Post on 20-Jun-2020
DNA Structure
DNA Structure
• Chromosomes are made of DNA.
• The DNA is wound around proteins called
Histones.
• When chromosomes are unraveled, the
DNA strands are called Chromatin.
• DNA is made of Nucleotides.
– Deoxyribose (5 carbon sugar)
– Phosphate group
– Nitrogen Base (A, T, G, C)
DNA Structure
• Sugar –
Phosphate
Backbone held
together by strong
Phosphodiester
Bonds (covalent)
• The Nitrogen
Bases are held
together by
Hydrogen Bonds
6
DNA Nucleotides
O=P-OO
PhosphateGroup
NNitrogenous base(A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
O
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7
Nitrogenous Bases• Double ring PURINES
Adenine (A)Guanine (G)
• Single ring PYRIMIDINESThymine (T)Cytosine (C)
T or C
A or G
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Table 1. Nitrogen Base Make-Up of Different Organisms’ DNA
(in Percentages)
Organism A T G C
Mycobacterium
tuberculosis
15.1 14.6 34.9 35.4
Yeast 31.3 32.9 18.7 17.1
Wheat 27.3 27.1 22.7 22.8
Sea Urchin 32.8 32.1 17.7 17.3
Marine crab 47.3 47.3 2.7 2.7
Turtle 29.7 27.9 22.0 21.3
Rat 28.6 28.4 21.4 21.5
Human 30.9 29.4 19.9 19.8
9
Base-Pairings
• Purines ONLY pair with Pyrimidines
• Three hydrogen bonds required to bond Guanine & Cytosine
CG
3 H-bonds
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10
T A
•Two hydrogen bonds are required to bond Adenine & Thymine
The hydrogen bonds between A=T and G=C create the twist in the double helix. Each twist of DNA is about 10 bases long.
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2 H-bonds
11
DNA
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
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DNA• One strand
begins with the 5’
end of the sugar
(5’ to 3’ =
Lagging Strand)
• The other strand
begins with the 3’
end of the sugar
(3’ to 5’ =
Leading Strand)
• Strands are
ANTIPARALLEL
(opposite)
13
Deoxyribose Sugar• Carbons are numbered clockwise 1’ to 5’
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
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Hydrogen
bonds
Nucleotide
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix