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