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Genetics > Lecture 16 > Flashcards

Lecture 16 Flashcards

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1
Q

What is the universal genetic code formed of?

A

61 amino acid coding codons
3 stop codons

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2
Q

Why is the universal genetic code degenerate?

A
  • All amino acids except Met and Trp are encoded by more than 1 codon
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3
Q

What codon used in initiation of protein synthesis

A

AUG

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4
Q

Types of RNAs in translation

A

mRNA (messenger RNA) - Encodes protein

rRNA (ribosomal RNA) - Forms part of the ribosome used to translate mRNA to protein

tRNA (transfer RNA) - Couples the region of the ribosome which bins the mRNA codon and amino acid

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5
Q

What is the shape of tRNA

A

Cloverleaf

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6
Q

Size of tRNAs

A

74-95 bases

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7
Q

What is the site of the attachment of amino acids on tRNA

A

CCA tail added post-transcriptionally

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8
Q

4 main regions of tRNA

A

3’ CCA tail

T-loop (Pseudouridine)

D-loop (Dihydrouridine)

Anticodon loop

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9
Q

What do D-loop and T-loop contain?

A

Modified bases

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10
Q

Attaching tRNA to cognate amino acid

A
  • Aminoacylation or charging
  • Carried out by aminoacyl-tRNA synthetases using ATP as cofactor
  • Produces aminoacyl-tRNA
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11
Q

Explain the aminoacylation reaction

A
  1. Amino acid binds to ATP
  2. Activates amino acid (PPi -> 2Pi)
  3. Transfer of amino acid to tRNA (AMP released)
  4. Amino acid attached to 3’ or 2’ OH group of 3’ terminal adenine nucleotide of tRNA
  5. Aminoacyl tRNA formed
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12
Q

Size exclusion

A

Amino acid too large

Can’t fit in amino-acylation site on synthetase

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13
Q

What provide basis for template recognition?

A

Codon-anticodon pairings

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14
Q

Explain codon-anticodon pairings

A
  • Based on normal Watson-Crick interactions
  • Binding between 1st base of anticodon and 3rd base of codon allows for other base pairings - wobbles
  • Wobbles cause basis of degeneracy
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15
Q

First amino acid

A

Methionine is first amino acid incorporated into proteins - may be removed later

tRNAfmet (prokaryotes) or tRNAimet (eukaryotes) is used at start

tRNAmmet used for elongation involving methionine

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16
Q

Prokaryotic ribosome subunits

A

50S subunit - 5S rRNA (120nts), 23S rRNA (2900nts), approx 34 proteins (L1-L34)

30S subunit - 16S rRNA, approx 21 proteins (S1-S21)

17
Q

Eukaryotic ribosome

A

60S subunit - 5S rRNA (120nts), 5.8S rRNA (160nts), 28S rRNA (4700nts), approx 49 proteins

40S subunit - 18S rRNA (1900nts), approx 33 proteins

18
Q

Initiation of translation

A
  1. Initiation factors bind to 30S ribosomal subunit - IF1 and IF3, IF2 binds to IF1
  2. 30S subunit binds to mRNA via the shine Dalgarno sequence binding to the anti-shine-Dalgarno sequence on 16S rRNA - releasing IF1 and IF3
  3. tRNAfmet binds to AUG start codon
  4. Large subunit then binds by GTP hydrolysis at mRNA, releasing GDP, Pi and IF2
19
Q

What are the 3 sites in ribosome complex?

A
  • Aminoacyl site (on large subunit)
  • Peptidyl site
  • Exit site
20
Q

Elongation in prokaryotes

A
  1. Tu loads next aminoacyl-tRNA into A site of ribosome
  2. Ribosomal peptidyl transferase catalyses peptide bond formation
  3. eEF2 promotes movement of mRNA:tRNA complex placing next codon into A site
  4. Class I release factors recognize stop codon
21
Q

What catalyses peptide bond formation?

A

Peptidyl transferase centre

22
Q

Why is peptidyl transferase not a protein

A
  • Carried out by RNA section of 50S subunit called ribozyme
  • Ribozymes involved in splicing of rRNA and removal of introns from mRNA
23
Q

Translocation

A
  1. Ribosome moves a codon in 3’ direction - requires EFG and GTP
  2. Peptidyl RNA moves from A site to P site
  3. Uncharged tRNA moves from P to E site
  4. EFG released (GTP hydrolysed)
24
Q

What is EFG a structural mimic of?

A

EFTu:tRNA

25
Q

Structural homology between EF-G and EF-Tu

A

EFG binds ribosome competitively with EFTu

Altered conformation forces movement of peptidyl-tRNA from A to P site and pushes deacylated tRNA to exit site

Ribosome moves 3 bases in 3’ direction on mRNA

26
Q

Termination

A

When one of 3 stop codons reached, no tRNA available to enter A site

Release factor binds to stop codon

RF1 stops codons UAA and UAG
RF2 stops UAA and UGA
RF3-GTP removes polypeptide by binding 50S subunit - forms hybrid state tRNA
RF3-GDP + Pi and RF1/2 released

27
Q

Recycling

A

Ribosome recycling factor and EFG-GTP promote complex disassembly by binding stop codon

RRF and EFG-GDP released and 50S ribosomal subunit disassociates

IF3 removes tRNAfmet and bind 30S subunit

Genetics (18 decks)

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