Meiosis and Sex Cells: Sexual Reproduction:
In higher organisms, plants and animals, each individual is diploid. A diploid organism has a
complete set of chromosomes in every cell and is 2n (diploid means 'double set'). The organism
gets one set from the mother and the other set from the father. The two partners produce gametes,
which are joined to produce an offspring. However, two problems must be solved with sexual
reproduction.
1) If fertilization occurs and the gametes join, why isn't the genetic material doubled?
2) How is it possible for each parent to give half of the genetic material?
The answer is meiosis, a process in which a diploid or double set of chromosomes is reduced to a
haploid (n), or a single, set of chromosomes. It is a process that guarantees that the number of
chromosomes remains stable from generation to generation. In humans the diploid number = 46
(2n = 46), the haploid number = 23 (n = 23); in fruit flies: 2n = 8, n = 4. Multicellular organisms
usually have two types of cells: 1) somatic and 2) gametes. Somatic cells are any type of cell that is
not a sperm or egg. These cells are diploid (2n). Gametes are cells that are sperm or eggs.
Gametes are haploid (n).
Homologous Chromosomes:
In humans there are 46 chromosomes. Each chromosome consists of a double helix molecule of
DNA. The DNA is folded with proteins to make up a chromosome. One chromosome represents
hundreds or thousands of genes, and each gene is a specific region of the DNA molecule. A gene's
specific location on the chromosome is called the its LOCUS. The 46 chromosomes are actually 23
pairs of chromosomes. The members of each pair are called homologous chromosomes
(homologues). The two homologues are functionally equivalent and contain the same kinds of
genes arranged in the same order.
One set of chromosomes that does not occur as homologue occurs in males; the X chromosome and
the Y chromosome are not homologues but pair up in meiosis. In females, there are two X
chromosomes that are homologues. These chromosomes are the sex chromosomes and the other 22
pairs of chromosomes are called AUTOSOMES.
During meiosis, three things happen to the homologues:
1) The homologues pair up.
2) The homologues exchange genetic information. This is called crossing over.
3) The newly scrambled chromosomes separate and go into different daughter cells in
such a way that each daughter cell
contains only one of each pair of homologues. These
cells are called gametes or sex cells.
Meiosis and life cycles:
Meiosis occurs at different times during the life cycle of different organisms. In protists and fungi,
meiosis occurs right after the fusion of the two mating cells. The mating cells are usually haploid
and the fusion produces a diploid cell. Immediate meiosis restores the haploid lifestyle.
In all plants, a multicellular haploid phase alternates with a multicellular diploid phase. The typical
fern is diploid and is called a SPOROPHYTE. The diploid sporophyte produces haploid spores
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through meiosis. These spores will grow into small haploid plants called a GAMETOPHYTE.
These produce male and female sex cells (gametes) via mitosis. The gametes will join to form a
diploid cell that will grow into the fern that you see. This alternation between diploid and haploid
is called ALTERNATION OF GENERATIONS.
Animals, including humans, are diploid organisms that produce haploid gametes. Two haploid
gametes will join to produce a diploid zygote. Most of the lifecycle in animals is in the diploid
state.
Mitosis vs. Meiosis:
1) Mitosis: occurs in haploid, diploid and polyploid cells.
Meiosis: occurs only in diploid and polyploid cells.
2) Meiosis: The nucleus divides twice producing four nuclei. The chromosomes replicate only
once, so each nucleus contains half of the number of chromosomes.
3) Each haploid chromosome is a new combination of old chromosomes because of crossing over.
Meiosis: There are two stages of Meiosis: Meiosis I and Meiosis II.
Meiosis I: Replication of chromosomes, crossing over of the chromosomes and reduction in the
chromosome number from diploid to haploid.
Meiosis I is often called the reduction division.
Premeiotic Interphase: G1, S (replication of the chromosomes), and G2.
Meiotic Prophase I: The first stage.
This is long and complex compared with mitotic prophase. In it:
1) Nuclear membrane disappears.
2) Spindle fibers form.
3) The chromosomes condense.
4) The homologous chromosomes pair up by touching each other in the appropriate places. First
there is a lot of random movement of chromosomes until the homologous chromosomes find each
other. It is important, for example, that chromosome #13 finds homologous chromosome #13.
When the two homologous touch each other in the same place, a specialized structure called the
Synaptonemal Complex holds the homologues together.
The meiotic cell of a human now has 23 genetic entities called tetrads, each packet containing four
chromatids and two centromeres. This is the point when crossing over occurs. A special enzyme
causes the chromatids to unwind, revealing the strands of DNA. A complex series of events
happen and the genetic material is exchanged between homologues.
Crossing over may occur at the INTRONS.
Several thousand base pairs of one strand pairs with the chromatid on another homologues. There
are breakages and the chromatids untangle themselves. Meanwhile other enzymes are repairing the
breaks in the DNA. This process makes new chromatids and is a source of genetic variation within
a population.
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After crossing over, the homologs begin to pull away from each other, except at the crossing over
points called the CHIASMATA (CHIASMA- singular).
Metaphase I:
In the first metaphase, the tetrads are brought to the metaphase plate. The synaptonemal complex is
lined up on the metaphase plate.
Anaphase I:
There is no separation of the centromeres, but the synaptonemal complex separates. This means
that the homologues separate and move to opposite poles. The first meiotic division reduces the
chromosome number by half.
Telophase I:
In this phase, the nucleus reorganizes and the nuclear membrane reforms. The chromosomes
decondense.
Cytokinesis I:
In this phase, the cytoplasmic division occurs.
Meiosis II: Division of the chromosomes, analogous to mitosis.
Meiotic Interphase: This involves G1 and G2 phases only. There is no S phase in this interphase.
This phase may be brief or last a long time.
Prophase II:
As in mitotic prophase, there are two sister chromatids attached to a centromere. The chromosomes
condense, the nucleus disappears, and the spindle apparatus forms.
Metaphase II:
Centromeres move to the metaphase plate during metaphase II.
Anaphase II:
During anaphase II, the kinetochores are broken down by enzymes, and sister chromatids separate
and move to the opposite poles.
Telophase II:
During telophase II, the nuclear membrane reforms and chromosomes decondense.
Cytokinesis II: the cytoplasm divides.
Summary of Meiosis:
From one pair of homologs, there are four, unique chromatids from prophase I, if crossing over has
occurred. Each unique chromatid ends up in one of the four cells that are the products of meiosis.
The amount of genetic material was reduced by one half in Meiosis I and divided in Meiosis II.
Each resulting cell (gamete) is haploid.
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Meiosis in Humans:
Meiosis in Males:
In the male each of these haploid cells is called a spermatid. This spermatid will undergo cellular
differentiation to become gametes (sperm).
Meiosis in Females:
Meiosis is begun but is only partly completed in human females shortly before birth. All oocytes
(oogonia) begin meiosis I and become primary oocytes. They remain frozen (in stasis) in the last
stage of meiotic prophase I. In humans, meiotic prophase I can last up to 50 years.
In spite of not continuing to metaphase I, the paired meiotic chromosomes are very active making
large amounts of ribosomes and mRNA.
By the time the primary oocyte is ready to be released. It is a large cell filled with yolk, mRNA,
ribosomes etc. The oocyte will not resume meiosis until released from the ovary. After ovulation,
the primary oocyte finishes meiosis I and produces two cells, the secondary oocyte and the primary
polar body. The primary polar body is basically a chromosomal trashcan. It cannot be fertilized
and will die.
The secondary oocyte begins meiosis II, but stops after prophase II and waits for a sperm. Meiosis
will not be completed unless the secondary oocyte meets a sperm and is fertilized. When this
happens, many changes occur in the oocyte including the completion of meiosis II. The cell
constituents are not divided evenly and most of the cytoplasm ends up in one cell. Only one cell
will develop into the egg (ovum). The normal division occurs, but one of the two daughter cells
has most of the cytoplasm. The other daughter cell is very small and becomes the secondary polar
body. The secondary polar body will die and acts as a chromosomal trashcan. The other bigger cell
is known as the ovum. The ovum is formed after fertilization and will become the baby.
The importance of meiosis:
1) Sexual reproduction is a reshuffling of the genes of all the successful individuals of the
population. There are virtually infinite possibility combinations of genes. There are three sources
of variation due to meiosis. a) Independent Assortment of Chromosomes: In the diploid cell, you
start off with 4 chromatids per homologous chromosome. At the end of Meiosis II, each gamete
has 1 chromatid. Each chromatid moved independently of one another. Each chromatid is also
different.
b) Crossing over: This process produces individual chromosomes (chromatids) that are a
combination between the two grandparents (the parent’s parents think about it). C) Random
Fertilization: a new genetic combination of sperm and egg occurs when the sperm meets the egg.
2) The reduction and division of the chromosomes in the egg and sperm makes fertilization possible
and enables the maintenance of a constant chromosome number within a species.
Problems with Meiosis:
1) Unequal exchange of chromosomal material during crossing over. One chromosome will
end up with more DNA than its homologue. This can delete some genes from
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chromosomes and give extra genes to the other chromosome.
2) Inversion of the chromosome during crossing over. The piece of chromosomal material
will ‘flip’ during the event.
3) Translocation of the piece that’s crossing over. The piece that is cut will float away from
the homolog and attach to a non-homologous chromosome. One gamete will have extra
chromosomal material and the other gamete will be missing a piece of a chromosome.
4) Nondisjunction: When the chromosomes don’t divide properly during anaphase I or II. The
cells will not have the correct number of chromosomes. The spindle fibers might not attach
properly to the kinetochore (or synaptonemal complex), the spindle fibers might not
contract properly during anaphase, the APC (enzyme that breaks down the kinetochore)
might not work properly or the synaptonemal complex might not separate. If this happens
in Meiosis I, then all 4 cell products will have an incorrect number of chromosomes. If this
happens in Meiosis II, then 2 of the 4 cells will have an incorrect number of chromosomes.
If a whole set of chromosomes is affected, this is called polyploidy.
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