12-2
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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
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• 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
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• DNA replication (in interphase of mitosis or meiosis)
– Prokaryotic cell:• Begins at a single point • Proceeds in two directions
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• DNA replication (in interphase of mitosis or meiosis)
– Eukaryotic cell:• Begins at many sites• Proceeds in two directions
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• 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
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• 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
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AHL
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• 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
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AHL
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