DNA ReplicationCell DNA is copied

What is meant by DNA REPLICATION:

Replicate means copy, or duplicate.

DNA in cells must be copied exactly.

During the process of DNA replication, a duplicate of the DNA molecule is made.Not a good copy

Why must DNA be Copied? Replication: DNA is duplicated during

interphase so that when cells divide, their daughter cells will have identical DNA.

During DNA replication

each strand of the double

helix is a template or model for a new

strand

The Replication Process1) H bonds between bases are broken

(enzyme – helicase)

2) DNA “unzips” between bases and two strands separate. (Binding proteins attach to the strands for support)

This happens at a portion of the molecule called the “replication bubble” The Point at either end of the bubble, where DNA unzips is a “replication fork.”

3) The principle enzyme involved in replication is DNA polymerase.

It allows free nucleotides in the nucleus to find their place and bond to the DNA template strand.

Remember Correct Base-Pairing

A bonds to

T

C bonds to

G

4) The DNA polymerase also proofreads the copied strand.

It can correct any mistakes in the copied DNA.

The end result of replication is two duplicate DNA molecules.

Since each of the two new DNA strands contains one old strand and one new strand, the process of replication is called “semi-conservative.”

Okazaki Fragments DNA is not actually replicated

the same on both strands of the molecule.

Because of the orientation of the nucleotides in each strand (antiparallel), DNA polymerase makes one strand in one piece, and the other in many pieces called Okazaki fragments.

Antiparallel strands

• DNA polymerase can only work from the 3’ – 5’ direction. Since the strands of DNA are “opposite” one another, the enzyme must move in a seemingly opposite direction on the 5’ to 3’ direction.

• The DNA strand that is replicated in one continuous piece is called the leading strand.

• The other DNA strand is replicated in a discontinuous manner at the 3’-5’ direction. This is called the lagging strand. The fragments of this strand are called Okazaki fragments. (named for Reiji Okazaki.

They are usually 1000 – 2000 nucleotides in length)

• The O. fragments are bonded by the enzyme ligase.

• RNA primers of about 10 nucleotides are in place before the DNA polymerase makes a fragment.

It can get complicated.

DNA Replication