This document discusses bacterial cytology, focusing on ribosomes, chromosomal material, and extrachromosomal plasmids. It describes the structure and function of ribosomes, which are composed of RNA and proteins and are the site of protein synthesis. It also discusses the nucleoid, which contains the bacterial chromosome and proteins. The chromosome carries all the genetic information and can be circular or linear. Some bacteria contain extrachromosomal DNA molecules called plasmids, which are small, circular pieces of DNA that are not essential but can provide advantages like antibiotic resistance.

1. Bacterial Cytology :
Ribosomes, Chromosomal and
Extrachromosomal material
Vishrut S. Ghare
(M.Sc Microbiology, SET)
Asst. Professor, S.B.B alias
A. Jedhe College, Pune

2. Ribosomes

3. Molecular structure of 70S Ribosome

4. Ribosomes are made up of ribosomal RNA and Proteins and
are the site of protein synthesis, and large numbers (10,000 to
20,000) are found in nearly all cells.
The cytoplasm of rapidly growing bacterial cells is often
packed with ribosomes, and additional ribosomes may be loosely
attached to the plasma membrane.
The cytoplasmic ribosomes synthesize proteins destined to
remain within the cell, whereas plasma membrane–associated
ribosomes make proteins that will reside in the cell envelope or
be transported to the outside.
Bacterial ribosomes are called 70S ribosomes and are
constructed of a 50S and a 30S subunit.
The S in these values stands for Svedberg unit.
This is the unit of the sedimentation coefficient, a measure of
sedimentation velocity in a centrifuge; the faster a particle travels
when centrifuged, the greater its Svedberg value.

5. The sedimentation coefficient is a function of a particle’s
molecular weight, volume, and shape.
Heavier and more compact particles normally have larger
Svedberg numbers.
Bacterial ribosomes are composed primarily of ribosomal
RNA (rRNA) molecules.
The small subunit contains 16S rRNA, whereas the large
subunit consists of 23S and 5S rRNA molecules.
Approximately 55 proteins make up the rest of the mass of the
ribosome: 21 in the small subunit, and 34 in the large subunit.
Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
Dimensions: 14-15 nm by 20 nm
Molecular weight: approximately 2.7 million
In prokaryotes during translation, ribosomes bind to m-RNA to
and makes m-RNA ribosome complex called polysome.

6. In addition to rRNA and proteins, ribosomes also contains some
divalent metallic ions, such as Mg++, Ca++ and Mn++.
Polysome

7. Chromosomal material:
Nucleoid/nuclear region/ nuclear body/ chromatin
body

8. •The nucleoid is an ellipsoidal region that contains the cell’s
chromosome and numerous proteins.
•Although the nucleoid is not separated from the cytoplasm by a
membrane, it is a distinct region of the cell, as observed by
microscopy.
•The nucleoid of a bacterial cell usually contains a single long,
continuous, and frequently circularly arranged or linear thread of
double-stranded DNA called the bacterial chromosome.
•This is the cell’s genetic information, which carries all the
information required for the cell’s structures and functions.
Unlike the chromosomes of eukaryotic cells, bacterial
chromosomes are not surrounded by a nuclear envelope
(membrane) and do not include histones.
•The nucleoid can be spherical, elongated, or dumbbell shaped.

9. Chromosome released from a gently lysed E. coli cell. Note how
tightly packaged the DNA must be to fit inside the cell.

10. •Most bacteria have a single chromosome, but some bacteria, such
as Vibrio cholerae and Borrelia burgdorferi (causative agents of
cholera and Lyme disease, respectively), have more than one
chromosome.
•Many bacteria carry a single copy of their chromosome
(monoploid), but others are known to be polyploid, often with
more than 10 copies per cell.
•Bacterial chromosomes are longer than the length of the cell.
•E. coli’s circular chromosome measures approximately 1,400 μm,
or about 230–700 times longer than the cell.
•To pack inside the small cell the chromosome gets supercoiled
structure with the help of proteins.
•E.coli genome: 4639 Kbp or 4.6x106 bp (contains 3000 genes)
•Mycoplasma genitalium: 580 Kbp
•Myxococcus xanthus: 9200 Kbp

11. Extrachromosomal material: Plasmids

12. Extrachromosomal material: Plasmids
In addition to the genetic material present in the nucleoid, many
bacteria contain extrachromosomal DNA molecules called
plasmids.
Plasmids are small, extra-chromosomal, often covalently closed
circular DNA molecules.
Plasmids are double-stranded DNA molecules that can exist
independently of the chromosome.
Both circular and linear plasmids have been documented, but
most known plasmids are circular.
Plasmids have relatively few genes, generally less than 30.
Their genetic information is not essential to the bacterium, and
cells that lack them usually function normally.
However, many plasmids carry genes that confer a selective
advantage to the bacterium in certain environments.

13. The existence of plasmids in bacterial cytoplasm was revealed
by Joshua Lederberg in 1952 while working on conjugation
process in bacteria.
Lederberg coined the term ‘plasmid’ to refer to the
transmissible genetic elements that were transferred from one
bacterial cell to another and determined the maleness in bacteria.
Some plasmids are present in many copies per cell, whereas
others are present in only one or two copies.
Borrelia burgdorferi carries 12 linear and 9 circular plasmids.
Plasmids replicate independently of the host chromosome, in
the sense of possessing their own origin of replication.
However, they do rely on chromosomally encoded enzymes for
their replication.
Plasmids differ from chromosomes in carrying only
nonessential (but often very helpful) genes.
Thousands of different plasmids are known.

14. Indeed, over 300 different naturally occurring plasmids have
been isolated from strains of Escherichia coli alone.
Plasmids have been widely exploited in genetic engineering.
Naturally occurring plasmids vary in size from approximately 1
kbp to more than 1 Mbp.
Under certain adverse conditions plasmids are an advantage to
cells.
Plasmids may carry genes for such activities as antibiotic
resistance, tolerance to toxic metals, the production of toxins, and
the synthesis of enzymes.
Plasmids can be transferred from one bacterium to another.

16. Types of Plasmids