17 - Genome structure, replication and expression*

Question 1

Shine Dalgarno sequence

Answer 1

region where ribosomes bind to initiate translation of protein

Question 2

How is chromosome organised

Answer 2

So that ori and ter sites are found at poles of cell

Question 3

Isomerase

Answer 3

Unwinds supercoil

Question 4

Catenated

Answer 4

Two circular chromosomes intertwined

Question 5

FtsK-XerCD recombination machinery

Answer 5

Resolves dimer and cleaves holliday junction

Question 6

Size of prokaryotic ribsome

Answer 6

70S
subunits:
- 30S (16S rRNA and 21 polypeptides)
- 50S (23S rRNA and 5S, 31 polypeptides)

Question 7

Describe the basics of DNA replication in prokaryotes

Answer 7

Isomerase unwinds supercoil
Helicase unzips DNA double helix exposing
nitrogenous bases, forming two replication forks
Strands are kept apart by single-stranded DNA binding proteins (SSB)
DNA is replicated 5’-3’
DNA polymerase binds single-stranded DNA, recruits deoxyribonucleoside
triphosphates and matches them to the template strand by complementary
base pairing and catalyses the phosphodiester bond

Antiparallel strands: Okazaki fragments are synthesised by DNA primase using the template strand, extended from the 3’ end by DNA polymerase III

Question 8

Describe the basics of DNA transcription

Answer 8

Initiation
* RNA polymerase binds promoter and initiates transcription at the start point

Elongation
* Successive addition of ribonucleosides to RNA strand

Termination
* Completed mRNA transcript released

Question 9

Describe the basics of DNA translation

Answer 9

Initiation
* Ribosome binds Shine Dalgarno sequence and facilitates tRNAformyl-Met alignment with the start codon
Elongation
* Ribosome moves along mRNA, new tRNAs align and peptide bonds
formed between amino acids
Termination
* Ribosome reaches stop codon, releases new polypeptide and
dissociates

Question 10

Draw and label a diagram of a bacterial gene

Answer 10

Features:
1. (-35) and (-10) box = promoter - region
where RNA polymerase binds to initiate
transcription
2. +1 sight the first point of translation
3. 5’ untranslated region
4. Shine Dalgardo sequence - region where ribosomes bind to initiate translation of protein
5. Start codon - ATG
6. Stop codon - TAA, TGA, TAG
7. 3’ untranslated region
8. Transcription termination signal

Question 11

Explain how the structure of the nucleoid changes with growth phase and how this affects transcription/translation

Answer 11

Nucleoid is tightly wound to allow for maximised transcription and translation in exponential/growth phase

In the stationary phase the chromosome is relaxed with little transcription and translation occurring

Question 12

Explain how the two copies of the bacterial chromosome are terminated and resolved during bacterial fission

Answer 12

Circular chromosomes are interlinked or catenated
A double-stranded break is introduced into the dsDNA by topoisomerase on one chromosome
The strands are joined to create a dimer at the two ter sites
Homologous recombination occurs at the ter to form Holliday junctions (strand exchange between each dsDNA pair)
The dimer is resolved by the FtsK-XerCD
recombination machinery which cleaves the Holliday junction
DNA ligase repairs the phosphodiester bonds in the backbones to create an uninterrupted backbone