1. Chapter 13: Meiosis & Sexual Life Cycles
Genetics & Inheritance
Genes- hereditary units
program specific traits in the offspring
Most program cells to synthesize specific enzymes & proteins
Cumulative action of proteins produces traits
Specific gene location on a chromosome = locus
Gametes- reproductive cells which transmit genes between generations

2. Asexual vs. Sexual Reproduction
Asexual Reproduction
All offspring’s genes originate from one parent
New offspring produced through mitosis
Sexual Reproduction
Offspring’s genes originate from two parents
Unique combinations of genes can be made so that each offspring varies from its siblings (exception- identical twins)

3. Fertilization & Meiosis alternate life cycles
Generation-to generation sequence of stages in an organism’s life from conception to reproduction
Example organism: Humans
Somatic cell (body cell) = 46 chromosomes
Homologous chromosomes = carry genes controlling the same inherited trait
22 pairs of autosomes
1 pair of sex chromosomes
Diploid cells = have all chromosome sets (2n)
Haploid cells = have one chromosome from each set (n); egg or sperm cells

8. Fertilization & Meiosis alternate life cycles
Fusion of egg & sperm nuclei = zygote (2n)
Mitosis
Used to develop zygote into sexually mature adult; Forms all somatic cells from zygote
Meiosis
Occurs in gonads
Reduces diploid number to half
Creates sperm & egg
Maintains diploid number for organisms; when fertilization occurs the diploid number is restored

10. Sexual Life Cycles
Alternation of fertilization & meiosis is characteristic of all sexually reproducing organisms
Timing of fertilization & meiosis in the life cycle varies between different species

11. Three types of sexual life cycles:
Animals
Gametes are only n cells
Gametes do not divide before fertilization
2n zygote divides by mitosis after fertilization

12. Three types of sexual life cycles
Alternation of generations
Plants & some algae
Has n & 2n multi-cellular stages
2n multi-cellular stage = sporophyte
Formed by ferilization of gametes
Makes spores by meiosis
n multi-cellular stage = gametophyte
Formed by mitosis of spores
Makes gametes by mitosis

13. Three types of sexual life cycles
Most fungi & some protists
Diploid zygote forms & immediately goes through meiosis forming haploid cells
Haploid cells formed during meiosis divide by mitosis to form a multi-cellular n adult organism
n adult forms gametes through mitosis
Only 2n cell is the zygote

15. Concept Check
How does the karyotype of a human male differ from that of a human female?
How does the alternation of meiosis & fertilization in life cycles maintain the normal chromosome number for that species?
What process mitosis or meiosis is more directly involved in gamete production:
In animals?
In plants & most fungi?

16. Meiosis Has two consecutive cell divisions
Results in 4 haploid daughter cells

17. Stages of Meiosis: Meiosis I
Prophase I
90% of meiosis spent in this phase
Chromosomes condense
Crossing over may occur between homologous chromosomes
Synapsis occurs holding homologues together
Tetrads form
Nucleoli disperse; nuclear envelope fragments; formation of spindles
Metaphase I
Tetrads arrange on metaphase plate

18. Stages of Meiosis: Meiosis I
Anaphase I
Chromosomes move toward poles (sister chromatids still attached)
Telophase I & cytokinesis
In some species nuclear envelope & nucleoli reform
Cleavage furrow (animals) or cell plate (plants) forms

19. Stages of Meiosis: Meiosis II
No replication of chromosomes between meiosis I & meiosis II
Prophase II
Spindle apparatus forms
Metaphase II
Chromosomes position of metaphase plate
Anaphase II
Sister chromatids move toward opposite poles
Telophase II & Cytokinesis
Nuclei form; chromosomes de-condense
Cleavage furrow or cell plate forms

22. Comparison of Mitosis & Meiosis
Meiosis reduces chromosome number by half; mitosis conserves chromosome number
Synapsis, crossing over, & tetrad formation occur in prophase I of meiosis
Tetrads take position on metaphase plate in metaphase I of meiosis
Homologues separate during anaphase I of meiosis; sister chromatids remain attached during meiosis I

25. Concept Check
Explain how mitosis conserves chromosome number, while meiosis reduces the chromosome number by half.
Compare the chromosomes present in metaphase of mitosis to those in metaphase II of meiosis.

26. Sexual Reproduction & evolution
Sexual reproduction produces genetic variation which can contribute to evolution
Genetic variation
Mutations
Independent assortment
Crossing over
Random fertilization

27. Genetic Variation: independent assortment
Random orientation of homologues during metaphase I leads to unique combinations of maternal & paternal chromosomes in the egg & sperm

29. Genetic variation: crossing over
Allows genes to exchanged between maternal & paternal chromosomes creating unique chromosomes

31. Genetic Variation: random fertilization
Each egg & sperm cell is unique
Which sperm fertilizes which egg contributes to extreme variation among offspring
2n = # of possible combinations per gamete

32. Evolutionary Significance of Genetic Variations
Populations evolve as a result of differential reproductive success among its variant members
Individuals with variations best suited to the environment reproduce more thus transmitting more of their genes
Success of variations dependant on environment

33. Concept Check
In fruit flies 2n= 8, while in honeybees 2n=32. Assuming no crossing over, is genetic variation higher among fruit flies or honeybees? Explain.
When would crossing over not contribute to increased genetic variation?
If a human cell had 22 autosomes & a Y chromosome what kind of cell is it?
What is the immediate product of meiosis in a plant?
What feature is unique to a plant life cycle versus an animal life cycle?