Meiosis


Meiosis-a two stage type of cell division in sexually reproducing organisms
Genes-unit of hereditary information
Asexual reproduction-type of reproduction involving only one parent producing identical offspring
Clone-genetically identical
Sexual reproduction-type of reproduction where two parents have offspring with a combination of genes from both parents
Life cycle-generation to generation of stages in reproductive history
Somatic cell-any cell in a multicellular organism (excluding gametes)
Karyotype-a method of organizing the chromosomes of a cell in relation to number size and type
Homologous chromosome-chromosome pairs of the same length.
Sex chromosomes-one of the pairs of chromosomes that determines the sex of an individual
Autosomes-a chromosome that is not directly involved in determining the sex
Gametes-refers to the eggs and sperm
Haploid cell-a cell containing only one set of chromosomes
Diploid cell-a cell containing two sets of chromosomes
Fertilization/Syngamy-union of haploid gametes
Zygote-when the gametes are united
Sporophyte-multicellular diploid form in organisms undergoing alternation of generations that produces haploid spores
Spores-in life cycle of plant that produce haploid cells generating a multicellular individual without fusing with another cell
Gametophyte-multicellular haploid form in organisms that produces haploid gametes


Meiosis I - The first division
Prophase I: Chromosomes condense, homologous chromosomes pair up forming tetrads, crossing over occurs where a section of a chromosome is traded for another section on a different chromosome (aka chiasmata), spindle fibers attach to the kinetochores (where the chromosome is connected to itself).
Metaphase I: Tetrads line up at the metaphase plate, microtubules attach to kinetochores.
Anaphase I: Homologous chromosomes are pulled to opposite sides.
Telophase I: Chromosomes continue to move until there is a haploid set at each end, chromosomes are now consisted of linked sister chromotids.
Cytokinesis: Sperates the cytoplasm, in animals a cleavage furows appears and in plants a cell plate forms, results in two daughter cells with no further eplication of chromosomes. (unlike mitosis, teh daughter cells are not identical)


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Meiosis II - The second division
Prophase II: Spindle apparatus forms and attaches to the kinetochores of every sister chromotid.
Metaphase II: Sister chromatids line up on the metaphase plate, the microtubules are attached twice to the sister chromatid on opposite sides.
Anaphase II: Sister chromatids are separated at the centromeres.
Telopase II: The separated sister chromatids reach opposite sides and nuclei forms around them.
Cytokinesis: separates the cytoplasm, ending with four haploid daughter cells. (sill not genetically identical and the chromosome number is reduced by half unlike mitosis.)
external image meiosis_2.jpg