12.1.
Halving the Chromosome Number
A. Sexual Reproduction
1. Meiosis is nuclear division reducing chromosome number from
diploid (2n) to haploid (n) number.
2. Haploid (n) number is
half the diploid number of chromosomes.
3. Requires gamete formation and
then fusion of gametes to form a zygote.
4. A zygote always has
a full or diploid (2n) number of chromosomes.
5. If gametes contained same number
of chromosomes as body cells, doubling would soon fill cells.
6. Belgian cytologist found Ascaris egg and sperm contain two chromosomes; body cells
would contain four.
B. Homologous Pairs of Chromosomes
1. In a diploid body cell, chromosomes occur as pairs.
a. Each set
of chromosomes is a homologous pair; each member is a homologous
chromosome or homologue.
b.
Homologues look alike; they have same length and centromere
position; have similar banding pattern
when stained.
c. A
location on one homologue contains the same types of gene which occurs at the
same locus on other
homologue.
2. Chromosomes duplicate just before
nuclear division.
a.
Duplication produces two identical parts called sister chromatids,
held together at centromere.
b.
Non-sister chromatids do not share the same centromere.
3. One member of each homologous
pair is inherited from either male or female parent; one member of
each
homologous pair is placed in each sperm or egg.
C. Overview of Meiosis
1. Meiosis involves two nuclear divisions and produces four
haploid daughter cells.
2. Each daughter cell has half the
number of chromosomes in the diploid parent nucleus.
3. Meiosis I is the
nuclear division at the first meiotic division.
a. Prior to
meiosis I, DNA replication occurs and each chromosome has two sister chromatids.
b. During
meiosis I, homologous chromosomes come together and line up (cause unknown) in synapsis.
c. During synapsis, the two sets of paired chromosomes lay alongside
each other as bivalents or a tetrad.
d. Crossing
over is an exchange of homologous segments between non-sister chromatids of bivalent during
meiosis I; results in genetic recombination.
e. After
crossing over occurs, sister chromatids of a
chromosome are no longer identical.
4. Meiosis II
a. No
replication of DNA is needed between meiosis I and II because chromosomes
were already doubled.
b. During
meiosis II, centromeres divide; daughter chromosomes
derived as sister chromatids separate.
c.
Chromosomes in the four daughter cells have only one chromatid.
d. Counting
the number of centromeres verifies that parent cells
were diploid, each daughter cell is haploid.
e. In the
animal life cycle, daughter cells become gametes that fuse during
fertilization; this restores the
diploid number in body cells.
12.2.
Genetic Recombination
A. Genetic Recombination
1. Due to genetic recombination, offspring have a different combination of
genes than their parents.
2. Without recombination, asexual
organisms must rely on mutations to generate variation among offspring;
this
is sufficient because they have great numbers of offspring.
B. Crossing-Over Introduces Variation
1. Crossing-over results in exchange of genetic material between non-sister chromatids.
2. At synapsis,
homologous chromosomes are held in position by a nucleoprotein lattice (the synaptonemal complex).
3. As the lattice of the synaptonemal complex breaks down at beginning of anaphase
I, homologues are temporarily
held
together by chiasmata, regions were the
non-sister chromatids are attached due to
crossing-over.
4. The homologues separate and are
distributed to separate cells.
5. Due to crossing-over, daughter
chromosomes derived from sister chromatids are no
longer identical.
C. Independent Assortment of Homologous Chromosomes
1. Independent assortment in a cell with only three pairs of
chromosomes is eight possible combinations.
2. In humans with 23 pairs of
chromosomes, the combinations possible are 8,388,608 possible combinations.
D. Fertilization
1. Meiosis increases variation.
2. When gametes fuse at
fertilization, chromosomes donated by parents combine.
3. Chromosomally different zygotes
from same parents are 70,368,744,000,000 combinations
possible
without crossing over.
4. If crossing over occurs once,
then 4,951,760,200,000,000,000,000,000,000 combinations of
genetically
different zygotes are possible for one couple.
12.3.
Meiosis Has Phases
A. Number of Phases
1. Both meiosis I and meiosis II have four phases: prophase, metaphase,
anaphase and telophase.
B. Prophase I
1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope
fragments; centrosomes
migrate away
from each other; and spindle fibers assemble.
2. Homologous chromosomes undergo synapsis forming bivalents; crossing over may occur at this
time
in which
case sister chromatids are no longer identical.
3. Chromatin condenses and
chromosomes become microscopically visible.
C. Metaphase I
1. During prometaphase I, bivalents held together by chiasmata have moved toward the metaphase plate.
2. In metaphase I, there is a fully
formed spindle and alignment of the bivalents at the metaphase plate.
3. Kinetochoares
are regions just outside centromeres; they attach to
spindle fibers call kinetochore spindle fibers.
4. Bivalents independently align
themselves at the metaphase plate of the spindle.
5. Maternal and paternal homologues
of each bivalent may be oriented toward either pole.
D. Anaphase I
1. The homologues of each bivalent separate and move toward opposite poles.
2. Each chromosome still has two chromatids.
E. Telophase I
1. Only occurs in some species.
2. When it occurs, the nuclear
envelope reforms and nucleoli reappear.
F. Interkinesis
1. This period between meiosis I and meiosis II is similar to interphase of mitosis.
2. However, no DNA replication
occurs.
G. Meiosis II
1. During metaphase II, the haploid number of chromosomes align
at metaphase plate.
2. During anaphase II,
centromeres divide and daughter chromosomes move
toward the poles.
3. At the end of telophase
II and cytokinesis, there
are four haploid cells.
4. Due to crossing-over, each gamete
can contain chromosomes with different types of genes.
5. In animals, the haploid cells
mature and develop into gametes.
6. In plants, the daughter cells
become spores and divide to produce a haploid adult generation.
7. In some fungi and alge, a zygote results from gamete fusion and immediately
undergoes meiosis;
therefore,
the adult is always haploid.
12.4.
Comparison of Meiosis with Mitosis
A. How Meiosis I Differs from Mitosis
1. DNA is replicated only once before both mitosis and meiosis; in mitosis
there is only one nuclear
division; in
meiosis there are two nuclear divisions.
2. During prophase I of meiosis,
homologous chromosomes pair and undergo crossing-over; this does
not occur
during mitosis.
3. During metaphase I of meiosis,
paired homologous chromosomes align at the metaphase plate; in
mitosis
individual chromosomes align.
4. During anaphase I in meiosis,
homologous chromosomes with centromeres intact
separate and move
to opposite
poles; in mitosis at this stage, sister chromatids
separate and move to poles.
B. How Meiosis II Differs from Mitosis
1. Events of meiosis II are same stages as in mitosis.
2. However, the nuclei contain the
haploid number of chromosomes in meiosis.
3. Mitosis produces two daughter
cells; meiosis produces four daughter cells.
12.5.
Viewing the Human Life Cycle
A. Life Cycle has Both Meiosis and Mitosis
1. Life cycle refers to all reproductive events between one
generation and next.
2. In animals, the adult is always
diploid [Instructors note: some bees, etc. have haploid male adults].
3. In animals, it occurs during
production of gametes; adult is diploid and gametes are haploid.
4. Mosses are haploid most of their
cycle; oak trees are diploid most of their cycle.
5. In fungi and some algae,
organisms you see is haploid and produces haploid gametes.
6. In males, meiosis is part of spermatogenesis,
the production of sperm, and occurs in the testes.
7. In females, meiosis is part of oogenesis, the production of eggs cells, and
occurs in the ovaries.
8. After birth, mitotic cell
division is involved in growth and tissue regeneration.
B. Spermatogenesis and Oogenesis in Humans
1. Spermatogenesis
a. In the testes
of males, primary spermatocytes
with 46 chromosomes divide meiotically to form two
secondary spermatocyes, each with 23
duplicated chromosomes.
b. Secondary
spermatocyes divide to produce four spermatids, also with 23 daughter
chromosomes.
c. Spermatids then differentiate into sperm (spermatozoa).
d. Meiotic
cell division in males always results in four cells that become sperm.
2. Oogenesis
a. In the ovaries
of human females, primary oocytes
with 46 chromosomes divide meiotically to form two
cells, each with 23 duplicated chromosomes.
b. One of
the cells, a secondary oocyte, receives
most cytoplasm; the other cell, a polar body, disintegrates
or divides again.
c. A
secondary oocyte begins meiosis II and then stops at
metaphase II.
d. At
ovulation, the secondary oocyte leaves the ovary and
enters an oviduct where it may meet a sperm.
e. If a
sperm enters secondary oocyte, oocyte
is activated to continue meiosis II to completion; result is
a mature egg and another polar body, each with 23 daughter chromosomes.
f. Polar
bodies serve to discard unnecessary chromosomes and retain most of the
cytoplasm in the egg.
g. The
cytoplasm serve as a source of nutrients for the developing embryo.