Genetic Variation

In mitosis, every daughter cell is exactly like the parent cell.

Meiosis and sexual reproduction, however, result in a reassortment of the genetic material.

This reassortment, called genetic recombination, originates from three events during the reproductive cycle:

1. Crossing over. During prophase I, nonsister chromatids of homologous chromosomes exchange pieces of genetic material.

As a result each homologue no longer entirely represents a single parent.

2. Independent assortment of homologues. During metaphase I, tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. Which chromosome goes to which pole depends upon the orientation of a tetrad at the metaphase plate. This orientation and subsequent separation is random for each tetrad. For some chromosome pairs, the chromosome that is mostly maternal may go to one pole, but for another pair, the maternal chromosome may go to the other pole.

3. Random joining of gametes. Which sperm fertilizes which egg is to a large degree a random event.

In many cases, however, this event may be affected by the genetic composition of a gamete. For example, some sperm may be faster swimmers and have a better chance of fertilizing the egg.

Why Cells Divide

There are two important factors that limit the size of a cell and motivate its division.

The first is the relative size of the surface area of the plasma membrane and the volume of the cell.

When a cell grows, the volume of a cell increases faster than the surface area enclosing it. This is because volume increases by the cube of the radius (volume of a sphere = (4⁄3)πr3, where r is the radius), whereas the surface area increases by only the square of the radius (surface area = 4πr2).

When the surface-to-volume ratio (S/V) is large, there is a large surface area relative to volume. Under these conditions, the cell can efficiently react with the outside environment.

For example, adequate amounts of oxygen (for respiration) can diffuse into the cell, and waste products can be rapidly eliminated.

When the S/V is small, the surface area is small compared to the volume. When this occurs, the surface area may be unable to exchange enough substances with the outside environment to service the large volume of the cell. This situation is alleviated by cell division.

Cell Size

Surface Area (length x width x 6)

Volume (length x width x height)

Ratio of Surface Area to Volume

Ratio of Surface Area to Volume in Cells

A second reason for dividing is the limited capability of the nucleus.

The genetic material (chromosomes) in the nucleus, collectively called its genome, “controls” the cell by producing substances which make enzymes and other biosynthetic substances. These substances, in turn, regulate cellular activities.

The capacity of the genome to do this is limited by its finite amount of genetic material. As the cell grows, its volume increases, but its genome size remains constant.

As the genome-to-volume ratio decreases, the cell’s size exceeds the ability of its genome to produce sufficient amounts of materials for regulating cellular activities.

In addition to surface-to-volume and genome-to-volume ratios, other factors that are cell specific influence the onset of cell division.

For example, many cells will stop dividing when the surrounding cell density reaches a certain maximum (density-dependent inhibition).

Density-dependent inhibition Normal cells proliferate in culture until they reach a finite cell density, at which point they become quiescent. Tumor cells, however, continue to proliferate independent of cell density.

Other cells, such as nerve cells, will rarely divide once they have matured.

When the cell cycle is interrupted and the cell stops dividing, the cell remains in an extended G1 phase (or G0 phase), never beginning the S or G2 phases until some internal or external cue initiates a resumption of the cell cycle.