The other form of cell division
Chapter 8
 Genes are passed from parents to offspring on
chromosomes.
 Human body cells contain 46 chromosomes
 23 from mother & 23 from father
 Homologous- Each chromosome has a
corresponding, or homologous chromosome.
 Chromosomes occur in pairs
 Diploid: 2 complete sets of each chromosome (2N)
(where N = single set of chromosomes)
 Haploid: 1 single set of chromosomes (N)
In humans:
 Body cells are diploid, and contain 46 chromosomes
 Gametes are haploid, and contain 23 chromosomes
each.
 Why?
Sperm (23) + egg (23) = you! (46)
Karyotype: The arrangement of chromosomes by
homologous pairs
homologous
chromosomes
 Picture taken during cell division
 Stained to show banding
pattern- location of genes
 Use size and banding
pattern to match pairs
Meiosis
 The cell division that results in half the number of
chromosomes in each cell
 Occurs in human gametes (sex cells)
 Let’s follow a cell through meiosis where 2N=4
 1. Interphase- DNA is replicated and cell prepares to
divide
 2. Prophase I – Each replicated chromosome pairs up
with its homologous chromosome forming a tetrad.
*Plus everything else that
we already learned occurs
in prophase.
 Crossing over- Chromatids of homologous
chromosomes cross over each other and exchange
some alleles.
 3. Metaphase I Paired homologous chromosomes line up across the
center of the cell.
 Spindle fibers attach to each tetrad.
tetrad
 4. Anaphase I- Spindle fibers pull apart each
homologous chromosome pair toward the poles.
Crossing over that occurred
 5. Telophase I- Separated chromosomes cluster on
either side of the cell and nuclear membrane reforms
around each.
 Cytokinesis- Cytoplasm divides and cell membrane is
pinched off forming two non-identical cells (due to
crossing over).
 The two cells now enter meiosis II
 They do not re-copy DNA, however some cells might go through G1 and
G2 checkpoints again.
 Meiosis II  just like the steps of mitosis
 1. Prophase II Nuclear membrane breaks down
 Chromosomes each consisting of 2 sister chromatids
condense to become visible (if not so already).
 Spindle fibers form and centrioles begin to move to
opposite sides of the cell.
 2. Metaphase II- Chromosomes line up in the middle
of the cell, spindle fibers attach to centromeres.
 3. Anaphase II- Sister chromatids split and spindle
fibers pull them to either side of the cell.
 4. Telophase/Cytokinesis II- Nuclear membrane
reforms and cytoplasm is divided during the pinching
off of the cell membrane.
 Four haploid daughter cells (not genetically identical)
 N=2
 The resulting 4 haploid cells are gametes (in humans)
A video of meiosis