2. What is Meiosis?
• Meiosis is a process where a single cell divides twice to produce four
cells containing half the original amount of genetic information.
• These cells are our sex cells – sperm in males, eggs in females.
• During meiosis one cell divides twice to form four daughter cells.
• These four daughter cells only have half the number of chromosomes
of the parent cell – they are haploid.
3. • Meiosis can be divided into nine stages.
• These are divided between the:
First time the cell divides (meiosis I)
Second time it divides (meiosis II)
4. Meiosis I
• Meiosis is the process by which replicated chromosomes undergo two nuclear
divisions to produce four haploid cells, also called meiocytes (sperms and
eggs).
• Diploid (2n) organisms rely on meiosis to produce meiocytes, which have half
the ploidy of the parents, for sexual reproduction.
• Halving the ploidy in meiocytes is essential for restoring the genetic content of
the zygote to that of the parents.
• Meiosis uses similar mechanisms as those employed during mitosis to
accomplish the separation and redistribution of chromosomes.
• However, several features, namely, the pairing and genetic recombination
between homologous chromosomes, are unique to meiosis.
5. 1. Interphase:
• The DNA in the cell is copied resulting in two identical full sets of
chromosomes.
• Outside of the nucleus are two centrosomes, each containing a pair of
centrioles, these structures are critical for the process of cell division.
• During interphase, microtubules extend from these centrosomes.
6.
7. Role of Asters
• Asters are vital to the processes of mitosis and meiosis.
• They are a component of the spindle apparatus, which also includes
spindle fibers, motor proteins, and chromosomes.
• Asters help to organize and position the spindle apparatus during
cell division.
8. 2. Prophase I:
• The copied chromosomes condense into X-shaped structures that can
be easily seen under a microscope.
• Each chromosome is composed of two sister chromatids containing
identical genetic information.
• The chromosomes pair up so that both copies of chromosome 1 are
together, both copies of chromosome 2 are together, and so on.
• The pairs of chromosomes may then exchange bits of DNA in a process
called recombination or crossing over.
• At the end of Prophase I the membrane around the nucleus in the cell
dissolves away, releasing the chromosomes.
• The meiotic spindle, consisting of microtubules and other proteins,
extends across the cell between the centrioles.
9. 3. Metaphase I:
• The chromosome pairs line up next to each other along the centre (equator)
of the cell.
• The centrioles are now at opposites poles of the cell with the meiotic
spindles extending from them.
• The meiotic spindle fibres attach to one chromosome of each pair.
10. 4. Anaphase I:
• The pair of chromosomes are then pulled apart by the meiotic spindle,
which pulls one chromosome to one pole of the cell and the other
chromosome to the opposite pole.
• In meiosis I the sister chromatids stay together. This is different to what
happens in mitosis and meiosis II.
11. 5. Telophase I and cytokinesis:
• The chromosomes complete their move to the opposite poles of the
cell.
• At each pole of the cell a full set of chromosomes gather together.
• A membrane forms around each set of chromosomes to create two
new nuclei.
• The single cell then pinches in the middle to form two separate
daughter cells each containing a full set of chromosomes within a
nucleus. This process is known as cytokinesis.
12.
13. Meiosis II
• Cells move from meiosis I to meiosis II without copying their DNA.
• Meiosis II is a shorter and simpler process than meiosis I.
• Also known as “mitosis for haploid cells."
• The cells that enter meiosis II are the ones made in meiosis I.
• These cells are haploid—have just one chromosome from each
homologue pair—but their chromosomes still consist of two sister
chromatids.
• In meiosis II, the sister chromatids separate.
14.
15. • During prophase II, chromosomes condense and the nuclear
envelope breaks down, if needed. The centrosomes move apart, the
spindle forms between them, and the spindle microtubules begin to
capture chromosomes.
• The two sister chromatids of each chromosome are captured by
microtubules from opposite spindle poles. In metaphase II, the
chromosomes line up individually along the metaphase plate.
• In anaphase II, the sister chromatids separate and are pulled towards
opposite poles of the cell.
16. • In telophase II, nuclear membranes form around each set of
chromosomes, and the chromosomes decondense. Cytokinesis splits
the chromosome sets into new cells, forming the final products of
meiosis: four haploid cells in which each chromosome has just one
chromatid. In humans, the products of meiosis are sperm or egg
cells.
17. How meiosis "mixes and matches" genes
• The gametes produced in meiosis are all haploid, but they're not
genetically identical.
• Each gamete has a unique "sample" of the genetic material present in
the starting cell.
• As it turns out, there are many more potential gamete types than just
the four shown in the diagram, even for a cell with only four
chromosomes.
• In a human cell, the random orientation of homologue pairs alone
allows for over 8 million different types of possible gametes.
18. The two main reasons we can get many genetically different gametes
are:
• Crossing over. The points where homologues cross over and exchange
genetic material are chosen more or less at random, and they will be
different in each cell that goes through meiosis. If meiosis happens
many times, as in humans, crossovers will happen at many different
points.
• Random orientation of homologue pairs. The random orientation of
homologue pairs in metaphase I allows for the production of gametes
with many different assortments of homologous chromosomes.
