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Review Questions Meiosis Review Questions Meiosis 1. Asexual reproduction versus sexual reproduction: which is better? Asexual reproduction is much more efficient than sexual reproduction in a number of ways. An organism doesn’t have to find a mate. An organism donates 100% of its’ genetic material to its offspring (with sex, only 50% end up in the offspring). All members of a population can produce offspring, not just females, enabling asexual organisms to out-reproduce sexual rivals. 2. So why is there sex? Why are there boys? If females can reproduce easier and more efficiently asexually, then why bother with males? Sex is good for evolution because it creates genetic variety. All organisms depend on mutations for genetic variation. Sex takes these preexisting traits (created by mutations) and shuffles them into new combinations (genetic recombination). For example, if we wanted a rice plant that was fast-growing but also had a high yield, we would have to wait a long time for a fast-growing rice to undergo a mutation that would also make it highly productive. An easy way to combine these two desirable traits is through sexually reproduction. By breeding a fast-growing variety with a high-yielding variety, we can create offspring with both traits. In an asexual organism, all the offspring are genetically identical to the parent (unless there was a mutation) and genetically identically to each other. Sexual reproduction creates offspring that are genetically different from the parents and genetically different from their siblings. In a stable environment, asexual reproduction may work just fine. However, most ecosystems are dynamic places. Conditions are in constant flux: changes in weather, climate, and landscape; new competitors, new diseases, new predators, new parasites, etc. In a changing world, it is dangerous for an organism to put all their genetic “eggs in one basket”. So, most organisms use sex to hedge their bets. By producing genetically unique offspring, parents have a better chance that a least some of their offspring will have the right set of traits to survive a change. So why are there boys? Females need males to produce offspring with greater genetic diversity. Biologists see sex as a way a living organism can maintain the evolutionary balance in ecosystems. Researchers studying parasite loads on sexual- and asexual-reproducing populations discovered that the sexual reproducers suffer less parasitism than their asexual counterparts. Life is an evolutionary arms race. Parasites, for example, can quickly adapt to host organisms with a single genotype. However, if the host genotype constantly changes and varies every generation, adaptation, for the parasite, becomes difficult. Sex bestows the benefit of a moving target onto these organisms. For a female then, a male is worth the trouble if his shared diversity helps her children evolve, survive and compete in a changing world. 3. So if sex is great for creating genetic diversity, then how can critters like bacteria and yeast maintain diversity while reproducing asexually 99% of the time? Microbes can rely mostly on mutations for their genetic diversity because they reproduce often. When you have thousands of generations per year, you can keep up with environmental change by relying solely on mutations. Slower reproducing organisms have to reproduce sexually because the just can’t get enough diversity through mutations alone. 4. Compare and contrast haploid and diploid cells. Diploid cells have two sets of chromosomes. Diploid is abbreviated “2n”. Somatic cells (body cells) like bone cells, muscle cells, neurons are all diploid. The gametes or sex cells (eggs and sperm) are haploid. They have only one set of chromosomes. Haploid is abbreviated “n”. Where did these sets of chromosomes come from? In human diploid cells, we inherited one set from mom and one set from dad. We all have one set and a backup. If you examined the karyotype of a human somatic cell, you would see 23 types of chromosomes. Karyotyping is a way of visually organizing the chromosomes. They are arranged by size, the longest type being chromosome #1. There are two of each type—one from mom and one from dad. Each pair is called homologous chromosomes. You’ll notice homologues have the same length, the same centromere location, the same banding pattern, and if you were able to look at the DNA sequence, you would discover that they have genes from the same trait at the same location on the chromosome. 5. What is meiosis? Meiosis is a special kind of cell division in eukaryotes. Meiosis creates haploid gametes. Often referred to as reduction division, meiosis starts with a diploid cell and ends up after two cell divisions with four haploid daughter cells. Meiosis in humans occurs in the ovaries of females and the testes of males. 6. List the three functions of meiosis. Meiosis has three functions. The first function is to keep the chromosome number constant generation after generation. If meiosis did not occur, the sex cells would remain diploid and, after fertilization, the offspring would have double the number of chromosomes (tetraploid—4n). If this were allowed to continue, there would be a doubling of chromosomes every generation. Not good. The second function is the creation of genetic variation in the sex cells and in turn diversity in the offspring. Meiosis creates sex cells that are genetically different from the parent cell and genetically different from each other. The third function is genetic integrity. Meiosis insures that every sex cell gets one complete set of chromosomes. For humans, that would be one of every 23 types of chromosomes. 7. Diagram the events in meiosis. The chromosomes are duplicated in interphase. During prophase I the chromosomes appear, the nuclear envelope fragments, the spindle fibers form and attach to the centromeres of the chromosomes, the nucleoli disappear just like prophase in mitosis. However, there are a couple of events in prophase I that are unique. A duplicated chromosome is also called a dyad (refers to the two chromatids). Homologous pairs of dyads are pulled together into close proximity to one another in a process called synapsis. In fact, in synapsis, the homologous dyads wrap around one another and form a structure we call a tetrad (refers to the four chromatids). While the homologous chromosomes are locked inside a tetrad, parts of the non- sister chromatids break off, exchange places, and reanneal. This is called crossing over. The first source of variation, crossing over creates genetically unique sex cells. Not only does crossing over recombine genes it also creates mutations if the breaks occur within a gene. Crossing over doesn’t just occur on one location on a chromosome. Crossing over events may span the entire chromosome. In metaphase I the pairs of homologous chromosomes are lined up side by side on the equator. This is different from mitosis where the chromosomes line up singly. Although crossing over is great at creating variation, the major gene shuffling takes place in metaphase I during a process called independent assortment. Each pair of chromosomes can line up in two different ways: mom on the left—dad on the right or visa versa. Whether a chromosome lines up on the left or right of the metaphase plate is unpredictable. How they line up dictates what genetic combination ends up in the daughter cells: the greater the number of chromosomes, the greater the number of possible combinations. The tetrad is split and the dyads are pulled to opposite poles during anaphase I. In telophase I, the chromosomes unwind, spindle fibers disappear, nuclear envelope reforms, and the nucleoli reappear. The cell divides into two (we call this interkinesis). Meiosis II begins with prophase II. As predicted the chromosomes appear, nuclear envelope fragments, spindle fibers appear, the nucleoli disappear. The spindle fibers attach to the chromosomes and move them toward the equator. In metaphase II, the dyads are lined up singly on the equator. The spindle fibers shorten, the sister chromatids split and are pulled to opposite poles in anaphase II. Telophase II reforms the nuclear envelope, breaks down the spindle fibers, uncoils the chromosomes, and the nucleoli reappear. Cytokinesis divides the daughter cells and meiosis is complete. We are left with four haploid cells; each one genetically different from each other and the parent cell. 8. Describe the three ways meiosis produces genetic variability. We have seen that meiosis creates variation three ways: crossing over, mutations caused during crossing over, and independent assortment. 9. Describe spermatogenesis and oogenesis. Spermatogenesis is the production of mature sperm. Sperm cells are made in the seminiferous tubules within the testes. The average adult male makes 300,000 new sperm cells every minute. The average human ejaculate contains 250-400 million sperm. Males begin spermatogenesis at puberty and continue throughout life. From start to finish, it takes 65-75 days for a mature sperm to be made. Spermatogenesis begins with a diploid stem cell called a spermatogonium. Stem cells can make other stem cells, as well as, differentiate and become a specialized cell. Spermatogonia are no different. A spermatogonium can divide (through mitosis) to make new spermatogonia. When a spermatogonium differentiates to become a sperm it first transforms into a primary spermatocyte (also diploid). The first meiotic division occurs and the primary spermatocyte divides into two secondary spermatocytes that are haploid. The second meiotic division occurs and the secondary spermatocytes divide resulting in a total of four cells called spermatids (all haploid). The spermatids specialize and transform into sperm. They build their tails, rearrange their cytoplasm, and stuff mitochondria into their mid-piece. Oogenesis is the production of mature eggs. Eggs are made in the ovaries of a female. Eggs production begins before a woman is born.
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