Unlike single celled prokaryotes, most organisms are diploid and multicellular. At a particular time in their life cycle certain cells will differentiate, via meiosis, into haploid cells. During sexual reproduction two haploid cells called gametes will unite, thus restoring the diploid chromosome number in the next generation. Meiosis is especially important because it produces genetic variation, the raw material for evolution.

I. Halving the chromosome number

A. Sexual reproduction

1. Requires gamete formation and then fusion of gametes (syngamy) to form a zygote

a. Gamete = a sex cell (sperm or egg)

b. Zygote = first diploid cell resulting from syngamy (fertilization)

2. If gametes contained same number of chromosomes as body cells, doubling would soon fill cells

B. Life cycles

1. Life cycle refers to all reproductive events between one generation and next

2. Mitosis = nuclear division that maintains a constant chromosome number

3. Meiosis = nuclear division reducing chromosome number from diploid (2n) to haploid (n) number

4. Meiosis occurs at different points during life cycle of various organisms

C. Three basic types of life cycles:

1. gametic meiosis - meiosis give rise to gametes which fuse together in syngamy (fertilization). Characteristic of animals and some types of algae. Therefore, in animals, sexual reproduction involves a regular alternation between meiosis and syngamy.

2. sporic meiosis - in true plants (and some algae) meiosis results in the production of haploid spores, cells that can grow directly into a new haploid individual. Therefore, in plants there is a regular alternation of generations between haploid and diploid individuals.

a. In most plants meiosis takes place within flowers.

3. zygotic meiosis - haploid individuals (or cells) fuse to form a diploid zygote which immediately undergoes meiosis. This is the simplest type of life cycle and occurs in unicellular haploid organisms such as some algae and all fungi.

D. Chromosomes occur in homologous pairs

1. In a diploid cell, chromosomes occur as pairs

a. Each set of chromosomes is a homologous pair; each member is a homologous chromosome or homologue

Homologous chromosomes - corresponding chromosomes, one from each parent, which contain the same genes

b. Homologues look alike; they have same length and centromere position; have similar banding pattern human karyotype

c. A locus (location) on one homologue contains the same types of gene which occur at the same locus on the other homologue

2. Chromosomes duplicate just before nuclear division

a. Duplication produces two identical parts called sister chromatids, held together at centromere

3. One member of each homologous pair is inherited from either male or female parent; one member of each homologous pair is placed in each sperm or egg

II. Overview of meiosis

A. Purpose of Meiosis

1. Meiosis keeps chromosome number constant across the generations

2. Makes sure that each gamete contains only one member of each homologous pair

B. Meiosis has two divisions

1. Since meiosis involves two nuclear divisions it produces four haploid daughter cells; each, containing half the total number of chromosomes as the diploid parent nucleus

2. Meiosis I - the first nuclear division

a. Prior to meiosis I, DNA replication occurs and each chromosome has two sister chromatids

b. During meiosis I, homologous chromosomes pair; come together and line up in synapsis

c. During synapsis, the two sets of paired chromosomes lay alongside each other as bivalents

d. While paired up the chromosomes have equal exchanges of genetic material; this is called crossing over

e. After crossing-over occurs, sister chromatids of a chromosome are no longer identical

3. Meiosis II - second nuclear division, nearly identical to mitosis

a. No replication of DNA needed between meiosis I and II because chromosomes were already doubled

b. During meiosis II, centromeres divide; daughter chromosomes derived as sister chromatids separate

c. Chromosomes in the four daughter cells have only one chromatid

C. Crossing-over produces genetic variation, the raw material for evolution

1. Crossing-over results in exchange of genetic material between nonsister chromatids

2. Due to crossing-over, daughter chromosomes derived from sister chromatids have different mix of genes

PHASES OF MEIOSIS

I. Prophase I

A. Nucleolus disappears; nuclear envelope fragments; and spindle fibers assemble

B. Homologous chromosomes undergo synapsis forming bivalents; crossing-over may occur at this time in which case sister chromatids are no longer identical

C. Chromatin condenses and chromosomes become visible

II. Metaphase I

A. Bivalents independently align themselves at the equatorial plane (metaphase plate)

B. This independent assortment during metaphase I is another form of genetic recombination; produces more genetic variation

C. The different possibilities can be calculated using the formula 2n where n = haploid chromosome number. E.g. in humans 8,388,608 different ways homologous pairs can line up (223)

D. Note centromeres are on opposite sides of the equatorial plane

III. Anaphase I

A. The homologues of each bivalent separate and move toward opposite poles

B. Each chromosome still has two chromatids

C. Note the centromeres do not divide

IV. Telophase I

A. Only occurs in some species

B. When it occurs, the nuclear envelope reforms, nucleoli reappear and chromosomes may decondense

V. Interkinesis

A. Period between meiosis I and meiosis II

B. No DNA replication occurs

THE SECOND MEIOTIC DIVISION IS NEARLY IDENTICAL TO MITOSIS

VI. Prophase II

A. If need be the nuclear envelope and nucleoli dissolve and chromatin condenses into chromosomes

VII. Metaphase II

A. The chromosomes align with their centromeres on the equatorial plane

VIII. Anaphase II

A. Centromeres divide and daughter chromosomes move toward the poles

IX. Telophase II

A. Nuclear envelope reforms, nucleoli reappear and chromosomes decondense

B. Cytokinesis produces four haploid daughter cells

X. Comparison of meiosis and mitosis:

A. Mitosis occurs more often because it allows growth and repair of body tissues in multicellular organisms; meiosis only occurs at certain times in the life cycle of sexually reproducing organisms

B. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only one nuclear division; in meiosis there are two nuclear divisions

C. There is no crossing-over in mitotic prophase; there is crossing-over in prophase I of meiosis

D. Duplicated chromosomes align on metaphase plate in mitosis; bivalents align on the metaphase I plate

E. Sister chromatids separate to form daughter chromosomes in anaphase of mitosis; homologous chromosomes separate in anaphase I of meiosis

F. Meiosis II is like mitosis except the meiosis nuclei are haploid

G. Mitosis produces two daughter cells; meiosis produces four daughter cells

H. In mitosis, two daughter cells have same chromosome number as parent cell; in meiosis, four daughter cells are haploid

I. In mitosis, the daughter cells are genetically identical to each other and to the parent cell; in meiosis, the daughter cells are not genetically identical to each other or to the parent cell

XI. Significance of Meiosis

A. Meiosis produces genetic variation

1. Without meiosis, chromosome numbers would continually increase

2. Meiosis ensures daughter cells receive one of each kind of gene; precisely halves the chromosome number

3. Independent assortment provides 2n possible combinations of chromosomes in daughter cells

4. In humans with 23 haploid chromosomes, 2n = 223 = 8,388,608 possible combinations.

5. Variation is added by crossing-over; if only one crossover occurs within each bivalent, 423 or 70,368,744,000,000 combinations are possible

6. Fertilization also contributes to genetic variation; (223)2 = 70,368,744,000,000 possible combinations without crossing-over

7. With fertilization and crossing-over, (423)2 = 4,951,760,200,000,000,000,000,000,000 combinations are possible

B. Advantages of Meiosis

1. Tremendous storehouse of genetic variation provides for adaptations to changing environment

2. Asexual organisms depend primarily on mutations to generate variation

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