AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

CENTROMERE POSITIONS

Size of the chromosome

• The size of the chromosome shows a remarkable variation depending upon the stage of cell division. The chromosomes are the longest and thinnest during interphase (resting stage) and hence not visible under light microscope. Chromosomes are the smallest and thickest during mitotic metaphase.
• Chromosome size is not proportional to the number of genes present on the chromosome.
• The location of the centromere on each chromosome gives the chromosome its characteristic shape.

Centromere position

• Chromosomes are classified according to the centromere position is at one end (acrocentric), closer to one end than the other (submetacentric) or in the middle (metacentric).
• Each chromosome has two arms, labeled p (the shorter of the two) and q (the longer).
• The p arm is named for "petite" meaning 'small'; the q arm is named q simply because it follows p in the alphabet. (According to the NCBI, "q" refers to the French word "queue")

To view "Centromere Position Animation"

Metacentric

• The centromere is localized approximately midway between each end and thereby two arms are roughly equal in length.
• Metacentric chromosome takes V shape during anaphase.

Submetacentric

• Centromere is submedian, giving one longer and one shorter arm.
• Submetacentric chromosome may be J or L shaped during anaphase.

Acrocentric

• The centromere is more terminally placed and forms very unequal arm length (The "acro-" in acrocentric refers to the Greek word for "peak").
• The p (short) arm is so short that is hard to observe, but still present.
• Acrocentric chromosome may be rod shaped during anaphase.

Telocentric

• Centromere lies at one end.
• Telocentic chromosome may be rod shaped during anaphase.

• According to the number of the centromere the eukaryotic chromosomes may be acentric (without any centromere), mono centric (with one centromere), dicentric (with two centromeres) or polycentric (with more than two centromeres).