 The
subunit of DNA is called the
 _nucleotide__.
• What are the 3 components of this subunit
called?
3
Components:
• Deoxyribose sugar
• Nitrogen base
• Phosphate
5
different types in genetic material:
• Adenine – Purine– 2 rings, 2 bonds
• Guanine – Purine– 2 rings, 3 bonds
• Cytosine – Pyrimidine– 1 ring, 3 bonds
• Thymine – Pyrimidine– 1 ring, 2 bonds
• Uracil (found only in RNA)
A
long sugarphosphate backbone
with a sequence of
bases sticking off is
formed.
 This may go on for
millions of nucleotides
 The
sugar may be one of two kinds
(shown without the pesky hydrogens)
5
1
4
3
2
 With
both DNA and RNA,
• bases different from one nucleotide to the next
• Gives nucleic acids the appearance of messages
in some strange molecular language.
 DNA
has deoxyribose sugar; RNA ribose
sugar.
 DNA contains thymine; RNA contains
uracil
 Watson
and Crick are credited with
putting all the pieces together (pun
intended) and coming up with this
structure.
 Each
of the base pairs
are nearly flat
 Watson and Crick
proposed stacking them
up like stairsteps, with 2
sugar-phosphate strands
around the outside
 One
complication:
• The 2 strands wind in opposite directions
• The sugars are “upside down” compared with
those of the other strand
 The
model clearly explains Chargaff’s
observation that the number of T’s is
equal to the number of A’s: T and A are
always paired together!
Chromatin – uncondensed DNA (normal form)
 4.6 X 107 bp DNA = shortest human chromosome

• 14,000 micrometers of DNA unpacked
• 2 micrometers during mitosis
• 7000 packing ratio

Nucleosomes – DNA wrapped around proteins
called “histones”
 Packing ratio of 6
16 of 8
© Boardworks Ltd 2010
DNA replication
The discovery of DNA’s structure by Watson and Crick
provided evidence that complementary base pairing was
key to DNA’s ability to replicate.
Scientists proposed that DNA “unzipped”
as hydrogen bonds between base pairs
were broken.
New polynucleotide strands could then
be synthesized using the originals as a
template.
Several hypotheses were proposed as
to the specific mechanism by which new
strands are created.
17 of 8
© Boardworks Ltd 2010
Mechanisms of DNA replication
18 of 8
© Boardworks Ltd 2010
Meselson–Stahl DNA experiment
19 of 8
© Boardworks Ltd 2010
DNA replication
20 of 8
© Boardworks Ltd 2010
DNA replication enzymes
21 of 8
© Boardworks Ltd 2010
DNA replication
22 of 8
© Boardworks Ltd 2010