Question

Give an account on the morphology of the eukaryotic chromosomes.

Answer 1

Morphology of the eukaryotic chromosomes:

• The chromosomes are thin, coiled, elastic, contractile, anastomose thread-like structures during the interphase (when no division of cell occurs) and are called chromatin threads which under low magnification look like a compact stainable mass, often called as chromatin substance or material.
• During metaphase stage of mitosis and prophase of meiosis, these chromatin threads become highly coiled and folded to form compact and individually distinct ribbon-shaped chromosomes.
• These chromosomes contain a clear zone called kinetochore or centromere along their length.
• The number and position of centromeres is variable, but is definite in a specific chromosome of all the cells and in all the individuals of the same species.
• Thus, according to the number of the centromere the eukaryotic chromosomes may be acentric (with out any centromere), monocentric (with one centromere), dicentric (with two centromeres) or polycentric (with more than two centromeres).
• The centromere has small granules or spherules and divides the chromosomes into two or more equal or unequal chromosomal arms.
• According to the position of the centromere, the eukaryotic chromosomes may be rod-shaped (telocentric and acrocentric), J-shaped (submetacentric) and V-shaped (metacentric) as have been shown in the figure below.

• During the cell divisions the microtubules of the spindle are get attached with the chromosomal centromeres and move them towards the opposite poles of cell.
• Beside centromere, the chromosomes may bear terminal unipolar segments called telomeres. Certain chromosomes contain an additional specialized segment, the nucleolus organizer, which is associated with the nucleolus.

• According to the classical cytological studies, each chromosome structurally consists of a limiting membrane called pellicle, an amorphous matrix and two very thin, highly coiled filaments called chromonema or chromonemata. Each chromonemata is 800A thick and contains 8-microfibrils, each of which in its turn contains two double helices of DNA. Both chromonematae remain intimately coiled in spiral manner with each other and have a series of microscopically visible bead-like swelling along its length called chromomeres. The early geneticists have attached great significance to the chromomeres and erroneously considered them as hereditary unit, hereditary or Mendelian factors or genes; but modern cytological investigations have confirmed that the chromomeres are not genes but the regions of super-imposed coils. The recent cytological findings have also condemned the view that chromosomes have pellicle, matrix and chromonemata.

Morphology of the eukaryotic chromosomes:

• The eukaryotic chromosomes differ from the prokaryotic chromosomes in morphology, chemical composition and molecular structure.
• The shape of the eukaryotic chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division.