Question

Explain the difference between non-disjunction during Meiosis I and Meiosis II. Which is more detrimental to the genetic compositionof the gametes? Explain your answer

Answer 1

Non disjunction is a phenomena when two homologous chromosomes fail to separate during cell division.

  As a result of this, one of the daughter cells gets an extra chromosome while the other one gets one chromosome less than the normal number.

Meiosis is a process of creating haploid gametes from diploid cells. If everything goes smooth during meiosis, chromosomes will be separated and distributed evenly to produce 4 haploid gametes. However, sometiimes chromosomes do not separate properly, resulting in unequal distribution of chromosomes.

Non -disjunction occurs during Anaphase I of Meiosis I. In human diploid cells there are 23 pairs of homologus chromosomes . After meiosis I , normally two cells are formed each with half the number of chromosomes i.e 23 chromosomes in each cell.

When anaphase I begins, the two chromosomes of each bivalent separate and start moving towards the opposite poles of the cell as a result of the action of the spindle fibres. Sometimes, the bivalent fail to separate , resulting in the unequal distribution of the chromosomes. During this whole process the sister chromatids remain remain attached at their centromere.

If non-disjunction occurs during anaphase II of meiosis II, then it means that at least one pair of sister chromatids failed to separate. In this scenario, 4 cells will be formed , of these two cells will have the normal haploid nmber of chromosomes, one cell will have an extra chromosome [ n + 1] and the other will have one less number of chromosome [ n - 1]. This phenomena is called aneuloploidy.

So, from the above points we can conclude that the key difference between non-disjunction in meiosis I and meiosis II is that during meiosis I, homologus chromosomes fail to separate while in meiosis II sister chromatids fail to separate.

  Genetically, non -disjunction can cause several syndromes or disease condititions which have no cure.

For example , trisomy [ three in number instead of the normal one pair] of the 21st chromosome results in Down's syndrome babies. This occurs when a gamete containing n + 1 chromosome fertilising a normal gamete. The resultant zygote contains 2n + 1 chromosomes [ 47 chromosomes] .

Another example is Turner's syndrome. This occurs due to non-disjunction of the sex chromosomes[ monosomy XO]. This results in gametes with n-1 chromosome , which after fertilisation produce a zygote with 2n-1 [ 45] chromosomes.

Some other examples are - Patau's syndrome [ trisomy of 13th chromosome], Edward's syndrome [ trisomy of 18th chromosome] , Klinefelters syndrome [47, XXY males] etc.