1. Meiosis and Sexual Life Cycles
Fred and George Weasley… Identical Twins

2. Heredity
Heredity: the transmission of traits from one generation to the next
Asexual reproduction: clones
Sexual reproduction: variation
A budding hydra

3. Karyotyping
Chromosomes are photographed, paired, and arranged into a karyotype
Homologous chromosomes- same length, centromere position, and staining pattern
Carry same types of genes
Arranged from longest to shortest
1-22 = autosomes
#23 = sex chromosomes (X and Y)

4. Human Life Cycle 23 pairs of homologous chromosomes (46)
1 pair of sex and 22 pairs of autosomes
Gametes are haploid (1N); all other cells are diploid (2N)
Fertilization results in a zygote
Meiosis: cell division to produce haploid gametes

5. Alternate Life Cycles

6. Plants and Some Algae Alternation of generations:
2N sporophyte produces 1N spores by meiosis
Spore can divide by mitosis to create a 1N gametophyte
Gametophyte makes gametes by mitosis which fertilize to become a 2N sporophyte

7. Most fungi and some protists
Meiosis produces 1N cells that divide by mitosis to produce 1N adults
Adults undergo mitosis to produce gametes
Fertilized gametes become a 2N zygote

8. Meiosis After chromosome replication
2 cell divisions (Meiosis I & Meiosis II)
Produces 4 daughter cells
1/2 chromosome number (1N)
Variation in gametes that are produced

10. Meiosis I Metaphase I Interphase Prophase I Anaphase I
Tetrads line up along the metaphase plate
Both sister chromatids of a chromosome are controlled by a single kinetochore
Anaphase I
Homologous chromosomes are pulled apart
Sister chromatids remain intact
Telophase I and Cytokinesis
Haploid set of chromosomes
Clevage furrow/Cell plate forms
In some species, chromosomes uncoil and nuclear envelope forms
Interphase
Chromosomes and centrosomes replicate
Prophase I
Chromosomes condense
Homologous chromosomes pair up and crossing over occurs
Synaptonemal complex (protein) forms between homologues, forming a tetrad
Centrosomes move to opposite ends of the cell
Spindle fibers attach to kinetochores and begin moving tetrads to the metaphase plate

12. Meiosis II Prophase II MetaphaseII Anaphase II
Mitotic spindle forms again
Spindle fibers attach to kinetochores and chromosomes begin moving toward the metaphase plate
MetaphaseII
Chromosomes line up on metaphase plate as in mitosis
Anaphase II
Chromosomes separate at the centromere
Telophase II and Cytokinesis
Nuclei form
Chromosomes decondense
Each of the 4 daughtercells are genetically distinct from the parent cell

13. Meiosis vs. Mitosis Synapsis/tetrad/chiasmata (prophase I)
Homologous vs. individual chromosomes (metaphase I)
Sister chromatids do not separate (anaphase I)
Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

14. Genetic Variation Independent assortment:
homologous chromosomes position and orient randomly (metaphase I)
In humans, about 8 million different combinations
Recombinant chromosomes (from crossing over)
chromatids are no longer identical
More possibilities for genetic variation

15. Crossing Over Synapsis- homologous chromosomes come together
Synaptonemal complex holds them together
Form a tetrad
Arms of homologues overlap and form a chiasma
Arms of homologues exchange at gene lines
May occur multiple times
Produces recombinant chromosomes

16. No wonder siblings can be so different!
Random fertilization:
1 sperm (1 of 8 million possible combinations) x 1 ovum (1 of 8 million possibilities) = 64 trillion diploid combinations!
With the variation from crossing over, the number of possibilities is astronomical
Prince Harry and Prince William
George W and Jeb Bush
Venus and Serena Williams
The Jonas Brothers