What is Chromosomes ?
The terminology used to describe DNA replication may sometimes be confusing. Originally, the term "chromosome" referred to the structures seen through the light microscope in cell division. However, in recent years, the term has been used to describe the linear DNA strand containing genes, whether it is visible in the light microscope or not. You may read that a chromosome consists of a linear strand of DNA, yet later find that threads of replicated DNA in interphase cells are also called chromosomes even though they have been replicated two or more times and contain two or more DNA strands. Sister chromatids are usually said to consist of a single chromosome until the centromere is divided at anaphase, when the chromatids become two chromosomes.
In order to observe chromosomes at different mitotic stages, cells can be treated with a hypotonic (dilute salt or low osmotic pressure) solution to disperse the chromosomes. This makes them easier to examine with a light microscope, especially when chromosomes align at the metaphase plate. images of chromosomes obtained in this way have given us much information about the genetic content of cells and genetic recombination.
The number, size, and type of chromosomes in a cell form its karyotype. Each species has a characteristic karyotype. In humans, there are 23 pairs of chromosomes, or a total of 46. Each is characterized by a different length and shape, pattern of banding, and position of the centromere. In humans, each chromosome has been numbered and photographed in normal and abnormal cells, so deviations can be detected by comparing karyotypes.
One chromosome from each parent matches another similar chromosome from the other parent. Together they form a homologous pair. Members of homologous pairs are the same in size and appearance, but differ from other pairs. Each homolog carries information for the same types of traits, but not necessarily the exact same information that determines those traits.
Under normal conditions, some cells of certain organisms can contain anywhere from one to hundreds of copies of the single or haploid set of chromosomes. Nuclei containing more than two sets are called polyploid, and the number of sets, or ploidy level, is represented by a number followed by the letter n, For example, nuclei containing three sets are called triploid (3n); those with four tetraploid (4n).