A topic in biology that many students find challenging (and is known to appear on the DAT) is the number of chromosomes and chromatids present during the various stages of meiosis and mitosis in eukaryotes. To first clarify this topic, it is first essential to understand some basic definitions.
Chromatin is the general packaging of DNA around histone proteins – this arrangement of DNA helps to condense DNA to fit within the nucleus of the cell. Throughout most of the cell cycle, DNA is packaged in the form of chromatin. However, during mitosis and meiosis, chromatin exists in an additional level of organization known as a chromosome. Chromosomes are an even denser packaging of chromatin that are visible with a light microscope, particularly during metaphase. Chromosomes can exist in duplicated or unduplicated states. At the beginning of mitosis, for example, a chromosome consists of two sister chromatids – chromatids are the term used to describe the chromosome in its duplicated state. Let’s try to tie all of this information together and see how it applies to chromosome and chromatid count during the various stages of cell replication.
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):
As the above image shows, 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. Visualized below:
As you can see, 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:
Below is a table summarizing the chromosome and chromatid number during meiosis II in humans:
A quick tip: notice that during the stages of meiosis and mitosis, the chromatid count never changes. Only the number of chromosomes changes (by doubling) during anaphase when sister chromatids are separated. During meiosis I, neither the chromosome number nor the chromatid number change until after telophase I is complete.
Full summary chart: