Ch. 12.2: Replication of DNA

• Section objective:• Summarize DNA

replication

Replication of DNA

• Before a cell can divide by mitosis or meiosis, it must first make a copy of its chromosomes.

• The DNA in the chromosomes is copied in a process called DNA replication.

• Without DNA replication, new cells would have only half the DNA of their parents.

• DNA is copied during interphase prior to mitosis and meiosis.

• It is important that the new copies are exactly like the original molecules.

Replication of DNA• DNA replication depends on specific base pairing• In DNA replication, the strands separate

– Enzymes use each strand as a template to assemble the new strands

Parental moleculeof DNA

Nucleotides

Both parental strands serveas templates

Two identical daughtermolecules of DNA

Replication of DNA

• Semiconservative replication:• Parental strands of DNA separate, serve as

templates and produce 2 molecules of DNA that have one strand of parental DNA and one strand of new DNA

Copying DNA• Matching bases allows DNA

to be easily copied

DNA replication• DNA Helicase: enzyme responsible for

uncoiling the double helix and unzipping the weak hydrogen bonds between the base pairs

DNA replication

DNA polymerase

• Enzyme– DNA polymerase – adds new bases

DNA basesin nucleus

DNAPolymerase

Copying DNA Build daughter DNA strand

use original parent strand as “template”

add new matching bases synthesis enzyme =

DNA polymerase

• Adding bases – can only add

nucleotides to 3 end of a growing DNA strand• need a “starter”

nucleotide to bond to

– strand only grows 53

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

energy

energy

energy

Replication energy

3

3

5

5

• Leading strand- elongates as DNA unwinds

• Lagging strand –elongates in opposite direction . Synthesized discontinuously into small segments called Okazaki fragments

• DNA ligase (an enzyme)links these sections

Limits of DNA polymerase III can only build onto 3 end of an

existing DNA strand

Leading & Lagging strands

5

5

5

5

3

3

3

53

53 3

Leading strand

Lagging strand

Okazaki fragments

ligase

Okazaki

Leading strand continuous synthesis

Lagging strand Okazaki fragments joined by ligase

“spot welder” enzyme

DNA polymerase III

3

5

growing replication fork

New copies of DNA• Get 2 exact copies of DNA to split between

new cells

DNA polymerase

DNA polymerase