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MEIOSIS
Two kinds of reproduction
Asexual reproduction
(1 parent)
Offspring inherit parent’s genes
Clones (identical copies of parent)
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Sexual reproduction
(2 parents)
Offspring differ from parents and each another
Different combinations of alleles
Different details of shared traits
Slightly different forms of the same gene
Each specifies a different version of gene product
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During sexual reproduction….
Meiosis, gamete formation, and fertilization
All chromosomes are duplicated during interphase, before meiosis
Two divisions, meiosis I and II,
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divide parental chromosome number by two
End result gamete is haploid Meiosis and fertilization shuffle parental alleles
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Offspring inherit new combinations of alleles
What meiosis does?
Meiosis
– Nuclear division mechanism that precedes gamete formation in eukaryotic cells
– Halves parental chromosome number
• Fertilization
– Fusion of two gamete nuclei
– Restores parental chromosome number
– Forms zygote (first cell of new individual)
• Prophase I is long
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Meiosis I is a continuous process with five distinct phases
Chromosomes condense and align tightly with their homologues
Each homologous pair undergoes crossing over
Microtubules form the bipolar spindle
Prophase I (cont.)
One pair of centrioles moves
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to other side of nucleus
Nuclear envelope breaks up
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Microtubules growing from each spindle pole penetrate nuclear region
Microtubules anchors homologous pair
Metaphase I
homologous chromosomes aligns at equator region of the spindle
Anaphase I
Microtubules separate each chromosome from its homologue, moving to
opposite spindle poles
As anaphase I ends, one set of duplicated chromosomes nears each spindle pole
Telophase I
Two nuclei form
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Typically, the cytoplasm divides
All chromosomes are still duplicated
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Each still consists of two sister chromatids
Meiosis II
The second nuclear division
Sister chromatids of each chromosome are pulled away from each other
Each is now an individual chromosome
Anaphase II and Telophase II
anaphase II one chromosome of each type
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moves toward opposite spindle poles
end of telophase II,
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four haploid nuclei, each with unduplicated chromosomes
Results in haploid daughter cells
When cytoplasm
divides,
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four haploid cells
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Two events variation in traits in sexually reproducing species
– Crossing over during prophase I of meiosis
• Chromosome shuffling during metaphase I of meiosis
• Nonsister chromatids of homologous chromosomes undergo crossing over
– They exchange segments at the same place along their length
• Each ends up with new combinations of alleles not present in either
• parental chromosome
• Life cycles of plants and animals
• Multicelled diploid and haploid bodies
• Plants
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Sporophyte: A multicelled plant body (diploid) that makes haploid spores
Spores give rise to gametophytes (multicelled plant bodies in which
haploid gametes form)
Animals
Germ cells in the reproductive organs give rise to sperm or eggs
Fusion of a sperm and egg at fertilization results in a zygote
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Introducing Variation in Offspring
Three events cause new combinations of alleles in offspring:
Crossing over during prophase I (meiosis)
Random alignment of maternal and paternal chromosomes at metaphase I
(meiosis)
Chance meeting of gametes at fertilization
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All three contribute to variation in traits
Differences in mitosis and meiosis
Both require bipolar spindle to move and sort duplicated chromosomes
– mitosis, and may have evolved from meiosis
Mitosis maintains parental chromosome number
– Duplicates genetic information
– Occurs in body cells
• Meiosis halves chromosome number
– Introduces new combinations of alleles in offspring
– Occurs only in cells for sexual reproduction