1. Sperm & Eggs & Variation..OH MY!
MEIOSIS
Sperm & Eggs & Variation..OH MY!

2. Sexual Reproduction requires Cells Made by Meiosis
What if a new individual was formed through mitosis?

3. Meiosis – Key Terms allele gamete amniocentesis genome
asexual reproduction
autosome
binary fission
chorionic villi sampling
crossing over
diploid
exon
fertilization
gamete
genome
haploid
homologues
intron
karyotype
nondisjunction
placenta
reduction division
sexual reproduction
zygote

4. Genome Genome: Complete complement of an organism’s DNA.
includes genes (control traits) and non-coding DNA organized in chromosomes

5. Genes Eukaryotic DNA is organized in chromosomes
genes have specific places on chromosomes
exon: portion of a gene that is translated into protein (more in chapter 5)
intron: non-coding segment of DNA, often found within an exon; removed before transcription

6. Heredity
Heredity – way of transferring genetic information to offspring
Chromosome theory of heredity: chromosomes carry genes
Gene – “unit of heredity”

7. Reproduction Asexual splitting budding
parthenogenesis (egg develops w/o fertilization)
occurs naturally in some invertebrate animal species (e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles, and very rarely birds)

8. Sexual reproduction Fusion of two gametes to produce a single zygote.
Introduces greater genetic variation, allows genetic recombination.
With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

9. Chromosomes Karyotype: Make a Karyotype
ordered display of an individual’s chromosomes
collection of chromosomes from mitotic cells
staining can reveal visible band patterns, gross anomalies
Make a Karyotype

10. KARYOTYPING Obtain some cells from the individual
Culture them in a test tube with nutrients
Treat cells w/chemical that stops them exactly midway through cell division (chromosomes are coiled thickly and more visible than usual)
Cells are placed on a microscope slide and a stain is added that binds to the chromosomes, making them visible
Chromosomes are arranged by size and shape and displayed on a monitor or in a photograph

11. Karyotyping

14. HomologOUs CHROMOSOMES
Chromosomes exist in homologous pairs in diploid (2n) cells.
One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome).
Homologous chromosomes are similiar but not identical. Each carries the same genes in the same order, but the alleles (alternative form of a gene) for each trait may not be the same. 
Exception: sex chromosomes (X, Y)

15. HomologOUs CHROMOSOMES

16. In humans …
23 chromosomes donated by each parent (total = 46 or 23 pairs).
Gametes (sperm/ova):
contain 22 autosomes and 1 sex chromosome
haploid (haploid number “n” = 23 in humans)
Fertilization results in zygote with 2 sets of chromosomes - now diploid (2n).
Most cells in the body produced by mitosis.
Only gametes are produced by meiosis.

17. Chromosome numbers
All are even numbers – diploid (2n) sets of homologous chromosomes.
All even, as all are diploid, contain pairs of chromosomes.

18. Meiosis – key differences from mitosis
Meiosis reduces the number of chromosomes by half.
Daughter cells differ from parent, and each other.
Meiosis involves two divisions, Mitosis only one.

19. Meiosis – key differences from mitosis
Meiosis I involves:
synapsis
homologous chromosomes pair up
chiasmata form (crossing over of non-sister chromatids)
metaphase I: homologous pairs line up at metaphase plate
anaphase I: sister chromatids do NOT separate
overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome

23. Animation

24. Meiosis I Prophase 1 Metaphase 1
each chromosome duplicates and remains closely associated (sister chromatids)
crossing-over can occur during the latter part of this stage
Metaphase 1
homologous chromosomes align at the equatorial plate

25. Meiosis I Anaphase 1 Telophase 1
homologous pairs separate with sister chromatids remaining together
Telophase 1
two daughter cells are formed with each daughter containing only one chromosome of the homologous pair

26. Meiosis II Second division of meiosis: Gamete formation Prophase 2
DNA does not replicate
Metaphase 2
chromosomes align at the equatorial plate

27. MEIOSIS II Anaphase 2 Telophase 2 centromeres divide
sister chromatids migrate separately to each pole
Telophase 2
cell division is complete
4 haploid daughter cells

28. Mitosis vs. meiosis

30. Meiosis creates genetic variation
During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over
no daughter cells formed during meiosis are genetically identical to either mother or father
during sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring

31. Independent assortment

32. Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations

33. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!

34. Crossing over
Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids.
Crossing over produces recombinant chromosomes.

35. HOW SEX IS DETERMINED IN HUMANS
Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives. 
For this reason, we know that nothing on the Y chromosome is absolutely necessary.

36. SEX DETERMINATION IN OTHER SPECIES

37. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!

38. Meiosis & sexual life cycles
Life cycle = sequence of stages in organisms reproductive history; conception to reproduction
Somatic cells = any cell other than gametes, most of the cells in the body
Gametes produced by meiosis

39. Meiosis & sexual life cycles
Generalized animal life cycle

40. NONDISJUNCTION Unequal distribution of chromosomes during meiosis
Resulting gametes zero or two copies of a chromosome instead of a single copy

41. Down's Syndrome

42. TOO MANY OR TOO FEW CHROMOSOMES

43. Sex is costly!
Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required
Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.)
Genetic costs: in sex, we pass on only half of genes to offspring.
Males are an expensive luxury - in most species they contribute little to rearing offspring.

44. But …
More genetic diversity: more potential for survival of species when environmental conditions change.
shuffling of genes in meiosis
crossing-over in meiosis
fertilization: combines genes from 2 separate individuals

45. MITOSIS VS MEIOSIS REVIEW
In what cellular processes is mitosis involved? In what cellular processes is meiosis involved?
In what type of cells does mitosis occur? In what type of cells does meiosis occur?
How many times does DNA replicate in mitosis? How many times does DNA replicate in meiosis?
How many cellular divisions occur in mitosis? How many cellular divisions occur in meiosis?

46. MITOSIS VS MEIOSIS REVIEW
How many daughter cells are formed by mitosis? How many daughter cells are formed by meiosis?
What is the chromosome number in daughter cells formed by mitosis from diploid parent cells? What is the chromosome number in daughter cells formed by meiosis from diploid parent cells?
In mitosis, are daughter cells identical to or different from parent cells? In meiosis, are daughter cells identical or different from parent cells?
In mitosis, when do synapsis and crossing over occur? In meiosis, when do synapsis and crossing over occur?

47. CREDITS cchs.churchill.k12.nv.us/marshk/Notes/meio sis.ppt
Genetic Science Learning Center, University of Utah,
place/labbench/lab3/concepts2.html