Transcription Protein Synthesis

7/29/2019 Transcription Protein Synthesis

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TRANSCRIPTION &

PROTEIN SYNTHESIS

By

Dr Gnanajothy Ponnudurai



LECTURE OUTLINE

A . TRANSCRIPTION

B . THE GENETIC CODE

B . PROTEIN SYNTHESIS

i . Components required to make a polypept ide

i i . Steps in volved in trans lat ion

C. PROTEIN MATURATION AND MODIFICATION

D. TARGETTING OF POLYPEPTIDES TO SPECIFIC

DESTINATIONS IN THE CELL

E. REGULATION OF GENE EXPRESSION IN EUKARYOTES



AIMS AND OBJECTIVES:

After com plet ing th is top ic you should be able to:

Descr ibe what is transcr ipt ion and post-t ransc r ipt ional

modi f icat ion

Discuss the character ist ics of genet ic co de

Discus s the components requi red for prote in s ynthesis .

Descr ibe the events that occu r dur ing protein syn thesis.

Explain the proc ess of p rotein maturat ion and

modi f icat ion

Explain the impo rtance of regu lat ion o f gene expression

in eukaryotes



OVERVIEW OF TRANSCRIPTION AND

TRANSLATION

Flow of genetic information: DNA RNA

protein

Transcription : synthesis of RNA under thedirection of DNA.

Intermediate in flow of information from DNA to

protein is messenger RNA (mRNA).

Translation : synthesis of polypeptide under the

direction of mRNA.

The sites of translation are ribosomes.



Central Dogma of Molecular

Biology

DNA

RNA

Protein

Replication

Tanscription

Translation



OVERVIEW OF TRANSCRIPTION AND

TRANSLATION

Eukaryotic cell:

RNA is first synthesized as pre-RNA, whichundergoes modifications called RNA processingbefore leaving the nucleus as mature RNA.

Transcription occurs in nucleus whereas translationoccurs in the ribosomes in cytoplasm.

Prokaryotic cell :

No RNA processing

Transcription and translation can occur simultaneously





RNA SYNTHESIS

(TRANSCRIPTION OF GENES)

A closer look at Transcription

Transcription is the DNA-directed synthesis of

RNA. Transcription is catalysed by the enzyme RNA

polymerase.

Only one DNA strand is transcribed (copied).

RNA polymerase does not require a primer.





POSTTRANSCRIPTIONAL MODIFICATION

OF RNA (RNA PROCESSING)

Split genes and RNA splicing

Most eukaryotic genes are interrupted by long,noncoding regions called introns (intervening

sequences), interspread among coding regionscalled exons.

Both introns and exons are transcribed to form a

pre-RNA

RNA splicing involves removing the introns and joining the exons.





THE GENETIC CODE

There are only 4 nucleotide bases to specify 20 amino

acids.

4 nucleotide bases can generate 64 possible triplet

combinations. In an mRNA transcribed from a gene, these base triplets

are called codons.

The genetic code is a collection of codons.

Of the 64 codons, 61 code for the 20 amino acids.

3 codons code for termination of protein synthesis





THE GENETIC CODE

There is redundancy in the genetic code, but no ambiguity.

The genetic code is nearly universal.

The ‘start’ signal or initiation codon AUG sets the reading

frame.

(Since AUG also stands for methionine, polypeptide chainsbegin with methionine when they are synthesized. An

enzyme may subsequently remove this starter amino acid

from a chain)

The genetic code is nonoverlapping.





PROTEIN SYNTHESIS (TRANSLATION)

Translation is the RNA-directed synthesis of a polypeptide



DNA

Transcription

Primary transcript pre-tRNA pre-rRNA pre-mRNA

Mature RNA tRNA rRNA mRNA

Structural and regulatory Protein

functions. No translation

Eukaryotic transcription and translation



Components required to make a polypeptide

1. Amino acids

20 different amino acids




2. Messenger RNA (mRNA)

Template for synthesis of polypeptide chain




3. Transfer RNA (tRNA)

tRNAs are transcribed from DNA templates.

tRNA is the carrier of amino acid in the cytosol.

A tRNA molecule has amino acid attachment site (bindsand carries amino acids) and anticodon (base triplet that

binds to complementary codon on mRNA)






4. Aminoacyl-tRNA synthetases

Enzymes required for the attachment of amino acids to

their corresponding tRNA.

The process is driven by hydrolysis of ATP.




5. Ribosomes

A ribosome consists of 2 subunits, a large subunit and a

small subunit.

Ribosomes = Proteins + ribosomal RNA (rRNA) Ribosome has two binding sites for tRNA molecules: A

site and P site.

In eukaryotes, ribosomes are either free in the cytosol or

on the endoplasmic reticulum.






6. Protein factors

Initiation factors, elongation factors, release factor




7. Energy source.

For attachment of amino acid to tRNA energy is provided

by ATP.

For chain inititation and elongation energy is provided byGTP.



Steps involved in translation

Initiation

Formation of initiation complex

• mRNA

•

tRNA carrying Methionine (the 1st

amino acid of thepolypeptide)

• 2 subunits of ribosomes

• proteins called initiation factors

Initiation codon (AUG) on mRNA is positioned in P site of

ribosomes. Anticodon of Met-tRNA pairs correctly with initiation codon

AUG.






Elongation

tRNA carrying the 2nd amino acid binds to A site of ribosomes.

Enzyme peptidyl transferase transfers Met from its tRNA, tothe second amino acid in A site, forming a peptide bond.

Ribosome moves by the distance of 1 codon, towards 3’-endof mRNA.

tRNA without the amino acid is released back into thecytosol.

Movement of ribosome shifts dipeptide-tRNA from A site to Psite

3rd amino acid-tRNA binds to empty A site.




Termination.

Termination requires stop codon.

Stop codon occupies A site on ribosome.

Release factor binds to A site. Peptidyl transferase cuts completed polypeptide from tRNA

in P site.

mRNA, tRNA, release factor leave the ribosome. Ribosome

dissociates into subunits.

PROTEIN MATURATION AND



PROTEIN MATURATION AND

MODIFICATION

A gene determines primary structure of a polypeptide, andprimary structure determines the three dimensionalconformation.

Post-translational modification may be required before a

protein can be functional:• Disulfide bond formation and protein folding

• Proteolysis : Enzymes may remove one or moreamino acids from the polypeptide, or cleave thepolypeptide

• Glycosylation: certain amino acids are modified byattachment of carbodydrates

• Attachment of lipids

• Protein phosphorylation

•

Assembly of various protein subunits



REGULATION OF GENE EXPRESSION

IN EUKARYOTES

Gene expression – gene is expressed when it is

transcribed and translated to yield product.

Regulation of gene expression permits only a small

fraction of total genes in a cell to be expressed at anygiven time. Remaining genes are inactive.



REGULATION OF GENE EXPRESSION

IN EUKARYOTES

Regulation of gene expression is required for:

Adaptation – cells adapt to environmental changes

by turning expression of genes ‘on’ and ‘off’.

Development & differentiation – physical and

physiological changes that occur are due to

variation in gene expression. Different proteinssynthesised in different quantities

Cells conserve energy by making proteins only

when required

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Transcription Protein Synthesis