The nucleus is the largest and most prominent organelle in the animal cell.
Almost all eukaryote cells have a nucleus — red blood cells in mammals and phloem cells in plants are an exception.
Every nucleus is surrounded by a nuclear envelope. This consists of two membranes that are separated by a gap of 20 to 40 nm.
Nucleus may be irregular or spherical in shape and about in diameter. It contains the cell’s DNA, which carries information that allows the cell to divide and carry out all its cellular processes.
Nucleus controls the life and activities of the cell.
In animal cells Nucleus is generally present in the central part. In plant cells Nucleus is pushed towards periphery due to a large central vacuole.
DNA, RNA and proteins (including enzymes) forms the chemical composition of nucleus.
Nucleus consists of nuclear membrane, nucleoli, nucleoplasm and chromosomes.
The number of nuclear pores is highly variable. For example the undifferentiated cells (e.g., eggs) have many pores (about 30,000 per nucleus) while the differentiated cells (e.g., erythrocytes) have only 3 or 4 pores/nucleus. Each pore has a definite structure.
Nucleolus is a darkly stained structure within the nucleus. Nucleolus has no membrane. There are two regions of a Nucleolus:
- The Peripheral Granular area: It contains the materials from which ribosomal subunits are formed.
- The Central Fibrillar area: It has large molecular weight RNA and rDNA.
The heredity material is in the form of chromosomes, which controls all the activities of the cell. Chromosome is formed of DNA & proteins.
Nucleus is visible when the cell is in non-dividing stage. It contains chromatin network and soluble sap called nucleoplasm. Dark-staining chromatin, consisting of tightly packed DNA, is known as Heterochromatin. The lighter, more loosely packed material is called Euchromatin. Euchromatin contains the DNA that is being actively read to produce proteins. In heterochromatin, the DNA is packed together, and is not being read.
Nucleus is stained with basic dyes because of the chromatin material.
During cell division chromatin material is converted into darkly stained thread like structures called chromosomes.
A chromosomes is made of arms and centromeres.
Centromere is the place on the chromosome where spindle fibres are attached during cell division.
Each chromosome consists of two identical chromatids at the beginning of cell division which are held together at centromere.
A chromatid is exact replica of the chromosome.
The information to control cell activities is present on the chromosomes in the form of genes.
The chromosome number varies which may be 2n = 2 to 2n = 1200 (Pteridophytes).
The number of chromosomes in all individuals of the same species remains constant generation after generation.
Some examples of chromosome No. are: Frog = 26, Chimpanzee = 48, Fruit fly (Drosophila melanogaster) = 8, Wheat = 42, Onion = 16, Potato = 48, Garden pea = 14. Penicillium (a fungus) has two chromosomes (one pair), corn 20, wheat 42, sugarcane 80, some ferns have more than 500 pairs, mosquito 6, fruit fly 8, frog 26, honeybee 32, mouse 40 and human cells have 46 chromosomes (23 pairs).
The number of chromosomes in normal body cells are diploid (2n) while the germ cells (sperms and eggs) have haploid chromosome number (n).
Examples: Human germ cells (eggs and sperms) = 23 chromosomes. Drosophila germ cells = 4 chromosomes.