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.
A prokaryote cell is surrounded by a plasma membrane and a cell wall made of peptidoglycan. This E. coli cell is undergoing cell division in a process called binary fission. The DNA (red) has been duplicated and partitioned into the daughter cells in their nucleoid areas.
Surrounding the plasma membrane, bacteria possess a cell wall composed of varying amounts of peptidoglycan, a polymer made of amino acids and sugars.
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.
An eukaryotes cell such as an animal cell has several membrane-bound organelles. The genetic material DNA resides mainly in the nucleus, which is surrounded by a double membrane with numerous openings called nuclear pores. When the cell is not dividing, DNA is complexed with proteins in dispersed fibers called chromatin, which condense into visible chromosomes during cell division. The nucleolus is the region where ribosomal RNA (rRNA) is made and ribosomes are initially assembled. Other organelles reside in the cytoplasm. Surrounding the cell is a plasma membrane. In plant cells, a cell wall made of cellulose surrounds the membrane.
Colorized image of human mitotic chromosomes as seen with a scanning electron microscope. A complete complement of visualized chromosomes for an individual is called a karyotype.
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.
This is a human male karyotype, prepared from a diploid somatic cell in metaphase. All but the X and Y chromosomes are present in homologous pairs. Each chromosome is a double structure, constituting a pair of sister chromatids joined by a common centromere.
Humans (Homo sapiens have a haploid number of 23: each somatic cell contains 23 homologous pairs of chromosomes.
Somatic cells undergo a cell
: an actively dividing stage called
alternates
with an
.
In a typical somatic cell cycle, the period of active cell division is mitosis and the interval between cell divisions is interphase. After mitosis, the cells enters the first gap phase, G1, and may then become nondividing (G0) or continue to S (DNA synthesis), G2, and undergo mitosis again.
Under ideal conditions, an eukaryotic cell can complete a cell cycle in about 16 hours, of which about 1 hour is spent in mitosis.
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
.
From the G2 phase of interphase, where chromosomes are uncoiled and dispersed in a diffuse chromatin, cells enter prophase, the first stage of mitosis.
In prophase, the centrioles divide and move apart, the nuclear envelope breaks down, and the chromatin condense and become visible as chromosomes. The centrioles in animal cells organize microtubules into a series of spindle fibers. Sister chromatids are connected by a protein structure called the kinetochore at the centromere.
In
, the chromosomes move to the equatorial plane of the cell.
In prometaphase, the chromosomes move to the equatorial plane, or metaphase plate, of the cell.
In
, the chromosomes are aligned at the equatorial plane.
In metaphase, the chromosomes are aligned at the equatorial plane, bound to spindle fibers at kinetochores.
In
, sister chromatids separate from each other and migrate to opposite poles.
In anaphase, sister chromatids separate from each other (disjoin) and migrate to opposite poles. Each centromere divides, resulting in two kinetochores. Each chromatid is attached to one kinetochore, and is pulled to a pole by shortening the spindle fibers to which the chromatid is attached. The separated sister chromatids are called daughter chromosomes.
Centromere locations and designations of chromosomes based on centromere location. Note that the shape of the chromosome during anaphase is determined by the position of the centromere.
In
, the separated daughter
complete migration to opposite poles of the cell, and
divides the cell in two.
In telophase, The separated daughter chromosomes complete migration to opposite poles of the cell and the chromosomes uncoil to their interphase structure. The nuclear envelope reforms to complete karyokinesis. Cytokinesis begins, dividing the cytoplasm and produce two daughter cells.
* In animal cells, the plasma membrane forms a cleavage furrow that pinches the cell in two.
In plant cells, a cell plate grows at the equator, then a new cell wall forms between the daughter cells.
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.
DNA synthesis occurs during interphase before the beginning of meiosis I but does not occur again before meiosis II. In prophase I, homologous chromosomes pair up into tetrads in a process called synapsis, and crossing over occurs, where genetic information is exchanged between nonsister chromatids of the homologues. Crossing over produces recombinant chromosomes and contributes to genetic variation in sexual reproduction. metaphase I
In metaphase I, the paired tetrads line up on the metaphase plate, the sister chromatids of each chromosome held together by a single centromere, which does not divide. The terminal chiasmata reach the ends of the chromatids and separate. The alignment of maternal and paternal chromosomes with respect to the poles is random. anaphase I
Meiosis II
is an
division and each cell from Meiosis I produces two more
cells each with one
.
Meiosis II is similar to mitosis, except the cell undergoing division is haploid rather than diploid. In prophase II, each dyad has two chromatids attached to a common centromere. In metaphase II, chromosomes move to the metaphase plate, and centromeres start to divide. In anaphase II, the divided centromeres pull sister chromatids to opposite poles (disjunction). After telophase II and cytokinesis II, cell division is complete, producing monads. Each haploid daughter cell is a potential gamete and has one member of each pair of homologous chromosomes. Note that if disjunction fails (nondisjunction) in either meiosis I or
Meiosis
review and
comparison
with
mitosis.
Comparison of mitosis and meiosis. Note that meiosis has a reduction division followed by an equational division. As a result of the two divisions, the tetrads are reduced to monads.
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
.
Spermatogenesis in animals occurs in testes and starts when a diploid germ cell called spermatogonium grows and differentiates into a primary spermatocyte. Each primary spermatocyte undergoes meiosis I, producing two haploid secondary spermatocytes. continue
Spermatogenesis (continued) The secondary spermatocytes undergo meiosis II. The final products of Spermatogenesis from one primary spermatocyte are four equal-sized spermatids, which then differentiate into motile spermatozoa, or sperm cells.
Female
gametes are produced by
in the
.
Oogenesis in humans occurs in ovaries and starts when diploid germ cells called oogonia grow and mature into primary oocytes enclosed in small follicles. The primary oocytes start meiosis I, but stop at prophase I. Beginning at puberty, periodic hormone secretions induce a few primary oocytes to complete meiosis I, each producing a non-functional polar body and a haploid secondary oocyte where most of the cytoplasm from the primary oocyte is concentrated. continue
Oogenesis (continued) One of the secondary oocytes completes growth in the ovary and begins meiosis II, which is arrested at metaphase II. The mature follicle ruptures and releases the oocyte from the ovary in a process called ovulation. Completion of meiosis II does not occur until the oocyte is penetrated by a sperm, producing a second polar body and the ovum.
In plants, sexual reproduction alternates between a multicellular diploid
generation with a multicellular haploid
generation.
Life cycle of an angiosperm (flowering plant). A plant alternates between a multicellular diploid (2n) sporophyte and a multicellular haploid (n) gametophyte generation.
A mature plant is a multicellular diploid sporophyte with reproductive structures.
Anthers contain microsporangia in which germ cells divide by meiosis to produce microspores.
Ovaries contain megasporangia in which germ cells divide by meiosis to yield 4 megaspores each.
Microspores divide by mitosis to form multicellular male gametophytes (pollen grains), which contain sperm cells.
One of the 4 megaspores divides by mitosis to form a multicellular female gametophyte (embryo sac), which contains an egg cell in an ovule.
Fertilization (pollination) occurs when a sperm fuses with an egg, producing a diploid single-celled zygote.
The zygote develops by mitosis to produce the sporophyte.