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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
Fig. 12.2-1
Fig. 12.2-2
Fig. 12.3
Crossing Over
Fig. 12.4-p1
Fig. 12.4-p2
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
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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