DNA Structure

Occurrence• In eukaryotes: in nucleus• DNA in nucleus is associated with simple

proteins (deoxyribonucleo proteins)• Small quantity occurs in mitochondria and

chloroplast. (extranuclear DNA has no proteins associated).

C-value is the amount, in picograms, of DNA contained within a haploid nucleus (e.g. a gamete) or one half the amount in a diploid somatic cell of a eukaryotic organism.

C-value (quantity of DNA)

DNA Structure (an overview)

• DNA has three main components– 1. deoxyribose (a pentose sugar)– 2. base (there are four different ones)– 3. phosphate

#2. The BasesThey are divided into two groups

Pyrimidines and purinesPyrimidines (made of one 6 member ring)

ThymineCytosine

Purines (made of a 6 member ring, fused to a 5 member ring) AdenineGuanine

The rings are not only made of carbon (specific formulas and structures are not required for IB)

Purines

• 1. Adenine 2. Guanine

Pyrimidines

• Thymine Cytosine

DNA Double Helix and Hydrogen Bonding

1- 3

6 - 4

6 - 4

1 - 3

2 - 2

Phospho-diester bond

Glycosidic bondBond between nitrogenous base and deoxyribose sugar

P-5’- sugar (C-1)- N-1 (Py)=(Pu)- N-9- C- 1 (sugar-C-5’- P

•Adenine always pairs with thymine because they form two H bonds with each other

•Cytosine always pairs with guanine because they form three hydrogen bonds with each other

DNA Double Helix and Hydrogen Bonding

#6. Antiparallel strands

The strands run opposite of each other.

The 5’ end always has the phosphate attached.

5’

3’

3’

5’

• The DNA has a negatively charged backbone (because of the phosphate groups).

• Duameter= 20 A (2nm).• 1 turn of spiral = 34 A contains 10 bases.• The proteins (the histones) are positively

charged.• DNA is dexrorotatory.• The DNA and proteins are electromagnetically

attracted to each other to form chromatin

Charguff’s rule

• no. of A and T bp = G and C• Purines and pyrimidines are equal in amount.• Amount of A=T and G=C.• Base ratio A+T/G+C is constant for a species.• Deoxyribose sugar and phosphate occur in

equal proportions

The Structure of DNA.mp4