1. Sexual Reproduction and Meiosis
Anderson
Spring 2017
College of the Redwoods

2. Asexual Reproduction Involves a single parent
Offspring genetically identical to parent
All prokaryotes and some eukaryotes use this method
Binary Fission – parent cell splits into 2 identical cells
Fragmentation – parent organism breaks into pieces and each fragment becomes new organism
Budding – parent cell forms bubble-like bud that stays attached while it develops and grows, then detaches
Parthenogenesis – female produces an egg without being fertilized

3. Sexual Reproduction Involves 2 parents
Offspring in kind with parents but not genetically identical
Ultimate source of variation – different mutations are continually reshuffled from one generation to the next
Different genetic combinations occurs via meiosis – the division of contents of the nucleus to produce gametes

4. Sexual Life Cycles
Meiosis – reduces chromosome number by half in gametes
Fertilization – joins two haploid gametes to restore diploid configuration
3 main categories of life cycles:
Diploid-dominant
Haploid-dominant
Alternation of generations

5. Diploid-Dominant
Multicellular diploid stage is most obvious, no multicellular haploid stage
Nearly all animals
Germ cells – specialized cells that undergo meiosis to make gametes
Zygote – single celled diploid created by fertilization
Hermaphrodite – possess both male and female reproductive organs and makes both gametes, can self- fertilize

6. Haploid-Dominant
The body (mature, ecologically important form) is haploid
Most fungi and some protists
Black Bread Mold – hyphae (multicellular) meet and form zygospore (haploids fused to form diploid)
Zygospore nuclei undergo meiosis to form spores
Spores germinate and divide by mitosis to form multicellular fungi

7. Alternation of Generations
Blend of diploid-dominant and haploid-dominant extremes
Have both haploid and diploid multicellular stages
Found in some algae and all plants
Haploid – gametophyte (give rise to gametes)
Diploid – sporophyte (gives rise to spores)

8. Meiosis
Nuclear division of diploid (2 sets of chromosomes) cell to form haploid (1 set of chromosomes)
Parent and daughter cell are NOT identical, each daughter cell also NOT identical
Consists of 1 round of chromosome duplication and 2 rounds of nuclear division
Stages are analogous to stages of mitosis, but names with I or II to designate the round of division
Meiosis I has prophase I, prometaphase I, metaphase I, etc
Meiosis II has prophase II, prometaphase II, metaphase II, etc

9. Interphase
Meiosis preceded by interphase nearly identical to the phases preceding mitosis
G1 – cell growth
S – DNA of chromosomes replicated
Sister chromatids formed
Centrosomes replicate
G2 – cell undergoes final preparations

10. Prophase I Nuclear envelope begins to break down
Synapsis formed – tight pairing of homologous chromosomes
Crossing over – an exchange of chromosome segments between non-sister chromatids

11. Prophase I Crossing over is first source of genetic variation
Reciprocal exchange of equivalent DNA between maternal chromosome and paternal chromosome (recombinant sister chromatid)
When the chromosomes move into gametes, each will have some DNA from each parent

12. Prometaphase I Nuclear membrane broken down entirely
Microtubules assembled from centrosomes at opposite poles of the cell grow toward middle of cell
Attachment of spindle fiber microtubules to kinetochores
Each tetrad is attached to microtubules, one from each pole

13. Metaphase I
Homologous chromosomes arrange in center of cell with kinetochores facing opposite poles
The orientation of each pair of homologous chromosomes is random – independent assortment
Second form of genetic variation (crossing over 1st variation)
The possible number of alignments is 2n where n is the number of chromosomes per set
For humans, we have 23 chromosome pairs, so we have 223 possible gametes (without crossing over!)
Highly unlikely that 2 haploid cells will have the same genetic composition

14. Metaphase I

15. Anaphase I Spindle fibers pull linked chromosomes apart
Sister chromatids remain tightly bonded together
Homologous chromosomes are pulled apart

16. Telophase I and Cytokinesis I
Separated chromosomes arrive at opposite poles
In most organisms, chromosomes decondense and nuclear envelopes form around chromatids in telophase I
Cytokinesis separates cell contents (cleavage furrow or cell plate)
Each new cell is now haploid – only have one chromosome set (mostly maternal or paternal DNA) but each chromosome is still duplicated (sister chromatids)

17. Prophase II and Prometaphase II
Decondensed chromosomes from telophase I condense again
Nuclear envelopes fragment (if they formed during telophase I)
Centrosomes move to opposite poles
Each sister chromatid forms individual kinetochore that attaches to microtubules from opposite poles

18. Metaphase II and Anaphase II
Sister chromatids maximally condensed
Align individually at center of cell
Anaphase II
Sister chromatids pulled apart by spindle fibers
Moved towards opposite poles
Just like mitosis, except there’s only 1 sister chromatid for each chromosome

19. Telophase II and Cytokinesis II
Chromosomes arrive at opposite poles
Decondense
Nuclear envelopes form around chromosomes
Cytokinesis II
2 cells separate into 4 genetically unique haploid cells
Only have 1 copy of the single set of chromosomes
Each genetically unique due to crossing over and independent assortment

22. Errors in Meiosis
Inherited disorders arise when chromosomes behave abnormally during meiosis
Abnormalities in chromosome number
Chromosome structural rearrangements
Since even a small segment of chromosomes can span many genes, chromosomal disorders are characteristically dramatic or often fatal

23. Abnormalities in Chromosome Number
Duplication, entire loss, or changes in number of complete sets
Caused by nondisjunction – when homologous chromosomes or sister chromatids fail to separate during meiosis
Meiosis I – homologous chromosomes fail to separate
2 gametes lack that chromosome, 2 gametes have 2 copies
Meiosis II – sister chromatids fail to separate
One gamete lacks that chromosome, 2 normal gametes, 1 gamete with 2 copies of chromosome

24. Euploid – normal number of chromosomes
Trisomy
Monosomy
Aneuploid – error in chromosome number

25. Examples of Chromosome Number Abnormalities
Down syndrome – trisomy of chromosome 21
Characteristic physical features and developmental delays in growth and cognition
Incidence of DS correlated to maternal age
XXX – triplo-X individuals have 3 X chromosomes
Appear female but express developmental delays and reduced fertility
XXY – Klinefelter syndrome
Males with small testes, enlarged breasts, reduced body hair
X0 – only 1 X chromosome
Female with short stature, webbed skin in neck, hearing and cardiac impairments, and sterility

26. Chromosomal Structural Rearrangements
Partial duplications or deletions
Example: Cri-du-chat (“cry of the cat”) syndrome – deletion of most of the small arm of chromosome 5
Inversion (reversed orientation) or translocation (between 2 nonhomologous chromosomes)

27. Gametogenesis
Spermatogonium – germ cell that undergoes meiosis for males
Results in 4 unique sperm (haploid)
Oogonium – germ cell that undergoes meiosis for females
Results in 1 viable ovum (haploid)