The Chromosome - SS3 Biology Lesson Note

Chromosomes are structures within cells that contain genetic material (DNA) and carry the instructions for the development, functioning, and inheritance of living organisms. They are thread-like structures composed of DNA tightly wound around proteins called histones.

In most organisms, including humans, chromosomes exist in pairs known as homologous chromosomes. Humans have a total of 46 chromosomes, organized into 23 pairs. Of these 23 pairs, 22 pairs are called autosomes, and the remaining pair determines the individual's sex and is known as the sex chromosomes.

The sex chromosomes determine an individual's biological sex. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The X chromosome carries many genes involved in various aspects of development, while the Y chromosome carries a smaller number of genes, including those responsible for male sexual development.

Chromosomes are essential for the accurate transmission of genetic information from one generation to the next. During cell division, chromosomes replicate and condense into more visible structures. There are two types of cell division: mitosis and meiosis. Mitosis is responsible for the growth, development, and maintenance of body cells, resulting in the production of two genetically identical daughter cells. Meiosis is a specialized form of cell division that occurs in the cells involved in sexual reproduction (such as sperm and egg cells). It results in the formation of cells with half the number of chromosomes, ensuring that when fertilization occurs, the offspring will receive the correct number of chromosomes from each parent.

Abnormalities in chromosome number or structure can lead to genetic disorders. Examples include Down syndrome, which is caused by an extra copy of chromosome 21, and Turner syndrome, which occurs when a female has a single X chromosome (X0) instead of the usual XX configuration.