Cell is the structural and functional unit of life. A typical cell includes a cell membrane, cytoplasm and genetic material either within the nucleus or in the cytoplasm. The cytoplasm contains cell organelles such as mitochondria, endoplasmic reticulum lysosomes, vacuoles, ribosomes, golgi bodies.
Cell wall and chloroplasts are present only in plant cells while centrosome is present in animal cells.
Cell organelles: Each organelle performs a specific function in the cell.
- Well organised nucleus absent and genetic material lies in the cytoplasm.
- Membrane bound organelles absent.
- Examples: Bacteria, blue green algae.
- Genetic material enclosed within a nuclear membrane forming the nucleus.
- Organelles like Mitochondria chloroplasts, E.R. etc are present.
- Examples: Cells of fungi, plants, animals.
New cells are formed by cell division. Cell division replaces worn out cells, repairs injuries, helps in growth and in reproduction. There are two types of cell divisions: mitosis and meiosis. Prior to all divisions the chromosomes duplicate or replicate themselves and then enter cell division.
During mitosis, the two chromatids of the duplicated chromosome separate. Each chromatid is now a chromosome. One each of these two chromosomes moves into the two
Meiosis involves two successive divisions: In meiosis I, the chromosomes of the same (matching or homologous) pair move to two daughter cells. Thus, two cells with half the number of chromosomes are formed at the end of phase I. This is reduction division. However, each chromosome still has the duplicated copies with them.
In meiosis II, like mitosis the duplicated copies that is the chromatids of each chromosome separate and move to two new daughter cells. Thus, four haploid cells are formed. These daughter cells ultimately give rise to reproductive cells such as pollen grain, ovule, sperm and ovum.
Tissue is a group of similar cells performing the same function and having a common origin.