Download The Cell Cycle - Meiosis

The Cell Cycle - Meiosis
• In meiosis, the process is quite similar to
mitosis. However, another cell division
takes place in which there is no extra DNA
replication step. Instead of having a pair of
genes (as in a diploid cell), there is only one
copy of each gene (a haploid cell).
• This one copy of genetic information
produces gametes of either sperm or eggs.
Thus, only one copy of a gene is passed on
to each gamete. It is not until the sperm
and egg join that there will be two halves
of genetic information.
Development
• Many eukaryotic organisms do not exist as
one-celled bodies, and generally, the cells
multiply and develop into a more complex
organism. This development comes about by
way of two processes -- growth and
differentiation.
• During the growth process, cells reproduce
and grow more cells through mitosis. This
process occurs daily, and even many times a
day as cells continually need to be
replenished as others die.
• As cells form more complex organisms,
they must produce different types of cells
that result in a functioning integrated
organism.
• A group of cells of the same type form a
larger structure called a tissue (e.g. -- skin,
muscle).
• Multiple tissues can form an organ (e.g. eye, kidney). This variation among cell
groups is known as differentiation.
• Cells that are capable of differentiating
into many different cell types are called
totipotent.
Meiosis
• A specialized type of cell division that
occurs during the formation of gametes.
Although meiosis may seem much more
complicated than mitosis, it is really just
two cell divisions in sequence. Each of
these sequences maintains strong
similarities to mitosis.
• Meiosis I refers to the first of the two
divisions and is often called the reduction
division. This is because it is here that the
chromosome complement is reduced from
diploid (two copies) to haploid (one copy).
• Interphase in meiosis is identical to
interphase in mitosis. At this stage, there
is no way to determine what type of
division the cell will undergo when it
divides. Meiotic division will only occur in
cells associated with male or female sex
organs.
• Prophase I is virtually identical to prophase
in mitosis, involving the appearance of the
chromosomes, the development of the
spindle apparatus, and the breakdown of
the nuclear membrane.
• Metaphase I is where the critical
difference occurs between meiosis and
mitosis. In mitosis, all of the chromosomes
line up on the metaphase plate in no
particular order.
• In Metaphase I, the chromosome pairs are
aligned on either side of the metaphase
plate. It is during this alignment that the
chromatid arms may overlap and
temporarily fuse, resulting in what is called
crossovers.
.
• During Anaphase I, the spindle fibers
contract, pulling the homologous pairs away
from each other and toward each pole of
the cell.
• In Telophase I, a cleavage furrow typically
forms, followed by cytokinesis, the changes
that occur in the cytoplasm of a cell during
nuclear division; but the nuclear membrane
is usually not reformed, and the
chromosomes do not disappear.
• At the end of Telophase I, each daughter
cell has a single set of chromosomes, half
the total number in the original cell, that
is, while the original cell was diploid; the
daughter cells are now haploid.
• Meiosis II is quite simply a mitotic division
of each of the haploid cells produced in
Meiosis I.
• There is no Interphase between Meiosis I
and Meiosis II, and the latter begins with
Prophase II.
• At this stage, a new set of spindle fibers
forms and the chromosomes begin to move
toward the equator of the cell.
• During Metaphase II, all of the
chromosomes in the two cells align with the
metaphase plate.
• In Anaphase II, the centromeres split, and
the spindle fibers shorten, drawing the
chromosomes toward each pole of the cell.
• In Telophase II, a cleavage furrow
develops, followed by cytokinesis and the
formation of the nuclear membrane
• The chromosomes begin to fade and are
replaced by the granular chromatin, a
characteristic of interphase. When Meiosis
II is complete, there will be a total of four
daughter cells, each with half the total
number of chromosomes as the original cell.
.
• In the case of male structures, all four
cells will eventually develop into sperm
cells. In the case of the female life cycles
in higher organisms, three of the cells will
typically abort, leaving a single cell to
develop into an egg cell, which is much
larger than a sperm cell.
.
.
Genetic Diversity
• Independent Assortment
• The orientation of homologous
chromosomes on one side of the metaphase
plate or the other in Meiosis I is random.
• The number of possible orientations is 2n,
where n is the haploid number. For humans,
the number is 223 = 8.4 million
• Random fertilization
• Any of a male’s 8.4 million sperm can
fertilize any of a woman’s 8.4 million eggs.
The total number of combinations is over
70 trillion.
• Crossing over
• When crossing over is considered, the
number of combinations is nearly infinite.
• Genetic diversity contributes to
evolutionary change. If an offspring
inherits a combination of genes that gives
it a survival advantage, it is better able to
survive and pass on its genes. This means
the chance that the combination is passed
on increases. As a result, there is an
accumulation of favorable characteristics.
Nondisjunction Disorders
• If two homologous chromosomes move
toward the same pole, one daughter cell
will have an extra chromosome while the
other will be missing one.
• After fertilization, a zygote could have
three copies of a chromosome (trisomy) or
only one (monosomy), rather than the
normal two copies.
• Each cell of the organism will then have the
abnormal chromosome number.
Down syndrome
• Trisomy of chromosome 21
• Symptoms
• (1) Round, full face; enlarged tongue; short
height; large forehead
• (2) Low mental ability
• (3) Heart defects
• (4) Prone to Alzheimer’s and leukemia
• (5) Mostly sterile
• Frequency is 1 in 2500 for females under
30 years old. Frequency increases with
mothers age to 1in 100 for females over
30.
Turner’s syndrome
• Nondisjunction of the sex chromosome
• Females have only one X chromosome
• Symptoms
• (1) Fail to develop sexually; usually sterile
• (2) Short height; thick, wide neck
• (3) Normal intelligence
• (4) Frequency is 1 in 5000
Klinefelter syndrome
• Nondisjunction during the production of
the sperm or egg
• Individual has XXy sex chromosomes
• Appears male at birth because of the y
chromosome
• Testes fail to develop and sterility results.
• The two X chromosomes trigger the
development of breasts and other female
characteristics
• Frequency is between 1 in 1000 and 1 in
2000 births