1. SEXUAL REPRODUCTION

2. What is meiosis?
It is a special type of cell division that makes egg and sperm
How?
Chromosomes replicate
Each cell divides twice
This provides each cell with half the number of chromosomes

3. Let’s review some vocabulary first:
1. Homologous chromosomes (homologs):
A PAIR of chromosomes carrying the SAME GENES (ONE from each parent)
homologous = same information
Humans have 23 pairs
2. Sister Chromatids:
2 “identical” strands of DNA that make up each member of a homologous pair

4. 23 46 23 46 meiosis egg sperm 3. Diploid (2N):
having 2 of each chromosome type
ex. mitosis always STARTS and RESULTS in a diploid number (46)
ex. meiosis always STARTS with diploid number (46)
4. Haploid (N):
A cell (always a gamete) that contains ½ the number of chromosomes
Meiosis results in cells with ½ number of chromosomes (23)!
Draw and label
this diagram
in the box
of your notes 46 23
egg
meiosis 23 46
sperm
Haploid (n)
Diploid (2n) 4

6. 2n=46 egg n=23 5. Fertilization - THE JOINING OF SEX CELLS
½ genetic (egg) + ½ genetic (sperm) = 1 genetically whole organism
(n=23) (n=23) = (2n=46)
egg Sperm zygote . =
n=23
egg
sperm
n=23
2n=46
zygote

7. 6. Zygote
is a fertilized egg

8. Fertilization
Embryo
Zygote
Mitosis

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

10. There are two categories of chromosomes:
Sex chromosomes – X, Y
Autosomes – 1, 2, 3,….. (numbers 1-22 in humans)

11. XY chromosome - male
XX chromosome - female

12. 8. CROSSING-OVER:
Process when homologous chromosomes “cross” each other and exchange genes
Source of variation-gives new combinations of mom and dad’s genes in one chromosome
Reason why you aren’t exactly like your siblings!

13. Images of Chromosomes crossing over
Go to Section:

14. How does this explain: family resemblance & differences!
Why are the kids so similar to the parents but not exact?
Michael & Kirk Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

15. MEIOSIS: Cell Division to form Gametes
Video: (2 min)

16. Meiosis: SEX!!!!!
A form of cell division by which sex cells called gametes (egg and sperm), are produced.
Takes place in reproductive structures called gonads (ovaries and testes)
Diploid (2n)  haploid (n)
Each cell divides twice - (meiosis I and meiosis II)
Each sex cell has half the number of chromosomes

17. new gametes made by offspring
We’re
mixing things
up here!
Allows for genetic variation of offspring
gametes of offspring do not have same genes as gametes from parents
Consider the greater variation with 23 pairs of chromosomes = mixing and matching
from Dad
variation
from Mom
offspring
new gametes made by offspring 17

18. HOMOLOGOUS CHROMOSOMES SEPARATE
MEIOSIS I:
HOMOLOGOUS CHROMOSOMES SEPARATE
PRIOR TO MEIOSIS
MEIOSIS I
Tetrad (4 chromatids from homologous chromosomes)
Sister chromatids
Figure: 9.3, Left
Caption:
Exercise Label prophase, metaphase, anaphase, and telophase cells of meiosis I and meiosis II.
1. Chromosomes replicate in parent cell.
2. Synapsis of homologous chromosomes. Crossing over of non-sister chromatids.
3. Tetrads migrate to middle of cell.
4. Homologs separate.

19. SISTER CHROMATIDS SEPARATE
MEIOSIS II:
SISTER CHROMATIDS SEPARATE
MEIOSIS II
Figure: 9.3, Right
Caption:
Exercise Label prophase, metaphase, anaphase, and telophase cells of meiosis I and meiosis II.
6. Chromosomes begin moving to middle of cell.
5. Cell divides.
7. Chromosomes line up at middle of cell.
9. Cell division results in four daughter cells.
8. Sister chromatids separate.

20. 9. Gametogenesis
making gametes (sex cells- EGG and SPERM)

21. Basic diagram of Meiosis- Ova46
chromosomes
Oogenesis:
Formation of egg cells
(ova)
Results in 1 functional gamete and 3 polar bodies
Female Human
Pre-Gamete
Replication and Crossing-Over 46
different
chromosomes 46
different
chromosomes
First cell division
1 functional ovum and 3 polar bodies 23

22. Basic diagram of Meiosis- Sperm46
chromosomes
Male Human
Pre-Gamete
Spermatogenesis:
Formation of sperm cells
Results in 4 functional gametes
Replication and Crossing-Over 46
different
chromosomes 46
different
chromosomes
First cell division
4 functional sperm
After 2nd cell division 23 23 23 23

23. ADVANTAGES OF SEXUAL REPRODUCTION
1. Sexual reproduction increases individual variation within a species
Asexual Reproduction (Mitosis):
Results in offspring = to parents
Sexual Reproduction (Meiosis):
Results in offspring that are not = to parents
2. Genetic diversity may produce some organisms that are adaptable to non favorable conditions:
Susceptibility to pathogens
Environmental changes

24. DISADVANTAGES OF SEXUAL REPRODUCTION
Sex Slows Rate of Reproduction!
Sexual reproduction should decline in frequency relative to asexual reproduction
Sexual Reproduction:
Only females produce offspring, therefore fewer are produced
Asexual Reproduction:
All individuals can produce offspring

25. 2. Errors In Inheritance:
Genetic disorders occur for various reasons:
Nondisjunction:
Failure of a pair of chromosomes to separate during gamete formation
Abnormal numbers of one or more chromosomes is usually, but not always, fatal

27. Includes Genetic Disorders Like:
Down’s syndrome (chromosome #21)

28. Comparing Mitosis & Meiosis – 1 min
Animation

29. KEYS TO MEIOSIS Original cell has complete set of chromosomes
(Diploid or 2N)
2 divisions result in 4 DAUGHTER CELLS (gametes: 1 ova or 4 sperm)
Genetic material is exchanged during CROSSING OVER
CHROMOSOMES “criss-cross” EXCHANGING GENES
Daughter cells (gametes) are genetically different
Daughter cells (gametes) are now HAPLOID or N ( ½ the original # )
Ex. If original cell has 46 chromosomes, gametes that result from meiosis will have 23 chromosomes
OCCURS ONLY IN MULTICELLUAR, SEXUALLY REPRODUCING ORGANISMS

31. COMPARISON OF MITOSIS AND MEIOSIS
Characteristic
Mitosis
Meiosis (SEX)
Function
Growth, development, replacement, repair
Make gametes (egg and sperm)
Occurs in
Body (Somatic) cells
Immature reproductive cells
Results in
2 daughter cells (clones of each other)
4 daughter cells (all different)
Chromosome #
Daughter cells are Diploid
(2n) = 46 chromosomes
Daughter cells are Haploid
(1n) = 23 chromosomes
Reproduction Type
Asexual – 1 parent
Sexual – 2 parents
Stages
Drawing
1 cell division (IPMAT)
2 cell divisions (PMAT x 2) 46 46 23 46 23 46 46 46 23 23