UNCG Biology 112 Supplementary Notes — Lecture Section

These brief notes supplement and reinforce some of the material presented in our lectures. All these topics are covered thoroughly in the text as well, and you should be sure to look there for additional information.

Supplementary Notes on Photosynthesis

Green plants are autotrophs: they manufacture their own food from simple compounds. Animals, by contrast, are heterotrophs, and must ingest their food. All life ultimately depends upon the solar energy gathered and converted into food by autotrophs.

Green plants make their food through the process of photosynthesis which takes place in the chloroplasts, the green organelles that are found in many plant cells, especially cells in the leaves. In its most basic form, photosynthesis works like this:

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Carbon dioxide from the air and water from the soil are broken apart using light energy, and the parts are rearranged into a simple sugar, with oxygen given off as a waste product. Most of the oxygen we breathe is plant waste.

The actual steps are far more complex than this of course, and took many years to work out. There are two main sets of reactions involved: the light reactions and the Calvin cycle. In the light reactions, water is split, waste oxygen is given off, and two chemical “batteries” (ATP and NADPH) are charged up. In the Calvin cycle, these batteries are used to synthesize sugar from carbon dioxide and hydrogen, and then the empty batteries are returned to the light reactions for recharging. The carbon thus used is said to have been “fixed” (i.e., removed from the air and incorporated into organic matter).

The special molecule that is at the heart of photosynthesis is chlorophyll, which absorbs individual photons and converts them into chemical energy. Chlorophyll reflects green light, but absorbs most other wavelengths (especially blue/purple). Each chlorophyll molecule is built around a magnesium atom. Magnesium deficiency can be a serious nutritional problem in plants.

Supplementary Notes on Genetics

With hindsight, we can say that Mendel proved inheritance was particulate, not blending. Because the phenotype/genotype relationship is complex, however, it was not clear for a long time (until the mid-20th century) that Mendel’s work applied to all organisms.

The particulate theory was refined into a chromosomal theory of inheritance by Thomas Hunt Morgan and many others. Morgan chose the fruit fly Drosophila melanogaster as his experimental organism, and was able to locate particular genes on particular chromosomes. His first great discovery located an allele for white eyes on one of the sex chromosomes. A “place” on a chromosome is called a locus (Latin for “place”; plural: loci).

Mendel had proposed that the two copies of each gene that are present in every cell (to use modern language) sort themselves independently into the gametes, and all of the characters he studied do indeed behave that way, because as it turns out they are all on different chromosomes. Genes that are on the same chromosome, however, don’t obey this “law of independent assortment”—they are instead said to be linked.

Sometimes during meiosis there are errors in chromosomal replication, resulting in too many or too few chromosomes. Most such errors are lethal, but some are not. One well-known human example is Down Syndrome, also known as Trisomy 21. Individuals with this condition have three copies of chromosome 21 (instead of the usual two), and have characteristic body shapes and usually suffer mental retardation.

In female mammals (which have two X chromosomes), one of the X chromosomes shuts off during development, and the other carries on all the needed metabolic functions during the life of the animal. This phenomenon is called X-inactivation. Which of the two X chromosomes shuts off is random: in some cells it is the maternal copy, in some the paternal copy. A coat color gene on the X chromosome in cats produces the calico pattern in some female cats—these cats are heterozygous for that color gene, and in some parts of the body one allele is expressed, while in other parts of the body the other allele is expressed.

Be sure you know the results of the simple Mendelian crosses:

    PP × pp (homozygous dominant by homozygous recessive)
    PP × Pp or pp × Pp (homozygous by heterozygous)
    Pp × Pp (heterozygous by heterozygous)

If you know the results of these crosses, you can easily calculate the probability of any outcome from a more complex cross (such as PpYyrr × ppYYRr) by calculating the outcome for each character individually (Pp × pp or Yy × YY) and then multiplying them together. This is the rule of multiplication.

Some human diseases follow simple Mendelian patterns: cystic fibrosis, Tay-Sachs disease, and sickle-cell anemia are all caused by recessive alleles. Individuals with only one copy of the allele (heterozygotes) are called carriers and are usually healthy, but they may pass on the allele to their children. Huntington’s disease is a rare example of a lethal disease caused by a dominant allele; it persists in the population only because the allele’s effects do not appear until later in life, after the person may already have had children.

In general, however, the relation between an organism’s genotype and its phenotype is not as simple as it is in these examples. Phenomena like pleiotropy (multiple phenotypic effects from a single gene), incomplete and codominance, and others we have not mentioned complicate the simple Mendelian picture. Furthermore, every genotype has a norm of reaction: a range of phenotypes that it can produce depending upon the environment it is raised in. A homozygous tall pea plant that is raised in a very poor environment may actually be shorter in phenotype than a homozygous dwarf plant raised in a very good environment.

Here are some simple Mendelian genetics problems. Test yourself with these (questions like them will be on the exam). Remember that there are three ways to express fractions; you should always use the one that the question asks for:

           ordinary fractions:    0   1/4   1/2   3/4     1
                  percentages:   0%   25%   50%   75%  100%
    proportions (frequencies):  0.0  0.25  0.50  0.75   1.0

Let P stand for the dominant allele for purple flowers in peas and p stand for the recessive white allele. Given this cross: PP × Pp

    1. What fraction of the offspring will be purple?
    2. What fraction of the offspring will have the homozygous dominant genotype?
    3. What percentage of the offspring will have the heterozygous genotype?
    4. What proportion of the offspring will be white?

Let Y stand for the dominant allele for yellow seed color in peas and y stand for the recessive green allele. Given this cross: yy × Yy

    5. What percentage of the offspring will be green?
    6. What fraction of the offspring will be homozygous?
    7. What fraction of the offspring will be heterozygous?
    8. What proportion of the offspring will be yellow?

Given the above alleles, consider this cross: PPYY × ppyy

    9. What percentage of the offspring will be purple-flowered and yellow-seeded?
   10. What percentage of the offspring will be white-flowered and green-seeded?
   11. What fraction of the offspring will be purple-flowered?
   12. What fraction of the offspring will have green seeds?

Let C^r stand for an allele for red flower color in snapdragons, and C^w stand for an allele for white flower color. These alleles are co-dominant. Consider this cross: C^rC^w × C^wC^w

   13. What fraction of the offspring will be red?
   14. What fraction of the offspring will be pink?
   15. What fraction of the offspring will be white?
   16. What is the genotype of a pink-flowered plant?

Supplementary Notes on Evolution

The biological species concept has two main components: the interbreeding that occurs within species and the reproductive isolation that exists between species. We saw that various kinds of isolating mechanisms exist, some acting prezygotically and some postzygotically.

How do species originate? The two main modes are allopatric speciation and sympatric speciation. Allopatric speciation is the principal mode in animals; both modes occur in plants, with sympatric speciation occurring typically by polyploidy.

The origin of species by allopatric speciation is analogous to the origin of human languages: geographically separated populations accumulate differences until they can no longer interbreed (origin of new species) or no longer communicate (origin of new languages). This is a very close analogy, but there are differences of course; human languages do not have a genetic basis, for example.

Evolution within species and populations is typically called “microevolution.” Large-scale phenomena in the evolutionary history of life fall under the heading “macroevolution.”

You have probably seen traditional classification systems that group organisms into more and more inclusive collections of related species. From least inclusive to most inclusive the commonly-used levels are: species, genus (plural: genera), family, order, class, phylum, kingdom.

The study of the diversity and evolutionary relationships of organisms—their evolutionary “family tree” or phylogeny—is called systematics. Systematists study the evolutionary tree of life (phylogeny) at all levels, from the relationships of similar species to the relationships of major branches of life (animals, plants, fungi, and so on).

Biologists use a formal system of naming species that you have probably seen before also. It is called the “binomial” (two-name) system. The formal scientific name of every species has two parts, such as Homo sapiens, Felis leo, or Quercus alba. Notice the standard form: the first word (the name of the genus) is always capitalized, and the second word (the name of the species itself) never is. The scientific names of species are always either underlined or italicized; so Homo sapiens and Felis leo are also correct, but Felis Leo, homo sapiens, and homo Sapiens, are all incorrect. The single word for the species never appears by itself (sapiens or leo by themselves would always be wrong, for example), although sometimes the name of the genus is abbreviated when it is clear from the context (H. sapiens or F. leo).

We learn about the evolutionary history of life in a number of ways. One is by comparing the similarities and differences among organisms and looking for homologies (inherited similarities that indicate common ancestry). Homology is inherited similarity.

We also learn about the history of life from evidence in the fossil record. Geology as a modern science began in the early 1800s, and at first depended on the relative dating of fossils and other geologic events (x happened first, then z, then w). In the 20th century methods of absolute dating (x happened 140 million years ago, z happened 30 million years later, and then w happened 10 million years after that) became more thoroughly developed. Radiometric dating is the most common method of absolute dating now used; it is based on the decay rates of radioactive elements in rocks. The work of geologists has allowed us to assemble a detailed geological time scale for the history of the earth (which begins about 4.6 billion years ago). Four major periods on this time scale you should know in order are: Precambrian, Paleozoic, Mesozoic, and Cenozoic.

One of the great advances in geology in the 20th century was the acceptance of continental drift. Geologists had always believed the general positions of the continents had been stable through earth history, although sea levels had changed, mountains had risen, and so on. By the 1960s evidence accumulated to show that this was not correct. Over the course of earth history the continents have been in different positions, and at one point they were all connected together into a single landmass (“Pangaea,” meaning “all earth”). Continental drift is still going on and it can actually be measured very precisely today (the rate varies; something in the range of a foot per year is common).

Another advance in geology in the last 30 years has been the acceptance of the idea that asteroid impacts have had a major effect on the history of life on earth. It had been assumed that these were common in the very early history of the earth, but not in the last billion years or so. It is now clear, however, that there have been major impacts within the last few hundred million years, and that these have caused mass extinctions. The best studied example is a major asteroid impact on the edge of what is now the Gulf of Mexico about 65 million years ago that may have exterminated many species in North America, including many of the larger species of dinosaurs.

You can think of systematics as the study of biological relatives in the genealogical sense: evolutionary ancestors, cousins, descendants, and so on. Our next main topic is ecology, which is the study of biological neighbors and their “social” interactions in local neighborhoods.