Lec 5_Prok and Euk replication

Question 1

Gene Dosage effect

Prok

Answer 1

Genes at origin of replication are overepresented in cell relative to genes at terminus of replication
- because of the formation of multiple forks

highly expressed genes e.g rRNA genes
- positioned near the origin of replication

summary: gene required in high frequency situated close to ori

Question 2

Gene strand bias

Answer 2

genes occur on both DNA strands in opposite directions
positioning of genes are not random depends on:
- levels of transcription needed
- accessibility of genes to RNA Polymerase

BUT replication forks move in one direction (5 -> 3):
if genes are encoded on leading strand: DNA and RNA polymerases move in the same direction = avoiding head-on collisions
CO-ORIENTED COLLISION

If genes are encoded on the lagging strand = DNA and RNA polymerases move in opposite direction and will head-on collide

genes required at a high frequency = located on leading strand`

therefore >85% of rRNA operons are positioned on the leading strand of bacterial genomes.

Question 3

Two consequences of these head-on collisions

Prokaryotic

Answer 3

1. Disruption of RNA transcription
   - deleterious for highly expressed genes
   - hampers replication
   - negatively affects production of highly needed RNA molecules
2. Production of truncated mRNA transcripts
   - yield truncated proteins :
   * non functional
   * interfere with functions of other proteins and processes
   - deleterious to cell

Question 4

GENE REQUIRED AT HIGH FREQUENCY

Answer 4

1. situated close to ori

2. located on the leading strand

Question 5

Eukaryote replication

Answer 5

ARS (Autonomously Replicating Sequences)

• consensus sequence
• acts as origin of replication

A.T region
- 14 bp core

additional sequences - adjacent to 14bp core
- imperfect copies

replication machinery

• similar to prok
• ATP required to initiate process

Takes place in the S phase cycle
- EUK: large no of replicons
- individual replicons - relatively small
replicons do not have a termini
- two replication forks moving towards one another simply merge when they reach each other

Question 6

Eukaryotic replicons

• replication forks
• replicon

Answer 6

• EUK: large no of replicons
• individual replicons - relatively small
  replicons do not have a termini
• two replication forks moving towards one another simply merge when they reach each other

Question 7

Eukaryotic: chromosomal replication takes place

Answer 7

vertically not horizontally

Question 8

Yeast cell cycle

_ draw it too

Answer 8

separate at start and join before cytokinesis
happens simultaneously

1. Chromosomal cycle
   - initiation of replication
   - completion of S phase
   - Nuclear division (M phase) = 2 complete nuclei
2. Centrosome cycle
   - Duplication (G1) and separation (S) of spindle pole bodies (SPB)
   
   • centromers in higher eukaryotes
     
     • separation of two sets of chromosomes (Nuclear division, M - mitosis )
3. Cytoplasmic cycle
   - Bud emergence (S)
   - Nuclear migration into a bud at the start of (M)
   - formation of two cells

Question 9

Mitochondrial DNA Replication + Differences

Euk

Answer 9

NB Differences

• origins or replication on two strands occur at different locations
• DNA replication only initiates on the one strand

Steps

1. Replication starts on H (heavy) strand
   - leads to displacement of the complementing strand L (Light) strand
   - Forms D loop
2. Single D loop expands
3. When D loop passes L origin
   - replication starts from L strand in the opposite direction
4. Complete duplex formation
   - H strand replication finishes before L strand replication
5. Gaps in new strands are sealed

Question 10

FOR A2 JUST KNOW PROKAYTIC REPLICATION
will mark euk as perfectly known

keep prok at the green!!!

Answer 10

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