Animations/websites

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

• http://www.nobelprize.org/educational/medicine/dna_double_helix/

DNA replication

DNA length

• DNA molecules are very long– Example: E. coli bacteria-

4,639,221 base pairs• In order to fit in side of

the nucleus of a cell the DNA must be folded very small

• A human cell contain 100 times the b.p. found in bacteria (3.2 billion b.p.)

Chromosome Structure

• The nucleus of a cell contains more then 1 meter of DNA

• What is a chromosome made up of?Review!

• Chromosomes contain both DNA and protein• Protein= histones

Chromosome Structure

• DNA + Histone protein= Nucleosome

• Nucleosomes pack together to further condense

Location of Replication

• Where is DNA stored in the cell?• Can DNA leave the nucleus? (why?)

– DNA replication takes place inside of the Nucleus

• Why does DNA need to Replicate?• When does DNA replicate?– Replication takes place before cell division

DNA Replication

• DNA is copied to make 2 identical DNA strands• Each strand of the original double-stranded

DNA molecule serves as template for the production (semi-conservative)

How it works

• DNA replication is carried out by a series of Enzymes

Steps to replicationStep 1

1. The DNA molecule is unwound and prepared for synthesis by helicase (replication fork)

• Helicase- separates DNA strand

Step 2-Single- stranded Binding proteins

2. Single-stranded binding proteins (SSB)- keeps strands apart during replication

3. Elongation- different for each side of the DNA strand

Steps to replication

DNA directionality Review!

• The strands are oriented in opposite directions.

• This is the only way the bases can line up to form the hydrogen bonds

5’ 3’5’3’ refers to the order of the carbons in the deoxyribose sugar

5’ 3’ strandStep 3 A

• Called the leading strand• RNA primase adds

primer• DNA polymerase can

“read” the template– continuously add

nucleotides (into fork)

• Nucleotides are added according to the base pairing rule

DNA Polymerase

• DNA polymerase moves along the template strand in a 3'-5' direction– daughter strand is formed in a 5'-3' direction.

• Nucleotides are added according to the base pairing rule

3’ to 5’ strandStep 3B

• The new strand is called the lagging strand• Cannot be read by DNA polymerase (oriented

in the opposite direction)

Replication takes place AWAY from fork

Lagging Strand

1. RNA Primase reads the sequence and adds primer2. DNA polymerase lengthens the segment– Formation of Okazaki fragments

3. Helicase unzips more of the strand

Okazaki fragment

Lagging Strand

• DNA ligase connects the fragments together

DNA polymerase proofreads as it adds

nucleotides

“Key players”

• DNA polymerase- adds new nucleotides• RNA primase- starts replication on upside

down strand• DNA ligase- connects new fragments on

upside down strand