IMAT 2017 Q23 [Testis Cell Undergoing Meiosis]

Which of the following occurs in a healthy human testis cell before it undergoes meiosis?

  1. DNA content of the nucleus doubles
  2. spindle fibres form the spindle
  3. crossing over

A. 1, 2 and 3
B. 2 and 3 only
C. 1 and 2 only
D. 1 only
E. 1 and 3 only

The correct answer is D, but first, what is meiosis ?

  • It is a type of cell division, just like mitosis.

  • Its purpose is to reduce the number of chromosomes in gametes and, ultimately, to allow sexual reproduction : indeed, since we humans have 46 chromosomes, it is not possible for the child to receive 46 chromosomes from both parents.

  • Therefore, each diploid cell undergoes 2 rounds of division, and ultimately, yields 4 haploid cells :

    • A diploid cell has 2 complete sets of chromosomes ;
    • A haploid cell only has 1 set of chromosomes ;
  • It is divided into 4 different phases and 2 different cell divisions :

    • Prophase I and II

    • Metaphase I and II

    • Anaphase I and II

    • Telophase I and II

      [diagram coming soon]

What we want to do is to determine which event happens before meiosis. Knowing that, let’s review each item one by one :

  1. Like we’ve established previously, before entering mitosis, each cell has 2 sets of each chromosome. Similarly, we know that during meiosis, 1 mother cell yields 4 daughter cells. To make this possible, just before the cell enters meiosis, it duplicates its DNA and therefore, this item is correct.

  2. The spindle is a microtubule-based scaffold that ensures even chromosome distribution between daughter cells. Its action is necessary during the anaphase I and II of meiosis and not before. Therefore, this item is incorrect.

  3. Crossing over is the process that produces new and random combinations of genes through their exchange between 2 non-sister chromatids of homologous chromosomes. It happens during meiosis, and more precisely, during prophase.

For that reason, only the first item is correct, making D the right answer.