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Lecture Outline
In fertilization, a sperm from a male joins an egg from a female.
• Sperm and eggs are gametes.
• Gametes have half the number of chromosomes as the rest of the cells in that organism.
• Meiosis is the process whereby gametes are made.
The nuclei of the sperm and egg fuse to form an embryo, the new individual; the original chromosome number is restored.
I. How Does Meiosis Occur? (Fig. 12.4)
A. Chromosomes come in distinct packages.
1. Walter Sutton studied gamete formation in lubber grasshoppers and observed the karyotype of grasshopper cells. (Fig. 12.1)
a. Karyotype is the number and type of each chromosome in a cell. (Fig. 2.2)
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, which makes the cell diploid.
2. Subsequent work by others confirmed that homologs differ in size and shape and that homologs carry slightly different versions of the same genes, called alleles.
B. The concept of ploidy
1. Diploid organisms have two alleles for each gene, one on each of the homologous chromosomes.
2. Haploid organisms do not contain homologous chromosomes and have only one allele for each gene.
3. The number of chromosomes of distinct type and structure in a cell is called the haploid number (n) of that species. (Table 12.1)
4. A cell’s ploidy is the number of sets of chromosomes present. (Table 12.2)
C. An overview of meiosis
1. Prior to meiosis, the identical copies, sister chromatids, are joined at the centromere. (Figs. 12.3, 12.4, and 12.5)
2. A replicated chromosome is a single chromosome made up of two sister chromatids.
a. Students should be able to draw the same chromosome in unreplicated and replicated states, explain why both structures represent a single chromosome, and then label the sister chromatids in the replicated chromosome.
3. Meiosis is two cell divisions. (Fig. 12.4)
a. During meiosis I, the homologs separate and go to different daughter cells.
(1) The daughter cells are haploid after meiosis I.
(2) Each chromosome still has two sister chromatids.
b. During meiosis II, the chromatids separate and go to different cells.
(1) The cells produced by meiosis II have one copy of each chromosome; they are haploid.
(2) Each chromosome in the cell is unreplicated.
c. Meiosis produces haploid cells that go on to develop into gametes through gametogenesis.
4. Meiosis is a reduction division.
a. The chromosome number is reduced by half in the gametes.
b. When gametes fuse in fertilization, a diploid zygote containing two sets of chromosomes results. (Fig. 12.5)
c. Each zygote receives one set of its chromosomes from its mother and one set from its father.
5. Meiosis in the context of an organism’s life cycle (Fig. 12.6)
a. Meiosis in a diploid adult results in haploid gametes that fuse during fertilization to form a diploid zygote.
b. Students should be able to use Figure 12.6 to draw a life cycle for humans.
D. The phases of meiosis I
1. Prophase I: synapsis (Fig. 12.7)
a. Replicated chromosomes pair up with their homolog (synapsis).
b. One homolog is paternal in origin; the other is maternal in origin.
c. Each pair forms a tetrad composed of two replicated homologous chromosomes, for a total of four chromatids. (Fig. 12.7)
d. Chiasmata form between non-sister chromatids, one to several per homologous pair. (Fig. 12.7)
e. Thomas Hunt Morgan (1911) hypothesis: Paternal and maternal homologs undergo a physical exchange of DNA at each chiasma. This process is called crossing over.
2. Meiosis I: metaphase I (Fig. 12.7)
a. Homologous pairs migrate to the center (metaphase plate) of the cell.
b. Each tetrad is positioned randomly, without regard to which side of the metaphase plate the paternal or maternal homolog is on.
3. Meiosis I: anaphase I (Fig. 12.7)
a. Homologous pairs are pulled to opposite poles of the cell.
b. Each chromosome moves independently of every other chromosome.
4. Meiosis I: telophase I (Fig. 12.7)
a. The cytoplasm separates to form two daughter cells.
b. A random mixture of paternal versus maternal homologs ends up in each daughter cell (independent assortment).
c. Each daughter cell is haploid, with one full set of chromosomes, but each chromosome consists of two chromatids.
E. The phases of meiosis II
1. Meiosis II: prophase II (Fig. 12.7)
a. Each chromosome, consisting of two chromatids attached at the centromere, migrates to the center of each daughter cell.
2. Meiosis II: metaphase II (Fig. 12.7)
a. Chromosomes line up at the metaphase plate.
3. Meiosis II: anaphase II (Fig. 12.7)
a. Sister chromatids separate at the centromere.
b. Chromatids are pulled to opposite poles of the cells.
4. Meiosis II: telophase II (Fig. 12.5)
a. The cytoplasm separates to form a total of four haploid daughter cells.
5. Summary of meiosis (Figs. 12.4, 12.7, and 12.8)
a. Meiosis occurs in sexually reproducing organisms.
b. Meiosis reduces the chromosome number in half, from 2n to n.
c. Meiosis precedes the formation of gametes.
d. Crossing over (recombination) produces chromosomes unlike either parent.
e. Fertilization brings together haploid gametes to restore the diploid state.
6. Comparison of meiosis and mitosis (Table 12.3; Fig. 12.8)
a. Homologous chromosomes pair in meiosis and not in mitosis.
b. Meiosis reduces the number of chromosomes; mitosis does not.
c. Students should be able to describe the consequences for meiosis if chromosomes do not pair.
F. A closer look at prophase I (Fig. 12.9)
1. During synapsis, two replicated homologs align along their length.
2. When homologs synapse, they form a synaptonemal complex.
3. Crossing over happens along the length of the paired homologs.
a. Non-sister chromatids are broken at the same point and attached to each other, and a portion of the chromatid is exchanged.
b. As a result, segments of maternal and paternal chromosomes are swapped.
c. Students should be able to use two chains of paper clips to simulate non-sister chromatids and then simulate a crossing-over event between them.
4. Students should be able to look at Figure 12.8 and identify the haploid number, ploidy, and phase of meiosis illustrated in each cell in the figure.
II. The Consequences of Meiosis
A. Sexual reproduction versus 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. Chromosomes carry the hereditary material.
2. A gene is a chromosome segment that contains the instructions for an inherited trait.
3. Every chromosome has a linear array of many genes.
4. Since more than one form (an allele) of a given gene can exist in a population, different forms of the corresponding trait may be inherited.
C. Independent assortment produces genetic variation.
1. 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. 12.10)
2. The number of different possible combinations of paternally derived and maternally derived chromosomes in the daughter cells is 2n, where n = the number of chromosomes in the haploid state.
3. Students should be able to explain how genetic variation would be affected if maternal chromosomes always lined up together on one side of the metaphase plate during meiosis I and paternal chromosomes always lined up on the other side.
D. The role of crossing over
1. During prophase of meiosis I, homologous chromosomes exchange DNA at chiasmata.
2. This crossing over “mixes up” the maternal and paternal genes on each of the homologs.
3. Crossing over introduces an incalculable amount of genetic variation: At least one chiasma forms per homologous pair.
E. How does fertilization affect genetic variation?
1. Fusion of male and female gametes occurs randomly.
2. The 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.
a. Self-fertilization (Fig. 12.11)
(1) Male and female gametes that fuse are from the same organism.
(2) Offspring are still genetically different from parents due to independent assortment and crossing over occurring prior to gamete formation.
b. Outcrossing
(1) Male gametes are from a different individual than female gametes.
(2) This event maximizes genetic variability.
III. Why Does Meiosis Exist?
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 and sexually.
3. Sexual reproduction is the major mode of reproduction in multicellular, species-rich lineages (insects, vertebrates, and flowering plants).
B. The paradox of sex
1. Many organisms reproduce asexually.
2. John Maynard Smith (1978) proposed that natural selection should eliminate sexual reproduction from a population, based on a mathematical model.
a. Asexual reproduction is more efficient because 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. 12.12)
c. Paradox: Stable populations of sexually reproducing organisms do exist.
C. The purifying selection hypothesis
1. Asexual reproduction passes damaged genes to all offspring.
2. In sexually reproducing species, only half of the offspring inherit the damaged allele; this means that some of the offspring do not inherit the damaged gene at all.
3. Natural selection favors the success of those offspring that did not inherit the damaged gene; this is called purifying selection.
4. Purifying selection steadily reduces the numerical advantage of asexual reproduction.
D. The changing-environment hypothesis
1 Sexual reproduction provides more genetic diversity.
2. A population of genetically diverse offspring is more likely to survive changing environmental conditions than a population of genetically identical organisms.
3. Testing the changing-environment hypothesis: Lively’s studies on New Zealand snails (Fig. 12.13)
a. Both asexual and sexual snails exist in the population.
b. The snails are parasitized by many species of trematode worms.
c. Trematodes eat snail reproductive organs; infected snails cannot reproduce.
d. Hypothesis: Snails that reproduce only asexually should be common in habitats where trematodes are rare. Snails that reproduce sexually should be common in habitats where trematodes are abundant.
e. Results: The frequency of sexual reproduction was higher in areas where trematodes were more abundant.
IV. Mistakes in Meiosis
A. How do mistakes occur?
1. Nondisjunctions are chromosome separation mistakes that can occur during meiosis I or II.
2. Both members of a homologous chromosome pair may migrate to the same daughter cell. (Fig. 12.14)
a. One daughter cell has two copies of the chromosome after meiosis I (n + 1).
b. The other daughter cell lacks a copy of the chromosome after meiosis I (n – 1).
c. Following gamete formation, if the n – 1 daughter cell is fertilized by normal sperm, the zygote is 2n – 1: monosomy.
d. Following gamete formation, if the n + 1 daughter cell is fertilized by normal sperm, the zygote is 2n + 1: trisomy.
e. Cells that have too many or too few chromosomes are aneuploid.
3. Sister chromatids do not separate in anaphase II.
B. Why do mistakes occur?
1. Random error (Table 12.4)
a. There is no evidence of an inherited tendency for meiotic mistakes. Example: Down syndrome does not run in families.
b. Trisomies are more frequent with smaller chromosomes. Example: trisomy-21 Down syndrome.
c. With the exception of trisomy-21, most human trisomies involve sex chromosomes.
2. Maternal age. Example: trisomy-21. (Fig. 12.15)
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