1. Lesson 1: Sexual reproduction
Fusion of two gametes (sex cells) 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.
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3. The process resulting in the creation of sex cells (GAMETES)
MEIOSIS
The process resulting in the creation of sex cells (GAMETES)
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4. Haploid = 1 set of chromosomes
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5. Chromosome numbers: Ploidy = number of copies of each chromosome.
Haploid = 1 set of chromosome
Diploid = 2 sets of each chromosome
All sexually reproducing organisms usually have even numbers of chromosomes
: diploid (2n) sets of homologous chromosomes!
All even, as all are diploid, contain pairs of chromosomes.
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6. Chromosomes exist in homologous pairs in diploid cells.
Homologues
Chromosomes exist in homologous pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes, they have homologues.
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7. Sockosome Example
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8. Homologues Chromosomes in Humans
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9. In humans cont’d…
23 chromosomes donated by each parent (total = 46 or 23 pairs).
Gametes (sperm & ova or egg):
Contain 22 autosomes and 1 sex chromosome.
Are haploid (haploid number “n” = 23 in humans).
Fertilization results in zygote with 2 haploid sets of chromosomes - now diploid.
Diploid cell; 2n = 46. (n=23 in humans)
Most cells in the body produced by mitosis.
Only gametes are produced by meiosis.
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10. Are There Advantages of Diploidy in Sexually Reproducing Organisms?
Back up instructions if a problem arises with the first set
Greater diversity of genes that may impact the development of the organism (greater diversity = greater chance for survival in changing environmental conditions – EVOLUTION)
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11. Roadblock activity…
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17. Sockosome Example – Mitosis Vs. Meiosis
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20. Line Up and Expect to Be Divided!!! Mitosis and Meiosis Comparison…
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29. MEIOSIS II Resembles MITOSIS
2 CELLS
(2 N EACH)
MEIOSIS II Resembles MITOSIS
4 CELLS
(1 N EACH)
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32. Sockosome Example – Mitosis Vs. Meiosis
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33. Animation
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34. MITOSIS VS. MEIOSIS
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36. 2N or 4N??
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37. Mitosis vs. meiosis
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38. 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.
Meiosis I involves:
Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids).
In Metaphase I, homologous pairs line up at metaphase plate.
In Anaphase I, sister chromatids do NOT separate.
Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome.
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39. MEIOSIS AND 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.
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40. Independent assortment
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41. Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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42. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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43. >64 trillion combinations for a diploid zygote!!!
Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!
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46. Crossing over
Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids.
Crossing over produces recombinant chromosomes.
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48. Harlequin chromosomes
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50. >>>64 trillion combinations for a diploid zygote!!!
Add the variability of crossing over to the variable of all of the different combinations and sexual reproduction equals…
>>>64 trillion combinations for a diploid zygote!!!
SEXUAL REPRODCTION = DIVERSITY
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51. 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.
Generalized animal life cycle
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53. 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.
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54. 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
DNA back-up and repair.
Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged.
Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene.
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55. Problems! Oh No!
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56. Down's Syndrome
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58. 2. How does metaphase of mitosis differ from metaphase I of meiosis?
Study Questions
1. What happens as homologous chromosomes pair up during prophase I of meiosis?
2. How does metaphase of mitosis differ from metaphase I of meiosis?
3. What is the sole purpose of meiosis?
4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis?
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59. 5. Compare mitosis and meiosis on the following points:
a. number of daughter cells produced.
b. the amount of DNA in the daughter cells in contrast to the original cell.
c. mechanism for introducing genetic variation.
6.  What is a zygote and how is it formed?
7. Draw a nucleotide and then draw a 10 nucleotide linear sequence of DNA.
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60. References
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