1. Ch. 6 Sexual Reproduction
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2. Review: Chromosomes = packages of DNA
Body (somatic) cells have 2 versions of each chromosome
One from mom & one from dad
Humans have 23 pairs; 46 total
Reproductive cells (gametes) have only 1 version

3. DNA from a human somatic cell

4. Egg
Sperm

5. Cell Division Two types of division:
Mitosis – makes new somatic (body) cells
Meiosis – makes gametes (egg/sperm) used in sexual reproduction

6. 6.1 Meiosis
Meiosis = process that produces cells with ½ the original number of chromosomes
Has 2 separate nuclear divisions
Important aspect of sexual reproduction

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Sexual Reproduction
Sexual reproduction = reproduction that requires the participation of two individuals each contributing a gamete
Produces genetic diversity
Variation produces individuals that may have advantages over one another.
See pages
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Role of Gametes
Body cells have a diploid chromosome number (chromosomes are in pairs)
Gametes have half (haploid) the normal number of chromosomes
Ex) In humans, sperm & egg each have 23 chromosomes so when fertilization occurs:
23 (egg) + 23 (sperm) = 46 (zygote)
See page 190
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Zygote (fertilized egg) develops into an embryo, and on into a fetus
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Gamete Formation
Before meiosis, DNA replication must occur
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Stages of Meiosis
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See pages

12. Meiosis Events Meiosis I
Matching chromosome pairs (homologous chromosomes) move to opposite ends (poles) of the cell - two daughter cells result.
See pages

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Crossing Over
In meiosis I, chromatids of chromosome pairs can cross over each other & exchange DNA segments
increases genetic possibilities
produces more variation
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14. Meiosis II Chromatids of each chromosome are pulled apart
Resulting in four haploid cells
These final cells develop into gametes

15. Independent Assortment
Pairs of chromosomes separate independently
creates different combinations of chromosomes in daughter cells

16. (c) McGraw Hill Ryerson 2007
Review of Events:
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17. Meiosis Details Gametes do not form equally in males & females
See pages
Gametes do not form equally in males & females
In males, all 4 cells develop into sperm.
In females, 1 cell gets most of the cytoplasm & becomes the egg.

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Chromosome mutations sometimes occur spontaneously
During meiosis, parts of chromosomes can be deleted, inverted, duplicated or moved to another spot (translocated).
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19. Chromosome mutations can occur because of mutagens
Chromosome changes, sometimes leading to genetic disease or death, can be cause by mutagens such as radiation or chemicals.

20. Down syndrome karyotype:
Failed separation of chromosomes has serious consequences:
a gamete may end up with no chromosome or too many
Zygotes that result from these gametes rarely survive, and if they do, they will have serious genetic disorders.
Ex. Turner syndrome (XO), Klinefelter syndrome (XXY), Down syndrome
Karyotypes allow geneticists can view an individual’s chromosomes.
Down syndrome karyotype:
See pages
Take the Section 6.1 Quiz