Meiosis
Objectives
1. Describe the stages of meiosis.
2. Explain why meiosis occurs.
3. Compare and contrast mitosis and
meiosis
Pg 275-278
Think! Think!! Think!!!
Meiosis is the process in which one
cell divides into four daughter cells
that have half the number of
chromosomes as the parent cell.
Why would your body produce
cells that have half the number of
necessary chromosomes?
Think! Think!! Think!!!
Your body has two types of cells: body
cells and gametes.
Body cells – have 46 or 23 pairs of
chromosomes
Gametes – sperm and egg cells that have
only 23 chromosomes
Gametes join with other gametes and
together they have 46 chromosomes.
Meiosis
Meiosis is the process in which one cell divides into four
daughter cells that have half the number of chromosomes
as the parent cell. In addition, the daughter cells are
different from each other and different from the parent cell.
Diploid – two sets of chromosomes (body cell)
Haploid – one set of chromosomes (gamete)
Stages of Meiosis
The end result of meiosis is four cells, so
there must be two divisions. First one cell
divides into two (Meiosis I) and then each
of those cells divide into two (Meiosis II).
Meiosis I consists of Prophase I, Metaphase
I, Anaphase I and Telophase and
Cytokinesis I. Meiosis II consists of
Prophase II, Anaphase II and Telophase II
and Cytokinesis II.
Meiosis
http://www.sumanasinc.com/webcontent/ani
mations/content/meiosis.html
Mitosis vs. Meiosis
Mitosis
Meiosis
Creates two cells from one, identical to each
other and identical to the parent cell
Creates four cells from one, different from
each other and from the parent cell
Makes diploid daughter cells
Makes haploid daughter cells
Occurs in body cells
Occurs in gametes
Has only one set of phases
Has two sets of phases
Sister chromatids split apart and one
chromosome of each pair goes to each
daughter cell
Homologous sister chromatids pair up to
form a tetrad and two sister chromatids go to
each daughter cell, then the sister chromatids
split apart
Chromosomes do not “cross over”
Chromosomes do “cross over”
Meiosis
Homologous – similar
Homologous chromosomes pair up. They are
homologous because one was inherited from mom
and one was inherited from dad.
Tetrad – two sets of homologous sister chromatids
joined together.
Crossing Over
When the tetrad is formed, the “legs” switch
pieces.
Mitosis vs. Meiosis
http://www.pbs.org/wgbh/nova/body/howcells-divide.html
Meiosis
During meiosis, the chromosomes are
“shuffled” like a deck of cards because of
crossing over. This means that every child a
parent has will be different from all the
other children that could possible be
conceived.
Crossing over spreads biological variation
throughout the population. Variation makes
a population strong!