junkct

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Ok so I'm having a bit of trouble understanding the whole concept of hfr cells. The way I see it, usually the F factor in F+ bacteria is on an extrachromosomal plasmid, which replicates and transfers to the F- bacteria during conjugation (ONLY the F factor plasmid can be transferred).

I'm also reading that this F factor can incorporate itself into the bacterial chromosome in some bacteria (which are called hfr bacteria), so that during conjugation the MAIN chromosome can actually replicate and transfer genes to an F- bacterium. However, in this situation, the F factor does NOT always get transferred.

Is this correct?? Or am I missing something? Clarification would be greatly appreciated! thanks!
 

engineeredout

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I copied this straight from notes from my bio class, I think it clarifies everything for you.


Conjugation is bacterial sex. This name
refers simply to the fact that it is a mechanism SPECIFICALLY DESIGNED for transfer of DNA
from one bacterial cell to another. Long protrusions called sex pili extend from some bacterial
cells. These are hollow tubes made of proteins, through which DNA can travel from one cell to the
other. A piece of DNA called an F (fertility) factor confers the ability to make sex pili. The F
factor can exist either in the chromosome or free as a circle called a plasmid. A plasmid is a small
circular piece of DNA that can replicate separately from the chromosome, because it has an origin
of replication. An F+ cell has an F factor as a plasmid; an Hfr cell has the F factor integrated into
the chromosome, and an F- cell has no F factor. (A cell can switch between the F+ state and the Hfr
state when the F factor either integrates into the chromosome, or pops out to form a plasmid.)
During conjugation, DNA is transferred from either an F+ cell or an Hfr cell to an F- cell. This
occurs differently, depending on whether the donor cell is an F+ cell or an Hfr cell. If it's an F+
cell, the F plasmid starts replicating. One end of the new copy inserts into the sex pilus and travels
through it. The whole new linear F plasmid passes into the recipient cell, and then it forms a circle
(new F+ plasmid) when its ends are joined. Now both the donor and recipient cells have F factors,
and are both F+! When an Hfr cell is the DNA donor, because the F factor is integrated into the
chromosome, part of the chromosome is replicated and transferred through the sex pilus. Normally,
the sex pilus breaks before the whole new copy of the chromosome can be transferred, and only a
fragment is transferred. This fragment can undergo crossing over with the cell's chromosome, so
that part of the fragment is integrated into the chromosome. This process can be of medical
importance because some plasmids carry genes for antibiotic resistance. These code for proteins
that allow pathogenic bacteria to survive in the presence of antibiotics that would normally kill​
them. Antibiotic resistance genes can be transferred between different pathogenic bacteria.

 

junkct

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I copied this straight from notes from my bio class, I think it clarifies everything for you.


.
gracias my fellow engineer! Exactly what I've been looking for! One question: so when an Hfr cell begins transferring it's chromosome, does it always transfer the F factor? Or only sometimes? I can't remember if the F factor is the FIRST gene that gets transferred during conjugation of Hfr cells.
 
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futuredoctor10

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so when an Hfr cell begins transferring it's chromosome, does it always transfer the F factor? Or only sometimes?
When an Hfr cell transfers its chromosome, it only sometimes transfers the F factor. We learned in my Intro Bio course the F factor is rarely transferred - usually only some of the chromosomal genes will be transferred.

I can't remember if the F factor is the FIRST gene that gets transferred during conjugation of Hfr cells.
Nope not first- it is at the end I think.
 

fly1346

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Engineer: you go to stony brook rite, in BIO202 with Deboarh Brown ;) Howd you do in that class?
 
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