Chapter 5 : Microbial genetics and evolution

Microbiology is the study of single celled creatures or organisms. They are divided in ,Eukaryotes and Prokaryotes

Eukaryotes  : Fungi ,Protozoa

Prokaryotes : Bacteria,Archie

Because bacteria are most diverse collection of organisms, it is very hard to say something universally about bacteria. So, this discussion can be inexact concerning all bacteria.

Generally, bacteria are small microorganismes (eg: E coli measures approximately 0,5 in width by 2 μm in length). They have one chromosome, generally circular DNA molecule, but they are very diverse.

Bacteria can be represented schematically:

The center is the nucleoid which is a single chromosome in all bacteria surrounded by cytoplasm, there is an inner membrane and a rigid cell wall.

And some bacteria contain another membrane on the outside (50%) = outer membrane (gram stain is on outer membrane).

Bacteria are divided by fission: for ex E coli is divided every 20’  ⇒1000 (10 generations) in 200′ , 1000000 (20 générations) in 400′ ……….

Bacteria has one DNA molecule and it is typically a circle. And in every case, there is a point on that molecule, where the DNA replication initiates. it is called origin of replication and when it is initiated it progresses in two directions and go around to meet each other at the bottom and at that moment the chromosome is completely replicated.

The speed of DNA polymerase of E coli is 1000 base pairs/sec ,The error rate is 1 Base/generation.

Now, the E coli DNA has 4639221 base pairs ,How long E coli takes to replicate his chromosome? = 70 minutes .It divides par 20 minutes but replicates its DNA per 70 minutes!! How it is possible?? Part of the response is bacteriophage

The DNA is really pressurized in the head of phage ,(Pressure = 6 mega Pascale = 60 atmosphere  remember that with a champagne bottle after shaking there is 6 atmospheres).

Phage life style

The second mechanism is shown by lambda phage ,When lambda penetrates, it will see the conditions inside the bacteria; if conditions are bad so it integrates bacterial DNA and stays there until the conditions change.

So, the decision making between giving lytic or lysogenic, is probably the most well understood, of molecular switches in all of biology

In bacteria there is three kinds of exchange of DNA

1) Transformation

What haemophilus will do in order to explore evolutionary space, is it take up DNA from a solution : if that DNA comes from another haemophilus (there are changes in DNA and some are advantageous but some are not).

When it takes DNA from outside it see if it is advantageous or not so integrates or not.

What haemophilus has on it’s surface is a receptor protein that recognizes 11 base pairs sequences of DNA and this is common in haemophilus chromosomes and is uncommon an other chromosomes.

And how it can knows to take a good DNA and not a bad one? Normally, the 11 pairs are different from the phage’ s chromosomes, but sometimes they are the same.The second protection is: restrication-modification system. Each bacteria has an enzyme in it that recognizes a particular sequence of DNA and simply clives that sequence: 4 bases/sequences, 5 bases pairs/sequence, 6 or 8 bases pairs /sequences

CGATCG            GCTAGC

When DNA is methylated (CH3 ) the enzyme doEs not cut but otherwise  it  cut, so if it comes from haemophilus it is methylated, but in viruses and phages it is not.

2)      Conjugation

Plasmids is small circular DNA molécules ,F= fertility plasmid .How doses that work?

3)      Transduction: