MCB31. Spring 2008

Lecture 5. Feb 6

The first in-class quiz will be next Monday, Feb. 11. Takes about 12-15 min during the

latter portion of the lecture period. Use the web-site lecture notes as a study tool.

Stem Cells

A. During development most cells specialize, becoming muscle, nerve, blood, etc. When they specialize, or differentiate, they tend to cease cell division.

B. But some cells remain mitotically active, continuing to form daughter cells that differentiate, and some daughter cells that do not differentiate but remain

mitotically active. Such cells are called "stem cells". The growth of hair, and the formation of blood cells , both of which occur continuously throughout life, are examples of the activity of stem cells.

C. What are important differences between embryonic and adult stem cells?

Meiosis

A. Sexual reproduction involves the union of sex cells ( sperm and egg) from the male and the female. Both parents contribute chromosomes ( i.e., DNA). But it is impossible for the sperm and eggs each to have 46 chromosomes. That would lead to a doubling of chromosome number each generation.

B. Meiosis is a special kind of cell division, found only in formation of sperm and eggs It involves a single replication of DNA and the attendant chromosome duplication, followed by 2 divisions. The two divisions are quite orderly, and result in only one of each kind of chromosome type being distributed to each of the four daughter cells. This temporary halving of the chromosome number, followed by regaining the proper number by means of sexual reproduction, is a terrific way to generate genetic novelty.

C. There is another way novelty can be generated. When the chromosomes are duplicated during meiosis, the two representatives of each kind line up closely with each other, and when DNA replication occurs, there is occasionally a swapping of DNA between one strand of DNA and another. This is called "crossing over" and is another way to generate novelty.

Laws of Inheritance

I. Some background

A. Some qualitative appreciation of the fact that traits are passed from parents to offspring has been appreciated for centuries.

B . Farmers and those involved in animal breeding, especially of horses and dogs, were well aware that by carrying out appropriate matings that stable new phenotypes could be maintained.

C. Even in stable populations, occasional odd-balls would turn up. This phenomenon was noted formally by Correns around 1900 when he observed that in a field of thousands of flowers there would occasionally be one with a very different petal color. these were called “sports”. It helped Correns re-discover the work of Mendel.

II. One of the difficulties in learning about how heredity works was the popularity of the hypothesis of blending.

A. For instance, if you cross a red snapdragon with a white one (this is called the parental or P generation), the offspring (first generation or, F-1) are all pink.

B. But if you interbreed the pink F-1s with one another to produce a second (F-2) generation, you find red, pink and white snaps! There is a reappearance of parental characteristics. This kind of finding was crucial in Mendel’s formulation of the idea that heredity is accomplished by particulate, non-divisible, unit. We now know that the unit of heredity is the gene, a double helix of DNA.

III. Mendel observed the same phenomenon of reappearance of parental characteristics, and furthermore he carefully quantified the results of his experiments using pea plants.

In the characters Mendel studied ( e.g., tall vs. short plants) there was no evidence of blending, even in the F-1. One character ( e.g. tall) always dominated the F-1 totally.

If you cross tall and short peas all the F-1 are tall. But when he carried out an F-2, short plants re-appeared, and always in approximately 1/4 of the cases. Mendel proposed a model to explain how this could work. He postulated that there are two functional units of heredity ( i.e. genes) in each individual, and that the sex cells carry only one of each. So when fertilization occurs and the new plant arises, it receives one gene from each parent. Which of the two genes the F-1 receives from each parent is completely by chance.

Furthermore, some genes are more “powerful” than others, ie., they are dominant over the recessive form.

A. We now know Mendel’s model is correct. There are two genes ( or more properly, two versions of the same gene, technically called alleles) for each trait studied (one ‘version” on each of the two chromosomes) . The genetic constitution determining a character is called the genotype. The actual trait which is observed is called the phenotype. There are at least two gene versions for each trait, and the sex cells only receive, by chance, one of them. Meiosis is the process that deals out the genes to the sex cells.

B. If you study more than one trait at a time, as Mendel did, you may observe, as he did, that the distribution ( sometimes called segregation) of genes for the different characters is completely independent, one from the other. In other cases, however, the genes for different characters behave as if they are “linked”. We now know that linked genes are on the same chromosome, while genes that segregate independently are on different chromosomes. We shall come back to this phenomenon later.