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Molecular Genetics > lecture 22 > Flashcards

lecture 22 Flashcards

1
Q

ribosomes are the

A

protein factories within cells

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2
Q

ribosomes are present in the

A

cytosol of all cells
matrix of mitochondria
stroma of chloroplast

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3
Q

ribosome provides environemnt for

A

interaction of tRNAs and mRNA and catalyzes peptide bond formation

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4
Q

RNA-protein complex

A

ribosomal proteins (r-proteins) and ribosomal RNAs (rRNA)

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5
Q

eukaryotic ribosome

A

80S
- 60 S
- 40 S

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6
Q

bacterial ribosome

A

70 S
- 50S
- 30S

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7
Q

small subunit contains the

A

deconding center

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8
Q

decoding center

A

where aminoacetylated tRNAs “read” the genetic code by base pairing with each triplet codon in the mRNA

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9
Q

large subunit contains the

A

peptidyl transferase center

(active site)

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10
Q

peptidyl transferase

A

catalyzes peptide bond formation between adjacent amino acids in a growing polypeptide chain

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11
Q

the ribosome is a ribozyme

A

ribosomal RNA has a fundamental importance in catalyzing protein synthesis
- deproteinized ribosome retains peptidyl transferase activity
- there is no protein within the peptidyl transferase active site in the cyrstal structure of the 50S ribosomal subunit

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12
Q

ribosomal RNA, not protein

A

is responsible for catalysis of peptide bond formation

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13
Q

stages of translation

A
  • activation of amino acids
  • initiation
  • elongation
  • termination
  • protein folding
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14
Q

initation

A

involves assembly if active ribosome with mRNA and the first aminoacyl-tRNA before 1st peptide bond formaiton

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15
Q

elongation

A

all reactions from 1st to last peptide bond formation

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16
Q

termination

A

ribosome reaches a stop codon; release of completed peptide and dissociation of ribosome from mRNA

17
Q

ribosome has three tRNA binding sites

A

APE

18
Q

A (aminoacyl)

A

Aminoacyl tRNA binding

19
Q

P (peptidyl tRNA binding)

A

P binding site

20
Q

E (exit)

A

occupied by the tRNA molecule released after the growing polypeptide chain is transferred to the aminoacyl-tRNA

21
Q

shine dalgarno sequence (translaiton in prokaryotes)

A

positions the mRNA on the bacterial ribosome by binding to the 16S rRNA

also called the ribosome binding site (RBS)

  • consensus sequence - initation signal - 5 to 9 purines, situated 8 to 13 nucleodes on the 5’ side of the start codon
  • base pairing with a complementary pyrimidine rich sequence near the 3’ end of the 16S rRNA of the 30 S ribosomal subunit
  • mRNA-rRNA int. positions the initating 5’AUG sequence of the mRNA in the precise location on the 30S subunit where it is required for translation initaiton.
22
Q

all protein synteses start with same aa

A

methionine, which may be removed later

23
Q

AUG codon

A

represents Met, two tRNAs

24
Q

fMet-tRNA^fmet

A

charged, used only for initiation

25
Q

initation factors in bacteria

A

IF3
IF1
IF2

26
Q

IF3

A

prevents premature association of the 30S and 50S

27
Q

IF1

A

binds at the A site and blocks tRNA binding during initaiton

28
Q

IF2 (GTPase)

A

facilitates binding of charged initiator tRNA to partial P site

29
Q

translation initation in bacteria

A
  1. mRNA, fMet-tRNA^fmet, and IFs bind to 30S
  2. IF3 leaves when fMet-tRNA^fmet pairs with mRNA
  3. 50S joins in as IF2 hydrolyzes GTP and IF2-GDP and IF1 leave
  4. 70S initation complex is ready for elongation
30
Q

translation in eukaroytes

A

similar to prokaryotic

  1. Met-tRNAi^met (not formylated)
  2. more eIFs required (atleast 12)
  3. Met-tRNAiMet Always binds to 40S before mRNA binding
  4. ribosomes are recruited to mRNA by 5’ Cap (NO RBS)
31
Q

initiation sites in eukaryotic mRNA

A

NO SD sequence

  • modification at both ends are important for initation
  • 43S recognizes 5’ cap and then migrates along mRNA
  • leader region is scanned in the 5’-3’ direction
  • AUG is recognized by 43S complex
32
Q

Kozak sequence

A

enhance translation (increase translation efficiency)

Presence of purine three residues before the start codon and G residue immediately following the start codon

33
Q

translation initation in eukaryotes

A

displacement of eIFs from 40S (requires eIF5 and eIF5B)

  • hydrolysis of the eIF2-bound GTP (eIF2 dissocation)
  • association of 60S to form functional 80S ribosome
  • initaiton is now complete
34
Q

Poly A tail contributes to efficient translation by

A

“circularization of mRNA”

  • eIF4F: eIF4A, eIF4E, eIF4G
35
Q

circularization of the eukaryotic mRNA

A

eIF4G-PABP interaction

increase the efficiency of translation

Molecular Genetics (9 decks)

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