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DNA, Chromosomes & genes
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2007-2008
Watson and Crick
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1953 article in Nature
Watson and Crick
 Discovered that DNA

was a double helix
Based on x-ray
crystallography
photographs of DNA
taken by Franklin
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DNA
 Deoxyribonucleic acid
 Carries the genetic information of an

organism
Identical copies are found in every cell
of that particular organism
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nucleotide
PO4
DNA
 Made of repeating

subunits called
nucleotides
Sugar, phosphate &
nitrogenous bases
N base
CH2
O
ribose
OH
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Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic
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material.
”
Watson & Crick
Double helix structure of DNA
One complete turn of the helix is 3.4nm and is 10 nucleotides
long,
therefore there are 0.34 nm between base pairs
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Directionality of DNA
 You need to
PO4
nucleotide
number the
carbons!

it matters!
N base
5 CH2
This will be
IMPORTANT!!
O
4
3
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1
ribose
OH
2
The DNA backbone
 Putting the DNA
backbone together

refer to the 3 and 5
ends of the DNA
 the last trailing carbon
5
PO4
base
5 CH2
O
4
1
C
3
O
–O P O
O
5 CH2
2
base
O
4
1
2
3
OH
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3
Anti-parallel strands
 Nucleotides in DNA
backbone are bonded from
phosphate to sugar
between 3 & 5 carbons
5
3
3
5
DNA molecule has
“direction”
 complementary strand runs
in opposite direction
(antiparallel)

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Anti-parallel strands
 3’ end, ends with an OH
group (attached to the 3rd
carbon in the sugar ring)
 5’ end, ends with a
phosphate group (attached
to the 5th carbon in the
sugar ring)
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5
3
3
5
Bonding in DNA
5
hydrogen
bonds
3
covalent
phosphodiester
bonds
3
5
….strong or weak bonds?
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Biology
How
do the bonds fit the mechanism for copying DNA?
Base pairing in DNA
 Purines
adenine (A)
 guanine (G)

 Pyrimidines
thymine (T)
 cytosine (C)

 Pairing

A:T
 2 bonds

C:G
 3 bonds
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Chromosomes
 Chromosomes in interphase are called



chromatin
Chromatin = 40% DNA + 60% Protein
A single strand of DNA have
approximately 300 M nucleotide
DNA is highly coiled to fit in the
nucleus of a cell
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Chromosomes
 Every 200 nucleotides, DNA is coiled


around 8 positively charged proteins
called histones
Proteins are positive, DNA is negative
Each group of histones and DNA is
called a nucleosome
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Chromosomes
 A series of nucleosomes coil into
chromatin fibres
 These fibres then fold into the final
chromatin and are supercoiled
(see DNA packaging ppt)
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What is a gene?
 A specific sequence of nucleotides in
a section of DNA that performs a
specific function
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What is a gene?
 When genes (small sections of
chromatin) are active, the chromatin
isn’t tightly coiled or condensed, but
it is in an open configuration called
euchromatin.
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What is a gene?
 Heterochromatin – are condensed


portions of chromatin
Some portions of chromatin are
permanently condensed & their genes
are never expressed
Heterochromatin exists during cell
division
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http://learn.genetics.utah.edu/content/beg
in/tour/
Tour of the cell animation- do on your
own time!
Also check out DNAi.org website as well!
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