Understanding Meiosis: The Key to Genetic Diversity

What is Meiosis? Have you ever wondered why you are different from your siblings?

The answer lies within the amazing process of meiosis. 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

than the parent cell; they are haploid. Meiosis takes place in germ or sex cells called gametes;

eggs in females and sperm in males. Similar to mitosis, cells also pass through the Interphase,

G1, S and G2 phase before they enter Meiosis. Here are the distinct phases of meiosis:

INTERPHASE MEIOSIS 1 CYTOKINESIS 1

MEIOSIS 2 CYTOKINESIS 2 First at interphase.

In interphase, DNA is copied, resulting in two identical full sets of chromosomes to prepare for division.

Outside of the nucleus are two centrosomes, each containing a pair of centrioles. These structures are critical

for the process of cell division in next step at meiosis 1. Meiosis 1 takes place in following steps:

Prophase 1 Prophase 1 is typically the longest phase of meiosis. During prophase 1,

homologous chromosomes pair and exchange DNA (homologous recombination). The new combinations of DNA

created during crossover are a significant source of genetic variation, and result in new combinations of alleles,

which may be beneficial. The paired and replicated chromosomes are called bivalents or tetrads,

which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes

is called synapsis. At this stage, non-sister chromatids

may cross-over at points called chiasmata (plural), (singular: chiasma). Prophase 1 has historically been divided

into a series of substages which are named according to the appearance of chromosomes. Leptotene

In this stage of prophase 1, individual chromosomes, each consisting of two sister chromatids,

become "individualized" to form visible strands within the nucleus. Leptotene is of very short duration

and it's when their progressive condensation and coiling of chromosome fibers takes place. Zygotene

Chromosomes approximately line up with each other into homologous chromosome pairs through synaptonemal complex.

The paired chromosomes are called bivalent or tetrad chromosomes. Pachytene

This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Non-sister chromatids

of homologous chromosomes may exchange segments over regions of homology.

At the sites where exchange happens, chiasmata formed. Diplotene

The synaptonemal complex degrades and homologous chromosomes separate from one another a little.

However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata,

the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are at the transition to anaphase 1.

Diakinesis It closely resembles prometaphase of mitosis; the nucleoli disappear,

the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form. Synchronous Process

During these stages, two centrosomes, containing a pair of centrioles in animal cells,

migrate to the two poles of the cell. The microtubules invade the nuclear region after the nuclear envelope disintegrates,

attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome

along the attached microtubule. Microtubules that attach to the kinetochores are known as kinetochore microtubules.

Other microtubules will interact with microtubules from the opposite centrosome. These are called nonkinetochore microtubules

or polar microtubules. Metaphase 1 Homologous pairs move together

along the metaphase plate: the paired homologous chromosomes align along an equatorial plane

that bisects the spindle. Anaphase 1 Kinetochore microtubules shorten,

pulling homologous chromosomes, which consist of a pair of sister chromatids, to opposite poles.

Non kinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates

in preparation for division down the center. Telophase 1 The first meiotic division effectively ends

when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes

but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear,

and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin.

Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs,

completing the creation of two daughter cells. Sister chromatids remain attached during telophase 1.

Cells may enter a period of rest known as interkinesis or interphase 2. No DNA replication occurs

during this stage. Meiosis 2 Meiosis 2 is the second meiotic division

and usually involves equational segregation, or separation of sister chromatids. Mechanically,

the process is similar to mitosis, though its genetic results are fundamentally different.

The end result of meiosis 2 is production of four haploid cells, n chromosomes; (23 in humans).

The four main steps of meiosis 2 are: prophase 2, metaphase 2,

anaphase 2, and telophase 2. In prophase 2,

we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening

and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers

for the second meiotic division. In metaphase 2, the centromeres contain two kinetochores

that attach to spindle fibers from the centrosomes at opposite poles. This is followed by anaphase 2,

in which the remaining centromeric cohesion is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention

are now called sister chromosomes as they move toward opposing poles. The process ends with telophase 2,

which is similar to telophase 1, and is marked by the disassembly of the spindle and decondensation

and lengthening of the chromosomes. Nuclear envelopes reform and cleavage or cell plate formation

eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete

and ends up with four new daughter cells.