Page 169 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

• There are two major types of nitrogenous bases.

Purines • Adenine (A)
• Guanine(G)

Pyrimidin • Cytosine (C)
es • Thymine (T).

Base Pair Rule (Chargaff’s Rule)

The two DNA chains are held together by hydrogen bonds
between their nitrogenous bases.

(2 H bonds)

(3 H bonds)

• The backbone of DNA strand consists of alternating phosphate and

deoxyribose sugar

• The deoxyribose sugars are joined at both the 3'-

hydroxyl and 5'-hydroxyl groups to phosphate

groups in ester links, which are known as

"phosphodiester" bonds.

• DNA Composed of parallel strand and anti-

parallel strand

⮚ (one oriented 5’– 3’ and the other 3’ – 5’)

⮚ The two chains of the double helix have

complementary sequence

General Microbiology & Immunology (PM 401) theoretical handbook Page 158