Meiosis

1. Meiosis Objectives: Describe what happens during each stage of meiosis. Compare and contrast the phases of meiosis to the phases of mitosis. Explain the effect of crossing over on genetic variability Describe alternation of generations in plants Vocabulary: Diploid * haploid * chromatids * chromatin * homologous chromosomes * interphase I & II * prophase I & II * metaphase I & II * anaphase I & II * telophase * gametes * somatic cells * autosomes * germ cells * tetrad * crossing over * alleles independent assortment * meiosis * zygote * nondisjunction * polyploidy * homologue * fertilization * spores * alternation of generations * sporophyte * gametophyte

2. Meiosis Meiosis is a process where the number of chromosomes is reduced from a diploid state (23 pairs of chromosomes - total of 46 - also called 2N) to a 1N, or haploid state (total of 23 chromosomes - no partners) during the formation of gametes (egg and sperm cells), also called germ cells. This prevents a higher and higher number of chromosomes each generation. However, scientists have been able to prevent the separation of some chromosomes in plants to produce flowers with more petals. This artificial build-up of chromosomes is called polyploidy. A similar attempt was made to increase the intensity of color in flowers by keeping extra chromosomes for color around. It failed. Instead, the flowers all turned white (absence of all color)! It was discovered that these extra chromosomes created a situation similar to what viral invasions create so the plant’s “immune system” attacked and destroyed the chromosomes. This information is now being used in the fight against age-related macular degeneration. By inserting excess numbers of the genes that cause blood vessel proliferation (a problem in the eye of AMD), we hope the immune system will destroy the genes calling for more blood vessels where they don’t belong.

3. During meiosis, some genetic exchange between chromosomes within a person can occur. That’s because each of us has a set of 23 chromosomes from Mom and another set of 23 chromosomes from Dad. Mom’s chromosome number 1 has a matching chromosome number 1 from Dad, etc. These are called homologous chromosomes, or homologues. They code for basically the same things because each chromosome has an allele (partner gene) on its homologue. (Ex: the gene for nose shape, but Dad’s genes may code for a flat nose tip while Mom’s code for a pointy nose tip.) Anyway, the homologous chromosomes line up during prophase I. This forms a tetrad (the homologues from Mom and Dad each have their sister chromatids still together, making available 4 copies of that chromosome number). If these homologues touch each other, a piece can break off from each and reattach on the opposing homologue. Therefore, chromosome #1 from Mom may trade a piece to the homologue from Dad. Each homologue now has a piece from the opposite partner. This increases genetic variability in offspring.

4. Phases of Meiosis InterphaseI - As with mitosis, this is the time when the chromatin (loose strands of DNA) is “unzipped” and copied, forming 2 copies of the original DNA. 1) Prophase I - The chromatin condenses around a histone protein to form chromosomes with an “X” shape. These are the identical sister chromatids joined by their centromeres. During prophase I, the homologous chromosomes (each with sister chromatids) pair up forming tetrads. This is when genetic exchange due to crossing over can occur. So, if chromosome #5 carried Mom’s genes for blue eyes and blonde hair and its homologue from Dad carried genes for brown eyes and brown hair. A new combination can occur with one chromosome now carrying the gene for blue eyes and brown hair and the other carrying the genes for brown eyes and blonde hair. 2) Metaphase I - Spindle fibers move the tetrads to the middle of the cell.

5. 3) Anaphase I - The homologous chromosomes separate to the opposite sides of the cell BUT the sister chromatids remain joined together. (Each chromosome still has 2 chromatids.) Because chromosome #1 from Dad might go to the same side of the cell as chromosome #2 from Mom, and so on, this is called independent assortment. Occasionally, during this phase nondisjunction occurs. In this case, the homologues for one set of chromosomes all travel together to the same side of the cell. Now one side has only 22 chromosomes and the other side has 24 chromosomes. 4) Telophase I - usually the cytoplasm divides creating 2 new cells, each with only 23 chromosomes (but still with sister chromatids). These are now haploid (1 N) cells because they have half the normal number of chromosomes. This process is sometimes called “reduction division”.

6. Interphase II - In the 2 recently formed cells, chromosomes unfold back to chromatin, but genetic material does NOT get copied. Additional proteins, etc. are made to support the formation of the next set of cells. 5) Prophase II - Chromatin within the 2 haploid cells condenses to form chromosomes again with their sister chromatids still intact. 6) Metaphase II - Chromosomes align in the center of the cell. 7) Anaphase II - Sister chromatids are separated at their centromeres and travel to opposite sides of the cell. Occasionally, the chromatids do not separate, this is another time nondisjunction can occur. 8) Telophase II - the nuclear membranes re-form. There are now 4 nuclei formed from the 1 original cell. Cytokinesis - the cytoplasm surrounding the 4 nuclei divides, forming 4 haploid daughter cells. (In mitosis, 2 diploid daughter cells were formed.)

7. The 4 haploid daughter cells, are germ cells or gametes (egg or sperm cells), not somatic (body cells, everything but gametes) cells. It is important to note that the first meiotic division in the formation of an egg actually occurs while a female child is still in her mother’s womb! Anything that might disrupt normal meiosis may not only affect the embryonic daughter but her future children (the grandkids)! Fertilization or the joining of the egg by the sperm creates a new 2N zygote (fused egg and sperm). Humans have 22 pairs of autosomes (chromosomes not involved in sex determination) and chromosome pair #23 in which a male will carry 1 “X” and 1 “Y” chromosome. Only sperm can donate a “Y” chromosome so only men determine a baby’s sex. In plants, the haploid and diploid conditions may exist as separate structures or generations. For example, ferns produce haploid (1N) spores on the bottom of their sporophyte (spore producing) fronds. The spores drop to the ground, form a tiny heart shaped gametophyte (gamete producing) generation that form egg and sperm in separate areas of the gametophyte. The sperm then swim through moist soil to fertilize the eggs. This is called “alternation of generations.”