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Chromosomes and DNA Replication
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DNA and Chromosomes
Many eukaryotes have 1000
times the amount of DNA as
prokaryotes.
Eukaryotic DNA is located
in the cell nucleus inside
chromosomes.
The number of
chromosomes varies widely
from one species to the
next.
DNA and Chromosomes
Eukaryotic Chromosome Structure
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
DNA and Chromosomes
DNA and Chromosomes
In prokaryotic cells, DNA is
located in the cytoplasm.
Most prokaryotes have a single
DNA molecule containing
nearly all of the cell’s genetic
information.
DNA and Chromosomes
Chromosome
E. Coli Bacterium
Bases on the
Chromosomes
The Components and Structure of DNA
I The Components and Structure of DNA
DNA is made up of nucleotides.
A nucleotide is a monomer of
nucleic acids made up of:
• Deoxyribose – 5-carbon Sugar
• Phosphate Group
• Nitrogenous Base
The Components and Structure of DNA
There are four
kinds of bases
in in DNA:
• adenine
• guanine
• cytosine
• thymine
The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:
–The percentages of guanine [G] and
cytosine [C] bases are almost equal in
any sample of DNA.
–The percentages of adenine [A] and
thymine [T] bases are almost equal in
any sample of DNA.
DNA and Chromosomes
Chromosome Structure
Eukaryotic chromosomes
contain DNA and protein,
tightly packed together to
form chromatin.
Chromatin consists of DNA
tightly coiled around
proteins called histones.
DNA and histone molecules
form nucleosomes.
Nucleosomes pack together,
forming a thick fiber.
The Components and Structure of DNA
DNA Double Helix
The Components and Structure of DNA
Watson and Crick discovered that
hydrogen bonds can form only between
certain base pairs—adenine and thymine,
and guanine and cytosine.
This principle is called “the base pairing
rule”.
II
DNA Replication
Each strand of the DNA double
helix has all the information
needed to reconstruct the
other half by the mechanism
of base pairing.
In most prokaryotes, DNA
replication begins at a
single point and continues
in two directions.
DNA Replication
In eukaryotic chromosomes, DNA
replication occurs at hundreds of places.
Replication proceeds in both directions
until each chromosome is completely
copied.
The sites where separation and
replication occur are called replication
forks.
Copyright Pearson Prentice Hall
Duplicating (replicating) DNA:
Before a cell divides, it duplicates its
DNA in a process called replication.
Replication ensures that each
resulting cell will have a complete
set of DNA.
During DNA replication:
1.The DNA molecule
separates into two strands.
Each strand of the double
helix of DNA serves as a
template for the new strand.
This is carried out by an
enzyme, DNA helicase, that
“unzips” a molecule of DNA at
the Hydrogen bonds between
base pairs and the two
strands of DNA unwind.
DNA Replication
2. It produces two new
complementary strands following
the rules of base pairing as each
nucleotide matches the exposed
base on each side of the ladder,
DNA polymerase joins individual
nucleotides to produce a DNA
molecule and then “proofreads”
each new DNA strand.
DNA Replication
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication Fork
Replication Fork
DNA Polymerase
DNA Replication
END OF SECTION
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15509/replication_basic.mp4