Why does mitosis not result in haploid cells, while it does in meiosis 2, when the sister chromatids separate?

Think about it. Haploid somatic cells (mitosis) wouldn't make sense, because you'd be missing half of your chromosomes. If your somatic cells were haploid and you needed to translate a certain gene into a protein and that chromosome is missing -- you're out of luck!

Meiosis, however, needs to produce haploid gametes. When egg and sperm combine, both of them are haploid and combine to form a DIPLOID zygote. If gametes were diploid, you'd have 46 pairs of chromosomes. Then, your offspring would have 92 pairs of chromosomes. That's far too many.

But to answer why meiosis results in haploid cells, the chromosomes are only duplicated in the M1 stage. The two newly formed cells are diploid, and their chromosomes DO NOT replicate again. Instead, each pair is pulled apart for a total of 23 chromosomes (haploid cells).

You can look at a diagram to see how the chromosomes actually separate during meiosis.

That is because it is >meiosis I < that separates homologous pairs and reduces the daughter cells to haploidy.

meiosis II then completes the process with the separation of the duplicated chromosomes.

Think of meiosis as having a short step added in the middle of mitosis. Between the replication of the DNA into dyads with sister chromatids and their separation is an additional meiosis I.

Meiosis I aligns the dyads into tetrads. Homologous pairs in synaptonemal complexes form tetrads. This stage separates the tetrads so each daughter gets one copy of the genome in duplicate sister chromatids.
Meiosis II separates the duplicate chromatids.

In mitosis there is only one round of division.  In G1 the cell contains a full complement of chromosomes -- it is 2n.  Then in the S phase of interphase, the DNA is replicated and then each chromosomes consists of two genetically identical sister chromatids.  Then in anaphase, the sister chromatids separate and each daughter cells ends up with a full complement of chromosomes.  2n again.

In meiosis there are 2 rounds of division, but only 1 replication of DNA.  So each daughter cell ends up with 1/2 as many chromsomes (n instead of 2n; haploid instead of diploid) as the parent cell.

In G1 the cell contains a full complement of chromosomes -- it is 2n.  Then in the S phase of interphase, the DNA is replicated and then each chromosomes consists of two genetically identical sister chromatids.  Then in anaphase I, the homologous chromosomes separate -- the sister chromatids remain together.  Thus before meiosis II even begins, the parent cell has already divided into 2 cells.  Then in anaphase II, in each of the 2 cells that resulted from meiosis I, the sister chromatids separate and each one goes into a different daughter cell. This is the second round of division; one more round of division than mitosis has, so each daughter cell ends up with only 1/2 as many chromosomes as in mitosis.


If it is still unclear, think of each double helix of DNA as a chromatid, and let's consider humans, who have 46 chromosomes.

[Mitosis: including interphase]
G1 = 46 "chromatids"
S phase = 92 "chomatids"

Anaphase = 92 "chromatids", with 46 "chromatids" heading towards each pole of the cell
Telophase/cytokinesis = 2 cells, each with 46 "chromatids"



[Meiosis: including interphase]
G1 = 46 "chromatids"
S phase = 92 "chomatids"

Anaphase I = 92 "chromatids", with 46 "chromatids" heading towards each pole of the cell
Telophase I/cytokinesis = 2 cells, each with 46 "chromatids"

Anaphase II = 2 cells: each cell with 46 "chromatids", with 23 "chromatids" heading towards each pole of the cell
Telophase I/cytokinesis = 4 cells, each with 23 "chromatids"