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BACKGROUND INFORMATION
Mitosis
Cell Theory states that, "All cells come from pre-existing cells". In single celled organisms,
such as bacteria and protists, cells divide to create two individuals. Prokaryotic cells
reproduce primarily by binary fission. The cell divides in two, with each daughter cell
receiving a complete set of genetic material. In eukaryotes, cell division is more complex.
In multicellular organisms, cell division is necessary for growth, and for the replacement of
dead or dying cells. Your body is comprised of trillions of cells. Yet, you started out as one
cell called a zygote. Through the process of mitosis, your cells have divided, allowing you
to develop into a multicellular organism. In the nucleus, eukaryotic DNA is organized into
chromosomes. Mitosis is a complex process that ensures the production of two nuclei with
identical sets of chromosomes from one cell. Cell division only accounts for a short portion
of a cell’s life cycle.
The Cell Cycle
Most cells in your body pass through a regular sequence of events
called the cell cycle. Mitosis occupies only a very small portion of the
cycle. Each dividing cell spends most of its time in interphase.
Interphase is a very active phase in
the cell cycle and can be divided into three phases. Cytoplasmic
growth and organelle replication takes place during the G1 period.
During the S period,
DNA is duplicated in preparation for mitosis. During G2, structures
directly involved in mitosis, such as microtubules, are synthesized.
The Chromosome
Chromosomes contain the genetic information of the cell. Since each
person is genetically different, the information in your chromosome is
unique to you. However, the structure of chromosomes is similar. Chromosomes are long
fibers of DNA molecules and proteins strung together. The arrangement of nucleotides in
DNA molecules dictates genetic information. During interphase, each chromosome is
extended as a very thin strand within the nucleus. In this form, individual chromosomes are
not distinguishable and the mass of chromosomes
is called chromatin. During the S phase of interphase, the DNA replicates itself, providing
two identical copies of the genetic information of the cell. During early mitosis, the
chromatin condenses (shortens and thickens) and each replicated chromosome
(duplicated chromosome) consists of two identical sister chromatids. The chromatids are
attached together at the centromere.
Meiosis
Meiosis is the cellular process that results in genetically unique individuals to arise. The
events of mitosis produce two cells with identical genetic information. In contrast, the
events of meiosis produce gametes (eggs and sperm), which are not genetically identical
to the parent cell. The gametes produced in meiosis are used in sexual reproduction. To
understand meiosis at the cellular level, it is important to understand how the
chromosomes are arranged in the cell.
Homologous chromosomes
Each cell in your body, except the gametes, has the same number of chromosomes. The
chromosome number is characteristic for each species; the human chromosome number is
46. All chromosomes in human cells belong to a pair. Therefore, there are 23 pairs of
chromosome in your cells. A cell with this full complement of chromosomes is said to have
a diploid, or 2N, number of chromosomes. (2N = 46 in human) This means that in each
diploid cell, half the chromosomes are maternal in derivation—they were contributed by
the egg cell. The other half of chromosomes are paternal and were contributed by the
sperm. The two corresponding chromosomes of each pair - one maternal and one paternal
are called homologous chromosomes. Homologous chromosomes are identical in size and
shape and point of centromere attachment. They also carry the same genes - units of
genetic information about specific characteristics. However, they may differ in the specific
genetic information carried on the chromosome. For example, you inherit information
about eye color from both parents, but the specific information (encoded on each
chromosome) you inherit from each parent may differ. During meiosis, the chromosome
number is reduced by half. Each of the resulting daughter cells receives one member of
each homologous pair of chromosomes. The resulting four daughter cells are haploid (or
N) in chromosome number. Meiosis in humans generates haploid gametes—eggs and
sperm. The human egg and sperm each contain 23 chromosomes, half the diploid (2N)
number of 46. During sexual reproduction, a haploid egg is fertilized by a haploid sperm,
forming a diploid zygote. Thus, sexual reproduction restores the diploid number. The
diploid zygote is the first cell of a new individual.
Genetic Variation
Meiosis allows for genetic variation in the resulting cells. How the homologous pairs align
on the metaphase plate during Metaphase I dictates the genetic information in the
daughter cells. Each pair of chromosomes lines up on the plate independent of how the
other homologous pairs line up. Therefore, the homologous chromosomes follow the
principle of independent assortment. This means that the greater the number of pairs, the
greater the number of possible chromosome combinations in each daughter cell.
Independent assortment is important for genetic variation. “Crossing-over” is another
event, which allows for genetic variation. During Prophase I, the sister chromatids are in
close proximity to each other, and to the sister chromatids of the homologous
chromosome. Due to this close proximity, the chromatids often exchange their tips. This
process is called “crossing over”. If sister chromatids exchange tips, there is no difference
in the genetic information on each chromatids, because they are identical. If a sister
chromatid exchanges the tip with the sister chromatid of the homologous pair, the result is
variation among all sister chromatids in the homologous pair. Due to crossing over, meiosis
produces 4 genetically unique cells from one parent cell.