Replication, Transcription & Translation

Question 1

Describe Base Pairing in DNA replication

Answer 1

• ability of each strand to act as a template for producing a complementary strand
• copy of genes produced before passing them on

Question 2

Define DNA polymerase

Answer 2

• catalyses the addition of nucleotides to the 3’ end of a growing strand of DNA using a parental DNA strand as a template

Question 3

Describe DNA helicase

Answer 3

uses the energy of ATP hydrolysis to unwind the DNA double helix ahead of the replication fork

Question 4

Describe the function of single-strand DNA binding protein

Answer 4

binds to single-stranded DNA exposed by DNA helicase, preventing base pairs from re-forming before the lagging strand can be replicated

Question 5

Describe the function of DNA topoisomerase

Answer 5

produces transient nicks in the DNA backbone to relieve the tension built up by the unwinding of DNA ahead of the DNA helicase

Question 6

Describe the function of sliding clamps

Answer 6

Keeps DNA polymerase attached to the template, allowing the enzyme to move along without falling off as it synthesises new DNA

Question 7

Describe the function of clamp loaders

Answer 7

uses the energy of ATP hydrolysis to lock the sliding clamp onto DNA

Question 8

Define Primase

Answer 8

synthesises RNA primers along the lagging-strand template

Question 9

Describe the function of DNA Ligase

Answer 9

uses the energy ATP hydrolysis to join Okazaki fragments made on the lagging-strand template

Question 10

Describe the Structure of the Replication Fork

Answer 10

• asymmetric structure
• DNA daughter strand that is synthesised continuously - known as the leading strand
• lagging strand - direction of nucleotide polymerisation is opposite to the overall direction of DNA chain growth
• synthesis of this strand is by a discontinuous ‘backstitching’ mechanism

Question 11

Describe DNA replication

Answer 11

• double helix is unwind by helicase & topoisomerase
• 1 DNA polymerase produces the leading strand copy
• another binds to the lagging strand - this enzyme makes discontinuous segments = Okazaki Fragments
• DNA ligase joins them together

Question 12

Describe Transcription

Answer 12

• process of synthesising an RNA transcript with the transfer of sequence information from a DNA template
• DNA molecule is transcribed into a complementary sequence of bases, mRNA, which is used for protein synthesis

Question 13

Describe tRNA

Answer 13

• deciphers the 3 nucleotide condons of mRNA in protein synthesis
• a range of tRNAs exist which bind specifically to 1/20 amino acids based on the anti-codon
• so correct amino acid is brought in during protein synthesis

Question 14

Describe rRNA

Answer 14

• Ribosomal RNA associates with a range of proteins to form the small & large sub-units of ribosomes
• these sub-units provide the framework by spatial & complementary structure to move along mRNA strand & to bind tRNA

Question 15

What are the 3 stages of transcription ?

Answer 15

1. Initiation
2. Elongation
3. Termination

Question 16

Describe the first stage of transcription - initiation

Answer 16

• RNA polymerase &transcription factors bind to promoter DNA
• RNAP creates transcription bubble which separates 2 strands of DNA helix
• 6 general transcription factors = TFIIA, TFIIB, TFIID, TFIIE, TFIIF & TFIIH

Question 17

Describe the 2nd stage of transcription - Elongation

Answer 17

• RNAP adds RNA nucleotides
• RNA sugar-phosphate backbone forms
• proofreading mechanism that can replace incorrectly incorporate bases

Question 18

Describe the 3rd stage of transcription - Termination

Answer 18

• hydrogen bond of the RNA-DNA helix break, freeing the newly synthesised RNA strand
• RNA may be further processed
• the RNA may remain in the nucleus or exit cytoplasm through nuclear pore complex

Question 19

Describe the structure of Ribosomes

Answer 19

Each ribosome has
- a binding site for mRNA
- 3 binding sites for tRNA –> A (aminoacyl) and P (peptide) sites & E (exit) site

Question 20

Describe the initiation of translation

Answer 20

• translation is initiated by a special Met-tRNAi - 40S subunit with Met-tRNAi
• Eukaryotic Initiation Factors = eIF

Question 21

Describe the termination of translation

Answer 21

• stop codon triggers release of the completed polypeptide chain - UAG, UAA, UGA
• Releasing factor = eRF
• eRF binds to A site in conjunction to GTP
• peptide group is transferred to water instead of aminoacyl-tRNA
• tRNA left in the P site is expelled with concomitant hydrolysis of GTP

Question 22

Describe A & P sites

Answer 22

cavities on ribosomes where charged tRNA molecules bind during peptide synthesis

Question 23

Describe E sites

Answer 23

Discharged tRNA’s leave the ribosome