translation

Question 1

What is the Genetic code?

Answer 1

o One nucleotide cannot code for an amino acid.
o mRNA read in blocks of 3=nucleotide = 4x4x4= 64 codons.
o Specific codons code for specific amino acids.

Question 2

What direction does translation occur?

Answer 2

o It is read in the 5’ to 3’ direction.

Question 3

What is the term used to describe the code? Most amino acids are coded by multiple codons

Answer 3

• degenerate code.

Question 4

What is the start codon?

Answer 4

o START = AUG(met)

Question 5

What is the stop codon?

Answer 5

o STOP = UAA, UAG, UGA

Question 6

Where does the genetic code apply? Is it universal?

Answer 6

o Genetic code is universal except for in mitochondria and chloroplasts.

Question 7

How does a tRNA read a codon?

Answer 7

• A-acids can’t recognise their own codon – adapter molecules are used.
  o Each tRNA carries an A-acid and recognises at least 1 codon for that A-acid.

Question 8

Draw a tRNA, what does it look like? LABEL EVERYTHING including where other things attach

Answer 8

see notes i put in dropbox or google for a picture
o tRNA structure has four loops: anticodon, DHU, TC and variable.
o A- acid attached to the 3’ end.
o Anticodon is complementary to the codon on the mRNA.

Question 9

What is charging the tRNA?

Answer 9

o Aminoacyl tRNA synthase attaches each amino acid to the correct tRNA molecule = charged tRNA.
o Synthesising a charged tRNA = 1ATP  AMP +PPi
o Reaction is driven by breakdown of PPi 2Pi

Question 10

Where does translation occur?

Answer 10

• Translation occurs on the ribosome:

o They are major cell constituents.

Question 11

What is the difference between proka and eucka ribosomes?

Answer 11

o Prokaryote ribosomes = 50+ proteins & 3 rRNAs.

o Eukaryote ribosomes = 80+ proteins & 4 rRNAs.

Question 12

Draw the structure of a ribosome

Answer 12

o Ribosomes contain a large and small subunit.

o Contain 3 sites to bind tRNAs

Question 13

What are the stages of translation?

Answer 13

cba to write out

Question 14

Describe initiation in prokaryotes

Answer 14

• Initiation of translation – prokaryotes:
o Sequence on the mRNA upstream from the start codon that tells ribosome where to start = Shine-Dalgarno (Ribosome Binding Site).
o Ribosome, mRNA, fMet-tRNA and initiation factors (Ifs) are needed.
o Assembly of these requires 1 GTP.
o The initiating tRNA(met) and the small ribosomal subunit and the mRNA form a complex.
o This is followed by the binding of the large ribosomal subunit.
o Translation = ready  elongation phase
.

Question 15

Describe initiation in prokaryotes

Answer 15

• Initiation of translation – eukaryotes:
o Eukareyoteic mRNAs don’t contain a Ribosome binding site sequence.
o The ribosome binds the 5’-Cap and then finds the first AUG codon.
o The ribosome, mRNA, MET-tRNA and eIFs are required.
o Assembly of these require 1ATP & 1GTP.
o This forms the small complex followed by binding of large ribosomal subunit.
o Translation = ready  elongation Phase.

Question 16

What are the differences in initiation between prok and euk?

Answer 16

o Eukareyoteic mRNAs don’t contain a Ribosome binding site sequence.
o Assembly of these require 1ATP.
o CHECK IF OTHERS I HAVENT READ IT PROPERLY
.

Question 17

Describe the translation elongation phase

Answer 17

o Ribosome contains 3 sites that bind tRNA: A(aminoacyl), P(peptidyl) and the E(exit) site.
o Initiating aminoacyl tRNA binds to P site.
o Growing peptide chain is connected via the tRNA to the P-site.
o Incoming aminoacyl-tRNA are delivered by an elongation factor to the A-site= uses 1GTP.
o Growing chain is transferred from the tRNA in the P-site to the amino group of the A-acid carried by the A-site tRNA.
o This peptidyl transfer reaction is catalysed by the peptidyltransferase activity in the ribosome forming a peptide bond between A-acids.
o Translocations which require an elongation factor and hydrolysis of GTP:
o The mRNA advances a codon.
o The peptidyl tRNA moves from AP-site & the empty tRNA moves from the PE-site.
o This cycle then repeats until STOP codon.

Question 18

Describe the termination phase

Answer 18

• Translation Termination phase:
o When STOP codon moves into A-site, no aminoacyl-tRNA can bind to the A-site.
o A release factor (RF) binds to the A-site leading to the release of the new protein from the ribosome.
o The mRNA contains a 3’ untranslated region.
o The ribosome then splits into its large and small subunit.

Question 19

In prok how are transcription and translation coupled?

Answer 19

o in space and time.
o Occur in same area of the bacteria cell and so as mRNA leaves RNA polymerase its becomes translated as soon as its made.

Question 20

How is this different in euk?

Answer 20

o In eukaryotes transcription & translation are separated in space and time because transcription occurs in the nucleus and translation in the cytoplasm – therefore occur at different times too.

Question 21

What is a polyribosome?

Answer 21

Wiki says: A polyribosome (or polysome) is a complex of an mRNA molecule and two or more ribosomes that act to form polypeptide chains during active translation.
- Most mRNA molecules are associated with polyribosomes(polysomes).

Question 22

What is the wobble hypothesis?

Answer 22

proposed by crick in 1966; some tRNA’s recognise more than one codon (e.g. GUA and GAC code Asp and both are recognised by the same tRNA with the anticodon CUG.
They recognise the same tRNA by varying the 3rd position, the “wobble base”.

Question 23

What is non-standard base pairing and what bases does it occur between?

Answer 23

there is flexibility in base pairing of the tRNA to the 3rd position in the codon. e.g. G in tRNA can recog C or U; Ionosine can recognise U, C or A..

Question 24

How many tRNAs are there in Ecoli vs Humans

Answer 24

Ecoli has 88, containing 40 different anticodons (ie several tRNAs have the same anticodon (there are 6 tRNA with the anticodon UUU).
Humans have 622 tRNAs (171 thought to be psuedogenes) containing 50 different anticodons. e.g there are 15 tRNA (lys) all with the anticodon UUU.

Question 25

Mutation in what gene leads to vanashing white matter/ childhood ataxia with central hypormyelination (VVM/CACH)? the disease WHERE THE BRAIN MELTS AWAY.

Answer 25

elF2B.

Question 26

Trinucleotide repeat expansion of what gene increases the risk of developing several cancers?

Answer 26

Release Factor gene.