Gas vacuoles are also known as pseudovacuoles or air vacuoles. Gas vacuoles are the characteristic feature of prokaryotes reported only in prokaryotes. Each gas vesicles are made of sub-microscopic hexagonal vesicles and are surrounded by a thin protein membrane. Gas vacuoles store metabolic gases and take part in buoyancy regulation.

A polysome consist of a cluster of ribosomes that are held, simultaneously by a strand of messenger RNA (mRNA) in rosette or helical group. They contain a portion of the genetic code that each ribosome is translating and are used in formation of multiple copies of same polypeptide. They are found in the cytoplasm during the process of active protein synthesis.

In metacentric chromosome centromere is median, i.e., centromere lie in the middle portion. Thus, forming two equal arms of chromosome.

The chromosomes may have additional constriction or secondary constriction at their ends as distal part of the arm formed by chromatin thread are known satellite chromosome. These constriction gives appearance of an out growth or small fragment.

These are also known as (sat) chromosomes or marker chromosome number. Chromosome 13, 14, 15, 16, 21 and 22 are satellite chromosomes.

Nucleolus is the round, naked and a slightly irregular structure, which is attached to the chromatin at a specific region called as Nucleolar Organizer Region (NOR) Nucleolus was first discovered by Fontana (1781).

(i) Nucleolus is the chief site for the ribosomal RNA synthesis.

(ii) It is the centre for the formation of ribosome components.

(iii) It is the colloidal complex that fills the nucleus.

(iv) It combines rRNA with proteins to produce ribosomal sub-units. After their formation, the ribosomes sub-units pass out and get established in the cytoplasm.

(v) It also receive and store ribosomal proteins formed in the cytoplasm.

(vi) These ribosomal proteins formed are the sites for protein synthesis in the cell.

(vii) Nucleolus is essential for spindle formation during nuclear division.