Question

If a human has 46 chromosomes and it undergoes meiosis this means it will produce haploid cells. (each cell having 23 chromosomes) However, if the cell replicates itself in interphase. Wouldn't we have 92 chromosomes once replicated? How would this translate in having a haploid # of chromosomes after meiosis I? If possible please draw a diagram I am super confused ;/

Answer 1

Meiosis is another type of cell division process. In humans it takes place in order to produce gametes for fertilization. So fundamentally, in meiosis, a diploid cell with two sets of chromosomes (23 pairs i.e, 46 chromosomes in humans) divides to produce a haploid cell with only one set of chromosomes (23 chromosomes)

But before the cell enters the meiosis phase it undergoes interphase which consists of the G1, S and G2 phase. Let us know a little about these phases first before proceeding to the meiosis phase. A newly formed cell has to grow in order to divide again. This interphase is the phase in a cell cycle where the cell grows and makes a copy of it's DNA. In the G1 phase the cell grows, it's size increases and synthesizes requirements the S phase. In the S phase, DNA is replicated which means an identical copy of a chromosome is formed with both the copies joined together by a common centromere known as a sister chromatid. ​​Thus all the 23 pairs or 46 chromosomes replicate and form a sister chromatid which is an identical copy of themselves. So there are still 46 chromosomes after the S phase but each chromosome has an identical copy of itself joned together with a common centromere. In the G2 phase the cell grows more and prepares itself for the meiosis phase.

The G2 phase is followed by meiosis I which consists of the prophase I, metaphase I, anaphase I and telophase I. Similarly, the meiosis II which follows right after meiosis I, consists of prophase II, metaphase II, anaphase II and telophase II. We need to concentrate on metaphase I and II to understand the question you have. Thus, in metaphase I,homologous chromosomes(not individual chromosomes) line up at the metaphase plate for separation. In anaphase I the homologous chromosomes are pulled apart to opposite ends of the cell whereas the sister chromatids still remain attached to each other. This is then followed by telophase I. So after meiosis I there are 23 chromosomes in each daughter formed. Futhermore, in meiosis II which takes place in each of the 2 cells previously formed in meiosis I, the 23 chromosomes line up at the metaphase plate in metaphase II and the sister chromatids are separated to form 2 cells with 23 chromosomes each. So ultimately at the end of the meiosis process a total of 4 haploid cells is formed.