Bio3400
Chapter 2
Mitosis and Meiosis
In
prokaryotes
such as
Escherichia coli
, the genetic material
is organized as a long circular molecule located in a
area.
In
eukaryotes,
DNA is transmitted from one generation of cells to the next by mitosis, which produces diploid
cells, and meiosis, which produces haploid
or spores.
Fusion of two gametes at fertilization results in a diploid
.
In somatic cells, chromosomes exist in
pairs.
Such cells are
(2n) and contain two copies, or
, of each
gene.
Somatic cells undergo a cell
: an actively dividing stage called
alternates
with an
.
In
the cell may become nondividing or engage in duplication of
, organelles, and other cell components in preparation for the next division.
Mitosis consists of
division (karyokinesis), which can be broken into five discrete stages, followed by
division (cytokinesis).
In
, the centrioles divide and move apart, the nuclear envelope breaks down, and the chromatin condense and become visible
.
In
, the chromosomes move to the equatorial plane of the cell.
In
, the chromosomes are aligned at the equatorial plane.
In
, sister chromatids separate from each other and migrate to opposite poles.
In
, the separated daughter
complete migration to opposite poles of the cell, and
divides the cell in two.
Mitosis produces two diploid daughter cells that are genetically identical.
Mitosis
review.
Meiosis produces haploid
and requires two rounds of cell division.
Meiosis I
is the chromosome
division and produces two
cells with two
still attached to the chromosomes.
Meiosis II
is an
division and each cell from Meiosis I produces two more
cells each with one
.
Meiosis
review and
comparison
with
mitosis.
Sexual reproduction produces genetic
by several
mechanisms.
In
independent
, each pair of chromosomes sorts its
into daughter cells independently, giving 2
23
(about
million) combinations in humans.
The random
fusion
of gametes during
produces a zygote with 2
23
x 2
23
, or over
trillion, combinations of chromosomes.
Crossing over produces
chromosomes that carry genes derived from different parents, increasing genetic variation even more.
In animals, sexual reproduction involves the
of haploid
to produce diploid reproductive individuals.
Male
gametes are produced by
in the
.
Female
gametes are produced by
in the
.
In plants, sexual reproduction alternates between a multicellular diploid
generation with a multicellular haploid
generation.