What is DNA?
• Deoxyribonucleic acid (DNA) is a nucleic acid that contains the
genetic instructions used in the development and functioning
of all known living organisms.
• This chemical substance is present in the nucleus of all cells in
most living organisms.
• DNA controls:
– All the chemi cal cha nges whi ch take pla ce in cells .
– The kind of cell whi ch is formed, (mus cle, blood, nerve etc) is
controlled by DNA.
– The kind of organism whi ch is produced (buttercup, gi ra ffe, sal mon,
human etc) is controlled by DNA.
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DNA Subunits
• DNA is a very large molecule made up of a
long chain of subunits.
• The subunits are called nucleotides.
• Each nucleotide is made up of:
– a sugar called deoxyribose
– a phosphate group -PO4 and
– an organic base
Ribose & Deoxyribose
• Ribose is a sugar, like glucose, but with only
five carbon atoms in its molecule
• Deoxyribose is almost the same but lacks one
oxygen atom
• Both molecules may be represented by the
following symbol
The Bases
The
Most
Common
Bases
Are
Two Kinds of Bases
Adenine (A)
• Pyrimidines are single
ring bases.
Thymine (T)
– Thymine and cytosine
each have one ring of
carbon and nitrogen
atoms.
Cytosine (C)
Guanine (G)
N
C
O C
C C
N
O
N
C
N
C
C
C
C
cytosine
thymine
• Purines are double
ring bases.
– Adenine and guanine
each have two rings of
carbon and nitrogen
atoms.
O
N
N
N
C
C
N
O
C
N
C
adenine N C
N
C
N
C
N C
C
guani ne
N
N
C
Nucleotides
Joined Nucleotides
Combine to form a nucleotide
• The deoxyribose, the phosphate, and one of
the bases combine to form a nucleotide.
• A molecule of DNA is
formed by millions of
nucleotides joined
together in a long
chain
PO4
PO4
PO4
Thymine (T)
PO4
adenine
phosphate
organic
base
sugarPO
phosphate 4
backbone
bases
deoxyribose
DNA Structure
• In fact, the DNA usually
consists of a double
strand of nucleotides
• The sugar-phosphate
chains are on the
outside and the strands
are held together by
chemical bonds
between the bases
bases
DNA Structure (cont.)
• Notice that the
bases always
pair up in the
same way
Adenine
sugar-phosphate
chain
Cytosine
T hymine
PO4
PO4
PO4
PO 4
PO4
PO4
PO4
PO4
Guanine
PO4
PO 4
PO4
PO 4
PO4
PO4
T
A
G
C
N
C
N
N
C
N
C
N
• Their amounts in a given DNA molecule will be about
the same.
C
• Guanine must pair with Cytosine
• The bases attract each
other because of hydrogen
bonds.
• Hydrogen bonds are weak
but there are millions and
millions of them in a single
molecule of DNA.
• (The bonds between
cytosine and guanine are
shown here.)
C
• Adenine must pair with Thymine
Hydrogen Bonds
O
Chargaff’s Rule
N
C
N
C
N
C
C
O
DNA Structure (cont.)
• The paired strands are
coiled into a spiral
called a double helix
• The structure of DNA was first identified by
James D. Watson and Francis Crick in 1953.
• The discovery was based upon data from
Maurice Wilkins and Rosalind Franklin.
DNA By The Numbers
• Each cell has about 2 m
of DNA.
• The average human has
75 trillion cells.
• The average human has
enough DNA to go from
the earth to the sun
more than 400 times.
• DNA has a diameter of
only 0.000000002 m.
History of DNA
DNA Fingerprint
• DNA is like a fingerprint, in that it is
unique to each individual and can be used
to identify them, even though it is not
visible to the naked eye.
The earth is 150 billion meters or
93 million miles from the sun.
Genetic Diversity
• Different arrangements of nucleotides in a
nucleic acid (DNA) provides the key to
diversity among living organisms.
• The DNA is stored as chromosomes.
• The “code” of the chromosome is the specific
order that bases occur.
A T C G T A T G C G G…
Chromosomes
Chromosomes can be represented in
different forms by using a variety of
microscopes:
A: Light microscope view of a chromosome
Banding: groups of genes stained light and
dark.
Puffing: areas of transcription (mRNA
production).
B: Scanning electron microscope (SEM) view of
sex chromosomes in the condensed state during a
cell division. Individual chromatin fibers are
visible.
C: Transmission electron microscope (TEM) view
of chromosomes lined up at the equator of a cell
during the process of cell division. These
chromosomes are also in the condensed state.
A
B
C
DNA Condensing
Chromosome States
Interphase:
Interphase
chromosome
This chromosome would
not be visible as a coiled
up struc ture, but unwound
as a region of dense
chromatin in the nucleus
(as in the TEM of the
nucleus above)
Chromosomes are
single-armed
structures during
their unwound
state during
interphase.
(a–e) Pro gressive imag es from atomic forc e microscopy show the compaction of DNA in yeast caus ed by a protein called AbF2.
Chromosome States
Replicated chromosome
Dividing cells:
Replicated
chromosome
prepared for cell di vision
Chromosomes are
double-armed
structures, having
replicated their
DNA to form two
chromatids in
preparation for
cell division.
Chromatin: a
Chromatin
compl ex of DNA
and protein
Centromere
Cell
Chromatid
A single chromosome may contain hundreds of
genes.
El
Rh
AMY
RB
MN
TYS
CBD
ABO
NP
1
4
HEMA
9
13
DNA
molecule
X
(doubl e helix
comprising
genes)
Numbers of Chromosomes
Chromosome
numbers vary
considerably among
organisms.
The numbers may
differ markedly even
between closely
related species:
Below a re the locations of s ome known genes on human
chromos omes :
Fy
Indi vidual atoms
Histone
proteins
Chromatid
Chromosomes Contain Genes
Chrom osome:
Chromosome Structure
Organisms
Chromosome No.
human
chimpanzee
gorilla
cattle
cat
goldfish
Drosophila
honey bee
Hydra
cabbage
beans
orange
garden pea
46
48
48
60
38
94
8
32 or 16
32
18
22
18, 27 or 36
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A Little DNA Humor