Meiosis – Advanced

How do you make a cell with half the DNA?

Meiosis, this process creates cells with half the DNA, allowing two cells to come back together, enabling the creation of
new organisms with complete chromosome numbers and ensuring genetic variation.

Meiosis
 Fertilization joins haploid gametes into diploid zygote.
 Meiosis is a cell division that produces haploid cells, that halves the number of chromosomes.
 It is specific to gamete producing cells in the gonads.
 It begins with a diploid cell and ends with four haploid cells. These cells differentiate into mature sperm or egg
cells.
 Only germ cells like spermatocytes and oocytes can undergo meiosis.
 Meiosis involves two nuclear and cell divisions without an interphase.
 Each division, meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase, and telophase,
followed by cytokinesis.

During meiosis, homologous chromosomes

separate and go to different daughter cells
to produce gametes with one chromosome
from each pair. This diagram shows just the
nuclei of the cells.

 A human cell prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex
chromosomes.
 At the end of meiosis, each haploid cell will have 22 autosomes (not pairs) and 1 sex chromosome, either an X or a
Y chromosome. One chromosome of each pair was inherited from mother and the other from father.

The Eight Phases

There are eight stages of meiosis, divided into meiosis I and meiosis II, in which cells go through the same four phases as
mitosis - prophase, metaphase, anaphase and telophase. However, there are important differences between meiosis I
and mitosis.

Meiosis I

- is referred to as a reductional division that separates homologous chromosomes, producing in two haploid
cells. The starting diploid cell has 46 chromosomes (23 pairs of homologous chromosomes), each with two
sister chromatids attached at the centromere. After cytokinesis, two haploid cells form.
o Prophase I
- is the longest phase of meiosis. It involves breaking down the nuclear envelope, condensing chromatin into
chromosomes, and centriole migration. Homologous chromosomes form pairs, allowing gene-for-gene
crossing-over, this process is also called as homologous recombination, which is crucial for genetic variation.

Prophase I can be divided into 5 distinct stages: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

1. Leptotene
- is the first prophase I stage, where chromatin condenses into visible chromosomes with two sister chromatids
and homologues, allowing synaptonemal complex begins to assemble.
2. Zygotene
- this involves chromosomes pairing with their homologue, forming homologous chromosome pairs called
synapsis. These pairs are highly specific and exact, with genes from the two chromosomes aligning along
the chromosome's length. These paired chromosomes are called bivalent or tetrad chromosomes.
3. Pachytene
- the stage when chromosomal crossover occurs, where non-sister chromatids of homologous chromosomes
exchange segments over homology regions, forming chiasmata and resulting in genetic recombination,
more common in autosomes than sex chromosomes.
4. Diplotene
- is when the synaptonemal complex degrades and homologous chromosomes disassociate slightly, while in
human fetal oogenesis, all developing oocytes develop to this stage and stop. This suspended state of the
 oocytes is referred to as the dictyotene stage and the eggs remains in this stage until released following
puberty.

5. Diakinesis
- chromosomes further condense. The chiasmata remain intact during this stage. The remainder of this stage is
similar to prometaphase of mitosis: the nucleoli disappear, the nuclear membrane disintegrates, and the
meiotic spindle begins to form.

o Metaphase I
- In metaphase I, 23 homologous chromosome pairs line up along the equator. During mitosis, 46 individual
chromosomes line up. In meiosis I, 23 pairs line up, moved by spindle fibers attached to centromeres. Gregor
Mendel suggested independent assortment of chromosomes.

o Anaphase I
- During anaphase I, spindle fibers shorten, separating homologous chromosome pairs. This occurs as
chiasmata are severed, pulling chromosomes apart. This results in 23 chromosomes at one pole and 23 at
the other, with sister chromatids attached at the centromere.
o Telophase I
- Telophase I, followed by cytokinesis, ends the first division of meiosis, resulting in two haploid cells with
unique chromosome combinations. The spindle fiber disassembles, and the nucleus reforms. Cells may enter
interkinesis or meiosis II, without DNA replication between them.

Meiosis II
- Meiosis II involves chromosome division, separating sister chromatids. Four haploid cells with 23
chromosomes result after cytokinesis, with each chromosome containing DNA equivalent to a mitotic cell.

o Prophase II
- the nucleolus disappears and the nucleus breaks down. The chromatin condenses into chromosomes. The
spindle begins to reform as the centrioles move to opposite sides of the cell.

o Metaphase II
- Metaphase II involves spindle fibers aligning 23 chromosomes along the cell's equator, rotating the new
metaphase plate 90 degrees perpendicular to meiosis I.

o Anaphase II
- a stage in cell division where chromatids are separated, similar to mitosis. The centromeres are cleaved,
resulting in sister chromatids moving to opposite cell poles. This process creates 23 chromosomes, each with
half the genetic material at the beginning.

o Telophase II
- Telophase II and cytokinesis end meiosis, causing the nucleus to reform and spindle fibers to break down.
Chromosomes uncoil into chromatin, and each cell undergoes cytokinesis, producing four haploid cells.

Significance of Mitosis
Cancer and Cell Cycle

Cancer is a disease that occurs when the cell cycle is no longer regulated, often due to DNA damage from
exposure to radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. They may form a
mass of abnormal cells called a tumor, that consume necessary nutrients and space, causing tissue and organ damage
and ultimately leading to death.
 These cells are cancer cells,
growing out of control and
forming a tumor.

Significance of Meiosis
It maintains constancy of chromosome number within a species, increases genetic variability and helps in the
process of evolution.