Review Questions

1. Asexual versus : which is better? is much more efficient than sexual reproduction in a number of ways. An doesn’t have to find a mate. An organism donates 100% of its’ genetic material to its (with , only 50% end up in the offspring). All members of a can produce offspring, not just , enabling asexual 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 because it creates genetic variety. All organisms depend on for . Sex takes these preexisting traits (created by mutations) and shuffles them into new combinations (). For example, if we wanted a rice 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 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 . 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 “ 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 . see sex as a way a living organism can maintain the evolutionary balance in ecosystems. Researchers studying parasite loads on sexual- and asexual-reproducing discovered that the sexual reproducers suffer less 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, , for the parasite, becomes difficult. Sex bestows the benefit of a moving target onto these organisms. For a 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 and 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 . Diploid is abbreviated “2n”. Somatic cells (body cells) like bone cells, muscle cells, neurons are all diploid. The or sex cells (eggs and ) are haploid. They have only one set of chromosomes. Haploid is abbreviated “n”.

Where did these sets of chromosomes come from? In 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 of a human somatic , 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 #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 location, the same banding pattern, and if you were able to look at the DNA sequence, you would discover that they have from the same trait at the same location on the chromosome. 5. What is meiosis? Meiosis is a special kind of in . 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 occurs in the of females and the testes of males.

6. List the three functions of meiosis. Meiosis has three functions. The first 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 .

During I the chromosomes appear, the fragments, the spindle fibers form and attach to the of the chromosomes, the nucleoli disappear just like prophase in .

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 ). Homologous pairs of dyads are pulled together into close proximity to one another in a process called . 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- 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 . Crossing over doesn’t just occur on one location on a chromosome. Crossing over events may span the entire chromosome. In 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 I.

In 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 . We have seen that meiosis creates variation three ways: crossing over, mutations caused during crossing over, and independent assortment.

9. Describe and . 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 and continue throughout . 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 (also diploid). The first meiotic division occurs and the primary spermatocyte divides into two secondary that are haploid. The second meiotic division occurs and the secondary spermatocytes divide resulting in a total of four cells called (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 is born. The stem cell oogonia have all ready transformed into primary . At birth, the primary oocytes are in prophase I (synapsis, tetrads, crossing over).

Women are born with 2 million primordial follicles (each one with a primary ). By puberty, the number of follicles has dropped to 300,000 to 400,000.

The first meiotic division also occurs at puberty forming two new daughter cells. One daughter cell, the secondary oocyte is large but the other, called a , is small and tiny. Unlike spermatogenesis, oogenesis only produces one from one primary oocyte. The other three daughter cells (polar bodies) are discarded. The egg has to be large (lots of cytoplasm) to support the developing .

The second meiotic division only occurs if the secondary oocyte is fertilized by a sperm. Without fertilization, meiosis never goes to completion. The final result of oogenesis is four haploid cells; one functional egg and three discarded polar bodies. 10. How many genetically different types of human gametes can be produced? We can calculate the genetic diversity produced by meiosis by calculating the number of possible combinations made during independent assortment. Humans have 23 pairs of chromosomes. Each pair in metaphase I has two possible combinations (one side or the other). Combine that 23 times and you get the following: 223 = 8 million

So, independent assortment can produce 8 million different sperm or 8 million different eggs. Therefore, the chances that one sperm is identical to another is 1 in 8 million. The chances that one egg is identical to another egg is also 1 in 8 million.

11. What are the chances that two offspring are genetically identical? We can calculate those odds by multiplying the number of possible combinations of a sperm with an egg. 223 x 223 = 64 trillion

The chances that two siblings will be genetically identical is 1 in 64 trillion (assume two different sperm and two different eggs). If we asked your parents to make another child genetically identical to you, they would only have a 1 in 64 trillion chance of doing it. Throw in the variation made by crossing over and the odds sky rocket. Truly, there is no one like you.