1. AP Biology
Meiosis Part 2

2. Interphase

3. Important concepts from previous units:
Evolution is “change over time”.
Sexual reproduction involves haploid sperm and egg gametes.
The DNA within the egg and sperm will create the next generation organism.

5. Mom and Dad’s DNA contribution The egg surrounded by sperm.

6. Meiosis - means “The process of gamete formation”
This process occurs in the cells of the sex organs of the organism. These organs are called Gonads.
This process has 2 divisions in the process after the S and G2 phases.
Remember , that the S phase doubles the number of chromosomes. In humans 46  92.
Meiosis I - This division is the separation of chromosome pairs. In humans, 92  46
Meiosis II - This division is the separation of sister chromatids. In humans, 46  23

7. (First Division)

8. (Second Division)

9. TWO Divisions

10. In this process, males produce 4 haploid sperm; each having 23 chromosomes.
In this process, females produce 1 haploid egg with 23 chromosomes. The other three cells degrade into structures called polar bodies during the process. These can be seen on the nucleus membrane in female cells, not males.
Stages to the process of Meiosis
These stages are very similar to the stages of Mitosis.
Three major differences, from Mitosis, are present to increase variation.
(Remember, Mitosis is normal cell division. It basically makes clones of the adult. No variation.)
Crossover (“genetic swapping”) occurs in Prophase I. (Creates variation.)
Chromosome pairs separate in Anaphase I. (Creates Variation.)
Sister Chromatids separate in Anaphase II. (Creates Variation.)

12. Spermatogenesis Epididymis Seminiferous tubule Testis Cross section
of seminiferous
tubule
Spermatogonium
Mitotic division, producing
large numbers of spermatogonia
Sertoli cell
nucleus
Differentiation and onset of meiosis I
Primary spermatocyte
(in prophase of meiosis I)
Meiosis I completed
Secondary spermatocyte
Meiosis II
Lumen of
Seminiferous tubule
Early
spermatids
Spermatids
(at two stages of
differentiation)
Differentiation
Sperm cells
Neck
Head
Midpiece
Tail
Plasma membrane
Acrosome
Nucleus
Mitochondria

13. Oogenesis in the ovaries
Ovary
Primary germ cell
in embryo
Differentiation
Oogonium
in ovary
Oogonium
Mitotic
division
Primary
oocyte
within
follicle
Primary
oocyte
Completion of meiosis I
and onset of meiosis II
Growing
follicle
Secondary
oocyte
First polar body
Ovulation
Entry of sperm
triggers
completion
of meiosis II
Mature follicle
Ruptured
follicle
Second polar body
Ovum
Ovulated
secondary oocyte
Corpus luteum
Degen-
erating corpus luteum

14. Crossover (“genetic swapping”) between homologous chromosomes.
This creates variation from the parent’s genome. They are then called Recombinant Chromosomes.
Synapsis – Chromosomes that are in a state of being intertwined together. (“syn” means “together”; “sis” means “process of”)
Tetrad - Four chromosomes twisted together (“tetra” means “four”… Like the game Tetris has four different shapes.)
Chiasmata – Where the chromosomes physically overlap making an “x”. (Chi is the Greek letter for X.)

16. Crossover in Prophase I

17. Major differences between Mitosis and Meiosis:
The number of divisions (Mitosis has 1; Meiosis has 2)
The final products of each process (Mitosis – “cloned” daughter cells; Meiosis – haploid gametes)
Crossover, in Prophase I, creates variation (No crossover in Mitosis)
Chromosome pairs vs. sister chromatids separating in the second division to reduce DNA to haploid state.

18. Mitosis vs. Meiosis

19. (before chromosome replication)
MITOSIS
MEIOSIS
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Propase
Prophase I
Chromosome
replication
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
Duplicated chromosome
(two sister chromatids)
2n = 6
Chromosomes
positioned at the
metaphase plate
Tetrads
positioned at the
metaphase plate
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate during
anaphase
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Anaphase I
Telophase
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Daughter cells of meiosis II
Sister chromatids separate during anaphase II

20. Sources of variation creation
Independent assortment of chromosomes. ( This happens 2x in Anaphase I and II.)
2 = Total number of possibilities (One goes one way; the other the other way in separation.)
n = number of variables; 23 = number needed to make a haploid set in humans.
2n = 223
For humans the total is about 8 Million possibilities for each parent with each division.
8 Million possible outcomes X 2divisions X 2 parents = 4,096,000,000 possible combinations for just 46 chromosomes!

21. Independent Assortment
Key
Maternal set of
chromosomes
Possibility 1
Possibility 2
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

24. Now add in Crossover (“genetic swapping”)
Amount of crossing over varies from tetrad to tetrad. If little crossover occurs, the offspring looks very much like the adult parent. If lots of crossover occurs, the offspring looks very different from the adult parent.
Random fertilization by a sperm. (There are millions released by the male. Which one will make it to the finish line?)
That makes you a 1 in 70 trillion possibility – YOU ARE PRETTY DARN SPECIAL!

26. 2^23

27. Evolution? As organisms became more complex, a more complex and more survival oriented way of reproducing came into existence over millions of years. The addition of a second division with a couple of slight changes in the same four steps (Prophase, Metaphase, Anaphase, and Telophase) creates the variation. The variation helps with survival and this would be beneficial in changing environments. Those that survive long enough get to reproduce and keep the species going. Those that don’t do not pass on those defective traits for surviving in that environment.

29. Volume of a sphere V= 4/3 π r3
Volume calculation, teach them this approach:
Step 1: Take the radius and cube it (For example 33 = 3 X 3 X 3 = 27)
Step 2: Take the cube answer and the multiply it by 4.
Step 3: Take the answer from step 2 and divide it by 3.
Step 4: Take the answer from step 3 and multiply it by 3.14