Oooc oc

Figure 3. Relative size of an E. coli cell and its chromosome.

factor (F plasmid) recombines with (splices itself into) the E. coli chromosome at a specific site. It then acts as a "molecular motor" to drive the transfer of the entire E. coli chromosome to a recipient (F_) cell. The transferred molecule can then recombine with the host chromosome, increasing the genetic diversity of the host. Transferring the entire chromosome takes approximately one hundred minutes, and thus the genetic map is divided into one hundred minutes (which were later defined as one hundred map units). As more and more genetic markers were found and mapped, it became apparent that the genetic chromosome map formed a circle, as shown in Figure 1.

The DNA sequence of the E. coli chromosome. E. coli was chosen as one of the genetic model organisms whose chromosome was to be sequenced as part of the Human Genome Project. Although it was not the first bacterial species to be completely sequenced, it was one of the most important ones. In 1997, Fredrick Blattner of the University of Wisconsin and colleagues published the sequence of 4,639,221 base pairs of the K-12 laboratory strain. E. coli is estimated to have 4,279 genes.

Many sets of genes on the E. coli chromosome are organized into operons. An operon is a set of functionally related genes that are controlled by a single promoter and that are all transcribed at the same time.

Comparative bacterial genomes. As of June 2002, the genomes of sixty-five different bacterial species had been completely sequenced. Several of these are listed in Table 1, along with the genomes' size and number of genes. Many of the species sequenced are human pathogens. Having the DNA sequence will prove useful in designing drugs and antibiotics to combat infections and bacterial toxins. DNA sequences may be found on the Internet, at the Genome Web site of the National Center for Biotechnology Information.

pathogens disease-causing organisms

Table 1.

CHROMOSOME SIZE

AND NUMBER OF GENES FOR

SEVERAL

BACTERIAL SPECIES SEQUENCED 1995-

2000

Chromosome size

Number

Year sequence

Bacterial species

(base pairs)

of genes

completed

Haemophilus influenzae

1,830,138

1714

1995

Mycoplasma genitalium

580,074

480

1995

Synechocystis sp.

3,573,470

3167

1996

Mycoplasma pneumoniae

816,394

1054

1996

Helicobacter pylori

1,667,867

1576

1997

Escherichia coli

4,639,211

4279

1997

Bacillus subtilis

4,214,814

4112

1997

Borrelia burgdorferi

910,724

851

1997

Aquifex aeolicus

1,551,335

1529

1998

Mycobacterium tuberculosis

4,411,529

3927

1998

Treponema pallidum

1,138,011

1036

1998

Rickettsia prowazekii

1,111,523

835

1998

Chlamydia trachomatis

1,042,519

895

1998

Chlamydiophila pneumonia

1,230,230

1054

1999

Thermotoga maritima

1,860,725

1858

1999

Campylobacter jejuni

1,641,481

1654

2000

Neisseria meningitidis

2,272,351

2079

2000

Buchnera sp.

640,681

564

2000

Bacillus halodurans

4,202,353

4066

2000

0 0

Post a comment