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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.