DNA Replication and Repair - - PowerPoint PPT Presentation

DNA Replication and Repair

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DNA Replication

• genetic information is passed on

to the next generation

• semi-conservative

A C T A G A C T A G A C T A G A C T A G T G A T C T G A T C A C T A G A C T A G T G A T C T G A T C T G A T C T G A T C

Parent molecule with two complementary molecules

Parental strands separate Each parental strand is a template

Each daughter DNA molecule consists of one parental and one new strand

Overview of replication

• DNA is unwound and stabilized
• Origins of replication: Replication bubble and replication fork

Initiation

• RNA primers bind to sections of the DNA and initiate synthesis

Priming

• Leading strand (5’  3’) synthesized continuously
• Lagging strand synthesized discontinuously then fragments are joined
• RNA primer replaced by DNA

Elongation

• Mismatch repair by DNA polymerase
• Excision repair by nucleases

Proofreading

Review of DNA structure

• double helix
• each strand has a 5’

phosphate end and a 3’ hydroxyl end

• strands run antiparallel

to each other

• A-T pairs (2 H-bonds),

G-C pairs (3 H-bonds)

STEP 1 Initiation at origins

• f replication

separation sites on DNA strands

• Depend on a specific AT-rich DNA sequence

– Prokaryotes – one site – Eukaryotes – multiple sites

• Replication bubble
• Replication fork
• Proceeds in two directions from point of
• rigin

The proteins

• f initiation

1. Helicase – unwinds double helix 2. Single-strand binding proteins – holds DNA apart 3. Topoisomerase – relieves strain by breaking, swiveling, rejoining strands

STEP 2 Priming

initiation of DNA synthesis by RNA

RNA primers bind to unwound sections through the action

• f primase

– leading strand –

• nly 1 primer

– lagging strand – multiple primers – replaced by DNA later

STEP 3 Elongation of a new DNA strand

lengthening in the 5’ 3’ direction

DNA polymerase III can only add nucleotides to the 3’ hydroxyl end Leading strand

• DNA pol III – adds nucleotides

towards the replication fork;

• DNA pol I - replaces RNA with

DNA Lagging strand

• DNA pol III - adds Okazaki

fragments to free 3’ end away from replication fork

• DNA pol I - replaces RNA with

DNA

• DNA ligase – joins Okazaki

fragments to create a continuous strand

STEP 4 Proofreading

correcting errors in replication

Mismatch repair

• DNA pol III – proofreads

nucleotides against the template strand Excision repair

• nuclease – cuts damaged

segment

• DNA pol III and ligase – fill the

gap left Telomeres at 5’ ends of lagging strands

• no genes, only 100 – 1000

TTAGGG sequences to protect genes

• telomerase catalyzes

lengthening of telomeres

DNA Replication and Repair

• 1. Summarize the central dogma in a diagram.
• 2. Define antiparallel and semiconservative in terms of

the structure of DNA.

• 3. Use the following terms associated with replication

and create a flowchart showing the different stages: replication bubble and replication fork, helicase, single-strand binding proteins, RNA primer, primase, leading strand, lagging strand, DNA polymerase III, DNA polymerase I, DNA ligase, Okazaki fragments, and 5’à 3’.

• 4. Differentiate between mismatch and excision repair.
• 5. What are telomeres and what role do they play in

protecting the integrity of the lagging strand of the DNA?

Modelling

• 1. By team, create a DNA strand that is at least

20 nucleotide pairs long with at least one stretch that has the sequence ATATAA

• 2. One member should be sketching the DNA

strand on the sheet provided

• 3. Indicate the 5’ end and the 3’ end for each

strand

Modelling

• 4. You are modeling eukaryotic DNA. How

would prokaryotic DNA be different?

• 5. Use the clay to create helicase,

topoisomerase, and single-strand binding proteins.

• 6. Show how these act in unwinding, stabilizing

and holding the strands apart.

Modelling

• 7. In real life, RNA primers are 7-10 nucleotides
• long. Create two 3-nucleotide long RNA

primers that would correspond to the sequence complementary strands closest to the two replication forks.

• 8. Create primase using clay and use it to attach

the RNA primers to the correct sequences on the complementary DNA strand.