Bacterial chromosome

 Bacterial Chromosome:

Bacterial genome is equivalent  to a single  chromosome of  euaryotes. Its DNA content is about one__ thousandth the DNA content of  eukaryotic  genome. Bacterial chromosome lies freely  inside the cytoplasm. A limiting membrane is absent. Histone proteins  are not associated  with this chromosome. Therefore, DNA of  procaryotes  is called naked.Polyamines have been found to be associated in certain cases.DNA   is circular. It is folded extensively  with  the  help of RNA. The bacterial chromosome is often called nucleoid. It is attached to a  specialized membranous  body called mesosome.The attachment is through two closely  placed Y_ shaped forks.


Under favourable conditions  bacteria multiple by binary fission . The time interval required by a newly formed daughter  bacterium to mature and divide into two is called generation time.It is as low as 20 minutes ( e.g., Escherichia coli) oras high as 15__ 20 hours ( e.g., Mycobacterium tuberculosis).

Bacterial chromosome replicates while attached to mesosome. There is a single origin of replication. Replication proceeds bidirectionally. By the time DNA replication is completed, the  cell  begins to elongate by growth of area between  the two attaching points of   the nucleoid. The mesosome divides in the process.With the movement of the daughter  mesosomes the daughter  chromosomes also move apart. Initially they are attached to mesosomes by a single Y_ fork. A new attachment point is produced nearby. 

Gene Recombination ( Exchange of Genetic Material):

Typical sexual reproduction is absent in bacteria because of the absence of haploid_ diploid alteration. However, gene recombination can occur by three methods __ conjugation, transformation and transduction. 

1): Conjugation:

It was first discovered in K__ 12 strain of Escherichia Coli by Lederberg and Tatum. They found that two different  types of auxotrophs growth together  on minimal medium produced an occasional prototroph. Cell contact was required. Bacteria showing conjugations  are dimorphic,i.e.,they have two types of cells, male ( F+) or donor and female ( F-) or recipient. The male cell possesses sex pili and fertility factor in its plasmid. Fertility factor contains  genes for producing sex pili and other characters needed for gene transfer. Sex pili are narrow protoplasmic outgrowths that occur all around the  cell. Both  sex pili and fertility factor are absent in female cells. If two types of  cells happen to come nearer,  a pilus of male cell grows in  size and establishes a protoplasmic bridge or conjugation tube with the  female cell. It takes 6__ 8 minutes. Gene exchange can occur by two methods: 

( a) Sterile Male Method: The plasmid having fertilized factor replicates. A copy of it gets transferred to the recipient cell through  the conjugation tube. The recipient cell also becomes donor.

(b) Fertile Male Method: The F+ Plasmid of the donor cell gets integrated to bacterial chromosome. The attachable plasmid is known as episome. The  point at which plasmid gets integrated to bacterial chromosome differs in different strains. Integration is possible  because  certain nucleotide sequences present in bacterial chromosome are compatible with sequences in plasmid DNA. The donor male or super male because it has a recombination frequency of 1000 times more as compared to normal F+ . Norintegrated F+ plasmids disintegrate in Hfr cells. The integrated F+ factor breaks the bacterial chromosome at one end of its attachment. The bacterial chromosome now undergoes replication. A copy of the freed end of bacterial chromosome ( end  distal  to F+ factor, also called zero end) passes into the recipient cell through the conjugation tube. Fertility factor is the last to do so. The Hfr chromosome is transferred  to the F- cell in a linear fashion even though it is a circular  chromosome. Generally whole of bacterial chromosome does not pass into recipient cell. F+ factor is very rarely  transferred  as conjugation is maintained for a brief period ( It takes about 100 minutes to inject a copy of the whole Hfr E.coil genome. Since conjugation is usually  interrupted by accident before this can occur, the distal Hfr genes are rarely transferred). Only a few genes are  transferred one in seven minutes,  two in nine minutes, three in ten minutes,  four in eleven minutes, etc. Conjugation produces an incompletely diploid " zygote " known as merozygote or partial zygote. The merozygote has complete genome of F-, called endogenote and an incomplete genome called exogenote derived from F+ or Hfr. The new  genes may replace the genes present  in the recipient cells ( those of the recipient cells disintegrate) or get added to them. As F+ factor is very rarely transferred,  most of the recombinants produced by Hfr remain F- ( c.f.F+ strains).

Hfr cells can revert to the F+ state. When this occurs, the sex factor is released from the chromosome and resumes its autonomous replication. Sometimes this detachment is not cleanly accomplished, so the F factor carries along with it some chromosomal genes. In this state this is termed as F' factor and the cell in which this has occurred  is called F' cell. When such primary  F' cells are mated with F-  recipients, the sex factor is transferred  very efficiently together with the added bacterial genes. The recipient cell then becomes a secondary  F' cell. This process whereby bacterial  genes are transmitted from donor to recipient as part of the sex factor has been termed sexduction. 

2): Transformation:

It is the absorption of DNA  segment from the surrounding medium by a living bacterium. The phenomenon was discovered by Griffith. Its mechanism was worked out by Avery et al. Receptivity for transformation is present for a brief period when the cells have reached the end period of active growth. At this time they develop specific receptor sites in the wall. Normally  E' coli does not pick  up foreign DNA but it can do in the presence  of calcium chloride.  After DNA  entry into a cell, one strand is immediately degraded by deoxyribonucleases, while the other strand is incorporated into the recipient   chromosome by displacing a homogeneous section of one of the recipient DNA  strands, which is then excised and degraded. Since complementary base pairing takes place between  one strand of donor DNA fragments and a specif region of the recipient chromosome, only closely  related  strains of bacteria can be transformed. The molecular size of DNA also affects transformation. The transforming  molecules usually contain on the average  about 20,000 nucleotide base pairs. Smaller DNA pieces  can also be absorbed  by the recipient cell, but a minimum  length  of about 450 base pairs seems to be essential  for transformation to occur.

3): Transduction:

( Also ready lysogenic cycle in viruses). It is the transfer of foreign genes  by means of viruses,  Transduction was first discovered by Zinder and Lederberg in Salmonella typhimurium. The process also occurs in E.coil and many other hosts. A virus may pick up gene of the host during its multiplication in the host cell. Such a virus is non__ virulent. It  passes over the gene of the previous  host to the new host.


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