GENE EXPRESSION

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OVERVIEW

Gene expression

Transcription

Post transcriptional modification

Transportation

Translation

Post translational modification

WHAT IS GENE & GENE EXPRESSION ?

Gene : is a stretch of DNA that encodes information.

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, the product is a functional RNA.

RNA PROTEINDNA

TRANSCRIPTION TRANSLATION

The “Central Dogma”

Eukaryote gene expression is regulated at six levels:

1. Transcription

2. RNA processing

3. mRNA degradation

4. mRNA transport

5. mRNA translation

6 . Protein degradation

A . TRANSCRIPTION

A . TRANSCRIPTION

“The production of messenger RNA (mRNA) from the DNA by the enzyme RNA polymerase “

Uses an enzyme RNA polymerase Proceeds in the same direction as replication (5’ to 3’) Forms a complementary strand of mRNA

It involves three steps : Initiation.

The DNA molecule unwinds and separates . RNA polymerase binds to the promoter of the template strand (also known as the 'sense strand' or 'coding strand'). The synthesis of RNA proceeds in a 5' to 3' direction, so the template strand must be 3' to 5'.

Elongation. RNA polymerase moves along the template strand, synthesising an mRNA molecule .In eukaryotes there are three RNA polymerases: I, II and III. The process includes a proofreading mechanism.

Termination.Stop codons UAA UAG UGA come termination occur

1.CONTROL REGIONS

A promoter.A region a few hundred nucleotides 'upstream' TSS

.It is not transcribed into mRNA, plays a role in controlling the transcription of the gene. Transcription factors bind to specific nucleotide sequences in the promoter region and assist in the binding of RNA polymerases.

Enhancers:Some transcription factors (called activators) bind to regions called 'enhancers' increase the rate of transcription. These sites may be thousands of nucleotides from the coding sequences or within an intron.

Silencers:Some transcription factors (called repressors) bind to regions called 'silencers' that depress the rate of transcription.

B.POST TRANSCRIPTIONAL

MODIFICATION

B.POST TRANSCRIPTIONAL MODIFICATION:

Capping changes the five prime end of the mRNA to a three prime end by 5'-5' linkage, which protects the mRNA from 5' exonuclease, which degrades foreign RNA. The cap also helps in ribosomal binding.

RNA editing is a process which results in sequence variation in the RNA molecule, and is catalyzed by enzymes.

Splicing removes the introns, noncoding regions that are transcribed into RNA, in order to make the mRNA able to create proteins. The two ends of the exons are then joined together.

Addition of poly(A) tail otherwise knownas polyadenylation. That is, a stretch of RNA that ismade solely of adenine bases is added to the 3' end,and protects from 3' exonuclease. In addition, a longpoly(A) tail can increase translation.

2.(RNA PROCESSING CONTROL)

. : RNA processing regulates mRNA production from precursor RNAs.

Two independent regulatory mechanisms occur:

Alternative polyadenylation = where the polyAtail is added

Alternative splicing = which exons are spliced Alternative polyadenylation and splicing can occur together.

Examples: Human calcitonin (CALC) gene in thyroid and neuronal cells

Fig. 18.14, Alternative polyadenylation and splicing of the human CACL gene in thyroid and neuronal cells.

Calcitonin gene-related peptide

D.TRANSPORTATION :

D.TRANSPORTATION :

Transcription

DNA

messenger

RNA

Gene

Nucleus

D.TRANSPORTATION :

after post transcription the mature mrna is transported from the nucleas to the cytoplasm for translation process

4. mRNA transport control:

Eukaryote mRNA transport is regulated.

Some experiments show ~1/2 of primary transcripts never leave the nucleus and are degraded.

Mature mRNAs exit through the nuclear pores.

5. mRNA degradation control:

• All RNAs in the cytoplasm are subject to degradation.

• tRNAs and rRNAs usually are very stable; mRNAs vary considerably (minutes to months).

• Stability may change in response to regulatory signals and is thought to be a major regulatory control point.

• Various sequences and processes affect mRNA half-life:

• AU-rich elements

• Secondary structure

• Deadenylation enzymes remove As from poly(A) tail

• 5’ de-capping

• Internal cleavage of mRNA and fragment degradation

C. TRANSLATION

C. TRANSLATION In translation the mature

mRNA molecule is used as a template to assemble a series of amino acids to produce a polypeptide with a specific amino acid sequence

RIBOSOME.The complex in the cytoplasm .

It is a mixture of ribosomal proteins and ribosomal RNA (rRNA), and consist of a large subunit and a small subunit.

Amino acids , Transfer RNA

Mature RNA transcript , translation factors

MECHANISM OF TRANSLATION

Transcription involves four steps:Initiation. The small subunit of the ribosome binds at the 5' end of the mRNA molecule and moves in a 3' direction until it meets a start codon (AUG). It then forms a complex with the large unit of the ribosome complex and an initiation tRNA molecule.Elongation. Subsequent codons on the mRNA molecule determine which tRNAmolecule linked to an amino acid binds to the mRNA.

An enzyme peptidyl transferase links the amino acids together using peptide bonds. The process continues, producing a chain of amino acids as the ribosome moves along the mRNA molecule.Termination.Translation in terminated when the ribosomal complex reached one or

more stop codons (UAA(ochre ), UAG(amber), UGA(opal).

3.mRNA translation control: Unfertilized eggs are an example, in which mRNAs (stored in the egg/no

new mRNA synthesis) show increased translation after fertilization).

Polyadenylation and 5’ caping allows efficianet translation

E.POST TRANSLATIONAL MODIFICATIONS:

E.POST TRANSLATIONAL MODIFICATIONS:

• The chemical modifications that take place at certain amino acid residues after the protein is synthesized after translation are known as Post Translational modifications.

• They are essential for normal functioning of the proteins.

• They occur mostly in ER and Golgi bodies.

FUNCTIONS:• Increase functional diversity of the proteome.• Generate heterogeneity in proteins.• Transport

PTM’S

• Phosphorylation

• glycosylation

• Acetylation

• Alkylation

• Methylation

• Glycylation

• Lipoylation

• sulphation

5 .POST-TRANSLATIONAL CONTROL-PROTEIN DEGRADATION:

• Proteins are degraded in subcellular organelles, such as lysosomes or in macromolecular structures called proteasomes.

• Proteins can be short lived or long lived e.g lens proteins in our eyes.

• Protein degradation in eukaryotes requires a protein co-factor called ubiquitin. Ubiquitin binds to proteins and identifies them for degradation by proteolytic enzymes.

• Amino acid at the N-terminus is correlated with proteins stability and determines rate of ubiquitin binding.

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