Depending upon the location of centromere in a chromosome, the chromosome is classified into the following four types :
(a) Telocentric: Centromere is present at the terminals.
(b) Acrocentric: Centromere is present near the terminals resulting in two short arms and two long arms.
(c) Sub- metacentric : Centromere divides the chromosomes into two unequal arms. It lies either below centre or above centre.
(d) Metacentric : Centromere divides the chromosomes into two equal arms with the centromere in the middle.
Characteristics of Prokaryotic cell are as follows:
1. The nucleus is not bounded by a membrane.
2. Their genetic material is naked and present in the form of nucleoid in the cytoplasm.
3. They are generally smaller and divide more rapidly as compared to the eukaryotic cell.
4. They are usually surrounded by cell wall except for mycoplasma.
5. Besides genomic DNA many prokaryotic cell have small circular DNA called plasmids outside the genomic DNA.
6. Eukaryotic organelles or membrane bound organelles are not found in the prokaryotic cells. The ribosomes found in these are of 70S type.
7. Mesosomes which are the invaginations of plasma membrane may be present.
8. They have special inclusion bodies.
Mesosome is a specialised membranous structure formed by the invagination of plasma membrane in prokaryotes. These extensions are
in the form of vesicles, tubules and lamellae.
The mesosome performs the following functions -:
1. Help in respiration, secretion processes by increasing the surface area of the plasma membrane and enzymatic content.
2. Helps in DNA replication and distribution to daughter cells.
4. Help in cell wall formation.