CHAPTER 11 NOTES
MEIOSIS
Mendel’s Law of Segregation
Definition:
a process of reduction division that
produces gametes in which the
number of chromosomes per cell
is cut in half (haploid) through the
separation of homologous
chromosomes.
Homologous chromosomes:
chromosomes that have the same
genes but different variations
because one comes from mom
and one from dad.
• A cell that contains both sets of
homologous chromosomes is
diploid, which means “two sets”.
(2n)
• A cell that contains only one set
of chromosomes is haploid. (n)
CELL CYCLE:
Interphase
Meiosis I
Cytokinesis
Meiosis II
Cytokinesis
INTERPHASE: just like interphase
before mitosis
• G1: cells grow, protein
production
• S: DNA replication
• G2: productions of new
organelles including centrioles
MEIOSIS I:
PROPHASE I:
• Chromatin coils into chromatids
and the homologous pairs of
chromatids come together to form
a tetrad.
• Crossing over
occurs where
portions of the
homologous
chromosomes
are exchanged.
• The rest of prophase is very
similar to mitosis.
Why is crossing over so important?
Crossing over allows for the
random mixing of genes (genetic
recombination) which adds
genetic variety to a species so that
no two individuals are exactly the
same.
METAPHASE I:
• Tetrads (doubled homologous
chromosomes) line up at the
equator of the spindle.
ANAPHASE I:
• Homologous chromosomes
(tetrad) separate and are pulled to
the poles (the centromeres do
NOT split).
TELOPHASE I:
• Chromosomes uncoil, spindle
breaks down. The nuclear
membranes and nucleoli reform.
• Cytoplasm divides (cytokinesis)
to form two cells. Cells are
haploid at the end of meiosis I
NOTE: There is NO interphase
between meiosis I and meiosis II.
Why is this important?
So that each new cell will only
have half (haploid) the number
of chromosomes as the original
parent cell.
PROPHASE II:
METAPHASE II:
ANAPHASE II:
TELOPHASE II/
CYTOKINESIS:
*just like mitosis
except that there
is only half the
number of
chromosomes.
Prophase II
Metaphase II
Anaphase II
Telophase II
What is the end result of meiosis?
Four genetically different haploid
cells that containing half the
number of chromosomes.
What was the end result of mitosis?
Two diploid cells genetically
identical to the original parent
cell.