DNA
 DNA is often called
the blueprint of
life.
 In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for many
reasons, e.g.,
 its central importance
to all life on Earth,
 medical benefits such
as cures for diseases,
 better food crops.
DNA Structure
 DNA consists of two molecules that are arranged
into a ladder-like structure called a Double
Helix.
 A molecule of DNA is made up of millions of
tiny subunits called Nucleotides.
 Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar
3. Nitrogenous base
Nucleotides
Phosphate
Nitrogenous
Base
Pentose
Sugar
Nucleotides
 The phosphate and sugar form the backbone
of the DNA molecule, whereas the bases form
the “rungs”.
 There are four types of nitrogenous bases.
Nucleotides
A
Adenine
C
Cytosine
T
Thymine
G
Guanine
Two Kinds of Bases in DNA
N
 Pyrimidines are
single ring bases.
N
O
C
C
N
 Purines are double
ring bases.
C
N
C
N
C
C
C
N
C
N
N C
Thymine and Cytosine are
pyrimidines
 Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
N
O
C
C
N
N
C
C
thymine
O
C
C
C
C
N
C
cytosine
Adenine and Guanine are
purines
 Adenine and guanine each have two
rings of carbon and nitrogen atoms.
O
N
N
N
C
C
N
C
C
C
C
N
N
N
Adenine
N
C
N
C
Guanine
C
C
N
Nucleotides
 Each base will only bond with one other
specific base.
 Adenine (A)
 Thymine (T)
 Cytosine (C)
 Guanine (G)
Form a base pair.
Form a base pair.
DNA Structure
 Because of this complementary base pairing,
the order of the bases in one strand
determines the order of the bases in the other
strand.
A
T
C
G
T
A
C
G
A
T
G
C
T
A
N
Hydrogen Bonds
C
C
N
N
C
O
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
 Hydrogen bonds are
N
N
C
 The bases attract
each other because
of hydrogen bonds.
N
C
N
C
C
C
N
O
Hydrogen Bonds, cont.
 When making
hydrogen bonds,
cytosine always pairs
up with guanine,
 And adenine always
pairs up with
thymine.
 (Adenine and
thymine are shown
here.)
O
N
O
C
C
C
N
C
C
AS Biology. Gnetic control of protein structure and
function
DNA Structure
 To crack the genetic code found in DNA we
need to look at the sequence of bases.
 The bases are arranged in triplets called
codons.
AGG-CTC-AAG-TCC-TAG
TCC-GAG-TTC-AGG-ATC
DNA Structure
 A gene is a section of DNA that codes for a
protein.
 Each unique gene has a unique sequence of
bases.
 This unique sequence of bases will code for the
production of a unique protein.
 It is these proteins and combination of proteins
that give us a unique phenotype.
DNA
Gene
Protein
Trait
Your Task
 Draw a flow chart to
show how to get from:
Looking Ahead: Properties of
DNA
 Property 1 - it must contain, in a stable form,
information encoding the organism’s
structure, function, development and
reproduction
 Property 2 - it must replicate accurately so
progeny cells have the same genetic makeup
 Property 3 - it must be capable of some
variation (mutation) to permit evolution
Replication of DNA and
Chromosomes
 Speed of DNA replication:
3,000 nucleotides/min in human
30,000 nucleotides/min in E.coli
 Accuracy of DNA replication:
Very precise (1 error/1,000,000,000 nt)
More 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.
The earth is 150 billion m
or 93 million miles from
the sun.