12-2

12-2 chromosomes and DNA replication

• DNA: – Prokaryotic cell:

• Naked, in cytoplasm, no nucleus• Single circular DNA• No complex with histone

– eukaryotic cell: • In nucleus, • In the form of chromosome

Chromosome

E. coli bacterium

Bases on the chromosome

• Chromosome structure: – Chromatin : DNA + proteins – Chromosome : supercoiled chromatin– Nucleosome

• beadlike structures in chromosome formed by DNA and histone molecules

• Help to fold long DNA into tiny cell nucleus

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

• DNA replication (in interphase of mitosis or meiosis)

– Prokaryotic cell:• Begins at a single point • Proceeds in two directions

• DNA replication (in interphase of mitosis or meiosis)

– Eukaryotic cell:• Begins at many sites• Proceeds in two directions

• DNA replication– Semi-conservative replication

• During DNA replication, the DNA double helix separates into two strands. Each strand serves as a template to produces two new complementary strands following the rules of base pairing.

Parent DNA

Copy

• DNA replication– Steps

• Unzip (form replication fork)• Strands assembled in the 5’ to 3’ direction

(Continuous and Discontinuous, form okazaki fragments)• 2 identical DNA molecules

replication fork

AHL

AHL

• DNA replication– Enzyme envolved

Helicase: unzips DNA.Primase: add short RNA primers.Single-strand binding protein : hold seperated strands apartDNA polymerase :

DNA polIII: attaches complementary nucleotides & proofreading

DNA pol I: replaces RNA primers with DNA nucleotides.

DNA ligase: seal okazaki fragments

AHL

AHL