Earth Science: Cells – Chromosomes
Chromosomes are threadlike structures that carry genetic information. In eukaryotes, chromosomes are located inside the nucleus. They consist of DNA and stabilizing proteins (histones).
Chromosomes continuously change shape from an uncoiled form to a condensed coiled form, depending on current functional requirements. While a cell is not in the process of division, they remain uncoiled. As soon as a cell starts dividing, they contract to form a dense and mobile shape that is visible even under a light microscope.
Chromosomes consist of two identical DNA double strands (sister chromatids), which are linked together at the centromere. After cell division, a chromosome is initially composed of only one chromatid. It will eventually double so that there are two identical chromatids again.
The number of chromosomes is always the same for a certain animal or plant species. For example, there are 46 chromosomes in a human cell and 44 of these can be aligned by shape and size to make 22 pairs (homologue chromosomes).
They are called autosomes, while the two remaining ones are called gonosomes (sex chromosomes). Men have one larger X chromo- some and one smaller Y chromosome, and women have two X chromosomes. A diploid set of chromosomes has two copies of every chromosome. Reproductive cells, eggs, and sperm, have only a single, or haploid set of chromosomes.
Genetic mutations and abnormalities
Occasionally, individual chromosomes are not replicated during cell division or individual chromosome pairs are not separated for distribution into daughter cells (nondisjunction). Such a change in chromosome number is known as genome mutation. The phenomenon of one too few chromosomes is called monosomy and one too many chromosomes is called trisomy (see in focus).
A change in entire sets of chromosomes is a type of euploidy. A genome mutation on any chromosome will result in a specific syndrome or condition. Some cause significant abnormalities, others may go undetected for life.
TRISOMY 21
Trisomy 21, or Down syndrome, is a genome mutation caused by the nondisjunction of chromosome pair number 21 during meiosis or due to a failed segregation during meiosis II that results in a gamete having two copies of chromosome 21 During fertilization, the spermatozoan adds its own chromosome number 21, so that the zygote now has three chromosomes.
Such a trisomic condition results in ab- normal physical and mental developments.
BASICS
THE NUMBER OF CHROMOSOMES varies in organisms, hut this is no indication of complexity. The cells of lampreys have 174 chromosomes and humans have 46.