cmb lect 7 2011 colour 2 slides per page

8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page

1/17

3/11/20

Lecture 7

Transcription (mRNASynthesis) and Translation

BIOSCI 101: Cellular and Molecular Biology

Transcription and Translation Transcription is the process of transfer of

information from DNA into RNA (reverseranscr p ase ac eves e reverse process

The existence of a messenger molecule whichtransferred genetic information from thenucleus to the cytoplasm was firsthypothesised by Jacob and Monod


Information transfer is completed bytranslating the nucleotide sequence of mRNAinto the amino acid sequence of a protein


2/17

3/11/20

Information flow from gene toprotein


Transcription and Translation (contd)

Synthesis of messenger RNA (transcription)

a) an enzyme called RNA polymerase binds to anucleotide sequence known as the promoter

- as RNA polymerase moves along the DNAmolecule the two strands of the moleculeseparate

b) the polymerase assembles ribonucleoside


- same base-pairing principle that governs DNAreplication (rA pairs with dT, rG with dC, rC withdG, rU with dA)


3/17

3/11/20

Stages oftranscription

BIOSCI 101: Cellular and Molecular BiologyCampbell 17.7

Elongation a closer look



4/17

3/11/20

Transcription and Translation (contd) Transcription (contd)

c) synthesis is in a 53 direction as the

35 direction

- as the enzyme moves along, the DNA helixrewinds, displacing the newly made mRNAmolecule

- transcription ceases when the polymerasereaches a termination signal and releases the


DNA template and mRNA product

d) the finished mRNA strand is antiparallel andcomplementary to the template strand fromwhich it was transcribed


Transcription (contd)

e) both strands may code for mRNA and somegenes may overlap

f) the average mRNA molecule is 900 - 1200bases long (sufficient to code for an averagepolypeptide 300 - 400 amino acids long)


- in prokaryotes mRNAs may code for morethan one polypeptide


5/17


6/17

3/11/20

RNA splicing (1)


RNAsplicing

(2)



7/17

3/11/20

Exons may encode protein domains


Translation the basic



8/17

3/11/20


Protein Synthesis (Translation)

a) mRNA

- the sequence of bases in DNA and then mRNAspecifies the amino acid sequence

- AUG = start codon, UAA, UAG and UGA = stopcodons (see next lecture)


- every mRNA molecule contains a ribosomalbinding site at its 5 end

Transcription and Translation (contd) Translation (contd)

- ribosomes bind to the 5 end of newlytranscribed mRNA

- the small ribosomal subunit has a bindingsite for the mRNA molecule

- the large subunit has three binding sites fortRNA molecules (the peptidyl site, the


aminoacyl site and the exit site)


9/17

3/11/20


Translation (contd)

- the peptidyl (P) site holds the tRNA carryingthe growing polypeptide chain

- the aminoacyl (A) site holds the tRNAcarrying the next amino acid to be added

- the exit (E) site is the site from whichdischarged tRNAs leave the ribosome


Anatomy ofa ribosome

tRNA

molecules

Growing

polypeptide Exit tunnel

E

Large

subunit

Small

subunit

mRNA5

3

A si te

P site Exit tunnel


Small

subunit

Large

subunitP A

mRNA

binding site

E site

E


10/17

3/11/20


Translation (contd)

c

- adapter molecules which bring amino acids intoregister on the message

- for each of the 20 amino acids there is at leastone specific tRNA molecule present in the cell

- the tRNA molecule possesses a site for


anticodon loop to pair with the mRNA codon

- specific enzymes (aminoacyl tRNA synthetases)recognise the tRNAs and attach the correct aminoacid

The structure of transfer RNA



11/17

3/11/20

Specific

amino acidsare joined to


Campbell 17.16


Translation (contd)

d) Synthesis of a polypeptide involves three stages:-

i) initiation

-commonly used start signal

- the smaller ribosomal subunit attaches to the 5 endof the mRNA

- the first tRNA molecule bearing the amino acid met,plugs into the initiator codon AUG on the mRNAmolecule


- the larger ribosomal subunit locks into place withthe met tRNA occupying the P site

- the A site is vacant and the initator complex is nowcomplete


12/17

3/11/20

Translation 1:Initiation



Translation (contd)

ii) elongation

- a second aminoacyl tRNA binds to the codonin the A site

- a component of the larger subunit catalysesthe formation of a peptide bond between thetwo amino acids brought together at the


. ,the first amino acid and its tRNA is broken


13/17

3/11/20


elongation (contd)- the ribosome translocates ie moves alongthe mRNA chain in a 5 to 3 direction and thesecond tRNA, with the mRNA and thedipeptide attached, is moved from A to P

- the first tRNA moves from P to E and isreleased from the ribosome

- a third tRNA moves into the A site andanother e tide bond is formed. This ste is


repeated until the polypeptide is complete

Translation 2:Elongation



14/17

3/11/20


Translation (contd)

iii termination

- the ribosome reaches a termination codon(UAA, UAG or UGA)

- release factor binds to the A site

- the polypeptide is cleaved from the last tRNAwhich is then released


- e m an r osoma su un s ssoc a e

Translation 3: Termination



15/17

3/11/20


Translation (contd)

- more than one ribosome binds to each mRNA

- a polyribosome consists of a single mRNA alongwhich are travelling a series of ribosomes towhich in turn are attached growing peptidechains




16/17

3/11/20


Translation (contd)

- many proteins are modified in the ER or theGolgi after they are made

- many hormone molecules mature via acleavage reaction

- glycosylation is the addition of carbohydrate


completed)

- signal sequences at the N-terminus of thepolypeptide target proteins destined for secretionto the ER and Golgi

Proteins may be targeted tothe ER



17/17

3/11/20

Test question

Which of the following is NOT directly involved inthe process of translation?

1. mRNA

2. rRNA

3. tRNA

4. aminoacyl-tRNA synthetase

5. ligase


cmb lect 7 2011 colour 2 slides per page

Documents