Lecture 12: Translation in Eukaryotes Flashcards
How are amino acids ‘activated’?
Activation for synthesis via:
1. attachment to tRNA via aminoacyl-tRNA
2. tRNA acts as an adapter to translated
mRNA to protein
what are ‘svedberg’ units a factor of?
- Size
2. mass
What percentage of bacteria’s dry weight is ribosomes
- ~25% (15,000 per cell)
a. ~65% rRNA, ~35% protein
b. rRNA forms the core
Is the structure of the small ribosomal subunit highly conserved?
Yes, across all domains of life
How do anticodons and condons bond
- Hydrogen bonds
2, Antiparallel (5’-3’, 3’-5’
How are tRNAs modified?
1. 7-15 bases that are post-transcriptionally
modified
a. affect structure
b. and binding between codon and
anticodon
2. INSOSINE MODIFICATION - affects
codon-anticodon interactionwhat are the structural characteristics of tRNAs
- ssRNA 73-93 nt
- twisted L shape
- most have:
a. G at 5’
b. CCA at 3’ - modified bases (methylation etc.)
- Regions
a. anticodon arm
b. T-gamma-C arm: Contains
pseudouridine, helps folding
c. D arm: contains dihydrouridine,
contributes to folding
d. amino acid arm: has AA esterified
via carboxyl group to the 2’-OH or
3’-OH of the A of ther terminal CCA
codon
what are the structural characteristics of tRNAs
- ssRNA 73-93 nt
- twisted L shape
- most have:
a. G at 5’
b. CCA at 3’ - modified bases (methylation etc.)
- Regions
a. anticodon arm
b. T-gamma-C arm: Contains
pseudouridine, helps folding
c. D arm: contains dihydrouridine,
contributes to folding
d. amino acid arm: has AA esterified
via carboxyl group to the 2’-OH or
3’-OH of the A of ther terminal CCA
codon
What is meant be the term ‘wobbling’ with regards to mRNA and tRNA interacions?
1. tRNAs can recognise more than one
codon for the AA they carry
2. "REDUNDANCY" - 3rd base of codon
can form non-canonical base pairs
with anticodon
3. "Wobble" allows recognition of more the
one codonWhat do XYU and XYC encode?
What do XYA and XYG encode?
Why is this?
- Always the same AA
- Usually the same AA
- X & Y show strong base pairing
- Enables rapid dissociation of tRNA
from mRNA = protein synthesis
FASTER - THIS IS HOW 32 tRNAs DECODE 61
CODONS
How is it aminoacyl-tRNA synthetases catalyse the attachment to corresponding AA
- EACH ENZYME is specific to ONE AA
- “second genetic code”
- Two step process to establish
covalent bond between tRNA and AA
a. E.g., Phe to rRNA^phe
b. ATP -> AMP + PPi
How do aminoacyl-tRNA synthetases recognise the right tRNAs
- nucleotide positions in tRNA recognised
a. “recognition points”
b. “structural features” - tend to be
universal
How is Ala-tRNA made?
- A single G-U pair of tRNA^ala required for
recognition by Ala-tRNA synthetase - Creates a small twist which is recognised
How does aminoacylation of tRNA occur?
- Step one: formation of aminoacyl
adenylate (remains bound to active
site) - Step two: aminoacyl group transferred to tRNA. Two classes of aminoacyl-tRNA synthetases:
a. Class 1 enzymes: aminoacyl group
initially transferred to 2’-hydroxyl group
of the 3’ terminal A residue, THEN to
the 3’-hydroxyl group by a
transesterification reaction
b. Class 2 enzymes: aminoacyl group
transferred directly to 3’-hydroxyl group
of terminal adenylate
How does base pairing between codon and anticodon work to incorporate AA?
- Charged tRNA chemically modified so
tRNA sequence is uncharged but AA
is charged
a. E.G., nickel hydride treatment of
tRNA^cys - cell-free translation system - Alanine
is inserted into polypeptide at codons
normally specifically cysteine
