1

2

Cell Division Cycle• New cells are produced by division of existing cells• If many new cells are needed, cells go through a cycle of

events again and again• This is called the cell division cycle

3

• The longest phase in this cycle is interphase• During interphase the cell carries out many biochemical

reactions and grows larger, increase numbers of organelles

• The DNA molecules in the chromosomes are not coiled up (chromatin) and the genes on them can be transcribed to allow protein synthesis

• If the cell is going to divide again, the DNA is all replicated• These and other processes make interphase a very active

period for a cell

DNA replication

chromosomes condense

cell grows and develops

4

Interphase:DNA Replication

• DNA replication is a way of copying DNA to produce new molecules with the same base sequence

5

antiparallel strands of

DNA

6

Base Pairing during Replication

Each old strand serves as the template for complementary new strand

7

• Semiconservative model of DNA replication– Each molecule formed by replication consists of one new strand

and one old strand conserved from the parent DNA molecule thus each new DNA molecule is a combination of one parent strand and one newly replicated strand

8

9

10

Replication begins at multiple sites at the

same time

E. Coli

11

Continuous and Discontinuous Assembly

• Strands can only be assembled continuously toward the replication fork in the 5’ to 3’ direction

• The discontinuous side must be assembled in chunks called Okazaki fragments

12

13

1.

2.

3.

4.

5.

6.

Summary of DNA replication:

14

• Step 1. Helicase enzyme unwinds the parent double helix. Hydrogen bonds are broken and strands are separated

• Step 2. Binding proteins stabilize unwound DNA• Step 3. Single strands act as templates for new

strands. Free nucleotides are present in large numbers around the replication fork. DNA polymerase synthesizes the continuous 5’3’ leading strand by forming hydrogen bonds between the complementary bases

• Steps 4. and 5. Primase enzyme and another DNA polymerase enzyme synthesize the lagging strand fragments (Okazaki fragments)

• Step 6. DNA ligase joins the Okazaki fragments together. The daughter DNA molecules each rewind into a double helix

DNA Replication Summary

15

Enzyme Roles and Names

Where does the energy come from for DNA Replication?

•Deoxynucleoside triphosphates (dNTP)•Base + deoxyribose + THREE phosphates

•The 2 extra phosphates are lost and this provides the energy to bond one nucleotide to another

16

17

histone protein

DNA strand

DNA wrapped around histones

DNA-histone complex folded

folding again

folding yet again

chromosome: very densely packed DNA

and proteins

structure of chromosome

DNA + proteins

18

DNA

gene = a discrete unit of hereditary information (DNA)

locus = location of a gene on a chromosome

19

20

DNA replication

chromosomes condense

cell grows and develops

• At the end of interphase when DNA replication has been completed, the cell begins mitosis

• In mitosis the nucleus divides to form two genetically identical nuclei

• Towards the end of mitosis, cytoplasm of the cell starts to divide, eventually two cells are formed, each containing one nucleus

• The process of dividing the cytoplasm to form two cells is cytokinesis

• The two cells begin interphase when mitosis & cytokinesis have been completed