How does DNA determine our traits?    Levetin Ch. 7 p.112-121, Ch. 15 p. 249-261

1. Genes are pieces of DNA located on the chromosomes.

2. DNA is located in the nucleus. Recall that the nucleus controls what the cell does.

3. DNA contains information on how to build proteins.

4. Proteins do the work of the cell. Therefore, how a cell functions is determined by which proteins are built.

Example: Hemoglobin is a protein that carries oxygen. Information on how to build hemoglobin is carried in the DNA. Incorrect DNA information leads to incorrect hemoglobin and the disease sickle-cell anemia.

5. The DNA language (Table 7.2, p. 118)

a. 4 letter alphabet: G,C, A,T

b. Words are 3 letters long

c. 64 words exist in the DNA language

6. What does a DNA word mean?

a. Each word in the DNA language stands for an amino acid

b. Amino acids are the building blocks of proteins

c. Example: Hemoglobin is a chain of 146 amino acids linked together

d. Since there are 20 different amino acids and 64 DNA words, some of the 64 words will represent the same amino acid.

7. The DNA language is universal in all organisms.

a. Bacteria, protists, fungi, plants, animals all use the same DNA words to stand for the same amino acids.

b. Evolutionary significance: All organisms are alike at the DNA level since life arose only once and we are all descended from an original cell.

c. Genetic engineering: Since DNA is the same, DNA can be moved between unrelated organisms and traits can be moved between unrelated organisms.

Examples: Human insulin is produced by E. coli bacteria (Product name is Humulin and it is prescribed for diabetes).  The gene for human insulin was put into an E. coli bacteria.

Another example: A firefly gene has been inserted into tobacco.  Although this has no practical function, it was one of the first examples of moving DNA between unrelated organisms.

8. How does the information in DNA lead to a protein being built?

a. DNA never leaves the nucleus.

b. Proteins are built in the area outside the nucleus (but inside the plasma membrane) called the cytoplasm.

c. DNA is used to build a molecule called RNA. RNA moves out of the nucleus into the cytoplasm

9. Transcription (fig. 7.9)

a. When DNA is used to build RNA, this is called transcription.

b. RNA is molecule similar to DNA but with the letters G,C,A,U. Notice that U in RNA replaces the T in DNA.

c. DNA only fits together with RNA one way. DNA is a template on which the RNA is built.

Example: If the letters in DNA are GCAT, the letters in RNA that will bind to it are CGUA.

d. The RNA molecule built from the DNA moves out of the nucleus into the cytoplasm

10. Translation (fig. 7.10)

a. Using the RNA message to build a protein one amino acid at a time is called translation.

b. The RNA molecule is read by special kind of RNA called tRNA. tRNA is like a truck that only carries one load. In this case, the load is a single amino acid.

c. The tRNA molecule reads a word on the RNA molecule and inserts an amino acid.

d. A long chain of amino acids forms a protein which then carries out the work of the cell. (fig. 7.10)

11. Genetic regulation

a. While DNA lasts for the life of a cell, the RNA molecule lasts only a few hours to a few days. DNA is used by the cell to build different proteins as they are needed.

b. DNA is identical in all of our cells (except for sperm and egg cells). For example, a cell in our skin would have the same DNA information as a cell in our heart or our brain.

Exercise: If DNA is isolated from the roots of a pea plant, would it have a gene for flower color?

c. The reason cells function differently is that different proteins are being transcribed and translated in the different cells. Biologists say that different genes are "turned on". For example, insulin is produced in cells within the pancreas but nowhere else.

d. Since some proteins are produced in some cells and not in others, genes are regulated. Genetic regulation can occur at the level of transcription, translation and even in the processing of the protein after it has been made. Genetic regulation is an area of intense study in biology today.

12. Mutations

a. Occasionally when new cells are made, an error occurs in the copying of the DNA. These copying errors are called mutations. Mutations can also arise by environmental damage to DNA.  For example, many cancers are caused by exposure of DNA to mutagens (like cigarette smoke!).

b. Sickle cell anemia arises from a point mutation--a one amino acid difference from normal hemoglobin caused by a single letter difference.

c. Mutations are usually harmful. However, they are occasionally beneficial and so are the basis for new genes and new traits.

13. Genetic engineering (fig. 7.12)

a. DNA can be moved between unrelated organisms to confer new traits.

b. Examples of plants currently growing with genes from other organisms:

c. Concerns (See the PBS Website "Harvest of Fear").

Allergies: What if you are allergic to peanuts and a gene is inserted from peanut into wheat?

Hybridizing: What if a gene for herbicide resistance is spread to a wild relative by conventional pollination?

Evolution of resistance: By adding an insect resistant gene to cotton, corn and potato, won't we select for insects that are resistant to the gene product?

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