Mitosis

Mitosis produces two daughter cells from one parent cell.

These daughter cells are genetically identical to each other and to the parent cell, and each has the same (diploid) number of chromosomes as the parent.

During interphase the cell is metabolically active and preparing to divide — it is not a 'resting' phase. In the G1 phase the cell grows and synthesises new organelles and proteins, and in the G2 phase the cell grows further and the replicated DNA is checked.

In the S (synthesis) phase, the cell's DNA is replicated. Each chromosome is copied so that it now consists of two identical sister chromatids joined at a centromere.

Interphase therefore ensures the cell has enough organelles and a complete, accurate second copy of its DNA before mitosis begins.

During anaphase, the centromere of each chromosome divides, so the two sister chromatids of each chromosome become separated.

The spindle fibres shorten, and this pulls the separated sister chromatids towards opposite poles of the cell (the chromatids move centromere-first, giving a characteristic 'V' shape).

As a result, an identical set of chromosomes (chromatids) is moved to each pole, ensuring the two new nuclei will be genetically identical.

Prophase. The chromosomes condense by coiling and shortening, so they become visible; each chromosome is now seen to consist of two sister chromatids joined at a centromere. A spindle of protein fibres forms across the cell between the two poles, and the nuclear envelope breaks down.

Metaphase. The chromosomes move to and line up along the equator of the cell (the metaphase plate). Each chromosome becomes attached to spindle fibres by its centromere.

Anaphase. The centromeres divide, and the spindle fibres shorten, pulling the sister chromatids of each chromosome to opposite poles of the cell. This separates the two identical copies of each chromosome.

Telophase. The chromatids reach the poles and decondense (uncoil), and a new nuclear envelope forms around each of the two groups of chromosomes, producing two genetically identical nuclei. The spindle breaks down.

Cytokinesis then divides the cytoplasm to give two separate, genetically identical daughter cells.

Mitosis is significant because it produces two daughter cells that are genetically identical to each other and to the parent cell, each with the full diploid number of chromosomes. This is possible because DNA is replicated exactly in the S phase, and mitosis then separates the identical sister chromatids equally between the two cells. Three roles depend on this:

Growth. Multicellular organisms develop from a single fertilised egg (zygote) into a body of many cells by repeated mitosis. Because each new cell is genetically identical, all the body cells carry the same genetic information, which is essential for coordinated development.

Repair and replacement of cells. Cells that are damaged, worn out or lost are continually replaced by mitosis — for example skin cells, the lining of the gut and blood cell precursors. Producing genetically identical replacement cells restores the tissue's structure and function, and allows wounds to heal.

Asexual reproduction. Some organisms reproduce asexually using mitosis (e.g. binary fission in bacteria-like single cells, budding in yeast and Hydra, runners and tubers in plants). The offspring are genetically identical clones of the parent. This is advantageous in a stable environment because a successful genotype is reproduced exactly without needing a mate.

Consequence of no genetic variation. Because every cell or offspring produced is genetically identical, mitosis introduces no genetic variation. In asexual reproduction this is a serious disadvantage if the environment changes: since all individuals share the same alleles, none may possess a variation that allows survival, so a single new disease, pest or environmental change could wipe out the entire population (there is no raw material for natural selection or adaptation). The same principle explains why a genetically uniform crop monoculture is so vulnerable. By contrast, the genetic variation generated by meiosis and sexual reproduction provides the differences on which natural selection can act, allowing populations to adapt.

(a) The mitotic index is calculated as:

$\text{mitotic index} = \dfrac{\text{number of cells in mitosis}}{\text{total number of cells}} \times 100$

Substituting the values:

$\text{mitotic index} = \dfrac{36}{480} \times 100 = 0.075 \times 100 = 7.5\%$

So the mitotic index is 7.5%.

(b) The student could count a larger number of cells (e.g. examine more fields of view or more slides). A larger sample size reduces the effect of random variation, giving a more reliable estimate of the proportion of cells in mitosis.

First, the student should cut off the tip (last few millimetres) of a growing root, because this is where the meristem is and the cells are dividing rapidly, so many cells will be in mitosis.

The tip is then warmed in dilute hydrochloric acid (HCl). The acid softens the tissue and breaks down the material between the cells, so the cells can later be separated and spread out.

Next, the tip is placed on a slide and stained with acetic (aceto) orcein. This stain binds to and colours the chromosomes (DNA), making them dark so the stages of mitosis become visible.

A coverslip is added and the tip is squashed by pressing down firmly. This spreads the cells into a single thin layer (one cell thick) so individual cells and their stages can be seen clearly under the microscope.

Finally, the slide is examined under a light microscope (low power first, then high power) to find and identify cells in prophase, metaphase, anaphase and telophase.

This is mitosis, because the daughter cells have the same (diploid) chromosome number as the parent and are genetically identical.

Two ways its outcome differs from meiosis:

• Chromosome number: mitosis produces diploid cells (the full chromosome number), whereas meiosis produces haploid cells (half the chromosome number).
• Genetic variation: mitosis produces cells that are genetically identical to the parent, whereas meiosis produces cells that are genetically different (genetically varied) from each other and from the parent.

A further difference is that mitosis produces two daughter cells while meiosis produces four.

(a) Trend. As the distance from the tip increases, the mitotic index decreases. The mitotic index falls from 18.0% at 1 mm (position A), to 6.0% at 5 mm (position B), to just 0.5% at 12 mm (position C) — a fall of 17.5 percentage points across the 11 mm measured.

(b) Interpretation. A high mitotic index means a high proportion of cells are actively dividing. Because the mitotic index is highest at the tip (18.0%), this shows that most cell division occurs at or just behind the root tip, in the region called the meristem. Further from the tip the mitotic index is very low (0.5% at 12 mm), so very few cells there are dividing — instead these cells are elongating and differentiating rather than dividing. The root therefore grows mainly by cells dividing at the tip and then elongating behind it.

(c) Comparison with a tumour. A tumour consists of cells that are dividing uncontrollably by mitosis, so it would have a high mitotic index, like the tip region (A). In both the meristem and a tumour, a large proportion of cells are in a recognisable stage of mitosis at any moment. The difference is that in the root tip the division is controlled and serves growth, whereas in a tumour the cell cycle is no longer properly regulated, so cells divide continuously and excessively.

Genetic identity depends on first making an exact copy of the DNA and then sharing the copies out precisely equally.

During the S phase of interphase, the DNA is replicated semi-conservatively. Because replication relies on complementary base pairing, each new strand is an exact copy of the original, so each chromosome now consists of two genetically identical sister chromatids joined at a centromere.

When mitosis begins, in prophase the chromosomes condense and the spindle forms; in metaphase the chromosomes line up at the equator and attach to spindle fibres by their centromeres, so each chromosome is positioned ready to be split equally.

In anaphase, the centromeres divide and the spindle fibres shorten, pulling one sister chromatid of each chromosome to each pole. Because the two chromatids are identical, each pole receives one identical copy of every chromosome — a complete, identical set.

In telophase a nuclear envelope re-forms around each identical set, and cytokinesis then divides the cytoplasm so that each daughter cell contains one of these identical nuclei.

As a result, provided no mutation occurs, the two daughter cells have identical DNA and the same diploid chromosome number as the parent cell.