Question

How many chromosomes and chromatics would a typical human cell have after duplication but before mitosis? 0 46 chromosomes, 92 sister chromatids o 23 chromosomes, 46 sister chromatids 46 chromosomes, 46 sister chromatids O 23 chromosomes, 23 sister chromatids

Answer 1

The answer is - (a) 46 chromosome and 92 sister chromatids

Explanation-

First, during the S phase of interphase, the genetic material of a cell is duplicated. A human has 46 chromosomes (a set of 23 you inherit from your mother, and a set of 23 from your father). After the genetic material is duplicated and condenses during prophase of mitosis, there are still only 46 chromosomes – however, they exist in a structure that looks like an X shape.For clarity, one sister chromatid is shown in green, and the other blue. These chromatids are genetically identical. However, they are still attached at the centromere and are not yet considered separate chromosomes. Thus, the above picture represents one chromosome, but two chromatids. During prophase and metaphase of mitosis, each chromosome exists in the above state. For humans, this means that during prophase and metaphase of mitosis, a human will have 46 chromosomes, but 92 chromatids (again, remember that there are 92 chromatids because the original 46 chromosomes were duplicated during S phase of interphase). It is helpful to see this visualized (for visual simplicity, a 2n=8 arrangement of chromosomes will be demonstrated, rather than the 2n=46

arrangement of chromosomes in humans)

there are 8 chromosomes present, but 16 chromatids. Similarly, in humans (2n=46), there are 46 chromosomes present during metaphase, but 92 chromatids.

It is only when sister chromatids separate – a step signaling that anaphase has begun – that each chromatid is considered a separate, individual chromosome. Pictured below, we see how the 2n=8 cell from above has progressed from having 8 chromosomes to 16 chromosomes.Now that the sister chromatids have separated, each chromatid is also considered a chromosome. During anaphase, we now have a total of 16 chromosomes and 16 chromatids – in short, each chromatid is now a chromosome. Similarly, in humans, there are 92 chromosomes present and 92 chromatids during anaphase. These numbers remain the same during telophase. It is only after the end of mitosis – when the dividing cells have fully separated and the membranes have reformed – that the normal chromosome number is restored to the cell. Below is a table summarizing the chromosome and chromatid number during mitosis in humans.

The chromosome and chromatid count during meiosis works a bit differently. Recall that there are two divisions during meiosis: meiosis I and meiosis II. The genetic material of the cell is duplicated during S phase of interphase just as it was with mitosis resulting in 46 chromosomes and 92 chromatids during Prophase I and Metaphase I. However, these chromosomes are not arranged in the same way as they were during mitosis. Rather than each chromosome lining up individually across the center of the cell, homologous pairs of chromosomes line up together (forming tetrads, also known as bivalents).For visual consistency, let us look at the hypothetical 2n=8 cell from earlier during metaphase I. Here, the homologous chromosome pairs have been color coded.When anaphase I begins, you may expect the chromosome number to change, but it does not. Remember – it is only after the sister chromatids separate that the chromosome number changes. Since anaphase I only separates the homologous chromosomes, neither the chromosome number nor the chromatid number changes during anaphase.the separation of homologous chromosomes does not change the chromosome number or the chromatid number. There are still 8 chromosomes and 16 chromatids. In fact, until the completion of meiosis I, the chromosome and chromatid numbers remain the same through all stages. Similarly in a human, we do not see a change in chromosome or chromatid number until the end of meiosis I (when division of the cell in two results in half the chromosome and chromatid count). Below is a table summarizing the chromosome and chromatid number during meiosis I in humans.

The second division of meiosis (meiosis II) appears similar to mitosis, with the only difference being that there are now half as many chromosomes as before. Continuing with the 2n=8 cell example from above, we will observe a cell during metaphase II.During metaphase II, the chromosomes are lined up individually across the center of the cell. Due to the reduction division of meiosis I, there are now half as many chromosomes (and chromatids) as there were before. When anaphase II begins, however, the sister chromatids split apart, which once again doubles the chromosome number.