Molecular Genetics
I) DNA STRUCTURE
AND
REPLICATION
A) DNA STRUCTURE
I) DNA Structure and Replication
DNA Structure
„ according to the model proposed by Watson and
Crick
‰ DNA consist of two strands of nucleotides
‰ each nucleotide contains a:
„
„
„
‰
deoxyribose sugar
phosphate group
nitrogen base
all are covalently (molecularly) bonded together
A) DNA STRUCTURE
I) DNA Structure and Replication
„
„
each strand of DNA has a backbone of sugar and phosphates
groups.
‰ the nitrogenous bases stick out from the backbone of each
DNA strand
the two strands of DNA form a structure that resembles a
twisted ladder.
‰ the base pairs are the rungs of the ladder
‰ the sugar-phosphate backbone are the struts.
‰ the twisted ladder is called a double helix
„ the double helix twists in a clockwise direction
A) DNA STRUCTURE
A) DNA STRUCTURE
I) DNA Structure and Replication
A) DNA STRUCTURE
I) DNA Structure and Replication
„
in the DNA molecule, the bases of one DNA strand are paired
with bases in the other strand.
‰ a purine is always paired with a pyrimidine
„ adenine (a purine) is always paired with thymine
(pyrimidine)
„ guanine (a purine) is always paired with cytosine (a
pyrimidine)
‰ this type of pairing is called complementary base pairing
‰ hydrogen bonds (between A-T and G-C) hold the double
helix together.
ƒ hydrogen bonds are weak but because there is a lot of
them the double helix is very stable
A) DNA STRUCTURE
Side Note:
Chargaff's Rule of Base Pairing
A) DNA STRUCTURE
Side Note:
Chargaff's Rule of Base Pairing
„
„
whatever the amount of adenine
(A) in the DNA of an organism, the
amount of thymine (T) is the same
‰ similarly, whatever the amount
of guanine (G), the amount of
cytosine (C) is the same.
the C+G : A+T ratio varies from
organism to organism among the
prokaryotes),
‰ within the limits of experimental
error:
A = T and C = G.
Relative Proportions (%)
of Bases in DNA
Organism
A
T
G
C
Human
30.9
29.4
19.9
19.8
Chicken
28.8
29.2
20.5
21.5
Grasshopper
29.3
29.3
20.5
20.7
Sea Urchin
32.8
32.1
17.7
17.3
Wheat
27.3
27.1
22.7
22.8
Yeast
31.3
32.9
18.7
17.1
E. coli
24.7
23.6
26.0
25.7
A) DNA STRUCTURE
A) DNA STRUCTURE
I) DNA Structure and Replication
„
the sequence of bases on any one strand of DNA can vary
greatly between species.
‰ the opposite strand will always have the complementary
sequence of bases.
‰ the two strands of nucleotides are antiparallel.
„ they run in parallel but in opposite directions to one
another.
‰ one strand will have a 5’ carbon and phosphate group
at one end and a 3’ carbon and the hydroxyl group of a
deoxyribose sugar at its other end.
‰ the antiparallel strand will have the opposite.
A) DNA STRUCTURE
A) DNA STRUCTURE
I) DNA Structure and Replication
5’-ATGCCGTTA-3’
3’-TACGGCAAT-5’
A) DNA STRUCTURE