DNA ReplicationDNA Replication

DNA Does 2 Important Things in a DNA Does 2 Important Things in a Cell:Cell:

1)DNA is capable of replicating itself. Every time a cell divides, each DNA strand makes an exact copy of itself.

2)DNA directs the production of proteins in the cell. DNA contains the instructions on how to make proteins.

The Cell CycleThe Cell Cycle

During the S phase of Interphase, the DNA strands (chromosomes) are replicated.

Why does DNA replicate?Why does DNA replicate?

DNA replication occurs once in the life of a cell—right before it divides into two daughter cells.

DNA strands replicate so that each new daughter cell has its’ own copy of the DNA instructions.

DNA strands are anti-parallelDNA strands are anti-parallel

The deoxyribose sugar has 5 carbons. Carbon 1 is attached to the nitrogen base. Carbons 3 and 5 are attached to Phosphate groups.

These are called 5’ and 3’ ends.

DNA Replication, Step 1DNA Replication, Step 1

1) Helicase unwinds the DNA molecule and opens up the hydrogen bonds between the base pairs.

The place where the DNA is opening up is called the Replication Fork.

Single-stranded binding proteins (SSBs) hold the DNA strands open.

DNA Replication, Step 2DNA Replication, Step 2

DNA Polymerase adds nucleotides to the strands, one base at a time.

The bases are added to their complimentary bases, according to the base-pairing rules:

A bonds to TC bonds to GThis will make 2 strands

of identical DNA

DNA Replication—a few more DNA Replication—a few more detailsdetails

DNA polymerase cannot add nucleotides to an existing “open” strand by itself.

First, a short piece of RNA (called a primer) must be added to the open strand by Primase.

Later on, the RNA primer will be removed and replaced with DNA.

DNA Replication—More detailsDNA Replication—More details

DNA Polymerase can only add nucleotides in the 5’ to 3’ direction.

Therefore, one of the new strands will be made continuously. This is called the leading strand.

The other strand must be made in pieces called Okazaki fragments. This strand is called the lagging strand.

DNA Replication DiagramDNA Replication Diagram

http://www.youtube.com/watch?v=teV62zrm2P0

DNA Replication AnimationsDNA Replication Animations

http://www.youtube.com/watch?v=zdDkiRw1PdU&feature=related

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

Quick QuizQuick Quiz

What is the purpose of each of these enzymes?

A. HelicaseB. DNA PolymeraseC. Single Stranded Binding proteins (SSBs)D. PrimaseWhy does DNA replicate with a leading

strand and a lagging strand—why don’t both strands replicate the same way?

Name each part of this diagram:Name each part of this diagram: