1. Asexual Life Cycles, Sexual Life Cycles and Meiosis
SGN 16

2. A mitotic cell cycle produces clones of the parent cell, regardless of whether the parent cell is haploid or diploid; mitosis preserves chromosome number and content between parent and daughter cells

3. Mitotic cell cycle - typical in the growth of multicellular organisms, whether haploid or diploid; all cells are clones
Mitosis can also be used in organismal reproduction, to make spores or gametes

4. Mitosis - used in asexual reproduction (binary fission, budding, fragmentation, spores, etc.)
Asexual reproduction – a new clonal organism arises from a cell or cells derived from the parent organism
Spores are specialized haploid cells that disperse, each to become an individual, haploid clone (common in fungus and primitive plants such as moss or ferns)

5. What are advantages and disadvantages of asexual reproduction?

6. Mitosis can also be used to produce gametes if the parent cells are haploid (example - sexual reproduction in fungus)
Gametes, used in sexual reproduction, must typically be haploid, as they combine to create a diploid cell
Sexual reproduction – unique haploid gametes, the product of meiosis, combine to create a genetically new diploid organism
In animals and many plants gametes are produced by a meiotic cell cycle

7. Sexual reproduction and meiosis
Diploid organisms that reproduce sexually must be able to make gametes by dividing chromosome number (diploid = 2n) in half (haploid = n)
Diploid organisms have paired chromosomes, called homologous chromosomes
Homologues have paired alleles (matching genes on homologous chromosomes that might be the same but might be slightly different)
Genes are segments of DNA that hold the information necessary to make a particular protein; so paired alleles hold info to make the same protein, but sometimes the info is slightly different
During gamete formation homologous chromosome pairs in the diploid parent cell are separated to make individual haploid daughter cells

8. Diploid and haploid cells
In most animals the zygote and the somatic cells are diploid; the only haploid cells are the gametes
Single homologues
Paired homologues

9. Most other types of organisms (plants, fungus, many protists) alternate diploid and haploid stages of their life cycle using meiosis to ½ the chromosome number
Plants alternate a gametophyte generation (n) with a sporophyte generation (2n); the haploid generation is much reduced in more common plants

10. Meiosis is a two stage/division process that, together with DNA replication and cytokinesis, reduces the chromosome number in half in the daughter cells

11. First division (Meiosis I) separates homologous pairs of sister chromatids (students should know details of Meiosis I)
Cytokinesis produces two daughter cells, which are haploid but still with sister chromatids attached

12. Early during Meiosis I (during prophase 1), homologous pairs of sister chromatids form tetrads, a process called synapsis
In tetrad nonsister chromatids become entwined (chiasmata) and crossing over can occur
Crossing over – because of chiasmata, chromosome ends can break off during separation of tetrads in Meiosis 1 and reattach on the opposite nonsister chromosome, producing “new” chromosome

13. Second division (Meiosis II) separates sister chromatids (students should know details of meiosis II) Cytokinesis produces four haploid cells

14. Compare mitosis and meiosis

15. Sexual reproduction is a huge success
Sexual reproduction is a huge success! Why is sexual reproduction so evolutionarily successful? The principle difference between asexual and sexual reproduction is that SR produces offspring that are genetically unique Why does evolution favor a system of reproduction that produces genetically unique organisms, even though sexual reproduction has its obvious costs?
Disadvantages of Sexual Reproduction
- Time and energy inefficient
Time/energy to mature organism and gametes
Time/energy to find mate and mate
(External fertilization avoids problem)
If dimorphic and intersexual, 50% of population doesn’t make babies
- Beneficial traits not always passed on and in reshuffling of genes less beneficial of genes may occur
- An individual will only pass on ½ of its genes!!

16. Genetic variation is produced from sexual reproduction in 3 principle ways
Independent assortment of homologous pairs
Meiosis separates homologues pairs randomly, leading to a huge number of genetic variants in the gametes
Crossing over
Random fertilization

17. Evolution, through natural selection, selects individuals that exhibit traits and behaviors that allow for that individual’s genes to be successfully passed down through the generations; these traits and behaviors have to be successful every generation, and across many generations – success means genes are passed down to the next generation, where they allow success and reproduction, and so on.
Therefore sexual reproduction/genetic diversity must have a payoff (promoting gene persistence) every generation and across the generations.
What is the payoff? What is the generation to generation advantage of genetic diversity? It has been suggested that as change happens (environmental change, predator introduction, for example) some parts of the population will be resistant to it because of the diversity seen within that population.
But environmental change and predator introduction are rare things in generally stable ecosystems. These might select for a thickened coat or better camouflage, but evidence suggests that these rare events could not select for something as complex and costly as sexual reproduction.
The challenge that could drive selection for sexual reproduction would have to be ever present and ever changing, requiring the continual presence of diversity.
What could that challenge be?

18. One hypothesis that is strongly supported… The advantage to genetic variation is in regard to infectious and parasitic disease

19. The advantage to genetic variation is in regard to infectious and parasitic disease; pathogens and hosts take part in a genetic “arms race”; creating new combinations of genes, and occasionally new genes (for example if crossing over occurs in the middle of a gene) allows hosts to develop variations each generation with some degree of resistance (resistance immunologically, behaviorally), even as pathogens are evolving new variation to become more infective
Therefore genes for sexual reproduction in the genome make the entire genome more successful

20. When meiosis goes wrong?