General Science Quiz: Cell Division: Mitosis and Meiosis

During this phase the DNA of the cell is copied so that the amount of DNA in the nucleus doubles, while the number of chromosomes remains unchanged because each chromosome now simply consists of two identical sister chromatids joined at the centromere.

In mitosis each centromere splits in anaphase and one chromatid of every chromosome goes to each pole, so the two daughter cells receive exactly the same number of chromosomes as the parent cell had, and exactly the same genetic information.

In this stage the condensed chromosomes, each consisting of two sister chromatids, are moved by the spindle fibres until their centromeres lie in one plane across the middle of the cell, known as the metaphase plate or equatorial plate.

Once the chromosomes are aligned on the metaphase plate and every kinetochore is attached to a spindle fibre, the centromere holding the two sister chromatids together splits.

An animal cell divides its cytoplasm by a cleavage furrow, in which the plasma membrane is pulled inward around the middle until the cell is pinched into two.

Meiosis begins with a single round of DNA replication and is then followed by two successive nuclear divisions. Meiosis I is the reduction division, in which homologous chromosomes separate and the two cells formed already carry half the parental chromosome number.

By this sub-stage the homologous chromosomes have already paired in zygotene and are held together as bivalents or tetrads, each made of four chromatids.

In the zygotene sub-stage of prophase I the two members of each pair of homologous chromosomes come to lie side by side along their whole length, matching point for point, and the structure that forms between them and holds them together is called the synaptonemal complex.

This single difference is the reason meiosis I is called the reduction division. In anaphase I the two homologous chromosomes of each bivalent are pulled to opposite poles, but each of them still consists of two sister chromatids joined at an undivided centromere.

Interphase occupies more than ninety-five per cent of the duration of the cell cycle, because in it the cell must grow to a sufficient size, synthesise proteins and RNA, duplicate its entire DNA and prepare the structures needed for division.

His detailed drawings of the stages are the basis of the account still given in textbooks.

If gametes carried the full chromosome number, fertilisation would double it in every generation. Meiosis halves the number when gametes are formed, so that the union of two gametes restores the original diploid number and keeps it constant from one generation to the next.