Page 170 - MNU microbiology theoritical 2025

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

Hints

• Double helix dsDNA
Two polynucleotide chains are twisting around each other in the form of a

double helix dsDNA. They called The parallel strand (3’ – 5’) anti-parallel
strands (5’ – 3’)

• As A and T forming two hydrogen bonds, G and C forming three hydrogen
bonds So DNA with high GC-content is more stable than DNA with low
GC-content

The structure of DNA, Supercoiling:

⮚ Size of E. coli: 4 µm

⮚ Number of nucleotides: 4,000,000 nucleotides

⮚ Length occupied by each nucleotide: 3,4 Å

⮚ The whole length of DNA should be= 4,000,000 X 3,4 Å = 13,600,000 Å =

0,00136 M = 1360 µm

⮚ How DNA with a size of 1360 µm could be condensed in a cell with a

size 4 µm ?????

To overcome this problem:

⮚ DNA can be twisted in a process called DNA
supercoiling.

⮚ Topisomerases enzymes are also

needed to relieve the twisting during

processes such as transcription and

DNA replication.

General Microbiology & Immunology (PM 401) theoretical handbook Page 159