1. Meiosis & Sexual Reproduction

2. Cell division / Asexual reproduction
Mitosis
produce cells with same information
identical daughter cells
exact copies
clones
same amount of DNA
same number of chromosomes
same genetic information
Aaaargh! I’m seeing double!

3. What is Sexual Reproduction?
When an organism is produced from 2 parents
In sexual reproduction, genetically different offspring are produced
In asexual reproduction, genetically identical offspring are produced

4. Lets look at an example to start
We are using fruit flies as an example
Every cell in a fruit fly has 8 chromosomes
This means that 4 chromosomes came from the female parent and 4 chromosomes came from the male parent

5. Homologous Chromosomes
These chromosomes are called homologous chromosomes
This means that the 4 that came from the male parent, have a matching chromosome from the female parent

6. Diploid cell
A cell that contains 2 sets of homologous chromosomes is called a diploid cell
Examples: Skin, hair, brain, cheek cells, etc.
One set of chromosomes is donated from each parents
You represent this with a 2N
N stands for number of chromosomes
BODY (SOMATIC) CELL = DIPLOID = 2N

7. Formation of sex cells Sex cells are called gametes
In males, the gametes are called sperm
In females, the gametes are called eggs

8. Haploid cells
Gametes, or sex cells, only contain half the number of chromosomes as regular cells in the body
Cells that only have half of the chromosomes are called haploid cells
Haploid cells are represented by N
GAMETES = HAPLOID = 1N

9. SEXUAL REPRODUCTION – Check for understanding
Which human cells are haploid?
What are these cells called?
What symbol do we use for these cells?
Which human cells are diploid?

10. SEXUAL REPRODUCTION – What happens?
The sperm and egg come together in a process called fertilization
When the sperm and egg join, we call the cell a zygote

11. Why is it like this?
Why it is important that gametes (sex cells) only have half the number of chromosomes?
Because when they are fertilized, a gamete from the female parent and a gamete from the male cell fuse to form a diploid cell

12. SEXUAL REPRODUCTION – Summary
1N 2N
Gametes Body cell
Haploid Diploid
Sperm (dad) and Zygote
Egg (mom)

13. SEXUAL REPRODUCTION – Check for understanding
What do we call the process where a sperm and an egg join?
What is a zygote?
Are sperm and egg haploid or diploid before fertilization?
Is the zygote haploid or diploid?

14. Steps For Calculating Chromosome Number
1. HAMMER out the question
2. Use your 1N and 2N columns to figure out what you have and what you are trying to find
3. Use the following rules to calculate chromosome number:
1N N MULTIPLY BY 2
2N N DIVIDE BY 2

15. EXAMPLE #1
A human skin cell has 46 chromosomes. How many chromosomes will be in a human egg cell?

16. 46 chromosomes/2 = 23 chromosomes
EXAMPLE #1
A human skin cell has 46 chromosomes. How many chromosomes will be in a human egg cell?
Skin cell = diploid = 2N
Human egg cell?
46 chromosomes/2 = 23 chromosomes

17. EXAMPLE #2
An egg cell of a cockroach has 5 chromosomes. How many chromosomes will be in the fertilized zygote?

18. 5 chromosomes x 2 = 10 chromosomes
EXAMPLE #2
An egg cell of a cockroach has 5 chromosomes. How many chromosomes will be in the fertilized zygote?
Egg Cell = haploid = 1N
Fertilized zygote?
5 chromosomes x 2 = 10 chromosomes

19. YOUR TURN:
Horses have 64 chromosomes in each body cell. How many chromosomes should be in each horse gamete?
A squirrel tail cell has 50 chromosomes. How many chromosomes are present in the squirrel’s sperm cell?

20. Similar to mitosis…but not the same!
MEIOSIS

21. Meiosis Meiosis is the process of cell division that forms gametes
It is broken into 2 distinct divisions
Meiosis I and Meiosis II

22. Overview of meiosis I.P.M.A.T.P.M.A.T 2n = 4 interphase 1 prophase 1
metaphase 1
anaphase 1
n = 2
n = 2
prophase 2
metaphase 2
anaphase 2
telophase 2
n = 2
telophase 1

23. Meiosis I
Before meiosis I begins, amount of DNA is replicated (doubled)

24. Preparing for meiosis 1st step of meiosis Duplication of DNA
Why bother?
meiosis evolved after mitosis
convenient to use “machinery” of mitosis
DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis)
2n = 6
single
stranded
2n = 6
double
stranded
M1 prophase

25. Meiosis I
Prophase I :
Homologous chromosomes come together to form a tetrad
Tetrad is two chromosomes or 4 chromatides

26. Homologous chromosomes
Prophase I
Homologous chromosomes
sister chromatids
Tetrad

27. Crossing over During prophase I crossing-over happens
Crossing over is when chromatids exchange parts of their genetic material

28. What are the advantages of crossing over in sexual reproduction?
3 steps
cross over
breakage of DNA
re-fusing of DNA
New combinations of traits
Sexual reproduction is advantageous to species that benefit from genetic variability. However, since evolution occurs because of changes in an individual's DNA, crossing over and chromosome segregation is likely to result in progeny that are less well-adapted than their parents. On the other hand, asexual reproduction ensures the production of progeny as fit as the parent since they are identical to the parent. Remember the adage, “if it's not broken, don't fix it.” There are several hypotheses regarding the evolution of sexual reproduction. One is associated with repairing double-stranded DNA breaks induced by radiation or chemicals. The contagion hypothesis suggests that sex arose from infection by mobile genetic elements. The Red Queen hypothesis theorizes that sex is needed to store certain recessive alleles in case they are needed in the future. Along similar lines, eukaryotic cells build up large numbers of harmful mutations. Sex, as explained by Miller's rachet hypothesis, may simply be a way to reduce these mutations. The “whole truth” is likely a combination of these factors. Regardless of how and why, the great diversity of vertebrates and higher plants and their ability to adapt to the highly varied habitats is indeed a result of their sexual reproduction.

29. Meiosis I cont…
At this point the cells go through the rest of the steps of cell division to form new cells
Unlike mitosis, this process does not produce identical daughter cells
Because of cross over these cells are all a little bit different

30. Meiosis 1 1st division of meiosis separates homologous pairs synapsis
single
stranded
Meiosis 1
1st division of meiosis separates homologous pairs
2n = 4
double
stranded
prophase 1
synapsis
2n = 4
double
stranded
metaphase 1
tetrad
reduction
1n = 2
double
stranded
telophase 1
Repeat after me!
I can’t hear you!

31. What does this division look like?
Meiosis 2
2nd division of meiosis separates sister chromatids
1n = 2
double
stranded
prophase 2
What does this division look like?
1n = 2
double
stranded
metaphase 2
1n = 2
single
stranded
telophase 2 4

32. Meiosis II cont…
You end up with 4 new cells that each contain half the number of chromosomes
HAPLOID CELLS

33. Meiosis 1 & 2

34. Mitosis vs. Meiosis

35. Mitosis vs. Meiosis Mitosis Meiosis 1 division
daughter cells genetically identical to parent cell
produces 2 cells
2n  2n
produces cells for growth & repair
no crossing over
Meiosis
2 divisions
daughter cells genetically different from parent
produces 4 cells
2n  1n
produces gametes
crossing over

36. Putting it all together…
meiosis  fertilization  mitosis + development
gametes 46 23 46 23 46 46 46 46 46 23
meiosis 46 46
egg 46 46 23
zygote
fertilization
mitosis
sperm
development

37. The value of sexual reproduction
Sexual reproduction introduces genetic variation
genetic recombination
independent assortment of chromosomes
random alignment of homologous chromosomes in Metaphase 1
crossing over
mixing of alleles across homologous chromosomes
random fertilization
which sperm fertilizes which egg?
Driving evolution
providing variation for natural selection
metaphase1

38. Variation from genetic recombination
Independent assortment of chromosomes
meiosis introduces genetic variation
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
offspring
from Mom
from Dad
new gametes
made by offspring

39. Variation from crossing over
Crossing over creates completely new combinations of traits on each chromosome
creates an infinite variety in gametes