1. Part II: Genetic Basis of Life
Meiosis and sexual reproduction
Meiosis occurs at different times in the life cycle of plants, animals, and fungi, but its phases are the same.
In humans, meiosis is a part of spermatogenesis and oogenesis.

2. Halving the Chromosome Number: Sexual Reproduction
Meiosis is nuclear division reducing chromosome number from diploid (2n) to haploid (n) number.
Haploid (n) number is half the diploid number of chromosomes.
Requires gamete formation and then fusion of gametes to form a zygote.
A zygote always has a full (2n) number of chromosomes.

3. Halving the Chromosome Number: Homologous Pairs
Diploid cells= chromosomes in pairs.
Each set of chromosomes is a homologous pair; each member is a homologous chromosome or homologue.
A location on one homologue contains same type of gene at the same locus on other homologue.

4. Halving the Chromosome Number: Homologous Pairs
Chromosomes duplicate just before nuclear division.
Duplication produces two identical parts called sister chromatids, held together at the centromere.
Non-sister chromatids do not share the same centromere.
One member of each homologous pair is inherited from either male or female parent.

5. Meiosis involves two nuclear divisions and produces four haploid daughter cells.
Each daughter cell has half the number of chromosomes in the diploid parent nucleus.

6. Prior to Meiosis I, DNA replication occurs and each chromosome has two sister chromatids.
During Meiosis I, homologous chromosomes come together and line up in synapsis.
During synapsis, the two sets of paired chromosomes lay alongside each other as bivalents or tetrad.

7. Crossing over is an exchange of homologous segments between non-sister chromatids of bivalent during Meiosis I.
This results in genetic recombinations.
After crossing over occurs, sister chromatids of a chromosome are no longer identical.

8. Halving the Chromosome Number: Meiosis II
No replication of DNA is needed between Meiosis I and Meiosis II b/c chromosomes were already doubled.
During Meiosis II, centromeres divide; sister chromatids separate.
Chromosomes in the four daughter cells have only one chromatid.
Each daughter cell is haploid, and in animal life cycle, becomes gametes.

9. Genetic Recombination
Due to genetic recombination, offspring have a different combination of genes than their parents.
Without recombination, asexual organisms must rely on mutations to generate variation among offspring.

10. Genetic Recombination: Crossing-Over
Crossing over of nonsister chromatids results in exchange of genetic material between non-sister chromatids; introduces variation.
At synapsis, homologous proteins are held in position by a nucleoprotein lattice (the synaptonemal complex).

11. Genetic Recombination: Crossing-Over
As lattice of synaptonemal complex breaks down at the beginning of anaphase I, homologues are temporarily held together by chiasmata, regions where non-sister chromatids are attached due to crossing-over.
Due to crossing over, chromatids are no longer identical.

12. Genetic Recombination: Fertilization
Meiosis increases variation.
When gametes fuse at fertilization, chromosomes donated by parents combine.

13. Phases of Meiosis Prophase I:
Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments; centrosomes migrate away from each other; spindle fibers assemble.
Homologous chromosomes undergo synapsis forming bivalents; crossing over may occur at this time.
Chromatin condenses.

14. Metaphase I:
During prometaphase I, bivalents held together by chiasmata have moved toward the metaphase plate.
Kinetochores are regions just outside centromeres; they attach to spindle fibers called kinetochore spindle fibers.
Bivalents independently align themselves at the metaphase plate of the spindle.

15. Phases of Meiosis Anaphase I:
The homologues of each bivalent separate and move toward opposite poles.
Each chromosome still has two chromatids.

16. Phases of Meiosis Telophase I: Only occurs in some species.
When it occurs, the nuclear envelope reforms and nucleoli reappear.
Interkinesis:
This period between meiosis I and II is similar to interphase in mitosis.
However, no DNA replication occurs.

17. Meiosis II:
During metaphase II, the haploid number of chromosomes align at metaphase plate.
During anaphase II, centromeres divide and daughter chromosomes move toward the poles.
At the end of telophase II and cytokinesis, there are four haploid cells.

18. Mitosis II continued:
Due to crossing-over, each gamete can contain chromosomes with different genes.
In animals, the haploid cells mature and develop into gametes.
In plants, the daughter cells become spores and divide to produce a haploid adult generation.

19. Comparison of Meiosis and Mitosis
Meiosis I differs from Mitosis:
DNA is replicated only once before both mitosis and meiosis; in mitosis there is only one nuclear division, in meiosis there are 2.
During prophase I of meiosis, homologous chromosomes pair and undergo crossing-over.

20. Comparison of Meiosis to Mitosis
During metaphase I, paired homologous chromo’s align at the metaphase plate: in mitosis individual chromo’s align.
During anaphase I, homologous chromo’s with centromeres intact separate and move to opposite poles; in mitosis at this stage, sister chromatids separate and move to poles.

21. Comparison of Meiosis to Mitosis
Meiosis II differs from Mitosis:
Events in Meiosis II are same stages as in mitosis.
However, the nuclei contain the haploid number of chromosomes in meiosis.
Mitosis produces two daughter cells; meiosis produces four daughter cells.

22. The Human Life Cycle
Life cycle refers to all reproductive events between one generation and next.
In animals, adult is diploid and gametes are haploid.
Mosses are haploid most of their cycle, oak trees are diploid.
In fungi and algae, organism you see is haploid and produces haploid gametes.

23. Spermatogenesis.
In males, meiosis is part of spermatogenesis or production of sperm.
In the testes of males, primary spermatogenesis with 46 chromo’s divide meiotically to form two secondary spermatocytes, each with 23 duplicated chromo’s.
Secondary spermatocytes divide to produce four spermatids (23 chromo’s).

24. Spermatogenesis Spermatogenesis continued:
Spermatids then differentiate into sperm.
Meiotic cell division in males always results in four cells that become sperm.

25. Oogenesis
Oogenesis is in the ovaries of human females, primary oocytes with 46 chromosomes divide meiotically to form two cells, each with 23 duplicated chromosomes.
One of the cells, secondary oocyte, receives most cytoplasm.
Other cell, a polar body, disintegrates or divides again.

26. Oogenesis
Secondary oocyte begins in meiosis II and then stops at metaphase II.
At ovulation, secondary oocyte leaves ovary and enters an oviduct where it may meet sperm.
If sperm enters oocyte, it is activated to continue meiosis II, resulting in mature egg and another polar body, each with 23 chromosomes.

27. The Human Life Cycle: Oogenesis
Polar bodies serve to discard unnecessary chromosomes and retain most of the cytoplasm in the egg.
The cytoplasm serves as a source of nutrients for the developing embryo.