Question

What is disjunction?

Answer 1

Hint: Disjunction is an important event in the cell division. It happens when the sister chromatids separate and move towards the opposite pole in the anaphase during meiosis.

Complete answer:
Meiosis is the process of cell division which takes place in the reproductive cells.
Stages of meiosis-
>Prophase 1- chromosome condenses and crossing over occurs
>Metaphase 1- homologous chromosomes move to the equator.
>Anaphase 1- Homologous chromosomes move apart towards the opposite poles. This pulling apart is called disjunction.
>Telophase 1 and cytokinesis- chromosome gathers at the poles and the cytoplasm divides.
>Prophase 2- This is the division of the haploid chromosome formed in meiosis 1. New spindle forms.
>Metaphase 2- The chromosomes arranged at the equator
>Anaphase 2- Centromeres divide and the chromatids move towards the opposite poles. This is called disjunction.
>Telophase 2 and cytokinesis-The cytoplasm divides and the nuclear envelope is formed.

Thus disjunction is the pulling apart of the chromosome towards the pole. The pulling is carried out by the microtubules. It happens in the Anaphase 1 and anaphase 2 of meiosis. If the disjunction fails in meiosis 1, the cells can have an extra chromosome or a missing chromosome.

Note:Nondisjunction is the failure of disjunction. It leads to aberrations. For example in case of Down’s syndrome. It is a trisomy of chromosome 21 which is very common in humans. The trisomy can lead to abnormalities and also abortions.