(a) What is a cell? (b) Describe briefly the structure of DNA. (c) Describe the process of mitosis (d) What is the importance of mitosis to living organisms...

(a) What is a cell?

A cell is the basic structural and functional unit of life; it is the smallest unit of a living organism that can carry out all the processes of life. All living things are made up of one or more cells.

(b) Structure of DNA

DNA (deoxyribonucleic acid) is the hereditary material that carries genetic information. Its structure is:

• It is a very long molecule made up of two strands twisted round each other to form a double helix (like a twisted ladder).
• Each strand is a chain of units called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• There are four bases: adenine (A), thymine (T), guanine (G) and cytosine (C).
• The two strands are held together by hydrogen bonds between the bases. Bases pair specifically: adenine with thymine and guanine with cytosine (complementary base pairing).
• The sugar and phosphate groups form the sides (backbone) of the ladder, while the paired bases form the rungs.

(c) Process of mitosis

Mitosis is a type of cell division in which a parent cell divides to produce two daughter cells, each with the same number of chromosomes as the parent. It occurs in stages:

1. Interphase: the cell grows and the DNA (chromosomes) is copied (replicated).
2. Prophase: chromosomes shorten, thicken and become visible; each appears as two chromatids joined at the centromere; the nuclear membrane begins to break down and spindle fibres form.
3. Metaphase: chromosomes line up singly along the equator (middle) of the cell, attached to the spindle fibres.
4. Anaphase: the centromeres split and the two chromatids of each chromosome are pulled apart to opposite poles of the cell.
5. Telophase: the chromatids reach the poles; a new nuclear membrane forms around each group; the chromosomes uncoil.
6. Cytokinesis: the cytoplasm divides, forming two identical daughter cells.

(d) Importance of mitosis

• It brings about growth by increasing the number of cells.
• It repairs damaged tissues and replaces worn out or dead cells (healing of wounds).
• It is the basis of asexual reproduction in many organisms.
• It ensures that daughter cells receive an exact and constant number of chromosomes, keeping the species true to type.