Mitosis & Meiosis

[NS_Tutor_Sophia]

A student recently asked us this question about mitosis and meiosis: "During mitosis, the number of chromosomes stays the same, but the number of sister chromatids changes. So when we say we have 46 chromosomes, does that mean there are about 92 sister chromatids (after S phase and before anaphase)? "

The short answer to this is that, immediately after DNA replication (S phase), you are right: there are 46 chromosomes and 96 sister chromatids. Let's break this down further.

First let's consider mitosis.

• Before DNA replication, imagine that there are 46 chromosomes, 23 red (paternal) and 23 blue (maternal), each with just one sister chromatid

• After DNA replication, there are still 46 chromosomes, but now 92 sister chromatids -- these are the classic "X" structures of condensed chromosomes that we typically envision

• During mitosis, the two sister chromatids on each chromosome are split apart such that each daughter cell ends up with 46 chromosomes (each with just one sister chromatid)

Now let's consider meiosis (see image in reply).

• Before DNA replication, there are 46 chromosomes, 23 red (paternal) and 23 blue (maternal), each with just one sister chromatid

• After DNA replication, there are still 46 chromosomes, but now 92 sister chromatids

• During meiosis I, the homologous chromosomes are split apart so that each daughter cell ends up with 23 chromosomes, some red and some blue (each with TWO sister chromatids, so 46 total)

• During meiosis II (which is analogous to mitosis), the two sister chromatids on each chromosome are split apart such that each daughter cell still ends up with 23 chromosomes, but each with just one sister chromatid

If you're ever in any doubt, take it from the top. The cell always starts with 46 chromosomes, which are replicated into 96 sister chromatids. In mitosis, the chromatids are split apart. In meiosis, homologous chromosomes and then sister chromatids are split apart. It's okay to draw this out to help you figure out how many chromosomes and sister chromatids are at each step! Instead of drawing out all 46 chromosomes, you can use a smaller number of chromsomes (say, 4), and then when this number is halved to 2, you know the number of chromosomes in a human cell will be halved to 23.

[NS_Tutor_Sophia]

Meiosis: