DNA ReplicationDNA Replication

Chromosome

E. coli bacterium

Bases on the chromosome

DNA is very long!... but it is highly folded packed

tightly to fit into the cell!

For example, a human cell

contains more than 1 meter of DNA

made of more than 3 X 109 base pairs!

Go to Section:

DNA Structure

•Double helix made of nucleotides – instructions for life

•Each nucleotide is made of three part – phosphate, a 5-carbon sugar and a nitrogen base

Nucleotide Base Pairing• Four bases – A (adenine), C (cytosine), G (guanine),

T (Thymine)– A will always bond with T– C will always bond with G

Eukaryotic Chromosomes

• Contain DNA and proteins called histones• Tightly packed DNA and proteins form

chromatin• During mitosis, the chromatin condenses to

form tightly packed chromosomes

Figure 12-10 Chromosome Structure of Eukaryotes

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Section 12-2

Go to Section:

Watson & Crick again

• Earlier: Discovered double helix of DNA• Then: DNA can be copied or replicated,

because each strand of the DNA double helix has all the information needed to reconstruct the other half by way of base pairing– The strands are complementary!

DNA Replication• The process of making a copy of the DNA• Occurs inside the nucleus of the cell• Occurs when the cell is going to divide so

each resulting cell will have a complete set of DNA

• During DNA replication, the DNA separates into two strands, then produces two new complementary strands following the rules of base pairing.

• Each strand serves as a template, or model, for the new strand.

• Replication occurs in both directions• The site where separation occurs is called

the replication fork

DNA Replication• The two strands of DNA unwind or “unzip”

breaking the hydrogen bonds and separating. Then each strand becomes the guide or “template” for the making of a new strand.

• A protein called an enzyme called DNA polymerase breaks the nitrogen base bonds and the two strands of DNA separate, polymerizes individual nucleotides to produce DNA and “proof reads” the new DNA.

• The bases on each strand pair up with new bases found in the cytoplasm

• Then the sugar and phosphate groups form the sides of each new DNA strand

• Each new DNA molecule contains an original DNA strand and a new DNA strand (semi conservative model)

Figure 12–11 DNA ReplicationSection 12-2

Go to Section:

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous basesReplication fork

Original strand

New strand

STEP 1• Two original strands of DNA

separates by unwinding• This occurs through the

action of an enzyme that breaks the hydrogen bonds between the strands

• The two areas on either end where the DNA separates is the REPLICATION FORK

Replication fork

STEP 2

• At the replication fork, DNA polymerase moves along the DNA Strands adding nucleotides

• As DNA polymerase moves along two strands of DNA form

Step 3

• DNA Polymerase continues until all the nucleotides have been added

• Two new identical molecules of DNA are formed

• Another enzyme connects all the pieces of DNA together

Proofreading• At the end of

Replication, DNA polymerase goes through the DNA to make sure there are no errors

• This prevents mistakes in the DNA sequences

• Mutations- Change in the sequence of the DNA

Like a Zipper….

Figure 12–11 DNA ReplicationSection 12-2

Go to Section:

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous basesReplication fork

Original strand

New strand