Each cell contains a lot of DNA, at actual size, a human cell’s DNA totals about 3 feet in length. A typical cell size is 10 µm. How to enclose this entire DNA into a cell? The answer is chromosome.
What is Chromosome? – Definition
The DNA of cell is packed into compact units, thread-like structures called “chromosomes”. A unique packing system is used for chromosome, it includes several steps starting with DNA double helix. Here, DNA is coiled and folded along with proteins to produce chromosomes. Each chromosome normally consists of one very long strand (or molecule) of DNA. The exception arises because of DNA replication, just before cell division, where each chromosome contains two strands of DNA.
Most of the higher organisms are diploid, they have two copies of each chromosome, but some are haploids. More information on the topic can be read at Ploidy.
The important terminologies related to topic are homologous chromosome and sex chromosome. In case of human chromosomes it can be seen that cell contains chromosome pairs, where both the members have exactly same size. These are known as homologous chromosomes. However, one pair do not follow this quote, these are sex chromosomes – X and Y chromosomes. At the time of meiosis, each daughter cell receives half number of chromosomes,one from each pair.
The chromosome number is the number of chromosomes in each cell of an organism. A gametic cell (sperm or egg cell) contains only one set of chromosome. It is named as haploid or 1n. Basic number is another concept, it is gametic chromosome number of a true diploid. Body cells or somatic cells of sexually reproducing organisms are diploid, represented by 2n.
Chromosome number is constant, fixed for particular for a given species. It has immense importance in concepts like phylogeny and taxonomy of the species.
Chromosome size is different for every species. Length and diameter of chromosome is measured in micrometer (µm), during mitotic metaphase. It has maximum length during interphase and minimum at anaphase. The general proportion is lesser the number of chromosomes more is their size. Plants esp. monocotyledonous have larger chromosomes compare to animals and dicotyledonous plants.
Shape of chromosome usually noted in anaphase. Position of centromere or primary constriction is considered while determining shape of chromosome.
Types of chromosome are listed on the basis of number of centromeres and position of centromere.
Structure of Chromosome, especially of eukaryotic chromosome is explained on this page, with the help of some important terminologies.
Material of Chromosome, chromatin – euchromatin and heterochromatin are explained on the page.