1. Meiosis and sexual reproduction
Chapter 12

2. Types of reproduction
In asexual reproduction, a single individual gives rise to offspring that are _________to itself and others In sexual reproduction, two individuals mix their _____________

3. Alleles
Somatic (body) cells of sexually-reproducing eukaryotes contain pairs of ________________________ chromosomes:
1 maternal and 1 paternal
Homologous chromosomes:
Carry genes (one from the mother and one from the father) of the same _________________________
Different forms of the same gene are called ________________
(How many chromosomes do you think humans have?
So how many pairs of homologous chromosomes would that be?.....

4. alleles
Paired genes on homologous chromosomes may vary in DNA sequences as __________________
Arise by mutation
Are the basis of differences in shared _________________
Offspring of sexual reproducers inherit new ________________of parental alleles
Results in new combinations of traits

5. Advantages of sexual reproduction
Asexual reproduction produces ______________, which is a good thing if the organism lives in a favorable, ____________________________. BUT…most environments are constantly ___________________ Sexual reproduction produces offspring with different combinations of traits that suits them perfectly for change. Chances of survival are better. Better change of avoiding mutations? Why?

6. Red Queen Hypothesis
In reference to Lewis Carroll’s book Through the Looking Glass
Queen of Hearts tells Alice, “It takes all the running you can do, to keep in the same place”
Organisms must constantly adapt and evolve in an ever-changing environment
Used to explain the evolution of ____________________________________

7. Meioses
A type of cell division that results in four ______________ cells each with _________the number of chromosomes of the parent cell, as in the production of ___________and plant spores.
Why do you want half the number of chromosomes in gametes and spores?
________________________________________________________________________
Asexual reproduction produces clones
Sexual reproduction mixes up alleles from two parents
Involves the fusion of gametes, mature reproductive cells
Meiosis: ____________________________________________________________
______________________________________________________________________

8. Meiosis Halves the Chromosome Number
Gametes: ____________________________________________________________
Haploid(n): ____________________________________________, half of diploid(2n)
Arise by germ cells______________________________________________________ __________________________________________________________________
Meiosis: nuclear division that halves the number of chromosomes.
Necessary for gamete formation
Gives rise to new combinations of parent alleles.
Gamete formation differs among plants and animals

9. meiosis
Meiosis sorts chromosomes into new nuclei twice Resulting in FOUR _______________nuclei. Meiosis I and Meiosis II

10. meiosis
Occurs in the gametes – ___________________________________________ In males, the diploid germ cell develops into _______________________ In females, a diploid germ cell becomes an __________________

11. meiosis
Chromosomes are duplicated before meiosis begins. During meiosis I, each chromosome in the nucleus pairs with its homologous partner. The nucleus contains two of each chromosome, so it is diploid (2n).
Homologous partners separate and are packaged into two new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are still duplicated
Sister chromatids separate in meiosis II and are packaged into four new nuclei. Each new nucleus contains one of each chromosome, so it is haploid (n). The chromosomes are now unduplicated

12. Fertilization (Fusion of two gametes)
Restores _____________________________________________________________ Forms a ________________________(First cell of new individual) If meiosis did not precede fertilization, the chromosome number would double with every generation Chromosome number changes can have drastic consequences in animals An individual’s set of chromosomes is like a fine-tuned blueprint that must be followed exactly to have normal functions

13. Meiosis I

14. Meiosis I : One diploid nucleus to two haploid nuclei1
Prophase 1 2
Metaphase 1 3
Anaphase 1 4
Telophase 1
plasma
membrane
spindle
nuclear envelope
breaking up
pair of homologous chromosomes

15. Prophase 1
Homologous chromosomes ______________________________ Pair up (__________________________) Swap segments (__________________________) Spindle microtubules attach to _________________ Nuclear envelope __________________________

16. Metaphase 1
Homologous chromosome pairs are aligned between spindle poles at the _______________________________________ Spindle microtubules attach the chromosomes of each pair to opposite spindle poles.

17. Anaphase 1
Homologous chromosomes _________________and begin heading toward the spindle poles

18. Telophase 1
A complete set of chromosomes cluster at both ends of the cell A ____________________________forms around each set _________________________________occurs NO DNA REPLICATION BETWEEN MEIOSIS 1 AND MEIOSIS 2 Why??

19. Meiosis Ii

21. Prophase 2
Chromosomes __________________________ Nuclear envelope ____________________________________ New ______________________ forms Spindle microtubules attach to each sister _______________________________________

22. Metaphase 2
Chromosomes align midway between spindle poles

23. Anaphase 2
Sister chromatids __________________________ Now unduplicated Heads to the spindle poles

24. Telophase 2
Chromosomes cluster at both ends of the cell New ___________________ forms around each set Four __________________ nuclei form _______________________occurs

25. meiosis

26. Variation
___________________________ _____________________________________________ ______________________________________

27. Crossing over in prophase 1
Early in Prophase 1 Chromosomes condense, drawn close to homologous partner Chromosomes align along their length Segments of DNA are swapped Alleles are shuffled between homologous chromosomes

28. Crossing over

29. Chromosome segregation (Separation)
When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei
For each chromosome pair, the ________________or _____________version is equally likely to end up in either nucleus
Human gametes have 23 pairs of homologous chromosomes
Each time a human germ cell undergoes meiosis the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes (Without crossing over)

30. Chromosome segregation (Seperation)
Crossing over increases these combinations
The chance that the maternal or paternal version of any chromosome will end up in a particular nucleus is 50 percent – Why?
Due to the way the spindle segregates the homologous chromosome during meiosis I
In prophase I, chromosomes are attached to spindle poles
Each homologous partner becomes attached to opposite spindle poles

31. Comparing meiosis and mitosis
There are striking parallels between the stages of mitosis and meiosis II Many more similarities exist at the molecular level Meiosis may have evolved by remodeling the existing mechanisms of mitosis or for repairing damaged DNA

32. No sex for 80 million years
In nature, there are a few all-female species of fish, reptiles, and birds but not mammals
Females have been reproducing themselves for 80 million years through cloning themselves
__________________reproduction is a poor long-term strategy because it lacks crossing over that leads to genetic diversity
Bdelloid rotifers: microscopic animals found in fresh water
Males are not present within the species
Females reproduce by ___________________________
Have diversified into 360 species
Can import genes from bacteria, fungi, protists, and even plants
About 10 percent of their active genes have been pilfered from other organisms