9 MEIOSIS
CHAPTER OUTLINE
Meiosis (p. 172)
9.1
9.2
9.3
Discovery of Meiosis (p. 172; Figs. 9.1, 9.2)
A. In 1882, Belgian cytologist Pierre-Joseph van Beneden discovered that the gametes of the
roundworm Ascaris contained two chromosomes while somatic cells contained four.
B. Fertilization
1. van Beneden proposed in 1887 that an egg and sperm could fuse to form a zygote.
2. The fusion of gametes is called fertilization or syngamy.
C. Meiosis
1. A reduction division is required to reduce the number of chromosomes to half so sexual
reproduction can occur.
2. This reduction division is known as meiosis.
D. The Sexual Life Cycle
1. Adult cells are diploid, and gametes are haploid.
2. Sexual reproduction joins haploid gametes to produce a new diploid individual.
The Sexual Life Cycle (p. 173; Figs. 9.3, 9.4)
A. Somatic Tissues
1. In the sexual life cycle, there is an alternation of diploid and haploid generations.
2. Normal diploid body cells are called somatic cells.
3. Somatic cells arise from the zygote and are genetically identical to the zygote.
B. Germ-Line Tissues
1. Cells that produce gametes are called germ-line tissues.
2. Germ-line cells will undergo meiosis to produce haploid gametes.
The Stages of Meiosis (p. 174; Figs. 9.5, 9.6, 9.7)
A. Meiosis has two divisions: meiosis I serves to divide the two versions of each chromosome;
and meiosis II separates the two replicas of each chromosome.
B. Meiosis I
1. Meiosis is similar to mitosis, except that it involves two divisions, meiosis I and meiosis
II, and the resulting cells are haploid, rather than diploid like those produced by mitosis.
2. Also, a phenomenon called crossing over occurs during prophase I of meiosis I when
pieces of nonsister chromatids exchange places to promote new genetic combinations in
the offspring.
3. Prior to meiosis I, during interphase, the DNA replicates and the cell readies itself for
cell division.
4. Meiosis I consists of four stages: prophase I occurs as the chromosomes condense,
homologues pair, and crossing over occurs; metaphase I involves formation of the
spindle apparatus and alignment of chromosome pairs along the center of the cell; in
anaphase I, homologous chromosomes are separated and pulled toward the poles; and
during telophase I, the chromosomes gather at each pole and prepare for the second
division.
5. During meiosis I, the alignment of homologous pairs along the center of the cell is
random, with different combinations of parental chromosomes possible for each daughter
cell. This process is known as independent assortment.
C. Meiosis II
1. Meiosis II, which also has four stages, follows after meiosis I, and the result is the
separation of the sister chromatids to form four haploid gametes.
D. The Important Role of Crossing Over
1. Because of crossing over, the sister chromatids of each chromosome cannot be pulled
apart during meiosis I.
Comparing Meiosis and Mitosis (p. 178)
9.4
How Meiosis Differs From Mitosis (p. 178; Figs. 9.8, 9.9)
Meiosis has two unique features: synapsis and reduction division.
Synapsis
1. The process of pairing throughout the length of the homologous chromosomes is called
synapsis.
2. Synapsis holds the homologous chromosomes close enough to allow crossing over to
occur.
C. Reduction Division
1. Since DNA only replicates once, before meiosis I, the two divisions of meiosis result in
halving the chromosome number in the daughter cells (gametes).
Evolutionary Consequences of Sex (p. 180; Fig. 9.10)
A. Sexual reproduction has the capacity to generate new genetic combinations.
B. Independent Assortment
1. The random alignment of homologues during meiosis I results in an astounding number
of possible kinds of gametes that can be produced.
C. Crossing Over
1. Crossing over allows for combinations of genes that may never have existed previously.
D. Random Fertilization
1. Since the new zygote is the product of two gametes, each with new variation within
them, fertilization adds even more genetic diversity.
E. Importance of Generating Diversity
1. The evolutionary process is revolutionary in that the pace of evolutionary change is
quickened by genetic recombination.
2. Sexual reproduction adds genetic versatility to the offspring, a trait favored in the
vertebrates.
A.
B.
9.5