Lecture Outline -- Meiosis, recombination, and sex
I.
What is sex?
A.
A sperm joins an egg 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.
B. The nuclei of the sperm and egg fuse to form an embryo, the new individual.
C. Diploid organisms have life cycles in which haploid and diploid phases
alternate.
II.
Process of Meiosis: definitions
A. Homologous chromosomes or homologs: the 2 copies of chromosomes of the same
type, one inherited from sperm, one from egg.
B. Alleles: slightly different forms of the same gene, on the homologs.
C. N = # of types of homologs, haploid number. 2N = diploid number.
D. Overview
1. Chromosomes replicate before meiosis begins.
2. There are two phases of meiosis, cells divide in each phase.
3. Meiosis I is a “reduction division”. One diploid cell divides to two haploid
cells. (The haploid cells might appear diploid, because the chromosomes
have replicated and each chromosome has 2 chromatids.)
4. In Meiosis II, sister chromatids separate, so that there is a single chromatid
(now called a chromosome) in each of four cells. Meiosis II is like mitosis in a
haploid cell; you start with two haploid cells and get 4.
5. Stages within each phase, learn prefixes:
a. Pro = before; line up
b. Meta = middle; go to middle
c. Ana = back; pull to poles
d. Telo = end; the last stage.
III. Process of Meiosis
A. Prior to meiosis, each chromosome is replicated; the identical copies, sister
chromatids, are joined at the centromere. (Figs. 12.3, 12.4, 12.5) REPLICATED
CHROMOSOMES ARE STILL CONSIDERED TO BE SINGLE CHROMOSOMES,
ALTHOUGH THEY CONSIST OF 2 CHROMATIDS.
B.
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.
4.
5.
Each pair forms a tetrad composed of two replicated homologous
chromosomes, for a total of four chromatids.
Chiasmata form between non-sister chromatids, one to several per homologous
pair.
Paternal and maternal homologs undergo a physical exchange of DNA at each
chiasma = “crossing over.”
C.
Meiosis I: Metaphase I
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.
D.
Meiosis I: Anaphase I
1. Homologous pairs are pulled to opposite poles of the cell.
2. Each chromosome moves independently of every other chromosome.
E.
Meiosis I: Telophase I
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.
F.
Meiosis II: Prophase II—Each chromosome, consisting of two chromatids attached
at the centromere, migrates to the center of each daughter cell.
G.
Meiosis II: Metaphase II—Chromosomes line up at the metaphase plate.
H. Meiosis II: Anaphase II
1. Sister chromatids separate at the centromere.
2. Chromatids are pulled to opposite poles of the cells.
I.
Meiosis II: Telophase II—The cytoplasm separates to form a total of four haploid
daughter cells.
WATCH ANIMATION IF POSSIBLE.
J.
Contrast Meiosis and Mitosis
Feature
Meiosis
Mitosis
Purpose
Sex
Growth
Ploidy of products
Haploid
diploid
Number of cell divisions
Two
One
Synapsis of homologs
Yes, prophase I
No
Chromosomes in
products
Independent assortment;
also recombination
Identical to parents
K.
Summary of Meiosis
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.
IV. Why is there sex -- Recombination and Variation
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). Many wild plants; apples & bananas.
V.
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. 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.
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.
2. 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.
3. 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.
Why Does Meiosis Exist? Continuing the Ultimate Explanation
A.
B.
Sexual reproduction is the major mode of reproduction in multicellular,
species-rich lineages (insects, vertebrates, flowering plants).
The Paradox of Sex
1.
2.
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.
The changing-environment hypothesis
a
Sexual reproduction provides more genetic diversity.
b. A population of genetically diverse offspring is more likely to survive
changing environmental conditions than a population of genetically
identical organisms.
c. 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.
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.
a. 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).
2. 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.
d.
Some types of nondisjunction allow survival, depending on
chromosome involved. Example: Down syndrome, trisomy of
chromosome 21. Klinefelter syndrome XXY Male sex organs, maybe
breasts. Sterile. Turner syndrome XO 1/5000 births. Sterile.
Important in plants! Many wild and crop plants are polyploid. Bananas
are triploid. Triploid watermelons are seedless. (In meiosis 1,
impossible for homologs to be evenly divided. Can’t make balanced
gametes.)