16.2 dna replication. layout of the eukaryote dna two dna strands are antiparallel –run in...
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16.2 DNA Replication
Layout of the Eukaryote DNA
• Two DNA strands are antiparallel– Run in opposite
directions– 3’ (three prime) – 5’
(five prime)– 5’ (five prime) – 3’
(three prime)
DNA Replication
• Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material
DNA Replication
• Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication
• In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
Fig. 16-9-1
A T
GC
T A
TA
G C
(a) Parent molecule
Fig. 16-9-2
A T
GC
T A
TA
G C
A T
GC
T A
TA
G C
(a) Parent molecule (b) Separation of strands
Fig. 16-9-3
A T
GC
T A
TA
G C
(a) Parent molecule
A T
GC
T A
TA
G C
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
(b) Separation of strands
A T
GC
T A
TA
G C
A T
GC
T A
TA
G C
Semiconservative Model
• Each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand
Fig. 16-10Parent cell
First replication
Second replication
(a) Conservative model
(b) Semiconserva- tive model
(c) Dispersive model
Matthew Meselson and Franklin Stahl
DNA in Prokaryotes and Eukaryotes
• Prokaryotes:– ring of chromosome– holds nearly all of
the cell’s genetic material
Prokaryote DNA Replication
• DNA replication begins at a single point and continues to replicate whole circular strand
• Replication goes in both directions around the DNA (begins with replication fork)
Fig. 16-12Origin of replication Parental (template) strand
Daughter (new) strand
Replication fork
Replication bubble
Two daughter DNA molecules
(a) Origins of replication in E. coli
Origin of replication Double-stranded DNA molecule
Parental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules
(b) Origins of replication in eukaryotes
0.5 µm
0.25 µm
Double-strandedDNA molecule
DNA Replication Overview
• DNA splits into two strands• Complementary base pairs fill in (A with T,
C with G)• Left with two DNA molecules
– Semiconservative model– Identical
Eukaryote DNA Replication
• Begins in hundreds of locations along the chromosome– Origins of replication
Initiation of DNA Replication
• Begins when the DNA molecule “unzips”– Replication fork– Replication “bubble”
• Hydrogen bonds between base pairs breaks
• Helicase• Single-strand binding proteins• Topoisomerase – relieves
pressure of DNA ahead of replication fork
Fig. 16-13
Topoisomerase
Helicase
PrimaseSingle-strand binding proteins
RNA primer
55
5 3
3
3
Synthesis of a New DNA Strand
• Each strand serves as a template for a new strand to form
• Primer of RNA– The primer is short (5–10 nucleotides long), and the 3 end
serves as the starting point for the new DNA strand
• Complimentary bases will attach• DNA polymerase
– E. coli – DNA polymerase III and DNA polymerase I– Humans – 11 different DNA polymerase molecules
Synthesis of a New DNA Strand
• RNA primer• Nucleoside
triphosphate– As each nucleotide is
added to the new strand, 2 phosphates are lost• Hydrolysis releases
energy to drive reaction
• Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
• dATP supplies adenine to DNA and is similar to the ATP of energy metabolism
• The difference is in their sugars: dATP has deoxyribose while ATP has ribose
• As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Synthesis of a New DNA Strand
• Antiparallel Elongation– Remember 3’ – 5’ and 5’ – 3’
• Replication in the 3’ to 5’ direction ONLY– MEANING the NEW strand of DNA will form starting
with the 5’ end
• Leading strand (only 1 primer needed – moves toward the replication fork)
• Lagging strand (many primers needed – moves away from replication fork)
Fig. 16-16b1
Template strand
5
53
3
Fig. 16-16b2
Template strand
5
53
3
RNA primer 3 5
5
3
1
Fig. 16-16b3
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
Fig. 16-16b4
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
Fig. 16-16b5
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
12
3
3
5
5
Fig. 16-16b6
Template strand
5
53
3
RNA primer 3 5
5
3
1
1
3
35
5
Okazaki fragment
12
3
3
5
5
12
3
3
5
5
12
5
5
3
3
Overall direction of replication
Important Enzymes
• Helicase, single-strand binding protein, topoisomerase• Primase
– Synthesis of RNA primer
• DNA polymerase III (DNA pol III)– Add new bases to DNA strand
• DNA polymerase I (DNA pol I)– Removes and replaces RNA primer from 5’ end
• DNA ligase– Links Okazaki fragments and replaces RNA primer from 3’ end
Topoisomerase Video
• http://www.youtube.com/watch?v=EYGrElVyHnU
The Finished Product
• Each DNA molecule has one original strand and one new strand
• Molecules are identical
DNA Replication Video
• http://www.youtube.com/watch?v=teV62zrm2P0
Fig. 16-UN5
Repair of DNA
• DNA polymerase– Proofreads and repairs damaged/mismatched
DNA• Nuclease
– Removes section of DNA that is damaged– DNA polymerase and DNA ligase replace
missing portion
Telomeres
• Found at the ends of each chromosome• Contain no genes• Sequence that can be cut short and will
not affect normal functioning• TTAGGG• Telomerase lengthens telomeres in
gametes
Telomeres
• The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions
• There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist
16.3 A chromosome consists of a DNA molecule packed
together with proteins
Chromosomes
Chromosome Structure
• Bacterial chromosome– double-stranded– circular – small amount of protein
• Eukaryotic chromosomes – Linear DNA molecules – large amount of protein
• DNA in bacteria is “supercoiled” and found in a region of the cell called the nucleoid
Chromatin and Histones
• Chromatin is a complex of DNA and protein, and is found in the nucleus of eukaryotic cells
• Histones are proteins that are responsible for the first level of DNA packing in chromatin– Form a tight bond because DNA is negatively
charged and the histones have a positive charge
Fig. 16-21a
DNA double helix (2 nm in diameter)
Nucleosome(10 nm in diameter)
Histones Histone tailH1
DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)
Fig. 16-21b
30-nm fiber
Chromatid (700 nm)
Loops Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber Looped domains (300-nm fiber)
Metaphase chromosome
Chromosome Organization
• Chromatin is organized into fibers• 10-nm fiber
– DNA winds around histones to form nucleosome “beads”
– Nucleosomes are strung together• 30-nm fiber
– Interactions between nucleosomes cause the thin fiber to coil or fold into this thicker fiber
Chromosome Organization
• 300-nm fiber– The 30-nm fiber forms looped domains that
attach to proteins• Metaphase chromosome
– The looped domains coil further– The width of a chromatid is 700 nm
Euchromatin
• Most chromatin is loosely packed in the nucleus during interphase – Condenses prior to mitosis– Euchromatin
Heterochromatin
• During interphase, a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin– Dense packing of the heterochromatin makes
it difficult for the cell to express genetic information coded in these regions