Meiosis and Alteration of Generations

Outline Overview  Asexual and Sexual Reproduction Phases of Meiosis  Meiosis I  Meiosis II Alternation of Generations

Overview Asexual Reproduction - Involves cells that are identical in their chromosomes with the cells from which they arose. Nearly all plants undergo sexual reproduction.  Flowering and cone-bearing plants ultimately result in formation of seeds. - In sexual reproduction gametes, egg, and sperm, fuse forming a zygote.

Overview Meiosis brings about the development of gametes that have only half the number of chromosomes of any cell developing within the zygote.  All living cells undergoing meiosis have two sets of chromosomes, one set from each parent. - Homologous Chromosomes.

Asexual and Sexual Reproduction

Interphase: As in mitosis, the meiotic division is preceded by interphase (G1, S, and G2 stages). G1 is Gap1 stage: cell increases in size. S is Synthesis stage: DNA (chromosome) replication takes place. G2 is Gap 2 stage: mitochondria divide and microtubules produced.

Phases of Meiosis Meiosis I (Reduction Division)  Prophase I - Chromosomes coil and condense. - Nuclear envelope and nucleolus disassociate. - Crossing-over occurs.  Chiasma

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Fig. 12.3

Crossing Over

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Phases of Meiosis  Metaphase I - Chromosomes align in pairs at equator. - Spindle formation completed.  Anaphase I - One chromosome from each pair migrates to a pole.  Telophase I  Original cell becomes two cells or two nuclei.

Phases of Meiosis Meiosis II (Equational Division)  Prophase II - Chromosomes of both groups become shorter and thicker.  Metaphase II - Centromeres become aligned along the equator. - New spindles are completed.

Phases of Meiosis  Anaphase II - Centromeres and chromatids of each chromosome separate and migrate to opposite poles.  Telophase II - Coils of the chromatids relax and chromosomes become longer and thinner. - Nuclear envelope and nucleoli reappear for each new group of chromosomes.

Alternation of Generations The original chromosomal complement, consisting of two complete sets of chromosomes, is restored when gametes unite and form a zygote.  Any cell with one set of chromosomes is haploid (1n).  Any cell with two sets of chromosomes is diploid (2n).

Alteration of Generations Occasionally, spindles may not form properly during meiosis, resulting in more than two sets of chromosomes.  Triploid (3n)  Tetraploid (4n) In plants, a complete life cycle involving sexual reproduction has an alternation of generations between a diploid sporophyte phase and a haploid gametophyte phase.

Alteration of Generations

First cell of any gametophyte generation is normally a spore, and the last is a gamete. Any gametophyte cell is usually haploid. First cell of sporophyte generation is normally a zygote, and the last cell is normally a sporocyte.

Alteration of Generations Any sporocyte cell is usually diploid. Change from sporophyte to gametophyte generation usually occurs as a result of meiosis. Change from gametophyte to sporophyte usually occurs as a result of fertilization (Syngamy).

Review Overview  Asexual and Sexual Reproduction Phases of Meiosis  Meiosis I  Meiosis II Alteration of Generations

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