Prokaryotes, including eubacteria and archaebacteria, have a single circular DNA molecule known as a bacterial chromosome, which is compacted through supercoiling facilitated by topoisomerases. The nucleoid area contains DNA and proteins, while most prokaryotic genes are organized in operons, allowing for coordinated regulation of related functions. Challenges arise during DNA replication and partitioning due to the chromosome's length relative to cell size.

1. Genome Organization in Prokaryotes
Prepared By-
Dr. Sangeeta Das
Assistant Professor
Department of Botany
Bahona College, Jorhat

2. Bacteria are very small unicellular organisms that
do not contain nuclear envelope, mitochondria,
endoplasmic reticulum, mitotic apparatus and
nucleolus etc., and divide by fission.
Bacteria have a rigid cell wall which surrounds their
cytoplasmic membrane.
Their cytoplasm contains ribosomes. mesosomes
and several granular inclusions.
About 1/5 of the cell volume is occupied by DNA,
the genetic material.
Genome Organization in Prokaryotes

3. Prokaryotes are composed of eubacteria (including
E. coli) and archaebacteria.
They have single circular DNA molecule called
bacterial chromosome.
DNA molecule is extremely long, as such is
compacted by a process called supercoiling.
This process is facilitated by enzymes called
topoisomerases. They introduce additional turns
into the DNA double helix causing the DNA strand
to wind up on itself.
Genome Organization in Prokaryotes

4. Topoisomerases act by breaking the DNA
polynucleotide and relating the two ends relative to
each other. The enzyme then rejoins the ends by a
process called positive supercoiling.
It also removes coiling in a process called negative
supercoiling by creating a turn in the opposite
direction.
Prokaryotes possess a nucleoid, a darker central
area containing DNA and proteins revealed by
electron microscopy and an outer area called
cytoplasm.
Genome Organization in Prokaryotes

5. Some of the proteins isolated from the nucleoid
resemble the histone proteins (found in eukaryotic
chromosomes) which may help in packaging DNA
into compact structure.
The length of prokaryotic chromosomes relative to
the cell dimension means that replication and
partitioning of the DNA molecules during cell
division is a difficult task.
Genome Organization in Prokaryotes

6. Genome Organization in Prokaryotes
Fig.: Supercoiling of circular DNA molecule in E. coli.

7. Almost all genes are present in bacterial
chromosome.
A few are present as small circular DNA molecules
called plasmids.
E. coli has about 4300 genes carried by a
chromosomal DNA molecule of 4.6 million base
pairs.
Some genes are arranged as families called operons
that encode proteins with related functions and are
regulated in a coordinated way.
Prokaryotic Genes