1. MEIOSIS NOTES

2. Do Now – Meiosis vs. Mitosis
What are the 6 phases of Mitosis?
During which phase of mitosis do the chromosomes line up across the center of the cell?
During mitosis, are the daughter cells identical to the parent cell or different? Explain your answer.

3. Reproduction
Asexual (relies on mitosis): all of the parents DNA goes to the offspring
Sexual (relies on meiosis): used to create sex cells

4. 2 TYPES OF CELLS
BODY CELLS
SEX CELLS

5. Types of Cells: Body Cells
All cells other than sex cells
-Also called somatic cells
-Made by mitosis
This scanning electron micrograph (courtesy of Dr. Marion J. Barnhart) shows the characteristic
biconcave shape of red blood cells.

6. Types of Cells: Body Cells
-Contain 2 copies of each chromosome
-called homologous chromosomes
-have diploid #, 2n
EX. Humans =46

7. Types of Cells: Sex Cells
*Also called gametes
*Eggs and sperm
*Made by meiosis
*Have haploid #, n
*EX. Humans =23

8. Sex Cells (sea urchin egg being fertilized)

9. Chromosome # Egg (haploid #) Sperm(haploid #) ZYGOTE (diploid #)
Ex. Humans
=46

10. Meiosis
The form of cell division by which gametes, with half the number of chromosomes, are produced.
Diploid (2n)  haploid (n)
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).

11. Meiosis occurs in 2 steps
MEIOSIS I AND MEIOSIS II

12. MEIOSIS I
Meiosis I (P1, M1, A1, T1)
-homologous chromosome pairs are divided
-two new daughter cells are produced
-reduction division

13. MEIOSIS II Meiosis II (P2, M2, A2, T2)
Two sister chromatids are split into two daughter cells
Analagous to mitosis because chromosome # stays the same

14. Meiosis Diagram

15. CROSSING OVER Occurs during Prophase I of meiosis
Chromosomes form and DNA is exchanged
ADVANTAGE: genetic variation

16.                                                               
Adapted from: Morgan T.H., Sturtevant A.H., Muller H.J., and Bridges C.B., "The Mechansim of Mendelian Heredity", 1915.
Crossing Over

17. Gamete Development in Males and Females
Meiosis occurs in testes at puberty and then occurs as needed
1 spermatocyte=4 sperm
FEMALES
Meiosis occurs in the ovaries before birth
1 oocyte=1 egg and 3 wasted cells

18. Sperm Formation

19. Egg Formation (unequal cytokinesis)

20. Comparing Mitosis and Meiosis
HOW MANY CELLS ARE MADE?
Mitosis 2
Meiosis 4

21. Comparing Mitosis and Meiosis
ARE RESULTING CELLS GENETICALLY THE SAME OR DIFFERENT?
Mitosis
Genetically the same
Meiosis
Genetically different

22. Comparing Mitosis and Meiosis
WHAT TYPES OF CELLS RESULT IN THE PROCESS? (BODY CELLS OR GAMETES)
Mitosis
Body cells
Meiosis
Gametes

23. Comparing Mitosis and Meiosis
WHAT ARE THE ADVANTAGES OF EACH PROCESS?
Mitosis
Growth and repair
Meiosis
Formation of sex cells and genetic variation

24. Fertilization n=23 egg 2n=46 zygote
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

25. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

26. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

27. Karyotype
A method of organizing the chromosomes of a cell in relation to number, size, and type.

28. Humans have 23 Sets of Homologous Chromosomes Each Homologous set is made up of 2 Homologues.

29. Autosomes (The Autosomes code for most of the offspring’s traits)
In Humans the “Autosomes” are sets

30. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?

31. In Humans the “Sex Chromosomes” are the 23rd set
Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male.
In Humans the “Sex Chromosomes” are the 23rd set
XX chromosome - female
XY chromosome - male

32. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

33. Crossing Over
Crossing over (variation) may occur between nonsister chromatids at the chiasmata.
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Chiasmata (chiasma) are the sites of crossing over.

34. Crossing Over - variation
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

35. Do Now
Take out your Meiosis notes.
Sit quietly.