I.
II.
Why sex?
A.
The Ultimate Explanation
1. The evolutionary answer that explains why something happens
2. Most organisms don’t reproduce sexually.
3. So, why is sex advantageous to some organisms?
B.
The Proximate Explanation
1. The mechanistic answer that explains how something happens.
2. A sperm from a male joins an egg from a female in fertilization.
a. Sperm and eggs are gametes.
b. Gametes have half the number of chromosomes of the rest of the cells in
that organism.
c. Meiosis is the process whereby gametes are made.
3. The nuclei of the sperm and egg fuse to form an embryo, the new individual.
How Does Meiosis Occur? The proximate explanation (Fig. 12.3–Overview)
A. Sutton—Observations of Gamete Formation in Lubber Grasshoppers
1. Observed the karyotype of grasshopper cells (Fig. 12.1)
a. Karyotype = number and type of each chromosome in a cell. (Box 12.1, Fig.
12.2)
b. Body cells contain 24 chromosomes, but only 12 distinct types, differing in
size and shape.
c. Two copies (homologs/homologous chromosomes) of each type are
present in each cell, making the cell diploid.
2. Subsequent work by others confirmed:
a. Homologs differ in size and shape.
b. Homologs carry slightly different versions of the same genes, called alleles.
c. The number of chromosomes in a “set” is called the haploid number (n) of
that species. (Table 12.1)
d. Most plants and animals have two or more copies of each chromosome
(diploid = 2n, triploid = 3n, tetraploid = 4n, etc.).
B.
Prior to meiosis, each chromosome is replicated; the identical copies, sister
chromatids, are joined at the centromere. (Figs. 12.3, 12.4, 12.5)
C.
Meiosis I: Prophase I—Synapsis (Fig. 12.4)
1. Replicated chromosomes pair up with their homolog (synapsis).
2. One homolog is paternal in origin; the other is maternal in origin.
3. Each pair forms a tetrad composed of two replicated homologous
chromosomes, for a total of four chromatids. (Fig. 12.4)
4. Chiasmata form between non-sister chromatids, one to several per homologous
pair. (Figs. 12.6, 12.7)
5. Thomas Hunt Morgan (1911) hypothesis—Paternal and maternal homologs
undergo a physical exchange of DNA at each chiasma = “crossing over.”
D.
Meiosis I: Metaphase I (Fig. 12.4)
1. Homologous pairs migrate to the center (metaphase plate) of the cell.
2.
Each tetrad is positioned randomly, without regard to which side of the
metaphase plate the paternal or maternal homolog is on.
E.
Meiosis I: Anaphase I (Fig. 12.4)
1. Homologous pairs are pulled to opposite poles of the cell.
2. Each chromosome moves independently of every other chromosome.
F.
Meiosis I: Telophase I (Fig. 12.4)
1. The cytoplasm separates to form two daughter cells.
2. A random mixture of paternal vs. maternal homologs ends up in each daughter
cell (independent assortment).
3. Each daughter cell is haploid, with one full set of chromosomes, but each
chromosome consists of two chromatids.
G.
Meiosis II: Prophase II—Each chromosome, consisting of two chromatids attached
at the centromere, migrates to the center of each daughter cell. (Fig. 12.4)
H. Meiosis II: Metaphase II—Chromosomes line up at the metaphase plate. (Fig. 12.4)
I.
Meiosis II: Anaphase II (Fig. 12.4)
1. Sister chromatids separate at the centromere.
2. Chromatids are pulled to opposite poles of the cells.
J.
Meiosis II: Telophase II—The cytoplasm separates to form a total of four haploid
daughter cells. (Fig. 12.4)
K.
Summary of Meiosis (Table 12.2; Figs. 12.3, 12.5, 12.7)
1. Meiosis occurs in sexually reproducing organisms.
2. Meiosis reduces the chromosome number in half, from 2n to n.
3. Meiosis precedes the formation of gametes.
4. Crossing over (recombination) produces chromosomes unlike either parent.
5. Fertilization brings together haploid gametes to restore the diploid state.
III. What Are the Consequences of Meiosis? Beginning the Ultimate Explanation
A. Sexual Reproduction vs. Asexual Reproduction
1. Sexual reproduction is preceded by meiosis and results in offspring that have a
combination of both maternal and paternal traits.
2. Asexual reproduction is based on mitosis, and results in offspring that are
identical to the parent (clones).
B.
Chromosomes and Heredity
1. A gene is a chromosome segment containing the instructions for an inherited
trait.
2. Every chromosome has multiple genes.
3. Since more than one form (an allele) of a given gene can exist in a population,
different forms of the corresponding trait can be inherited.
C.
Genetic variation in sexually reproducing organisms results from three key events
in meiosis and fertilization.
1. Independent assortment
a.
2.
3.
During metaphase of meiosis I, each homologous pair of chromosomes
lines up at the plane of cell division randomly with respect to which side of
the plane the paternal or maternal member of the pair is on. (Fig. 12.8)
b. The number of different possible combinations of paternally derived and
maternally derived chromosomes in the daughter cells = 2n, where n = the
number of chromosomes in the haploid state.
Crossing Over/Genetic Recombination
a. During prophase of meiosis I, homologous chromosomes exchange DNA
at chiasmata.
b. This “mixes up” the maternal and paternal genes on each of the homologs.
c. It introduces an incalculable amount of genetic variation—at least one
chiasmata forms per homologous pair.
Fertilization
a. Fusion of male and female gametes occurs randomly.
b. Amount of possible genetic variability introduced is calculated by
multiplying the number of possible gametes produced by the female
parent times the number of possible gametes produced by the male parent.
(1) Self-fertilization (Fig. 12.9)
(a) Male and female gametes that fuse are from the same organism.
(b) Offspring are still genetically different from parents due to
independent assortment and crossing over occurring prior to
gamete formation.
(2) Outcrossing
(a) Male gametes are from a different individual than the female
gamete.
(b) This event maximizes genetic variability.
IV. Why Does Meiosis Exist? Continuing the Ultimate Explanation
A.
Sexual reproduction occurs in only a small fraction of all organisms.
1. Bacteria, Archaea, and a few animals reproduce only asexually.
2. Most algae, fungi, some land plants, and most vertebrates reproduce both
asexually (Fig. 12.10) and sexually.
3. Sexual reproduction is the major mode of reproduction in multicellular,
species-rich lineages (insects, vertebrates, flowering plants).
B.
The Paradox of Sex
1. John Maynard Smith (1978) proposed that natural selection should eliminate
sexual reproduction from a population, based on a mathematical model.
a. Asexual reproduction—No males are required, so a population of all
females can produce more offspring than one in which some individuals
are male.
b. If a population is composed of asexual and sexual individuals, the asexual
individuals should increase in frequency and the sexual individuals
should decrease. (Fig. 12.11)
c. Paradox—Stable populations of sexually reproducing organisms do exist.
2. The changing-environment hypothesis
a
Sexual reproduction provides more genetic diversity.
b.
c.
C.
A population of genetically diverse offspring is more likely to survive
changing environmental conditions than a population of genetically
identical organisms.
Testing the changing-environment hypothesis—Lively’s studies on New
Zealand snails. (Fig. 12.12)
(1) Both asexual and sexual snails exist in the population.
(2) The snails are parasitized by many species of trematode worms.
(3) Trematodes eat snail reproductive organs; infected snails cannot
reproduce.
(4) Hypothesis: Snails that reproduce only asexually should be common
in habitats where trematodes are rare. Snails that reproduce sexually
should be common where trematodes are abundant.
(5) Results: The frequency of sexual reproduction was higher in areas
where trematodes were more abundant.
When do meiosis and fertilization occur during the life of an organism?
1. A life cycle is the sequence of events in the life of an organism that occur
between conception and the production of offspring.
2. Life cycles of sexually reproducing organisms are highly variable. (Fig. 12.13)
a. Most animals have predominantly diploid life cycles.
(1) The organism is diploid throughout all stages of the life cycle except
the gametes.
(2) Meiosis produces gametes, which undergo fertilization and form a
diploid zygote.
b. Most algae have predominantly haploid life cycles.
(1) The organism is haploid through most of the life cycle, except the
zygote.
(2) The zygote often is a tough dormant stage for survival of harsh
conditions.
(3) The zygote undergoes meiosis as it matures and becomes active.
3. Some life cycles alternate between a haploid and a diploid phase (alternation of
generations).
a. The organism undergoes transitions between a haploid adult phase and a
diploid adult phase.
(1) Meiosis occurs in the diploid phase, forming haploid spores.
(2) Spores germinate and grow into the haploid phase, which then
produces haploid gametes by mitosis.
(3) Gametes fuse to form a zygote that then grows into the diploid adult
phase.
b. Alternation of generations occurs in some algae, and all land plants.
VI. What are the consequences if there is a mistake in meiosis?
A. Nondisjunctions are chromosome separation mistakes that can occur during meiosis
I or II.
1. Both members of a homologous chromosome pair may migrate to the same
daughter cell. (Fig. 12.14)
a.
2.
B.
One daughter cell will have two copies of the chromosome after meiosis I
(n + 1).
b. The other daughter cell will lack a copy of the chromosome after meiosis I
(n – 1).
c. Following gamete formation, if the daughter cells are fertilized by normal
sperm, it will be aneuploid:
(1) Some zygotes will have three copies of the chromosome
(2n + 1 = trisomy).
(2) Other zygotes will have only one copy of the chromosome
(2n – 1 = monosomy).
Consequences of Nondisjunctions
a. Severe—Addition or deletion of whole chromosomes is frequently lethal.
b. Recent study of 119 pregnancies that ended in embryonic or fetal death:
(1) 38% were due to atypical chromosome complements; 36% of these
were due to nondisjunctions.
(2) Nondisjunctions may occur as often as in 10% of all meiotic divisions.
c. Some types of nondisjunction allow survival, depending on chromosome
involved. Example: Down syndrome, trisomy of chromosome 21
Why do meiotic mistakes occur?
1. Hypothesis—Errors are random accidents.
2. There is no evidence for inherited tendency for meiotic mistakes; Down
syndrome, for example, does not run in families. (Table 12.3)
3. Risk factors are maternal age; maternal errors are also more frequent than
paternal errors are. (Fig. 12.15)