Chapter 11 (Meiosis: The Cellular Basis of Sexual Reproduction)

Independent Assortment

A pair of homologous chromosomes consists of one maternal chromosome and one paternal chromosome. Independent assortment of chromosomes is the second major source of genetic variability in meiosis. In metaphase I, one of each homologous pair is randomly attached to the spindle of each pole. • The 23 chromosome pairs of humans allow 223 different combinations of maternal and paternal chromosomes.

Anaphase II and Telophase II

Anaphase II • Spindles separate the two chromatids of each chromosome and pull them to opposite spindle poles. • Chromatids (chromosomes) segregate to the two poles. Telophase II • Chromatids decondense to extended interphase state. • Spindles disassemble and new nuclear envelopes form around the chromatin. • The result is four haploid cells.

Variability

Asexual reproduction generates genetically identical offspring that are the products of mitotic divisions. Meiosis and fertilization both mix genetic information into new combinations - none of the offspring of a mating pair are likely to be genetically identical. Sexual reproduction generates the variability that is the basis of most inherited differences among individual sexually reproducing organisms - also important in evolution.

The Meiotic Cell Cycle (cont'd.)

During meiosis I homologous chromosomes pair and nonsister chromatids undergo a physical exchange of chromosome segments (crossingover). • Completion of meiosis I produces two cells, each with half the diploid number of chromosomes, with each chromosome still consisting of two chromatids. • In meiosis II, sister chromatids separate - daughter chromosomes segregate into four different cells, each with the haploid number of chromosomes.

Sources of Variability

During meiosis and fertilization, genetic variability arises from three sources: • Crossing-over between paired homologous chromosomes (genetic recombination). • Independent assortment of chromosomes segregated to the poles during anaphase I. • The particular sets of male and female gametes that unite in fertilization.

Crossing-Over between Homologous Chromosomes

Exchange of chromosome segments between homologous chromosomes (crossing-over) occurs during prophase I. If there are genetic differences between the homologs, crossing-over can produce new allele combinations in a chromatid. After the second meiotic division, four nuclei are produced, each receiving one of the four chromatids (chromosomes).

Mechanisms That Generate Genetic Variability

Genetic variability is the main evolutionary advantage of sexual reproduction. Variability increases the chance that at least some offspring will survive and reproduce in changing environments.

Meiosis in Some Fungi and Algae (cont'd.)

Haploid spores germinate to produce haploid individuals (gametophytes), which grow by mitotic divisions. Gametophytes form positive and negative gametes by differentiation of some of the cells produced by the mitotic divisions. Because the gametes are produced by mitosis, all the gametes of an individual are genetically identical.

The Meiotic Cell Cycle

Meiosis is a two-part process of cell division in sexually reproducing organisms (meiosis I and meiosis II). The duplicated chromosomes in the parental cell are distributed to four daughter cells, each of which has half the number of chromosomes of the parental cell. • DNA replicates, and the chromosomal proteins are duplicated in premeiotic interphase, producing identical sister chromatids of each chromosome.

Meiosis

Meiosis separates homologous pairs, reducing the diploid (2n) number of chromosomes to the haploid (n) number. Each gamete produced by meiosis receives one member of each homologous pair. • Humans have 46 chromosomes in their diploid cells, which make up 23 homologous pairs (2n). • A human egg or sperm cell contains 23 chromosomes, one of each pair (n).

Metaphase I

Metaphase I • Spindle microtubules align the tetrads on the equatorial plane (metaphase plate) between the two spindle poles.

Fertilization

When the egg and sperm combine in sexual reproduction to produce the zygote - the first cell of the new individual - the diploid number of 46 chromosomes (23 pairs) is regenerated. The processes of DNA replication and mitotic cell division maintain the diploid number in the body cells as the zygote develops.

Meiosis in Plants

All plants (and some algae and fungi) alternate between haploid and diploid generations - depending on the organism, either generation may dominate the life cycle. Fertilization produces the diploid generation (sporophytes). Certain cells of mature sporophytes undergo meiosis, producing haploid spores, which grow by mitotic divisions into a generation of haploid gametophytes. Gametophytes produce haploid egg and sperm nuclei.

Anaphase I

Anaphase I • The two chromosomes of each homologous pair separate and move to opposite spindle poles as the spindle microtubules contract. • Each chromosome still has two sister chromatids.

Meiosis in Animals

In animals, the diploid phase dominates the life cycle, the haploid phase is reduced, and meiosis is followed directly by gamete formation. In males, each of four nuclei produced by meiosis is enclosed in a separate cell which differentiates into a sperm - in females, only one of the four becomes a functional egg. Fertilization restores the diploid phase - only sperm or eggs are haploid.

Sex Chromosomes in Meiosis

In females, the two X chromosomes are fully homologous - in males, X and Y chromosomes are partly homologous. In meiosis, an X chromosome from the mother is able to pair with either an X or a Y from the father. A gamete formed in a female (an egg) may receive either member of the X pair - a gamete formed in a male (a sperm) receives either an X or a Y chromosome.

The Mechanisms of Meiosis

In humans and other animals, meiosis takes place in the primary reproductive organs - the gonads. Meiosis in mature gonads of the male (testes) produces spermatozoa (sperm) - the gametes of the male. Meiosis in mature gonads of the female (ovaries) produces ova (eggs) - the gametes of the female. • Gametogenesis is the cellular mechanisms of gamete formation.

Chromosome Segregation Failure

In nondisjunction, the spindle fails to separate the homologous chromosomes (in meiosis I) or the sister chromatids (in meiosis II). One pole receives both chromosomes, while the other pole has no copies of that chromosome. • Zygotes that receive an extra chromosome have three copies of one chromosome instead of two.

Meiosis in Some Fungi and Algae

In some fungi and algae, the diploid phase is limited to a single cell, the zygote, produced by fertilization. Immediately after fertilization the diploid zygote undergoes meiosis to produce the haploid phase - mitotic divisions occur only in the haploid phase. During fertilization two haploid gametes (+ and -) fuse to form a diploid nucleus, which undergoes meiosis to form haploid spores.

Prophase II, Prometaphase II, and Metaphase II

Prophase II • Chromosomes condense and a spindle forms. Prometaphase II • Nuclear envelope breaks down; spindle enters the former nuclear area; spindle microtubules attach to the two kinetochores of each chromosome. Metaphase II • Spindle microtubules align chromosomes on the metaphase plate.

Prophase I

Replicated chromosomes (sister chromatids) fold and condense in the nucleus. The two chromosomes of each homologous pair undergo pairing (synapsis), forming tetrads. Chromatids of homologous chromosomes physically exchange segments (crossing-over). Sites where crossing-over has occurred are marked by thickened spots (crossovers or chiasmata).

Molecular Insights: Mammalian Gamete Formation

Research Question: What molecular signal determines germ cell fate in mammals? Conclusion: The timing of the initiation of meiosis in mice (and probably other mammals) is determined by the regulation of retinoic acid presence by the Cyp26b1-encoded enzyme.

Sex Chromosomes

Sex chromosomes are pairs of chromosomes that are different in male and female individuals of the same species. Example: Human sex chromosomes • Cells of females contain an XX pair. • Males have an XY pair. • Development into a male depends on the presence of a gene on the Y chromosome. • In the absence of a Y chromosome, an XX individual develops into a female.

Why It Matters ...

Sexual reproduction is the production of offspring through the union of male and female. gametes (e.g., eggs and sperm) • Dependent on meiosis, a specialized process of cell division that, in animals, produces gametes. • Gametes have half the number of chromosomes present in the somatic cells (body cells) of a species. • At fertilization, the nuclei of an egg and sperm cell fuse, producing a zygote - the chromosome number is restored.

Telophase I and Interkinesis

Telophase I • A brief stage in which there is little or no change in the chromosomes. Telophase I is followed by an interkinesis in which the single spindle of the first meiotic division disassembles and microtubules reassemble into two new spindles for the second division.

Random Joining of Male and Female Gametes

The chance union of gametes increases the variability of sexual reproduction. The possibility that two children of the same parents could receive the same combination of chromosomes is 1 chance out of (223)2 or 1 in 70,368,744,000,000 (~70 trillion). • The only exception is identical twins, which arise from mitotic division of a single fertilized egg.

Prometaphase I

The nuclear envelope breaks down. The spindle enters the former nuclear area. The two chromosomes of each pair attach to kinetochore microtubules leading to opposite spindle poles. Sister chromatids remain attached to each other.

The Time and Place of Meiosis in Organismal Life Cycles

The time and place at which meiosis occurs follows one of three major patterns in the life cycles of eukaryotes. The differences reflect the portions of the life cycle spent in the haploid and diploid phases and whether mitotic divisions intervene between meiosis and the formation of gametes.

Homologous Chromosome Pairs

The two representatives of each chromosome in a diploid cell constitute a homologous pair - they have the same genes, arranged in the same order in the DNA of the chromosomes. One chromosome of each homologous pair, the paternal chromosome, comes from the organism's male parent. The other chromosome, the maternal chromosome, comes from the female parent.

Alleles

The versions of each gene (alleles) present in the members of a homologous pair may be the same or different. For a gene that encodes a protein, different alleles might encode different versions of the protein, or no protein at all. • Each individual (except for identical siblings) has a unique combination of alleles in each homologous pair. • Mixing mechanisms of meiosis and fertilization give each individual his or her unique combination of inherited traits.

Crossing-Over (cont'd.)

Two nuclei receive unchanged chromosomes (parental chromosomes) and two receive chromosomes with new combinations of alleles (recombinant chromosomes). Therefore, crossing-over is a mechanism for genetic recombination - it produces genetic recombinants One or more crossing-over events may occur in the same chromosome pair, involving the same or different chromatids.