How Reproductive Cells are
Produced
What is the Function of Meiosis?
• Meiosis is a special type of cell division that
occurs only in reproductive organs.
• Meiosis reproductive cells are called gametes.
• The gametes, either egg or sperm, are haploid
(n), which means they contain only one copy
of each chromosome that the diploid (2n)
parent cell contains.
• Reproduction involves the union of 2 cells to
form a zygote.
• The zygote contains chromosomes from both
parents, but it does not contain double the
number of chromosomes found in a normal
somatic cell.
• The first part of meiosis reduces the number
of chromosomes from diploid to haploid.
– This is referred to as reduction division.
• Each sperm or egg cell contains 22 autosomes
and one sex chromosome (X in eggs and X or
Y in sperm)
Phases of Meiosis
• Meiosis has 2 sequences of phases: Meiosis I
and Meiosis II.
Interphase
• The chromosomes replicate before Meiosis I
begins.
• After replication, each chromosome is made
up of a pair of identical sister chromatids that
are joined by a centromere.
Meiosis I
Prophase I
• Similar chromosomes, called homologous
chromosomes, pair to form homologous pairs.
Homologous chromosomes are made up of the
same genes, but are not identical.
• The homologous pair is made up of 4 chromatids
is called a tetrad.
Meiosis I
• Although homologous chromosomes contain the
same genes, they may have different forms of
these genes, which are the alleles.
• During the pairing process, crossing over of
chromatids can occur, in which non-sister
chromatids exchange genes.
• This allows for the recombination of genes in
each chromosome contributes to genetic
variation.
• Crossing over allows individual chromosomes to
contain some genes from the individual’s
mother’s side, and some from the father’s side.
Meiosis I
Metaphase I
• A spindle fibre attaches to the centromere of
each chromosome, and is connected to the
centrioles at the opposite ends of the cell.
• The chromosomes line up in homologous pairs
along the equator, on either side (instead of
single file as in mitosis).
• Chromosomes that come from one parent are
not all positioned on the same side of the
equator. They are positioned randomly –
independent assortment.
Meiosis I
Anaphase I
• The homologous chromosomes separate and
move to opposite ends of the cell. They are
pulled apart by the shortening of the spindle
fibres.
• The centromere does not split, as it does in
mitosis. Sister chromatids are held together.
Meiosis I
Telophase I
• The cytoplasm is divided, the nuclear
membrane forms around each group of
homologous chromosomes, and 2 cells are
formed.
• Each of these new cells contains one copy of
each chromosome. Since each chromosome
already consists of 2 chromoatids, a 2nd
replication does not take place between
telophase I and prophase II.
Meiosis II
• Phases are identical to mitosis.
• The two cells from telophase I go through
prophase II, metaphase II, anaphase II, and
telophase II.
• Each cell beginning meiosis II is haploid, but
consists of replicated chromosomes (each
consisting of 2 chromatids).
Meiosis II
• At the end of meiosis II, the daughter cells are
still haploid, but each cell contains single
unreplicated chromosomes (no longer made
up of two chromatids attached together).
• The daughter cells at the end of meiosis II are
called gametes in animals, and either gametes
or spores in plants.
Gamete Formation
• Gametogenesis – the formation of gametes;
end result of meiosis.
• Spermatogenesis – production of male
gametes
• Oogenesis – production of female gametes
Spermatogenesis
• Meiosis in mature males takes place in the
testes – male reproductive organs.
• Spermatogonium – starting diploid cell of
spermatogenesis; enlarges and undergoes
meiosis I and II
• The final product is 4 haploid sperm cells.
Each sperm cell has the same number of
chromosomes and the same amount of
cytoplasm.
• Following meiosis II, the sperm cells develop
into mature sperm. Each cell loses cytoplasm
and the nucleus forms into the head. A long
tail-like flagellum is developed for movement.
Oogenesis
• In females, meiosis takes place in the ovaries
– female reproductive organs.
• Oogonium – diploid cell; enlarges and
undergoes meiosis I and meiosis II
• At the end of meiosis I, the cytoplasm is not
equally divided between the 2 daughter cells.
• The cell that receives most of the cytoplasm is
called the primary oocyte. The other cell is
the polar body and not a viable sex cell.
• As the primary oocyte undergoes meiosis II,
the cytoplasm is again not divided equally.
• Only one cell becomes an egg and contains
most of the cytoplasm. The other cell is
another polar body.
• The purpose of these unequal divisions of the
cytoplasm is to provide the ovum with
sufficient nutrients to support the developing
zygote in the first few days following
fertilization.
Meiosis and Genetic Variation
• Variation is dependent upon 2 processes in
meiosis:
1. The combination of genes created by
crossing over in prophase I (usually 2 or 3
crossovers occur per chromosome)
1. How each pair of homologous chromosomes
line up during metaphase I.
• Both crossing over and random segregation
work to shuffle the chromosomes and the
genes they carry. This reassortment is called
genetic recombination.
• Variation among individuals in a species is
important because certain combinations of
genes can help an organism survive better
than other combinations of genes can.
Errors in Meiosis
• Changes in the structure of a chromosome,
also called mutations, can be passed from one
generation to the next when that gamete
combines with another gamete to form a
zygote.
Errors in Meiosis
• The failure of chromosomes to separate
properly is called nondisjunction and can
occur during meiosis.
• This error results in the addition or deletion of
one or more chromosomes from a gamete.
Errors in Meiosis
Errors in Meiosis
• Down Syndrome – trisomy of chromosome #21
– Round, full face; enlarged and creased tongue; short
height; large forehead
• Turner Syndrome – a female with a single X
chromosome
– Appear female, but don’t develop sexually; short
height; thick necks
• Kleinfelter Syndrome – 2 X chromosome and 1 Y
chromosome
– Appears to be male at birth, but begins producing
high levels of female hormone as he enters sexual
maturity; sterile