1. Meiosis & Sexual Reproduction

2. Aaaargh! I’m seeing double!
Cell division
Mitosis
produce cells with same information
____________________________
exact copies
_______________________
same amount of DNA
___________________________
same genetic information
Aaaargh! I’m seeing double!

3. The Human Life Cycle Gametes vs. Somatic Cells
____________: Sperm or ovary cells
Somatic Cells: Any other cell besides sperm or ova (muscle, nerve, etc.)
________________________________________________
Human gametes contain 23 chromosomes

4. + 46 46 92 How about the rest of us?
What if a complex multicellular organism (like us) wants to reproduce?
__________________________
Do we make egg & sperm by mitosis?
What if we did, then…. 46 + 46 92
egg
sperm
zygote
Doesn’t work!

5. Human female karyotype
46 chromosomes
23 pairs

6. Human male karyotype
46 chromosomes
23 pairs

7. Homologous chromosomes
Paired chromosomes
both chromosomes of a pair carry “matching” genes
control same inherited characters
_____________________________
single stranded homologous chromosomes
diploid 2n
2n = 4
double stranded homologous chromosomes

8. Chromosomes A chromosome is a long coiled-up strand of DNA
Humans have 23 pairs of homologous chromosomes in each somatic cell
Homologous Chromosomes:
Chromosomes that have the same length,
centromere position, and staining pattern
Carry genes controlling the same inherited characteristics (ie. Eye color, attached ear lobes)
One chromosome of each pair comes from each parent
Only exception…
_______________________________________
Female sex chromosomes (XX) -homologous

9. How do we make sperm & eggs?
Must reduce 46 chromosomes  23
_________________________________ 23 46 23 46
egg 23
meiosis 46 23
zygote
fertilization
sperm
gametes

10. Meiosis
Big Picture:
Replication of chromosomes (DNA) followed by two consecutive cell divisions
Results in 4 different daughter cells, each with only half as many chromosomes as the parent (haploid)

11. Steps of meiosis Meiosis 1 Meiosis 2 interphase prophase 1 metaphase 1
anaphase 1
telophase 1
Meiosis 2
prophase 2
metaphase 2
anaphase 2
telophase 2
1st division of meiosis separates homologous pairs
(2n  1n)
“reduction division”
2nd division of meiosis separates sister chromatids
(1n  1n)
* just like mitosis *

12. Meiosis: production of gametes
Alternating processes, alternating stages
chromosome number must be reduced
diploid  haploid
_______________
humans: 46  23
meiosis reduces chromosome number
makes gametes
fertilization restores chromosome number
haploid  diploid
____________

13. Meiosis Reduction Division
special cell division in sexually reproducing organisms
_______________
diploid  haploid
“half”
________________
sperm, eggs
Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse the two!

14. Preparing for meiosis 1st step of meiosis _________________
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

15. Meiosis I starts with diploid cell with duplicated chromosomes
ends with separation of homologous chromosomes in 2 daughter cells which are each haploid but with duplicated chromosomes (reductional division)

16. Meiosis I: Separating Homologous Chromosomes
Prophase I:
Chromosomes begin to condense (you can see them!)
_______________:
Homologous chromosomes come together (4 sister chromatids total)
Chiasmata
Chromatids cross over at various places, known as chiasmata
An exchange of genetic information occurs
_______________________________
Spindle microtubules begin to form

17. What are the advantages of sexual reproduction?
Crossing over
What are the advantages of sexual reproduction?
3 steps
________________
breakage of DNA
New combinations of traits

18. Meiosis I: Separating Homologous Chromosomes
Metaphase I:
Chromosomes are arranged on the metaphase plate in homologous pairs

19. Meiosis I: Separating Homologous Chromosomes
___________:
The spindle fibers move the chromosomes toward the poles
Each homologous chromosome of a tetrad goes to opposite poles

20. Meiosis I: Separating Homologous Chromosomes
Telophase I:
The chromosomes reach the poles of the cell
_______________
Animals: cleavage furrow
Plants: cell plate

21. Meiosis II: Separating Sister Chromatids
Prophase II:
Centrioles duplicate
Chromosomes move towards middle
Metaphase II:
_______________________________________________

22. Meiosis II: Separating Sister Chromatids
Anaphase II:
Centromeres of sister chromatids separate
Sister chromatids move toward opposite poles of the cell
Telophase II/Cytokinesis:
Nuclei form at opposite poles of the cell
Cytokinesis occurs
______________________________________________

23. The value of sexual reproduction
Sexual reproduction introduces genetic variation
genetic recombination during meiosis
independent assortment of chromosomes
random alignment of homologous chromosomes in Meiosis 1
crossing over
random fertilization
which sperm fertilizes which egg?
Driving evolution
variation for natural selection
metaphase1

26. Meiosis 1

27. Meiosis 2

28. Mitosis vs. Meiosis

29. Variation from genetic recombination
Independent assortment of chromosomes
________________________________
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