DNA REPLICATION

SERWELAS, CLAUDINE M.ESQUIVEL, CHERRY LYN

AAPD2-FGENETICSDR. LOS BANOS

What does DNA look like?• DNA has..• A sugar/phosphate

backbone • 4 Chemical bases

Sugar

The bases• It is composed of 4

chemical bases: Adenosine (A), Thymine (T), Cytosine (C) and Guanine (G).

• A always pairs with T• C always pairs with G• The pairs are held

together by hydrogen bonds (WNC’s).

What does DNA do?

• DNA is the genetic code.• It determines our physical characteristics:

from our hair color to what we are allergic to.• Our DNA codes for 20 amino acids which are

the building blocks of life.

What Is DNA Replication

• DNA Replication is the process in which the DNA within a cell makes an exact copy of itself.– Why does DNA replicate?

– During which phase of the cell cycle does DNA replicate?

DNA Replication models

The Three Possible DNA Replication Models

• Conservative- would leave the original strand intact and copy it.

• Dispersive-would produce two DNA molecule with sections of both old and new along each strand.

• Semiconservative –would produce DNA molecule with both one old strand and one new strand.

DNA Replication

Replicationbubble

Replicationfork

Replicationfork

Hydrogen bond

Replication occurs during Interphase

DNA replication is the process where an entire double-stranded DNA is copied to produce a second, identical DNA double helix.

DNA Replication

• Helicase unwinds the double helix starting at a replication bubble.

• The two strands separate as the hydrogen bonds between base pairs are broken.

• Two replication forks form and the DNA is unwound in opposite directions.

DNAhelicase

DNA Replication

•Helicase has completed unwinding the DNA strand.

•Single strand Binding Proteins (SSB) keep the two strands from re-annealing (coming back

together).

DNA Replication

Primase RNA Primer

•Primase is an RNA polymerase that makes the RNA primer.

•These primers “tell” the DNA polymerase where to start copying the DNA.

Leading Strand

Lagging Strand

DNA Replication

DNA Polymerase

• The DNA polymerase starts at the 3’ end of the RNA primer of the leading stand CONTINUOUSLY.• DNA is copied in 5’ to 3’ direction.• DNA polymerase copies the lagging strand DIS- continuously.

Leading Strand

Lagging Strand

3’ 5’

5’ 3’

Direction of Replication

Direction of Replication

DNA Replication

• The dis-continuous pieces of DNA copied on the lagging strand are known as Okazaki fragments.

DNA Replication

Another DNA Polymerase removes the RNA primers and replaces them with DNA.

DNA Replication

Finally the gaps in the sugar phosphate backbone are sealed by DNA ligase

There are now 2 identical double helices of DNA.

ligase

REACTION:

o The DNA occurs simultaneously forming sister chromatids.

o Nucleotides should always be in pair.o Nucleotides are held together with loose

hydrogen bonds.o Every cell in our body has the same copy of

DNA, and the DNA will copy itself trillion of times in our lifetime.

o Every copy of the DNA contains half of its original strand.

o DNA replication is semiconservative with each existing strand serving as template for synthesis of new strand.

o Replication begins at specific location called REPLICATION.

o On one strand (leading strand) synthesis is continuous

o On the other strand (lagging strand) synthesis is discontinuous.

o The 2 strands is producing a seriesOf okazaki fragments that must be Ligased together

DNA Replication ANIMATION

• http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html