Which of the following occurs in a healthy human testis cell before it undergoes meiosis?
- DNA content of the nucleus doubles
- spindle fibres form the spindle
- 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
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The correct answer is D, but first, what is meiosis ?
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It is a type of cell division, just like mitosis.
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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.
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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 ;
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It is divided into 4 different phases and 2 different cell divisions :
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Prophase I and II
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Metaphase I and II
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Anaphase I and II
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Telophase I and II
[diagram coming soon]
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What we want to do is to determine which event happens before meiosis. Knowing that, let’s review each item one by one :
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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.
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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.
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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.
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spindle fibres are formed during prophase 1, i believe. why anaphase? i seem not to understand it.
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it’s action is necessary during anaphase ie pulling away homologous chromosomes.
Hello! I’m having some difficulties understanding why Point 2 regarding the spindle fibers is incorrect and was hoping someone could clarify if they have the chance.
According to my understanding and research, mitotic division of the germline epithelium of the seminiferous tubules occurs prior to the two meiotic divisions (Gametogenesis | BioNinja). Since I believe spindle fibers are involved in mitosis (Mitosis | BioNinja) which appears to occur before meiosis, I was wondering if someone could clarify what I’m misunderstanding and what makes Point 2 incorrect? Thank you!
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I think 2 is incorrect because the spindle fibers don’t form the spindle, but the centriols form the spindles. So 2 is incorrect.