Humans are diploid organisms with a total of 46 chromosomes (2n = 46). They have 23 distinct chromosomes and each chromosome has a homologous chromosome, giving a total of 46 chromosomes. Upon completion of DNA replication in S phase, each chromosome gains an identical sister chromatid that is joined to the original chromosome at the centromere; however, this whole entity is still considered a single chromosome. When a cell enters prophase of mitosis, there are a total of 46 chromosomes, each with a sister chromatid. In metaphase, the cell still has 46 chromosomes and these chromosomes align along the midline of the cell. In anaphase, the sister chromatids get pulled apart to opposite ends; therefore, the sister chromatid separates from the chromosome and becomes its own chromosome. This means that in anaphase there are a total of 92 chromosomes in the cell.

These 92 chromosomes get pulled to opposite ends where 2 new daughter cells (with 46 chromosomes and no sister chromatids) are produced.

Mitosis and meiosis are both processes that involve cell division. In mitosis, a diploid parent cell divides and gives rise to two identical, diploid daughter cells. In meiosis, a diploid parent cell divides and gives rise to four identical, haploid daughter cells. Meiosis is divided into meiosis I and meiosis II. Meiosis I is unique because its prophase (prophase I) involves exchange of genetic material between chromosomes. This process is called recombination. Meiosis II is similar to mitosis. Note that both meiosis I and II have prophase, metaphase, anaphase, and telophase.

Metaphase in both mitosis and meiosis involves the alignment of the cell’s nuclear material along the midline. In mitosis, sister chromatids line along the midline. This means that in humans there will be a total of 46 columns of chromosomes along the midline (46 chromosomes with their respective sister chromatids). In meiosis I, however, it is slightly different. Recall that prophase I involves tetrad formation. This means that homologous chromosomes (with their respective sister chromatids) pair up along the midline, reducing the amount of columns of chromosomes in metaphase I to half of metaphase in mitosis. This means that in humans there will be only 23 columns of chromosomes in metaphase I.

Ploidy number refers to the amount of homologous chromosomes present. In humans, there are two sets of homologous chromosomes; therefore, humans are diploid. As mentioned mitosis produces two identical, diploid daughter cells (each cell has homologous chromosomes). In meiosis I two identical, haploid daughter cells are produced. These daughter cells have 23 chromosomes with sister chromatids; however, they lack their homologous pair and are, therefore, haploid. These two daughter cells undergo meiosis II and produce the final products of meiosis, which are four haploid daughter cells. Remember that the daughter cells of meiosis II will not have a sister chromatid because the sister chromatids are pulled to opposite poles during anaphase II.

Asexual reproduction is cell division that involves no sexual recombination. Recall that mitosis in eukaryotes produces two identical daughter cells with identical genetic make up. This is because there is no sexual recombination during mitosis.

Meiosis, on the other hand, undergoes sexual recombination during prophase I and is considered a form of sexual reproduction, which increases genetic variation. Meiosis occurs in the gonads, the site of sperm (in testes) and oocyte (in ovaries) production. DNA replication and transcription are not involved in cell division and reproduction.

During mitosis, the nuclear envelope breaks down to allow the formation of chromosomes. Since all concentration gradients are dependent upon the impermeability of a membrane, when this envelope breaks down, the concentration gradient weakens and disappears.