How Does DNA Copy Itself?

12.3 DNA Replication

DNA Replication – What and Why

Replication = DNA making copies of itself– DNA must be copied before a cell can divide– Each new cell will have

a complete set of DNA

History of Replication

Watson & Crick – realized that complementary base pairing provided a way for DNA to copy itself - base pairing could allow a new strand to be built on an old strand - 3 possible models of DNA replication

3 Models of Replication: Semi-conservative Replication

Each strand in a DNA molecule is used as a template to build a new strand using complementary base pairing Results in new molecule with one original DNA strand and one new strand

3 Models of Replication: Conservative Replication

Leaves the original DNA intact and produces another molecule of DNA identical to the first

3 Models of Replication: Dispersive Replication

Creates two molecules that are a mix of new and old DNA interspersed along each strand of the molecule

The Question is Answered

1957 Matthew Meselson & Franklin Stahl -conducted an experiment proving replication is semiconservative - each DNA molecule has one new strand and one old strand

DNA Replication: Process

@Replication begins when the enzyme DNA helicase opens the DNA forming replication bubbles@

DNA Replication: Process

Multiple replication bubbles are opened simultaneously allows the molecule to be replicated quickly

DNA Replication: Process

The nitrogen bases on the original DNA strands are exposed in the replication bubbles. They serve as a template to build new DNA strands

DNA Replication

The ends of the replication bubbles known as the replication fork is where replication begins

DNA Replication – DNA Polymerase

@The enzyme DNA polymerase brings new nucleotides to the

replication fork@ - it pairs them according to base

pairing rules A pairs with T C pairs with G

DNA Replication – Leading Strand

The process of replication proceeds from 5’ to 3’ leading strand- replication is continuous – it is built toward the

replication fork

DNA Replication – Lagging Strand

lagging strand- replication occurs in short segments - called Okazaki fragments - the new DNA strand grows away from the replication fork

DNA Replication- The Big Picture

Each Bubble has 2 Forks – each fork has a leading and lagging strand

DNA Replication

The process continues until 2 complete copies of the DNA are produced

@Each copy of the DNA has 1 strand of DNA

from the original DNA and one new strand that was produced by replication@

Known as semi-conservative replication

DNA Replication

DNA Replication

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter14/dna_replication.html

Telomeres

– The tips of chromosomes are known as telomeres.

– Over time, DNA may actually be lost from telomeres each time a chromosome is replicated.

– @An enzyme, telomerase, solves this problem by adding short, repeated DNA sequences to telomeres@

Prokaryotes vs. Eukaryotes

Prokaryotes- starts from a single point and goes in two directions until the entire chromosome is copied.

Eukaryotes- replication begins at dozens or even hundreds of spots on the DNA, going in both directions until each chromosome is completely copied.

Which enzyme is used to separate the DNA?

A. HelicaseB. LigaseC. PolymeraseD. Telomerase

Which enzyme adds nucleotides to the strands?

A. HelicaseB. LigaseC. PolymeraseD. Telomerase

Which enzyme prevents DNA from being lost during replication?

A. HelicaseB. LigaseC. PolymeraseD. Telomerase

How is Prokaryotic DNA different from Eukaryotic?

Prokaryotic DNA is circular and replication begins at 1 point.

Eukaryotic DNA is linear and replication begins at many points.