Meiosis and Sexual
Meiosis - A reduction division required to reduce the
number of chromosomes to half so sexual reproduction can occur.
Genes code for proteins, including enzymes
Genes are located on chromosomes,
which are single long strands of DNA.
A gene's specific location on a chromosome
is called its locus (pl: loci).
Reproduction can be asexual
Asexual - by mitosis - resultant organisms are clones,
genetically identical to the parent and to other offspring.
- Includes many single-celled organisms,
as well as some invertebrates (Fig
13.1, and movie of hydra budding) and many plants. Aphids,
water fleas, and rotifers reproduce by parthenogenesis,
a process in which the eggs develop without fertilization and
the offspring are haploid. If these haploid offspring
reproduce, their eggs are formed by mitosis, not meiosis. Many
plants reproduce by fragmentation or runners. This
is useful in agriculture.
The sexual life cycle
- Involves genetic contribution from
both parents. Provides much greater genetic variation within
a species. There are many variations of sexual life cycles (Fig
13.5), but we will only concern ourselves
with the human life cycle.
The human life cycle (Fig
Somatic cells - all
the cells of the body except the germ or stem cells which
are those that produce gametes (sperm and eggs). Somatic
cells all contain paired chromosomes. (Diploid or 2n)
Germ or stem cells are diploid
and will undergo meiosis to produce four haploid (1n)
Humans have 23 pairs, or 46, chromosomes
that vary by size, shape, and appearance. 22 pairs are called
autosomes. The other pair are the sex chromosomes.
These are the X and Y chromosomes. Other species have other arrangements
for sex determination.
Arranging these chromosomes, by pairs,
according to size and shape, results in a display called a karyotype.
13.3). Different species show different
karyotypes, varying in shape, size and number of chromosomes.
In humans, the gametes will have 22 autosomes
and one sex chromosome. At fertilization (syngamy), two
gametes will combine to cause the resultant zygote (fertilized
egg) to have 44 autosomes (22 pairs) and 2 sex chromosomes (1
pair). The zygote is diploid, as were the parents.
Interphase - (Movie)
- chromosomes and centrosomes duplicate as in mitosis.
The Stages of Meiosis -
Overview - Fig
Meiosis I - serves to
divide the two versions of each chromosome (fig
13.7, left page).
Prophase I (Movie)
- Chromosomes with sister chromatids
form and homologous chromosomes pair, forming tetrads.
Unlike mitosis, synapsis occurs and the chromatids of
homologues cross at chiasmata.
- Crossing over occurs
when these pieces of homologous chromosomes (fig.
13.10) (nonsister chromatids) exchange
places to promote new genetic combinations in the offspring.
- The rest of prohase I is similar to
- - centrosomes
move apart and spindle forms
- - nuclear envelope
and nucleoli disperse
- - spindle attaches
to kinetochores and begins moving chromosomes
- Chromosomes now arranged on the metaphase
plate. Independent assortment - the alignment of homologous
pairs along the center of the cell is random, with different
combinations of parental chromosomes possible for each daughter
- Opposite homologues are attached to
microtubules attached to opposite centromeres.
Anaphase I (Movie)
- Opposite homologous chromosomes, consisting
of paired sister chromatids, are pulled toward opposite ends.
Telophase I (Movie of telophase I and cytokinesis)
- Each pole now has a haploid chromosome
set, but each chromosome consists of sister chromatids.
- Division of cell itself. Often
occurs at same time as telophase I.
Meiosis II - separates
the two sister chromatids to form four haploid daughter cells
or gametes, each with a different genetic make-up. fig
13.7, right page
- Spindle forms, microtubules attach to chromosomes and pull
them toward the meaphase II plate
- Chromosomes line up on metaphase II plate
- Microtubules are attached to kinetochores
- Centromeres of sister chromatids separate and the sister
chromatids move toward opposite poles.
- They are now consided chromosomes.
- Nuclei reform and cytokinesis occurs.
- There are now four daughter cells, each haploid, and different
genetically from each other.
- May be sperm or ova (image).
II and cytokinesis movie.
A comparison of mitosis and meiosis. Image
and summary Fig.13.8b
Origins of genetic variation
in sexually reproducing populations.
- Sexual reproduction has the capacity to generate new genetic combinations.
Independent Assortment - The random alignment of homologues
during meiosis I results in an astounding number of possible
kinds of gametes that can be produced.
Crossing over allows for combinations of genes that may
never have existed previously.
Random Fertilization - Since the new zygote is the product
of two gametes, each with new variations within them, fertilization
adds even more genetic diversity.