Do Now
• Please hand your homework in the
bin.
• Then Answer in your notebooks:
– How do we get half of our DNA from
each parent, and still only have 46
chromosomes?
Meiosis
Chapter 10.1 and 11.3
Why does sex exist?
• Sexual reproduction creates genetic
variability
Let’s Brainstorm…
How do we get our different traits?
What is the difference
between fraternal and
identical twins?
Let’s define the following
words…
• Meiosis:
– Sexual reproduction-production of
gametes
• Gamete:
– Sex Cells (egg and sperm) haploid
• Somatic Cells:
– Body Cells - diploid
• Haploid:
– Half the chromosome number (n)
– 1 of each pair of chromosomes (23)
• Diploid:
– 2 of each type of chromosome (2n)
– Twice the haploid number
• Fertilization:
– Sperm enters egg
• Zygote:
– Fertilized egg (diploid)
Chromosomes and
Chromosome Number
 Human body cells have 46 chromosomes
 Each parent contributes 23 chromosomes
• Homologous chromosomes —one of two
paired chromosomes, one from each
parent
Homologous pair of
chromosomes
Centromere
Sister chromatids
One duplicated
chromosome
Homologous Chromosomes
 Same length
• Same centromere
position
• Carry genes that control
the same inherited traits
Karyotype
Meiosis
• The sexual life cycle in
animals involves
meiosis
 Meiosis produces
gametes.
 When gametes
combine in
fertilization, the
number of
chromosomes is
restored.
DNA
Replication
Growth
Prepare
Division
2N
2N
Mitosis
N
N
N
N
Meiosis
2N
Do Now
• Label A, B, and C
• (Hint same size,
same shape…)
Meiosis
• 2 Divisions
– Meiosis I (separation of homologous
chromosomes)
– Meiosis II (separation of sister
chromatids)
Interphase
• Same as Mitosis (G1, S, G2)
• DNA- Chromatin
• Centrioles- located near the nucleus
Prophase I
• Pairing of homologous chromosomes
occurs. (Synapsis)
• Each chromosome consists of two
chromatids.
• The nuclear envelope breaks down.
• Spindles form
Prophase I (cont.)
• Crossing over produces
exchange of genetic information.
• Crossing over —chromosomal
segments are exchanged
between a pair of homologous
chromosomes.
Metaphase I
• Chromosome centromeres attach to
spindle fibers
• Homologous chromosomes line up at
the equator.
Anaphase I
• Homologous chromosomes separate
and move to opposite poles
Telophase I
• The spindles break down.
• Chromosomes uncoil and form two
nuclei.
• The cell divides.
Prophase II
• A second set of phases begins as the
spindle apparatus forms and the
chromosomes condense.
Metaphase II
• A haploid number of chromosomes
line of at the equator
• Sister chromatids line up
Anaphase II
• The sister chromatids are pulled
apart at the centromere by
spindle fibers
• Move toward the opposite poles
of the cell.
Telophase II
The chromosomes reach the poles, and
the nuclear membrane and nuclei
reform.
Cytokinesis
 Cytokinesis results in four haploid
cells
 Each with n number of
chromosomes.
• https://www.youtube.com/watch?v=rq
PMp0U0HOA
Genetic Variability
• Depending on how the
chromosomes line up at the
equator, four gametes with
four different combinations of
chromosomes can result.
• Genetic variation also is
produced during crossing
over and during fertilization,
when gametes randomly
combine.
Genetic Variability Animation
• http://highered.mcgrawhill.com/sites/0072437316/student_vie
w0/chapter12/animations.html#
Review Spermatogenesis and
oogenesis
Do Now: Label the phases of meios
Do Now – Period 3
• How many
autosomes?
• How many sex
chromosomes?
• What is the sex
of this patient?
• Monosomy,
Trisomy?
• Is this individual
normal?
Do Now
• Match the description to the phase of meiosis.
1. Sister Chromatids are pulled
apart. _____
2. Homologous chromosomes line
up in the middle of the cell._____
A. Prophase I
B. Metaphase I
3. 2 daughter cells are formed ____ C. Telophase I
4. Nuclear membrane disappears__ D. Anaphase II
5. Nuclear membrane reforms
around 4 cells ____
E. Anaphase I
6. Homologous chromosomes are
pulled apart ____
F. Telophase II