Question

Translation: Prokaryotes vs. Eukaryotes 

 Characteristics of the code

 In the cell, translation occurs in the

 mRNA life

 Presence/use of aminoacil-tRNA synthetases

 Characteristics of the ribosomal subunits (importance?)

 Attachment of small subunit during initiation

 Amino acid specified by the initiation codon

 Factors used during initiation, elongation and termination

 Presence of polyribosomes

Answer 1

	PROKARYOTES	EUKARYOTES
Characteristics of the code	To facilitate binding by a ribosome, many prokaryotic ORFs contain a short sequence upstream (on the 5’sside) of the start codon called the ribosome binding site (RBS) in mRNA also called Shine–Dalgarno sequence.	1)Eukaryotic mRNAs Are Modified at Their 5’ and 3’ Ends to Facilitate Translation. (i)Eukaryotic mRNAs recruit ribosomes using a specific chemical modification called the 5’ cap, which is located at the extreme 5’ end of the mRNA called 5’ cap. The 5’cap is a methylated guanine nucleotide that is joined to the 5’end of the mRNA via an unusual 5’-to-5’ linkage. (ii) The presence of a poly-A tail at the Extreme3’ end of the mRNA 2)In some mRNAs, of eukaryotes, a purine three bases upstream of the start codon and guanine immediately downstream (5’-G/ ANNAUGG-3’) is present. This sequence is referred to as the Kozak sequence.
In the cell, the translation occurs in	Cytoplasm	Cytoplasm
mRNA life	Relatively lifetime of mRNA is shorter.	Relatively lifetime of mRNA is longer.
Presence /use of aminoacyl-tRNA synthetase	Aminoacyl-tRNA synthetase is present to charge tRNA with N-formyl methionine.	Aminoacyl-tRNA synthetase is present to charge tRNA with methionine.
Characteristic of the ribosomal subunit	They have a 70S ribosome. Larger subunit:50S Smaller subunit:30S Larger subunit contains 5SrRNA,23SrRNA and approximately 34 proteins. Smaller subunit contains 16SrRNA and 21 proteins.	They have an 80S ribosome. Larger subunit:60S Smaller subunit:40S Larger subunit contains 5.8SrRNA,5SrRNA,28SrRNA and 49 proteins. Smaller subunit contains 18SrRNA and approximately 33 proteins.
Attachment of small subunit during initiation	During initiation Each of the initiation factors binds at, or near, one of the three tRNA binding sites on the small subunit blocking the binding of charged tRNAs to the A-site, IF1 binds directly to the portion of the small subunit that will become the A-site. IF2 binds to IF1 and reachesover the A-site into the P-site to contact the fMet-tRNA f Met. Finally, IF3 occupies the part of the small subunit that will become the E-site. Thus, of the three potential tRNA-binding sites on the small subunit, only the P-site is capable of binding a tRNA in the presence of the initiation factors. A Specialized tRNA Charged with a Modified Methionine called fMet-tRNAfMet binds the P-site without previously occupying the A-site. The mRNA is recruited to the small subunit of mRNA.The association of the small subunit with the mRNA is mediated by base-pairing interactions between the RBS and the 16S rRNA. The 16S rRNA interacts with the RBS to position the AUG in the P-site. Binding of the mRNA to the small subunit always precedes association with the initiator tRNA.	During initiation, The small subunit is already associated with an initiator tRNA when it is recruited to the capped 5’ end of the mRNA. It then “scans” along the mRNA in a 5’ to 3’ direction until it reaches the first 5’-AUG-3’ which it recognizes as the start codon. Binding of the initiator tRNA to the small subunit always precedes association with the mRNA
Amino acid specified by the initiation codon	N-formyl methionine	Methionine
Factors used during initiation, elongation, and termination	Initiation-IF-1,IF-2,IF-3. Elongation-EF-TU, EF-TS, EF-G. Termination-RF-1,RF-2,RF-3.	Initiation- eIF1A, eIF3, eIF5, eIF4E, eIF4A, eIF4G, eIF4B, eIF5B. Elongation- EF-Tu, EF-G, EF-Ts. Termination-eRF1 and eRF3.
Presence of polyribosomes	Present in the cytoplasm.	Present polyribosomes are attached to the surface of the rough endoplasmic reticulum and the outer membrane of the nucleus.