Mitosis & Meiosis
Mitosis Review
Cell makes a copy of itself
1) Copy its DNA (part of a chromosome).
2) Copies are separated & sorted into two sides of the cell.
3) The cell then splits in two.
4) Part of each parent is carried to the two new cells.
5) Two EXACT daughter cells (same number of chromosomes)
Examples: skin, bones, internal organs
Meiosis – Sexual Reproduction
• Cell division that forms gametes (egg and sperm
cells)
• Gametes have half the # of chromosomes.
• Two divisions (meiosis I and meiosis II).
• Part of each parent is carried to the four new cells.
• Meiosis is similar to mitosis with some
chromosomal differences.
Mitosis:
Meiosis:
Each resulting cell still has chromosomes from Mom & Dad
Tips
• MITosis takes one cell and Makes It Two
• Meiosis has to do with sex
• From the cell’s point of view:
– mITosis results in Identical Twins CELLS
– mEioSis results in Egg and Sperm
Homologous Chromosomes
• Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
• Homologous pairs carry genes controlling the same
inherited traits.
• Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 1 pair of sex chromosomes
Homologous chromosomes
sister chromatids
paternal
Tetrad
sister chromatids
maternal
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
Two “X” chromosomes = female.
One “X” chromosome and one “Y” chromosome = male.
In Humans the
“Sex
Chromosomes”
are the 23rd set
XX chromosome - female
XY chromosome - male
Cell Division in Sex Cells
• Interphase I
• Meiosis I and Meiosis 2
• Similar to Mitosis EXCEPT changes in
chromosomes
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• Four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
spindle fiber
centrioles
TETRAD
Prophase I
• Homologous chromosomes come
together to form a tetrad.
• Tetrad is two chromosomes or
four chromatids (sister and
nonsister chromatids).
Homologous chromosomes
sister chromatids
paternal
Tetrad
sister chromatids
maternal
Prophase 1 – Crossing Over
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
• Crossing over causes variation
Crossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Metaphase I
• Tetrads align on the in center of cell on spindle
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
OR
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.
• Each pole now
has haploid setIof
Telophase
chromosomes.
• Cytokinesis occurs and two haploid daughter
cells are formed.
Meiosis II
• No interphase II
• Remember: Meiosis II is similar to mitosis
Prophase II
• same as prophase in mitosis
Metaphase II
• same as metaphase in mitosis
Anaphase II
• same as anaphase in mitosis
• sister chromatids separate
• Four haploid Telophase
daughter cells produced
II
gametes = sperm or egg
2n
diploid
n haploid
n haploid
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
Oogenesis
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Variation
• Important to population as the raw
material for natural selection.
• Question:
What are the three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
• A cell containing 20 chromosomes (diploid)
at the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes
• Meiosis animation:
http://www.youtube.com/watch?v=D1_mQS_FZ0&NR=1