lecture 5

Question 1

trandscription machinery

Answer 1

• RNA = single stranded; built on template of DNA
• RNA synthesis catalysed by RNA polymerase (pries DNA strand apart; hooks RNA nucleotide)
• follows same base pairing (Uracil for Thymine)

Question 2

transcription

Answer 2

• transcription unit = DNA stretch transcribed
• promoter = DNA sequence where RNA polymerase attaches
• comprises of 3 stages: initiation; elongation; termination

Question 3

initiation

Answer 3

• promoters signal initiation
• transcription factors mediate binding of polyerase initiating transcription
• full assemble transcription factors and pol.2 bound to promoter (transcription initiation complex)
• TATA box = promoter; crucial in forming initiation complex

Question 4

elongation

Answer 4

• RNA polymerase moves along DNA (untwists helix)
• transcription progesses (40 nucleotides/s)
• gene can be transcribed simultaneously by several RNA polymerases

Question 5

termination

Answer 5

• differ for bacteria and eukaryotes
• bacteria
• stops at end of terminator (RNA sequence)
• polymerase detaches
• eukaryotes
• continue through polyadenylation sequence (specifies a polyadenylation signal - AAUAAA)
• binds specific proteins cutting pre-mRNA free

Question 6

post transcription modification

Answer 6

• in nucleus
• both ends altered: 5’ = modified cap; 3’ = poly-A tail
• facilitate export of mRNA from nucelus
• protect mRNA from hydrolytic enzymes
• help ribosomes attach 5’
• coding portions (translated in amino acid sequences) = exons (expressed regions)
• noncoding portions = introns (intervening regions)
• RNA splicing removes introns, joing exons (creating mRNA moleucle with continuous coding sequence)

Question 7

RNA splicing

Answer 7

• carried out by spliceosomes (comprise various proteins, small nuclear RNAs recognise splice site)
• can occur without assistance (intron catalyses its on excision)
• enzyme-like catalytic RNAs (ribozymes) function as enzyme: pair with itself in 3D structure; contain functional groups; RNA can hydrogen bond with other nucleic acids)

Question 8

alternative splicing

Answer 8

• some genes can encode more than one type polypeptide (depend segments treated as exons)
• no. different proteins can produces exceeds no. of genes

Question 9

multidomain proteins

Answer 9

• modular architecture (consist of discrete domains)
• different exons code different domains in proteins
• introns increase probability of crossing between exons (facilitates evolution of new proteins)