TYPES OF CHROMOSOME-STRUCTURE&DEFINITION
Introduction
- Chromosomes are thread like structure made up of DNA tightly coiled many times around protein (histones) in the nucleus of a cell.
- Chromosomes were first described by Strasburger (1815), but the term ‘chromosome’ was first used by Waldeyer in 1888.
Types of chromosome
Chromosomes may be classified on the basis of number of centromere, location of centromere etc.
Autosomes and Sex chromosomes
Human chromosomes are of two types namely, autosomes and sex chromosomes. Genetic traits that are linked to the sex of the person are passed on through the sex chromosomes (allosomes), while the rest of the genetic information is present in the autosomes which are not sex chromosomes.
For example, Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell.
On the basis of location of centromere
Centromere is a small structure; at this point the two chromonemata are joined together. So, it divides chromosome into two arms. The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” (shown below)
- Telocentric– Telocentric chromosomes have the centromere at the very end of the chromosome or say centromere occupy the terminal position. Thus, the chromosome has just one arm. Such rod shaped chromosomes are found in species such as mice.
- Acrocentric- These are also rod-shaped chromosomes with centromere occupying a sub-terminal position. Thus, it makes one arm very long and the other very short. Human chromosomes 13, 15, 21, and 22 are acrocentric.
- Sub-metacentric– Here chromosomes are with centromere slightly away from the mid-point, as a result, the two arms are unequal. Human chromosomes 4 through 12 are sub-metacentric.
- Metacentric- Such chromosomes are V-shaped with centromere lies in the middle of chromosome or in the center, so that the two arms are almost equal. Human chromosome 1 and 3 are metacentric.
On the basis of number of centromere
- Monocentric- Such chromosomes are with one centromere.
- Dicentric- with two centromeres.
- Polycentric- means more than two centromeres.
- Acentric- Such chromosomes are without centromere. These are freshly broken segments of chromosomes.
(Courtesy- microbesnotes) - Diffused or non-located- with indistinct centromere diffused throughout the length of chromosome.
Read more…
CHROMOSOMAL THEORY OF INHERITANCE
HOMOLOGOUS CHROMOSOMES-AN OVERVIEW & FUNCTION
DIFFERENCE BETWEEN GENE AND ALLELE