Page 51 - Demo 1

MITOSIS vs. MEIOSIS

Mitosis occurs more commonly than meiosis because it happens in all
ssues during embryonic development and during growth and repair. The main
purpose of mitosis is to keep the number of chromosomes constant in all of
the cells of the body, so that every cell contains idencal genec material.

Meiosis, on the other hand, happens only at certain mes in the life cycle
of organisms that sexually reproduce, and only in specialized ssues. In humans,
meiosis is responsible for the producon of gametes that occur only in the testes
and ovaries. The purposes of meiosis are to provide variaon in gametes and to
keep the number of chromosomes constant generaon aer generaon. Aer
ferlizaon, the full number of chromosomes is restored.

Unlike mitosis, meiosis requires two consecuve nuclear divisions. It
results in four daughter cells because of these two successive divisions, called
meiosis I and meiosis II. The same four stages of mitosis – prophase,
metaphase, anaphase, and telophase – happen during meiosis I and meiosis II.
They differ, however, from each other and from mitosis in the manner that the
chromosomes align during metaphase.

The First Division – Meiosis I: Before the start of meiosis I, each
chromosome has already duplicated and is now composed of two sister
chromads. During prophase I, the nuclear envelope disintegrates and the
nucleolus disappears as the spindle appears. As the chromosomes condense,
homologous chromosomes or homologues that share the same size, shape,
and locaon of the centromere, of each pair come together and line up side by
side in an event known as synapsis. Synapsis produces a tetrad, an associaon
of four chromads (two homologous chromosomes with two chromads each).
When a tetrad forms, the non-sister chromads may exchange genec material
in an event called crossing-over. Synapsis and crossing-over do not happen in
mitosis.

During metaphase I, pairs of homologous chromosomes or tetrads line
up at the metaphase plate, also known as spindle equator. The tetrads then
separate during anaphase I. Following the reformaon of the nuclear envelopes
during telophase I and cytokinesis, the daughter nuclei are haploid (n), such
that each daughter cell contains only one chromosome from each homologous
pair. The chromosomes are now dyads, consisng of two sister chromads. No
replicaon of DNA takes place between meiosis I and meiosis II, a period
referred to as interkinesis.

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