Meiosis I
Prophase I |
Chromosomes condense, homologous pairs align, synapsis (precise alignment, ensires 1 copy of each gene in a daughter cell). Synapsed set called a tetrad (4 chromatids). Then, crossing-over occurs (exchange of chromosome segments between pairs of homologous chromosomes, or nonsister chromatids). Chiasma forms, visible structure from crossover. X and Y chromosomes pair and synapse thru terminal ends (PARs). |
2N |
Metaphase I |
Nucleoli and envelope broken down, centroiles w/ spinde enternuclear area, kinetechore microtubules attach to sister kinetochores, tetrads align at metaphase plate. |
2N |
Anaphase I |
Chromosomes of tetrad separate (now dyads). Sister chromatids remain attached at centromeres. DNA content is halved. |
2N-N |
Telophase I |
Spindles dissasemble, cytokinesis forms two haploid cells. |
N |
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Meiosis II
Prophase II |
Chromosomes condense and spindles form, kinetochores attach to tubules.. |
Metaphase II |
Alignment on metaphase plate. |
Anaphase II |
Centromeres separate, daughter chromosomes (still haploid) pulled to opposite sides. |
Telophase II |
Chromosomes decondense, nuclear envelope forms, cytokinesis. Four haploid cells produced, each with one chromosome from each homologous pair. |
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Meiosis Results
Generates haploid nuclei with half the number of chromosomes found in diploid cell. (2N -> N) Diploid number restored in fertilization.
Independent assortment of genes paternal and maternal chromosomes have an equal chance of aligning on one side of metaphase plate.
Number of chromosome arrangements is 2n-1, n= # chromosomes pairs (haploid number).
Number of chromosome combinations resulting from independent assortment is 2n (number of different gametes).
Number of kinds of genotypes is 3n. |
Ploidy
Haploid |
ONE copy of each chromosome. (N) |
Diploid |
TWO copies (homologues) of each chromosome. (2N) |
Homologous pairs |
Same gene loci, structure, and pair during meiosis. |
Genes
Allele |
Alternative forms of a single gene on the same locus that determine the same trait, but can produce different phenotypes. |
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Informative cheat sheet, well worth reading. It would be easier to understand if you included a short dictionary.
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