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Chromosomes and DNA
Replication
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Prokaryotic
DNA
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Prokaryotic cells lack
nuclei & many organelles
of Eukaryotes
Located in cytoplasm
 single circular DNA
molecule =
chromosome
Eukaryotic
DNA
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
~1000 times the amount
of DNA as prokaryotes
Located in nucleus as
chromatin
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DNA Length
DNA is very long
 E. coli contains 4,639,221 base pairs.
The length is roughly 1.6 mm
 a human cell's DNA totals about 2-3
meters in length (that’s just one cell!)
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Chromosome Structure
Eukaryotic chromosomes contain both
DNA and protein
 DNA + Protein (histones)  Chromatin
 DNA + histone  beadlike structure
called nucleosome
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Nucleosomes pack to form thick fiber,
shortened by system of loops & coils
(chromosome).
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Nucleosomes
Nucleosome Animation
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What do nucleosomes do?
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Able to fold enormous lengths of DNA into
the tiny space available in the cell nucleus
Histone proteins have changed very little
during evolution—probably because
mistakes in DNA folding could harm a cell's
ability to reproduce
http://www.johnkyrk.com/chromosomestruc
ture.html
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DNA Replication

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Each strand of DNA double helix has all info
needed to reconstruct the other half by
mechanism of base pairing
Each strand can be used to make the other
strand = complementary strands
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DNA Replication
in Prokaryotes
• Begins at a single point
Replication in
Eukaryotic
Chromosomes
• Occurs at hundreds of
places
• Proceeds, often in two
directions, until whole
• Proceeds in both
chromosome is
directions until each
replicated (bidirectional) chromosome is
completely copied
• Replication forks sites where separation
and replication occur
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Duplicating DNA

Before a cell divides, it duplicates its DNA in a
copying process called replication.

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During DNA replication:

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Ensures each cell will have a complete set of DNA
molecules
DNA molecule separates into 2 strands
Then produces 2 new complementary strands
following the rules of base pairing
Each strand of double helix of DNA serves as
a template, or model, for new strand
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How Replication Occurs

Carried out by a series of enzymes
1.DNA molecule unwinds
2.Enzymes “unzip” DNA by breaking the
hydrogen bonds between base pairs
3.Free-floating complementary bases
attach to the original (template) strands

DNA Replication Animation
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Enzymes in Replication

Helicase - unwinds and unzips the DNA

RNA Primer – provides DNA polymerase a

DNA polymerase – joins individual nucleotides

Ligase - binds the okazaki fragments together on

DNA to DNA animation
starting point
to produce a DNA molecule (a polymer)
 Also “proofreads” each new DNA strand; helps
maximize odds that each molecule is perfect
copy of the original DNA.
the lagging strand of new DNA
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CrashCourse: Replication
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