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What is DNA?
• Although the environment influences how
an organism develops, the genetic
information that is held in the molecules
of DNA ultimately determines an
organism’s traits.
• DNA achieves its control by determining
the structure of proteins.
• Within the structure of DNA is the
information for life—the complete
instructions for manufacturing all the
proteins for an organism.
The structure of nucleotides
• DNA is a polymer made of repeating
subunits called nucleotides.(the monomer)
• Nucleotides have three parts: a simple
sugar, a phosphate group, and a
nitrogenous base.
Nitrogenous base
Phosphate group
Sugar
(deoxyribose)
The structure of nucleotides
• in DNA there are four possible nucleotides, each
containing one of these four bases.
• The phosphate groups and deoxyribose molecules
form the backbone of the chain, and the nitrogenous
bases stick out like the teeth of a zipper.
• The Bases Bind the two sides of the chain together
with hydrogen bonds
Phosphate
group
Nitrogenous base
(A, G, C, or T)
Thymine (T)
Nucleotide
Sugar
(deoxyribose)
Sugar-phosphate backbone
DNA nucleotide
The 4 Unique Nitrogenous
Bases
Bases
• 2 Purines (larger):
– Adenine (A)
– Guanine (G)
• 2 Pyrimidines (smaller):
– Cytosine (C)
– Thymine (T)
Chargaff
• DNA composition: “Chargaff’s rules”
– varies from species to species
– all 4 bases not in equal quantity
– bases present in characteristic ratio
• Humans:A = 30.9%
T = 29.4%
G = 19.9%
C = 19.8%
A=T and C=G
Rosalind Franklin (1920-1958)
1953 | 1962
Structure of DNA
• James Watson and
Francis Crick
worked out the
three-dimensional
structure of DNA,
based on work by
Rosalind Franklin
and Maurice
Wilkens
Wilkins
Paired bases
• DNA structure
– double helix
• 2 sides like a ladder
• Bases match
together
– A pairs with T
– C pairs with G
DNA is a double-stranded
helix
• Watson and Crick
also proposed that
DNA is shaped like
a long zipper that
is twisted into a
coil like a spring.
• Because DNA is
composed of two
strands twisted
together, its shape
is called double
helix.
The structure of DNA
Hydrogen bond
Base
pair
Partial chemical structure
Ribbon model
Computer model
The importance of nucleotide sequences
Chromosome
The sequence of nucleotides
forms the unique genetic
information of an
organism. The closer the
relationship is between
two organisms, the more
similar their DNA
nucleotide sequences will
be.
• Scientists use nucleotide
sequences to determine
evolutionary relationships
among organisms, to
determine whether two
people are related, and to
identify bodies of crime
victims.
DNA Packing
DNA
double
helix
(2-nm
diamete
r
Histones
“Beads on
a string”
Nucleosome
(10-nm diameter)
Tight helical fiber
(30-nm diameter) Supercoil
(200-nm diameter)
700
nm
Metaphase
chromosome
Nucleosomes
• “Beads on a string”
– 1st level of DNA packing
– histone proteins
• 8 protein molecules
• positively charged amino acids
• bind tightly to negatively charged
DNA
8 histone
molecules
Replication of DNA
• Section objective:
• Summarize DNA
replication
Replication of DNA
• Before a cell can divide by mitosis or meiosis, it
must first make a copy of its chromosomes.
• The DNA in the chromosomes is copied in a
process called DNA replication.
• Without DNA replication, new cells would have only
half the DNA of their parents.
• DNA is copied during interphase prior to mitosis
and meiosis.
• It is important that the new copies are exactly like
the original molecules.
Replication of DNA
• DNA replication depends on specific base
pairing
• In DNA replication, the strands are
separated by an enzyme
– Enzymes then use each strand as a template to
assemble the new strands
Nucleotides
Parental molecule
of DNA
Both parental strands serve
as templates
Two identical daughter
molecules of DNA
Replication of DNA
• Semiconservative replication:
• Parental strands of DNA separate, serve
as templates and produce 2 molecules of
DNA that have one strand of parental
DNA and one strand of new DNA
Copying DNA
• Matching bases allows
DNA to be easily
copied
DNA replication
• DNA Helicase: enzyme responsible
for uncoiling the double helix and
unzipping the weak hydrogen bonds
between the base pairs
DNA replication
• Enzyme
– DNA polymerase
– adds new bases to
the old strands
DNA bases
in nucleus
DNA
polymerase
• Leading strand- elongates as DNA
unwinds
• Lagging strand –elongates in
opposite direction . Synthesized
discontinuously into small segments
called Okazaki fragments
• DNA ligase (an enzyme)links these
sections
New copies of DNA
• Get 2 exact copies of DNA to split
between new cells
DNA
polymerase
DNA
polymerase