Lecture 9-10

Question 1

Describe the strucutre of ribosomes

Answer 1

composed og large and small subunit each of which have several proteins pule 1 or more rRNA
Pro (L:23S + 5S, S:16s)
Euk (L:28s+5.8S, S:18S)
3 binding sites, A= acceptor (binds incoming amino acyl-tRNA, P= peptidyl transferase site (binds tRNA attached to gorwing polypeptide chain), E= exit site (uncharged rTNA exits ribosome

Question 2

Describe the biosynthesis of ribosomes

Answer 2

rRNAs are synthesized in the nucleolus in eukaryotes by RNA pol 1 (except for 5S) and are processed by cleavage and modification, sununits are assembled in nuleolus in euks by combin the processed rRNA with ribsomal sumbunits that were synthesized in the cytoplasm

Question 3

Describe tRNA charging

Answer 3

tRNAs tracibed by RNA pol III and procseed by cleavage and modification, specific amino-aclytRNA synthetases link psecific tRNAs (charging), carboxy-terminus of the amino acids is linked to the 3’ end of the tRNA

Question 4

Describe initation of translation in prokaryotes

Answer 4

i. 30S subunit binfs mRNA through interaction between Shine dalgarno sequence (AGGAGG) on the RNA and a complmentary sequence on the 16S rRNA alinging the AUG start codon (7 nt downstream from shin dalgarno) with the P site of the small sububit
ii. methionyl-turnfmet binds to P site ina comples with IF2-GTP
iii. initiation factors are relseased and the large subunit (50S) bind the small sununit (30S)

Question 5

Describe elongation in the tranlation process in prokaryotes

Answer 5

i. specific ainmpacly tRNA binds A site in a complex with EF-tu-GTP
ii. peptide bond formation between amin oacids on the tRNA in the p site and A site breaks the bon between the amin acids and the rTNA in the P site (catalysis mediated by the rRNA and uses energy ffrom GTP hyrolysis)
iii. ribosome moves to next codon towards 3’ end of the mRNA-peptidyl-tRNA moves from A site to P site, uncharged tRNA in the P site moves to the E site and exits, A site is empty and ready for next incoming aminoacyl-tRNA-EF-Tu comples (ribosome movement requires EF-G_GTP)
iv. elongation steps repeat until a stop codon is present in the A site

Question 6

Describe termination inthe translation process

Answer 6

i. one of two rlease factors (RE-1, RF-2) binds to a stop codon
ii. polypeptide chin is released from rTNA in the P site
iii. mRNA relseased and ribosome sununits dissociate

Question 7

Describe trnacionl in eukaryotes

Answer 7

inication involves the bindinf of initator of tRNA (tRNA f met) to the P site of ribosome small subunit (40S), which binds to the 5’ 7MG cap on the mRNA , initaiton factor are relased and the large subunit (60S) binds

iii. elongation process is the same as prokaryotes except: EF-1alpha-GTP = prokarote EF-Tu-GTP and Ef_2=prokaryotes EF-G
iv. a sinlge relsease facotr (eRF1) recognizes all 2 stop codons

Question 8

What are the futures of the genetic code

Answer 8

triplet
nonoverlapping: each nuclotide belongs to 1 codon
comma free: codons read consecutivelty
degerneate >1 code for most AA
ordered: == degeneracy: variable 3rd nucleotide
UNIVERSAL: codons have same meaning in all organisms

Question 9

What is the wobble hypothesis

Answer 9

Crick, suggests that lahtough there are 61 different codons specifiying 20 different AA isit not necessary to have 61 different tRNAs is base pariing between the 3rd base in the codon with the 1st base in the tRNA anitcodon is not stringent

Question 10

What does it mena tha the genetic code is degenerate and ordered

Answer 10

degeneracy is ordered, variation in 3rd codon, this minimizes the effects of mutation since mutation that change the 3rd base are less likely to alter the amino acids incorporated