Monday, 29 October 2007

Difference between Hfr and non-Hfr conjugation:
Hfr involves the main chromosomal molecule while non-Hfr involves the F plasmid.

1. An endonuclease enzyme will cause a nick to be formed at the ori T of the plasmid, opening up the circular the plasmid into a linear one.

2. A restriction enzyme will recognise the restriction sites on the main chromosomal molecule and cleave at these sites, causing the main chromosome to open up and allow the linear plasmid to integrate into the main chromosome.

3. A nick is then formed at the 5' end which is located in between the chromosomal molecule and the integrated plasmid. The 5' end consisting of the chromosomal molecule will get pulled over towards the F- cell across the conjugation bridge.

4. However, the conjugation bridge is very fragile and it breaks easily. When it breaks, the whole linear molecule that is in the process of transferring will break too, resulting in the 3' end of the linear molecule not getting a chance to be transferred over to the F- cell.

5. The transferred portion of the linear molecule will undergo homologous recombination with the main chromosomal molecule of the F- cell, exchanging alleles with each other.

6. However, the F- cell will not become a F+ cell as it does not possess a F plasmid or hence a pilus.



You have been painted head to toe by Corny @
9:29:00 pm



1. The F factor on the F plasmid in the F+ cell codes for the production of conjugation pilus will be used to "contact" the recipient F- cell.

2. The rolling circle replication occurs when a nick is formed at the origin of transfer (ori T) on the double-stranded F plasmid.

3. The 5' end of the outer strand gets pulled over towards the F- cell across the conjugation bridge. Meanwhile, at the 3' end of the outer strand, a new strand complementary to the inner strand is being synthesised by DNA polymerase as the the whole strand gets pulled over.

4. At the F- cell, after the whole nicked outer strand has been completely transferred, this linear strand will automatically recircularise to form the inner strand of a plasmid.

5. DNA polymerase will then synthesise an outer strand of the plasmid by using the recircularised inner strand as a template, overall forming a new F plasmid.

6. The F- cell now becomes a F+ cell.


You have been painted head to toe by Corny @
9:16:00 pm



1. The specific cell receptors of the bacterial host cell will recognise the tail fibres of the lambda phage and allow the phage to adsorp onto the cell surface receptor.

2. The lambda phage will inject its lambda DNA into the cytoplasm of the host cell through the receptor by facilitated diffusion.

3. The phage DNA will integrate into the host cell's main chromosomal molecule and remain dormant for a very long time until a certain stimulus will stimulate the prophage to excise itself from the host cell's main chromosome. (Lysogenic cycle)

4. Sometimes, the prophage will excise wrongly such that the excised portion will include a part of the host cell's chromosome and exclude a part of its own DNA.

5. The wrongly excised DNA will package itself into the capsid and the viral portion of the wrongly excised DNA will code for an enzyme that will degrade the host cell's cell wall, resulting in lysis of the cell wall to release the viral particle.

6. This viral particle will go on to infect another bacterial cell and the wrongly excised DNA that was packaged in the capsid is released into the cytoplasm of this host cell.

7. Since the wrongly excised DNA contains a portion of the bacterial DNA, this portion will either undergo homologous recombination (when different alleles are exchanged between similar genes on different chromosomes) or integrate into the host cell's chromosomal molecule.

8. This wrongly excised prophage will not do any damage to the host cell, instead it will be replicated along with the host cell's main chromosomal molecule during binary fission.

Note: If any part of this "model answer" is wrong, pls tell me and i will correct it.


You have been painted head to toe by Corny @
8:21:00 pm


I am her!
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