1. Topic 10.1 Meiosis
Essential idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.

2. 10.1 Meiosis
Nature of science: Making careful observations—careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. (1.8)
Understandings
Chromosomes replicate in interphase before meiosis
Crossing over is the exchange of DNA material between non-sister homologous chromatids
Crossing over produces new combinations of alleles on the chromosomes of the haploid cells
Chiasmata formation between non-sister chromatids can result in an exchange of alleles
Homologous chromosomes separate in meiosis I
Sister chromatids separate in meiosis II
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I

3. 10.1 Meiosis Applications and skills
Skill: Drawing diagrams to show chiasmata formed by crossing over

5. Review of Definitions Somatic cells: non sex cells
Sex cells (gametes): egg (ova), sperm
Autosome: chromosome that is not a sex chromosome (does not determine gender)
Sex chromosomes: dissimilar chromosomes that determine an individual’s sex. In humans: X, Y
Homologous chromosomes: pairs of chromosomes that have the same size, centromere position, staining pattern, location of genes

6. Review of Definitions
Fertilization: the union of two gametes to form a zygote
Zygote: a diploid cell that results from the union of 2 haploid gametes
Meiosis: special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization

7. Key differences between meiosis and mitosis
Meiosis is a reduction division: gametes have ½ number of chromosomes as parent cell
Meiosis creates genetic variation: 4 daughter cells genetically different from parent cell and from each other
Meiosis is 2 successive nuclear divisions

10. Stages of Meiotic Cell Division
Interphase I: precedes meiosis
Chromosomes replicate
Each duplicated chromosome consists of 2 identical sister chromatids attached at their centromere
Centriole pairs in animal cells also replicate into two pairs

11. Meiosis I Prophase I
Segregates the 2 chromosomes of each homologous pair and
reduces the chromosome number by one-half
Prophase I: 90% of the time required for meiosis.
Longer and more complex than prophase of mitosis.
Chromosomes condense- start to coil up and become shorter and thicker
Synapsis occurs- homologous chromosomes come together as pairs. Uses the synaptenemal complex to accomplish this

13. Prophase I continued
Each chromosome has 4 chromatids, so that each homologous pair in synapsis appears as a complex of 4 chromatids of a tetrad
In each tetrad, sister chromatids of the same chromosome are attached at their centromeres
Nonsister chromatids are linked by X-linked chiasmata, sites where homologous strand exchange or crossing over occurs
Chromosomes thicken further and detach from the nuclear envelope.
Centriole pairs move apart, toward poles, and spindle microtubules form between them
Nuclear envelope and nucleoli disperse

14. Chromosome Tetrad

15. Metaphase I
Metaphase I: tetrads (bivalents) are aligned on the metaphase plate, having moved there during this stage
Chromosomes continue to shorten and thicken
Spindle microtubules attach to the kinetochore region of the centromeres
Bivalents line up on the equator so that centromeres of homologues point towards opposite poles
Chiasmata slide towards the ends of the chromosomes causing the shapes of the bivalents to change
At the end, chromosomes start to move

16. Metaphase 1: 2n = 4

17. Anaphase I
Anaphase I: homologues separate and are moved towards the poles by the spindle apparatus. (Bivalents separate). This halves the chromosome number
Each chromosome consists of 2 chromatids (sister chromatids remain attached at the centromeres)
Because of crossing over, the 2 chromatids are not identical
At the end, chromosomes reach the poles

18. Telophase I and cytokinesis
Telophase I and cytokinesis: spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles
Each pole has a haploid set of chromosomes composed of 2 sister chromatids
Nuclear membranes form around the groups of chromosomes at each pole
Chromosomes uncoil partially
Cytokinesis occurs simultaneously forming 2 daughter cells. Cleavage furrows form in animal cells and cell plates form in plant cells

19. Meiosis I: reduction division – separation of homologous chromosomes

20. Meiosis II: Separation of sister chromatids

21. Interphase II Interphase II: May not really exist per se
Two cells either enter a brief period of interphase or immediately proceed to the second division of meiosis
DNA is NOT replicated

22. Prophase II
Prophase II: chromosomes become shorter and thicken again by coiling
Centrioles move to poles (animal cells)
Nuclear membrane breaks down
Spindle apparatus forms and chromosomes move towards the metaphase II plate

23. Metaphase II
Metaphase II: chromosomes align singly on the metaphase plate.
Spindle microtubules attach to the centromeres
Chromosomes line up on equator
Centromeres divide
Kinetochores of sister chromatids point towards opposite poles

24. Meiosis II continued
Anaphase II: sister chromatids separate and move toward opposite poles of the cell
Telophase II and Cytokinesis: nuclei form at opposite poles of the cell
Nuclear membranes reform around the groups of chromatids at each pole
Cytokinesis produces 4 cells total
Chromosomes uncoil, nucleoli appear
In most organisms, develop into gametes

27. Meiosis and Fertilization
Primary sources of genetic variation in sexually reproducing organisms
Sexual reproduction provides genetic variation by:
Independent Assortment
Crossing over during Prophase I
Random fertilization by gametes

28. Independent Assortment of chromosomes
Orientation of the homologous pair of chromosomes (one maternal and one paternal) is RANDOM chance that each gamete receives maternal or paternal derived chromosome
Each homologous pair of chromosomes orients independently of the other pairs at metaphase I. First meiotic division results in independent assortment of maternal and paternal chromosomes

29. Alternative arrangements of 2 homologous chromosome pairs

30. Independent Assortment Definitions
Independent Assortment: The random distribution of maternal and paternal homologues to the gametes OR
Independent Assortment: The presence of an allele of one of two genes in a gamete has no influence over which allele of another gene is present in the same gamete

31. Independent Assortment leads to Genetic Variation
Process produces 2n possible combinations of maternal and paternal chromosomes in gametes
If n = 2 then 22 = 4
If n = 23 (human) then 223 or 8,000,000 different possibilities before crossing over

32. Crossover
Definition: the exchange of genetic material between homologues (homologous portion of 2 non-sister chromatids trade places)
X-shaped chiasmata are the visible evidence of this process
Produces chromosomes that contain genes from both parents
In humans, an average of 3 crossovers/chromosome pair

33. Crossover
Most of the time, crossing over can occur without loss of genetic material because of the precise base to base pairing of homologues involving the formation of the synaptonemal complex, a protein structure that brings the chromosomes into close association

34. Results of crossing over during meiosis

36. Benefits of Crossing over
Chiasmata hold together homologues during prophase and metaphase I
Allows recombination of linked genes. Breaks up linkage groups/parental combinations

37. Crossing over results in an exchange of alleles
Parental combinations of linked genes cannot be broken up without crossing over
Crossing over occurs between non-sister chromatids of a homologous pair in prophase 1 between the loci of the 2 linked genes
Parentals: abc, ABC
Crossing over occurs between B and C
Position of chiasma formed by crossing over
The 4 chromatids separate into 4 nuclei produced by meiosis
Recombinants produced = abC, ABc

38. Linkage Groups
Definition: All the genes that have their loci on the same chromosome type form a linkage group

39. Example of Gene Linkage

40. Random Fertilization: Result of sexual reproduction
In humans an egg cell with 1 of 8,000,000 different possibilities will be fertilized by a sperm cell that is also 1 of 8,000,000 possibilities, resulting in a zygote that can have
1/64,000,000,000,000 possible diploid combinations

41. S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over
A chiasma is an X-shaped knot-link structure that forms where crossover has occurred
Draw two homologous chromosomes using two different colors- one from mom and one from dad- close to each other

42. S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over
Since the position of crossover is random you can draw it anywhere (and you can draw more than one)
The chromosomes will break where they will cross over
Draw an X-shaped chiasma

43. S 10.1.1 Drawing diagrams to show chiasmata formed by crossing over
Separate and draw the newly crossed over homologous chromosomes