Activating Strategy
• Two minute talk
– In 60 seconds, tell your partner everything you
know about meiosis.
– After 60 seconds, its your partners turn to add their
information to the discussion.
AP Lesson #38
EQ: What is the value of sexual
reproduction for living organisms?
What happens during
Meiosis 1? Interphase
• DNA copies during interphase
• 1st division of meiosis separates
homologous pairs
2n = 4
single
stranded
DNA copies
prophase 1
2n = 4
double
stranded
What happens during Meiosis 2?
• 2nd division of meiosis separates
sister chromatids
• What does this
division look like?
1n = 2
double
stranded
prophase 2
synapsis
metaphase 1
anaphase 1
tetrad
2n = 4
double
stranded
anaphase 2
1n = 2
double
stranded
What is the value of sexual reproduction?
– crossing over
metaphase 2
reduction
telophase 1
• Sexual reproduction introduces genetic variation
1n = 2
double
stranded
1n = 2
single
stranded
4
telophase 2
How does crossing-over create variation?
• DNA from one chromosomes switches places
with DNA from its homologous chromosome
• mixing of alleles
– genetic recombination
• independent assortment
– random fertilization
• which sperm fertilizes which egg?
• Driving evolution
– providing variation for natural selection
1
Activity #1
How is DNA traded?
1. Synapse
homologous
chromosomes
2. Any CrossingOver event
3. Compare your
gametes with
others
• Prophase 1: HC synapse and
sister chromatids intertwine
prophase 1
– Chiasmata
– homologous pairs swap
pieces of chromosome
chiasmata
synapsis
Why is crossing-over important?
• New combinations of traits on each chromosome
– creates an infinite variety in gametes
How does genetic recombination create variation?
• Independent assortment
– Chromosomes align independently during cell
division
– Each resulting gamete has different genes
and chromosomes
metaphase1
Activity #2
1. Draw a line down the
middle of your table
with chalk.
2. Line up your
homologous
chromosomes during
metaphase 1
3. Separate into gametes
4. Compare your gametes
5. How would this look
with 23 pair?
What does independent assortment create?
metaphase1
• gametes of offspring do not have same
combination of genes as gametes from parents
– random assortment in humans produces
223 (8,388,608) different combinations in
gametes
from Mom
from Dad
offspring
new gametes
made by offspring
2
How does random fertilization create variation?
• Sperm + Egg = ?
– any 2 parents will produce a zygote with over 70
trillion (223 x 223) possible diploid combinations
Activity #3
1. Take one of your gametes and go fertilize
someone!
2. Compare the zygote you created with other
groups.
3. What similarities do you see?
4. What differences do you see?
5. What does this all mean?
Sexual reproduction creates variability
But still allows us to maintain both genetic similarity &
differences.
Baldwin brothers
Summarizing Strategy
• Write a letter:
– Write a letter to me explaining why my new
born baby “Andrew” has different traits than
me.
Martin & Charlie Sheen, Emilio Estevez
Putting it all together…
Assessment
• HW: Chapter 13.3 – 13.4 Wkst
meiosis → fertilization → mitosis + development
gametes
46
meiosis
23
23
egg
23
46
23
zygote
fertilization
sperm
46
46 46
46 46 46
46
4646
mitosis
development
3