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8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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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
BIOSCI 101: Cellular and Molecular Biology
Information transfer is completed bytranslating the nucleotide sequence of mRNAinto the amino acid sequence of a protein
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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Information flow from gene toprotein
BIOSCI 101: Cellular and Molecular Biology
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
BIOSCI 101: Cellular and Molecular Biology
- same base-pairing principle that governs DNAreplication (rA pairs with dT, rG with dC, rC withdG, rU with dA)
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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Stages oftranscription
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.7
Elongation a closer look
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.9
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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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
BIOSCI 101: Cellular and Molecular Biology
DNA template and mRNA product
d) the finished mRNA strand is antiparallel andcomplementary to the template strand fromwhich it was transcribed
Transcription and Translation (contd)
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)
BIOSCI 101: Cellular and Molecular Biology
- in prokaryotes mRNAs may code for morethan one polypeptide
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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RNA splicing (1)
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.11
RNAsplicing
(2)
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.12
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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Exons may encode protein domains
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.13
Translation the basic
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.14
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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3/11/20
Transcription and Translation (contd)
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)
BIOSCI 101: Cellular and Molecular Biology
- 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
BIOSCI 101: Cellular and Molecular Biology
aminoacyl site and the exit site)
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
Anatomy ofa ribosome
tRNA
molecules
Growing
polypeptide Exit tunnel
E
Large
subunit
Small
subunit
mRNA5
3
A si te
P site Exit tunnel
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.17
Small
subunit
Large
subunitP A
mRNA
binding site
E site
E
8/14/2019 CMB Lect 7 2011 Colour 2 Slides Per Page
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
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
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.15
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Specific
amino acidsare joined to
BIOSCI 101: Cellular and Molecular Biology
Campbell 17.16
Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
- 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
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Translation 1:Initiation
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.18
Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
. ,the first amino acid and its tRNA is broken
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
repeated until the polypeptide is complete
Translation 2:Elongation
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.19
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
- e m an r osoma su un s ssoc a e
Translation 3: Termination
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.20
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.21
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Transcription and Translation (contd)
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
BIOSCI 101: Cellular and Molecular Biology
completed)
- signal sequences at the N-terminus of thepolypeptide target proteins destined for secretionto the ER and Golgi
Proteins may be targeted tothe ER
BIOSCI 101: Cellular and Molecular BiologyCampbell 17.22
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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
BIOSCI 101: Cellular and Molecular Biology