1. Meiosis

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

3. Eukaryotic DNA is organized in chromosomes.
Genes have specific places on chromosomes.

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

5. Reproduction
Asexual
Many single-celled organisms reproduce by splitting or budding.
Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).

6. Sexual Reproduction Fusion of two gametes to produce a single zygote.
Introduces greater genetic variation, allows genetic recombination.
Most zygotes have
gametes from two
different parents.

7. Human Zygote

8. Karyotype Ordered display of an individual’s chromosomes.
Collection of chromosomes from mitotic cells.
Staining can reveal
visible band patterns,
gross anomalies.

10. Homologues Chromosomes exist in homologous pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes

11. In humans … Gametes (sperm/ova):
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)

12. Each human cell (aside from red blood cells and gametes) contains a full set of 46 chromosomes.
What would happen if egg and sperm each had 46 chromosomes?
Mitosis will not work if offspring are to keep the same number of chromosomes as the parent

13. Meiosis
The process by which the chromosome number is halved and chromosomes are sorted and packaged to be passed on to an organism’s offspring. 

14. Each of the resulting reproductive cells, or gametes (sperm or egg), has only a single set of 22 autosomes plus a single sex chromosome, either an X or a Y.  A cell with a single chromosome set is called a haploid cell.

15. Fertilization fuses an egg with its 23 chromosomes and a sperm with its 23 chromosomes so we are back to 46.
The resulting fertilized egg, or zygote, contains the two haploid sets of chromosomes with genes originating in both the maternal and paternal family lines. 

16. As meiosis begins, each chromosome is mysteriously attracted to its special homologous partner.  The two #1 chromosomes--one from the paternal set and one from the maternal set--wrap tightly about each other in a process called synapsis. 

17. Since each #1 chromosome is already doubled, a tetrad of four chromosomes is created.  The same thing happens with the other chromosomes.  Each homologous pair forms its own tetrad. 

18. Two sets of divisions follow and the chromosomes are pulled apart. 
Meiosis I - the four chromosomes of each tetrad are first separated into twos
Meiosis II - then into ones.

20. Interphase I
Same as mitosis
Chromosomes double

21. Prophase I
Synapsis occurs – homologous pairs come together to form tetrads
Crossing over
Nuclear envelope disintegrates
Nucleolus disappears

22. Prophase I

23. Crossing over
Chiasma - Place where homologous chromosomes touch in crossing over

24. Metaphase I

25. Anaphase I

26. Telophase I

27. Meiosis II No cytokinesis No interphase usually
Similar to Mitosis but FOUR haploid gametes are formed, each containing one chromatid

28. Prophase II

29. Metaphase II

30. Anaphase II

31. Telophase II

32. 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, random fusion of the unique haploid gametes produces truly unique offspring.

33. Gametogenesis
Oogenesis – production of a mature egg cell (ovum), one per month from puberty to menopause
one egg cell is large, other three are smaller and absorbed into the ovary
Spermatogenesis – all four sperm cells are viable – produced from puberty for the rest of the male’s life

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

35. Before meiosis begins each chromosome in the pair has already doubled
Before meiosis begins each chromosome in the pair has already doubled.  The double maternal chromosome (black) and the double paternal chromosome (white) attach to the spindle.

36. As the cell divides, the double maternal chromosome and the double paternal chromosome move toward opposite poles. 

37. The pair in each daughter cell starts to pull apart in the direction of the arrows.  Another cell division occurs.

38. The final result is four cells, each containing the haploid number of chromosomes.  Two of the cells contain a chromosome from the maternal set and two contain a chromosome from the paternal set.