DNA Replication

State the part of the cell cycle DNA replication occurs in. Occurs during s phase (synthesis phase) of the cell cycle

Discuss the findings of Meselson and Stahl.

showed DNA replication is “semi-conservative”

Tracked normals and ‘heavy’ nitrogen isotopes during replication (light nitrogen percentage increases every time)

State the name of the phrase used to describe DNA replication. Semiconservative

Describe semi-conservative.

strands unwind

each strand is a template

new DNA nucleotides are added according to complementary base pairs

New DNA molecule has one original and one new strand

Same order of base pairs

Distinguish between origins of replication in prokaryotes and eukaryotes.

prokaryotes have 1 bubble (circular DNA) and replication occurs in both directions

eukaryotes have 1000s of bubbles (linear DNA) and all eventually meet

State the direction of DNA replication. 5’ to 3’

Explain the process of dna replication.

Primase creates RNA primer

complementary short segment of RNA

attachment point for DNA polymerase III

DNA polymerase III adds complementary DNA nucleotides in 5’ to 3’ direction

nucleotides always added to 3’ end of the new strand

The 2 newly created strands are called leading and lagging strands

“Leading Strand”:

runs 5’ to 3’

Its template strand is 3’ to 5’

created continuously in 5’ to 3’ direction

DNA polymerase III always adds DNA nucleotides in 5’ to 3’ direction

“Lagging strand”

runs 3’ to 5’

Template strand is 5’ to 3’

DNA polymerase I replaces RNA primer with DNA

DNA ligase links all unbonded segments

End Result: 2 new molecules of DNA

each has one parent strand and one new strand

Explain the process of DNA replication (8 marks)

Dna replication is semi conservative. DNA helicase separates the DNA strands (by breaking hydrogen bonds.) Single stranded binding proteins prevent it form rewinding. ON the leading strand, primerase will add an RNA primer. This allows DNA polymerase III to continuously add DNA nucleotides in 5’ to 3’ direction. On the lagging strand, multiple primers are needed. DNA polymerase III will discontinuously add nucleotides in the 5’ to 3’ direction, creating okazaki fragments. On both strands DNA polymerase I removes RNA and replaces it with DNA. DNA ligase will link any unbinded nucleotides. There are now 2 new molecules of DNA, each with an original strand and a new strand.

Describe PCR’s purpose.

produces large quantities of a small specific sequence of DNA in the laboratory

useful when only a small amount DNA is available

Ex: crime scene samples of blood, semen, tissue, hair, etc.

3 step process

Outline the step by step process of PCR

Denaturation: heat DNA sample to separate it

Annealing: DNA primer attach to opposite ends

Elongation: DNA Taq polymerase (heat-tolerant enzyme) copies the strands

AHL material:

state the functions of the enzymes in dna replication

DNA helicase: unwinds DNA (breaks hydrogen bonds)

Primerase: creates RNA primer

DNA Polymerase III: adds DNA nucleotides in 5’ to 3’ direction

DNA polymerase I: replaces RNA primers with DNA

DNA ligase: links all DNA nucleotides

DNA gyrase: straightens the strand after it's unwound

Single Strand binding proteins: prevent DNA strands from rewinding

Describe okazaki fragments.

short 5’ to 3’ segments of DNA

100-200 bases long

Added by DNA polymerase III in 5’ to 3’ direction

Compare the lagging and leading strands of DNA

Leading Strand

runs 5’ to 3’

continuously made

single primer

Lagging Strand

3’ to 5’

discontinuously made

okazaki fragments

multiple primers