Download 12-2 Chromosomes and DNA Replication

12-2 Chromosomes and
DNA Replication
Prokaryotic Cells
• No nucleus
• DNA in the cytoplasm
• Single circular DNA
molecule with all the cell’s
genetic info
Eukaryotic Cells
• 1000x the amount of DNA
• DNA in the nucleus, form of
chromosomes
• Chromosome # varies from species
to species
• Human 2N cells = 46 chromosomes
• Drosophila cells = 8
DNA Length
• E. coli have 4,639,221 base
pairs and about 1.6 mm long
all in one bacterium
• That’s like putting a 300 meter
long rope into a backpack
Chromosome Structure
• Nucleus of a human cell contains
> 1meter of DNA
• Eukaryotic chromosomes contain DNA
and protein tightly packed to form
chromatin
• Chromatin is DNA tightly coiled around
histones to form a nucleosome
• Nucleosomes fold huge lengths of DNA
into the tiny spaces in the nucleus
DNA Replication
• Structure of DNA explains how
it can be copied (replicated)
• Complementary strands have
all info needed to reconstruct
other half by base pairing
Prokaryotes
•DNA replication begins
at a single point and
proceeds
(in two directions)
Eukaryotes
• DNA replication occurs at
hundreds of places
• Proceeds in both directions until
each chromosome is completely
copied
• Replication forks – points where
separation and replication occur
• See page 298/Fig. 12-11
• Replication happens before a
cell divides—copies its DNA
• Two new complementary
strands made following base
pairing rules
• Each strand is a template
(model) for a new strand
TAGCAA
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How Replication Occurs
ENZYMES are IMPORTANT
DNA helicase unzips the DNA
DNA polymerase joins individual
nucleotides to produce a DNA
molecule
DNA polymerase also proofreads
each new DNA strand to ensure each
molecule is a perfect copy of the
original