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.
Sugar residues or carbohydrates attaches either with proteins or lipids usually making up less than $10 \%$ of the membrane weight, they can give rise to a wide variety of structures in relatively short chains. They give individual cell types a distinguishing features. Therefore, they may be involved in.
Cell Recognition e.g., surface of RBC have carbohydrates arranged in branched chains: differences in the arrangement give rise to different blood group antigens (i.e., AB and O ).
Cell surface differences are also responsible for the specificity of action of cells with hormones, drugs, viruses or bacteria. The cause of cell surface differences is related to characteristic surface due to carbohydrate component.