7 - nucleic acids (SL + HL)

Question 1

DNA replication process [8]

Answer 1

• helicase unwinds DNA double helix
• primase adds short RNA primer complementary to parent strand
• poly. III replicates from 5’ to 3’ end
• complementary base pairing between adenine+thymine and guanine+cytosine
• continuous replication on leading strand occurs towards the replication fork
• discontinuous replication on lagging strand forms okazaki fragments
• poly. I replaces RNA primers with DNA
• DNA ligase seals okazaki fragments by making phosphodiester bond

(disclaimer: RNA primer is added to both leading and lagging strands)

Question 2

features of ribosome that relate to its function in translation [6]

Answer 2

• translation leads to polypeptide synthesis
• 80s ribosome in euka has small and large subunit
• large subunit has 3 binding sites
• A, P, E sites (addition pairing exit)
• small subunit has binding site for mRNA
• 2 tRNA can bind to large subunit at the same time

Question 3

outline transcription in prokaryotes [6]

Answer 3

• transcription is copying a strand of DNA onto RNA
• RNA poly. binds onto promoter region of DNA
• template strand is anti-sense stand
• RNA poly. unwinds double helix
• complementary base pairing occurs (adenine to thymine, guanine to cytosine, adenine to uracil)
• transcription in 5’ to 3’ direction
• post-transcriptional changes do NOT occur in prokar