect the amount of
gene product produced.
Increase in copy number (gene amplification) is seen in some diseases and in
response to particular chemotherapeutic drugs.
e.g. methotrexate- an inhibitor of dihydrofolate reductase (DHFR)
• Arrangement of DNA:-
The process by which immunoglobulins are produced
by B lymphocytes involves permanent rearrangements
of the DNA in these cells.

12. Synthesis of specific light chain of the antibody-
(light chain)

13. • Transposons are mobile segments of DNA that move in an essentially
random manner from one site to another on the same or different
chromosome.
Movement is mediated by transposase and can be direct or replicative.
In eukaryotes, most transposons are transcribed to RNA.
Replicative transposition frequently involves an RNA intermediates made by a
reverse transcriptase.
Out of the 50% of transposons present in the human genome, 90% is
retrotransposons.
Transposition has the potential to alter gene expression and cause disease.
e.g. hemophilia A and Duchenne muscular dystrophy.

14. EPIGENETIC MECHANISMS
• The study of role of covalent modification of DNA and histone and
non-histone proteins and ncRNAs in gene regulation, particularly
chemical modification and/or molecules stably altering gene
expression patterns without altering the underlying DNA gene
sequence is known as epigenetics.
• Epigenetic mechanisms play key roles in the establishment,
maintenance, and reversibility of transcriptional states.
• A key feature is that multiple rounds of cell division can maintained
the transcriptional on/off states.
• It regulates gene expression by modulating chromatin structure via
histone modification or modification of DNA via methylation.

15. • There are two forms of epigenetic signals-
1. Trans epigenetic signal
2. Cis epigenetic signal

16. • There are three ways by which cis-epigenetic marks can be propagated
through a round of DNA replication.

17. • Epigenetic therapy of cancers:-
The enzyme DNA methyltransferase (DNMT) responsible for DNA methylation
is targeted for cancer therapy.
Inhibitors of DNMT like 5-azacytidine is used for the treatment of leukemia.
Inhibitors of histone deacetylase (HDAC) are also employed in the epigenetic
thaerapy of cancer.
Voristat is used for the treatment of T-cell lymphoma.

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