Watson and Crick
described the
structure of DNA:
1953
Died in 2004
Nucleic Acids are:
DNA (deoxyribonucleic acid):
- giant polymers that carry
instructions for making proteins
RNA (ribonucleic acid):
- interpret and carry out the instructions
coded in the DNA
Nucleic acids
store the
information to
make proteins
Monomers of nucleic acids:
nucleotides
A nucleotide consists of a:
Phosphate
Nitrogenous
base
Pentose sugar
Sugars in:
RNA
Ribose
DNA
Deoxyribose
Many nucleotides in
DNA :
Two nucleotides join together to form a
dinucleotide and many form a polynucleotide
DNA is like a ladder
Bases are like the
rungs of the ladder.
Backbones of RNA and DNA consist of:
alternating sugars
and phosphates
Bases:
- are attached to sugars
- project from the side
3´
5´
5´
5´
3´
T
T
A
G
C
C
G
T
A
A
T
A
A
C
G
G
C
A
T
T
A
C
G
C
T
A
T
G
C
T
A
G
C
T
A
Cartoon of
base pairing
A DNA molecule:
A
G C
consists of two
polynucleotide
strands each coiled
in a right-handed
helix (cylindrical
spiral)
C G
T
T
C
G
G
A T
A
T
A
A
DNA is a double
helix.
T
G
C
A
3´
G
T
3´
5´
Cartoon of
double helix
The two
complementary
polynucleotide
strands are held
together by
hydrogen bonding
between the
nitrogenous bases
of adjacent
nucleotides
Complementary base pairing:
 particular bases pair only with certain
bases e.g. adenine – thymine
 makes it possible to copy DNA
molecules faithfully
Chargaff’s rule:
A+G = T+C
e.g. If there is 31% adenine in DNA, find the
percentage of guanine.
A = T = 31 + 31 = 62%.
So C + G = 100 – 62 = 38%.
Guanine = 38/2 = 19%
The virus has single-stranded DNA as its genetic
material. Explain the evidence from the table
which suggests that the DNA is single-stranded. (2)
Organism
Human
Cow
Salmon
Rat
Virus
Percentage of each base
Adenine Guanine Cytosine Thymine
31.2
18.8
18.8
31.2
27.9
22.1
22.1
27.9
29.4
20.6
20.6
29.4
28.6
21.4
21.4
28.6
24.7
24.1
18.5
32.7
amounts of A and T /C and G/complementary bases
different; therefore no base-pairing;
Differences between
RNA & DNA
RNA
pentose sugar:
ribose
DNA
pentose sugar:
deoxyribose
bases:
adenine,
guanine,
cytosine
uracil
bases:
adenine
guanine
cytosine
thymine
RNA
ratio of:
adenine & uracil to
cytosine &
guanine varies
DNA
ratio of:
adenine & thymine to
cytosine & guanine is
one
RNA
DNA
manufactures in the found almost entirely
nucleus but found in the nucleus [occurs
throughout the cell in mitochondria &
chloroplasts]
interpret and carry
carry instructions for
out the instructions
making proteins
coded in the DNA
RNA
DNA
RNA strands are
continually
made, broken
down and reused
DNA is completely
protected by the body
RNA
DNA
RNA is synthesized from
DNA when needed
DNA is self-replicating
RNA
DNA
maybe temporary - permanent
existing for short
periods
only one basic form
3 basic forms:
messenger, transfer
and ribosomal RNA
THE
END