lecture 22

Question 1

ribosomes are the

Answer 1

protein factories within cells

Question 2

ribosomes are present in the

Answer 2

cytosol of all cells
matrix of mitochondria
stroma of chloroplast

Question 3

ribosome provides environemnt for

Answer 3

interaction of tRNAs and mRNA and catalyzes peptide bond formation

Question 4

RNA-protein complex

Answer 4

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

Question 5

eukaryotic ribosome

Answer 5

80S
- 60 S
- 40 S

Question 6

bacterial ribosome

Answer 6

70 S
- 50S
- 30S

Question 7

small subunit contains the

Answer 7

deconding center

Question 8

decoding center

Answer 8

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

Question 9

large subunit contains the

Answer 9

peptidyl transferase center

(active site)

Question 10

peptidyl transferase

Answer 10

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

Question 11

the ribosome is a ribozyme

Answer 11

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

Question 12

ribosomal RNA, not protein

Answer 12

is responsible for catalysis of peptide bond formation

Question 13

stages of translation

Answer 13

• activation of amino acids
• initiation
• elongation
• termination
• protein folding

Question 14

initation

Answer 14

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

Question 15

elongation

Answer 15

all reactions from 1st to last peptide bond formation

Question 16

termination

Answer 16

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

Question 17

ribosome has three tRNA binding sites

Answer 17

APE

Question 18

A (aminoacyl)

Answer 18

Aminoacyl tRNA binding

Question 19

P (peptidyl tRNA binding)

Answer 19

P binding site

Question 20

E (exit)

Answer 20

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

Question 21

shine dalgarno sequence (translaiton in prokaryotes)

Answer 21

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.

Question 22

all protein synteses start with same aa

Answer 22

methionine, which may be removed later

Question 23

AUG codon

Answer 23

represents Met, two tRNAs

Question 24

fMet-tRNA^fmet

Answer 24

charged, used only for initiation

Question 25

initation factors in bacteria

Answer 25

IF3 IF1 IF2

Question 26

IF3

Answer 26

prevents premature association of the 30S and 50S

Question 27

IF1

Answer 27

binds at the A site and blocks tRNA binding during initaiton

Question 28

IF2 (GTPase)

Answer 28

facilitates binding of charged initiator tRNA to partial P site

Question 29

translation initation in bacteria

Answer 29

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

Question 30

translation in eukaroytes

Answer 30

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**)

Question 31

initiation sites in eukaryotic mRNA

Answer 31

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

Question 32

Kozak sequence

Answer 32

enhance translation (increase translation efficiency) Presence of **purine** three residues before the start codon and **G residue** immediately following the start codon

Question 33

translation initation in eukaryotes

Answer 33

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

Question 34

Poly A tail contributes to efficient translation by

Answer 34

"circularization of mRNA" - eIF4F: eIF4A, eIF4E, eIF4G

Question 35

circularization of the eukaryotic mRNA

Answer 35

eIF4G-PABP interaction increase the efficiency of translation