DNA and genomics II

Question 1

what are the components of a gene

Answer 1

the promoter, transcription unit, and a termination sequence

Question 2

why is the genetic code a triplet code

Answer 2

with 3 nucleotides coding for 1 a.a., the protein can contain up to 4^3=64 diff codes; more than sufficient to code for 20 a.a

these 64 codons form the set of instructions that tells a cell the order in which a.a. are to be joined to form a protein

Question 3

key features of transcription

Answer 3

1. synthesis in 5’ to 3 direction
2. catalysed by RNA polymerase
3. formation of ssRNA
4. complementary base pairing
5. recognition sequence

Question 4

points of comparison between DNA and RNA

Answer 4

d/ss, size, stability, location, sugar residue, ratio of bases, bases, structural variation, forms, amount

Question 5

brief description of transcription

Answer 5

DNA is used as a template to synthesise the pre-mature mRNA/mRNA by complementary base pairing of gene nucleotides and is catalysed by RNA polymerase.

pre-mature mRNA will undergo post-transcriptional modification where a 5’ cap and poly-A tail is added

Question 6

two steps that occur at transcription’s termination stage

Answer 6

1. release of mRNA chain
2. RNA polymerase disassosciates
   –> terminates transcription

Question 7

define translation

Answer 7

process in which a sequence of ribonucleotides in mRNA is used as a template to synthesise a specific sequence of amino acid in a polypeptide by complementary base pairing

Question 8

how is the translation initiation complex formed

Answer 8

1. small ribosomal subunit and initiator tRNA assemble at start codon AUG downstream of 5’ end–> initiate translation
2. binding of large ribosomal subunit completes the ribosome and will form the translation initiation complex

Question 9

describe amino acid activation

Answer 9

1. each specific a.a. is covalently attached to a specific tRNA molecule with a specific anticodon at its 3’ CCA stem, forming an aminoacyl-tRNA; catalysed by 20 different enzymes, one for each a.a.; known as aminoacyl-tRNA synthetases
2. enzyme has specific complementary conformation and charge to the a.a. tRNA attached to and the specific tRNA with specific anticodon

Question 10

why simultaneous transcription and translation only occur in prokaryotes

Answer 10

prokaryotes do not have membrane-bound nuclei, thus the processes of transcription, translation occur simultaneously

Question 11

How does amino acid activation occur

Answer 11

• a specific amino acid is covalently attached to the 3’CCA stem of specific tRNA molecule with a specific anticodon by a specific aminoacyl tRNA synthase
• 20 different aminoacyl-tRNA synthetases; each has an active site which recognises a specific aa

Question 12

Explain the terminate stage

Answer 12

Termination begins when the stop codon on the mRNA enters the A site —> RELEASE factors enter the A site —> hydrolysis of the covalent (ester) bond between polypeptide chain and tRNA in P site

Polypeptide is released from ribosome and will fold into its secondary and tertiary structures—> ribosome disassembles into its subunits

Question 13

Enzymes involved in translation

Answer 13

1. Peptidyl transferase (adjacent amino acids on the tRNA molecule) 2. Aminoacyl-tRNA synthetase (hydrogen bonds)