Chapter 15 Outline - Meiosis
I.
Concept 13.1
A. Offspring acquire genes from parents by inheriting chromosomes
1. Inheritance of Genes
a) Genes account for phenotypical similarities between family members.
b) Inherited information is passed on in the form of each gene’s specific sequence fo
DNA nucleotides.
c) Gametes are the vehicles that transmit genes from one generation to the next
(during fertilization a male and female gamete unite).
d) Human DNA is packaged into chromosomes. Every human has 46 in each
somatic cell—cells excluding gametes and their precursors.
e) A genes specific location along the chromosome is called the locus.
2. Comparison of Asexual and Sexual Reproduction
a) In asexual reproduction (mitosis), a single individual passes copies of all its genes
to its offspring (e.g. single-celled eukaryotic organism).
b) In sexual reproduction, two parents give rise to offspring that have unique
combinations of genes inherited from the two parents.
II. Concept 13.2
A. Fertilization and meiosis alternate in sexual life cycles
1. Sets of Chromosomes in Human Cells
a) A micrograph of the 46 human chromosomes are arranged in pairs (homologous
chromosomes), starting with the longest chromosomes is called a karyotype.
b) The exception to homologs (autosomes) are the X and Y chromosomes (sex
chromosomes) which are structurally different and engage in sex determination.
c) Any cell with two chromosome sets are called diploid (2n).
d) Even though chromosomes are duplicated, they are still diploid because a sister
chromatid is merely a copy of the genetic information, not a separate set.
e) Gametes contain a single set of chromosomes and are called haploid cells (n).
f) The chromosome number does no correlate with the complexity of a species’
genome.
2. Behavior of Chromosome Sets in the Human Life Cycle
a) Gametes develop from specialized cells called germ cells in the gonads—ovaries
in females and testes in males.
b) Meiosis reduces the number of chromosome sets from one to two to
counterbalance the doubling that occurs at fertilization.
3. The Variety of Sexual Life Cycles
a) Meiosis occurs in germ cells during the production of gametes, which undergo no
further cell division prior to fertilization (Germ
cell>Meiosis>Fertilization>Zygote>Mitosis).
b) Plants and some algae exhibit a type of life cycle known as alternation of
generations. During the multicellular diploid stage, known as the sporophyte,
spores are produced meiotically. Unlike a gamete, a haploid spore does not fuse
with another gamete, but instead, divides
mitotically and generates a multicellular haploid
stage known as the gametophyte. The gametophyte
gives rise to gametes.
c) In a third type of life cycle, that occurs in some
fungi and protists, meiosis occurs without a
multicellular diploid offspring developing. This
gives rise to an either haploid multicellular
organism or a unicellular organism.
d) Only diploid cells can under go meiosis.
III. Concept 13.3
A. Meiosis reduces the number of chromosome sets from diploid to haploid
1. The Stages of Meiosis
a) A pair of homologous chromosomes are duplicated and the
copies are sorted into four haploid daughter cells.
b) Stages of meiosis
(1) Prophase 1
(a) Beginnings of
centrosome movement,
spindle formation and
nuclear envelope
disintegration.
(b) Homologs are
aligned gene by gene
and undergo crossing
over.
(c) Chiasmata are “x”
shaped regions where
crossing over has
occurred.
(d) Microtubules, from one pole or the other, attach to the kinetochores, one
at the center of each homolog.
(2) Metaphase 1
(a) Pairs of homologous chromosomes are aligned at the equatorial plate.
(b) Chromatids of opposite homologs are attached to separate kinetochore
microtubules.
(3) Anaphase 1
(a) Breakdown of the proteins that are responsible for sister chromatid
cohesion.
(b) Homologs move towards opposite poles.
(c) Sister chromatid cohesion persists at the centromere.
(4) Telophase 1 and Cytokinesis
(a) Each half of the cell has a complete haploid set of duplicated
chromosomes.
(b) Chromatids may include regions of nonsister chromatid DNA.
(c) In animal cells a cleavage furrow forms; in plants a cell plate forms.
(d) No chromosome duplication occurs between meiosis 1 and 2.
(5) Prophase 2
(a) Spindle apparatus forms.
(6) Metaphase 2
(a) Chromosomes, consisting of 2 sister chromatids, held together by proteins
called cohesins, align at the equatorial plate.
(b) Due to crossing over in meiosis 1, the two sister chromatids are not
genetically similar.
(c) Kinetochores of sister chromatids are attached to opposite poles.
(7) Anaphase 2
(a) Chromatids separate and move towards opposite poles.
(8) Telophase 2 and Cytokinesis
(a) Chromosomes begin to decondense.
(b) Second meiotic division produces four daughter cells. Each with a haploid
set of chromosomes (n).
(c) The four daughter cells are all genetically different.
2. Crossing Over and Synapsis During Prophase 1
a) In early prophase. 1, two member of a homologous pair associate loosely along
their lengths. When aligned to the corresponding genes, the DNA of the nonsister
chromatids are broken by proteins.
b) A synaptonemal complex holds the homolog together.
c) During this association, called synapsis, DNA breaks are closed by joining the
corresponding paternal and maternal segments, know as chiasmata.
d) At leas tone crossover point is needed so that the homolog pair stays together
through metaphase 1.
3. A Comparison of Mitosis and Meiosis
a) Events Unique To Meiosis During Meiosis 1
(1) Synapsis and crossing over do not occur
in prophase 1 of mitosis.
(2) Alignment of homologous pair at the
metaphase 1 plate, as compare to
individual chromosomes in mitosis.
(3) Separation of homologs, were as in
anaphase of mitosis, sister chromatids are
separated.
b) Chromatids are separated in two phases in
meiosis. First, during anaphase, the chiasma,
consisting of cohesin proteins and points of
crossing over, is cleaved by proteins that
separate the nonsister chromatids responsible
for homolog association. Second, in anaphase
2, cohesin proteins between sister chromatids
are cleaved.
c) Therefore crossing over and sister chromatid
cohesion play a major role in metaphase
equatorial plate alignment.
IV. Concept 13.4
A. Genetic variation produced in sexual life cycles
contributes to evolution
1. Origins of Genetic Variation Among Offspring
a) Independent Assortment of Chromosomes
(1) Each homolog may orientate its maternal
or paternal chromosome pair closer
towards a given pole, creating a 50% that
a daughter cell will either get a maternal chromosome or a paternal one.
(2) This results in a total of four combinations from a single diploid parent cell (if
n = 2, then, 2^2 = 4 combinations).
(3) In humans the resulting combinations adds up to about 84 million (if n =23.
then, 2^23 = 8,338,608 combinations).
b) Crossing Over
(1) Crossing over produces recombinant chromosomes that carry genetic material
from paternal and maternal origins.
(2) In humans, on average, one to three cross over events take place per
chromosome pair.
(3) This increases genetic variation of the gametes.
c) Random Fertilization
(1) In humans, each male and female gamete represent 2^23 possible
chromosome combinations, therefore 2^23 * 2 results in about 70 trillion
diploid combinations (this still discloses crossing over).
2. The Evolutionary Significance Of Genetic Variation Within Populations
a) Natural selection results in the accumulation of genetic variations favored by the
environment.
b) Mutations are the source of different alleles, which are then mine and match
through meiosis.
c) Sexual reproduction is more expensive energetically than asexual reproduction,
and it is less advantageous in a stable environment, as asexual reproduction
would ensure the perpetuation of successful combinations of alleles.
d) In the case of bdelloid rotifers, they attain genetic variation outside of sexual
reproduction.