Replication, transcription, translation

Question 1

Explain DNA replication

Answer 1

1) Before cell division:
Topoisomerase unwinds DNA
DNA helicases separates DNA apart-
expose two single DNA strands and create
two replication forks
2) DNA replication occurs simultaneously at
each form
Single stranded binding protein (SSB’s)
coat single DNA strands- prevent
reannealing
3) Primate enzyme uses original DNA
sequence on the parent strand to
synthesise a RNA primer
Primers necessary as DNA polymerase can
only extend a nucleotide chain- not start
one
4) DNA polymerase synthesises new DNA (via
complementary base pairing using free
floating nucleotides) strand
Extend RNA primer in 5’ and 3’ direction
5) Replication proceeds- RNAse H recognise
RNA primers bound DNA template
Removes primers by hydrolysing RNA
6) DNA polymerase then fills gap left by
RNAse H
7) DNA replication process completed when
ligase enzyme joins short DNA pieces
(Okazaki fragments) together form one
continuous strand.

Question 2

DNA replication

Answer 2

Question 3

What enzymes are involved in DNA replication?
Function?

Answer 3

Topoisomerase- unwind DNA
DNA helicase- seperate DNA
SSB’s- prevent reannealing
Primate enzyme- synthesis RNA primer
DNA polymerase- synthesise new DNA
RNAse H- hydrolyses primers
Ligase enzyme- join Okasaki form continuous

Question 4

Explain DNA transcription

Answer 4

Synthesis of mRNA from DNA- 4 stages

1. Preparation
   Topoisomerase unwinds double helix-
   relieving the supercoils.
   DNA helicase separates DNA apart-
   exposing nucleotides.
   Single stranded binding proteins (SSBP’s)
   coat single DNA strands to prevent DNA
   re- annealing
2. Production
   TATA sequence is recognition signal for
   starting
   AUG is the start codon (methionine)
   Free mRNA nucleotides line up next to
   complementary bases on template
   strand/antisense strand of DNA
   A-U & C-G
   Coding strand runs 5’ to 3’
   Template strand runs 3’ to 5’
3. Termination
   RNA polymerase 2 joins mRNA
   nucleotides
   Catalyses phosphodiester bonds between
   them
   Forms antiparallel mRNA strand (5’CAP
   head and 3’Poly A tail)- starting at a
   promoter
   P- specific sequence that RNA polymerase
   binds to- initiation of transcription.
   Transcription is stopped at termination- hairpin
4. Modification (splicing)
   Removal of introns
   Exons remain– coding part
5. mRNA leaves nucleus through nuclear
   pores

Question 5

Transcription

Answer 5

Question 6

Explain splicing?

Answer 6

Question 7

What enzymes are involved in transcription?

Answer 7

Topoisomerase- unwind double helix
DNA helicase- seperate DNA- expose nucleotide
SSBP’s- coat single DNA strands- prevent
reannealing
RNA polymerase 2- joins mRNA nucleotides

Question 8

What is the start codon for translation?

Answer 8

AUG- methionine

Question 9

What are the stop codons?

Answer 9

UAA
UAG
UGA

Question 10

Explain translation?

Answer 10

1) mRNA attaches to 80s ribosome
APE section in large subunit – A at the 5’
end of mRNA strand
Strand read 5’ to 3’

2) At ribosome mRNA (bases on mRNA are
read in 3’s - codons) sequence used as
template to bind to complementary tRNA
molecules at their anticodon (3 bases
complementary to codon on mRNA).

3) tRNA molecules are attached to specific
amino acids

4) Enzymes remove amino acid from tRNA
Amino acids linked by peptide bond
(created by condensation reaction)
creating a polypeptide chain

5) Stop codons
UAA
UAG
UGA

Question 11

Which way is the strand read and made in transcription?
Which way strand read form DNA replication

Answer 11

Read 3’ to 5’ made 5’ to 3’
Read up 3-5
Write down 5-3

Question 12

What reaction links amino acids together in translation?

Answer 12

Condensation reaction
Create polypeptide chain