MEIOSIS
Gametes are special cells used in sexual
reproduction. It is important that these cells contain 1/2 the
normal number of chromosomes than the regular body cells
(autosomes). When the sperm and egg unite the resulting zygote
will contain the normal number of chromosomes for that organism. A
special process called meiosis occurs in the reproductive organs
to allow this to occur. Meiosis is a bit more complex than mitosis although many of the terms and stages
show many similarities.
Before meiosis occurs the cell duplicates its
chromosomes.
Interphase
Prophase I : The nucleus disappears,
spindle fibers form , and the chromatid pairs form. Unlike mitosis, in meiosis the chromatid pairs pair up with their homologous partners
forming a tetrad. This tetrad contains 2 original homologous
chromosomes and their duplicates. The process of crossing over
occurs during this time allowing for genetic diversity in the
genome. Crossing over is a process
where pieces of two homologous chromosomes, switch places and end
up on the other chromosome. (1 cell )
>
Metaphase I : Tetrads line up along the
center of the cell. (1 cell )
Anaphase I : The tetrads split and move
in opposite directions. During this phase the chromatid pairs move
away from each other. (1 cell )
Telophase I : Two cells form each
containing a complete set of chromosomes in the form of chromatid
pairs. (2 cells )
Prophase II : The process continues
moving the chromatid pairs toward the center of the cell. (2 cells
)
Metaphase II : The chromatid pairs line
up along the center of the cell. (2 cells )
Anaphase II : The chromatid pairs split
moving the individual chromosomes toward the opposite poles. (2
cells )
Telophase II : Each of the two cells
will begin to split in half and produce 2 more cells bringing the
total to 4 cells. Each of these cells contain 1/2 the normal
number of chromosomes. ( 4 cells )
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