Chapter 9 Answers
How Genes Work
Visual Understanding
Figure 9.4
What are some of the possible problems you see if the cytosine indicated with the red arrow is accidentally replaced with an adenine?
First, the DNA molecule would no longer bind the two sides together properly. If it did manage to hold together until cell division and mitosis occurred, the two daughter cells would have two different copies of the DNA; one would have the old version, with cytosine, the second would have the new version with adenine.
Second, if it were to be read by an mRNA molecule, one of the codons might code for the wrong amino acid (depends on where in the codon the switch occurred). That would change the configuration of the future protein, which might not then fold into the correct shape, and would not work properly.
Table 9.1
Your friend Gorinda wants to know if there are ever mutations that don’t cause problems. What do you tell him?
Certainly. If a mutation is in a place that is not actually read to make an amino acid chain, then it may not cause any change at all. If the mutation falls at the end of a codon, it may still code for the same amino acid. Some mutations might code for a different amino acid, resulting in a final protein that actually works better, or faster, than the original, and so would be a positive mutation.
Challenge Questions
Genes Are Made of DNA
Based on the experiments and discoveries in the past two chapters, defend the statement, attributed to Sir Isaac Newton in 1676, (though some say that Bernard of Chartes said it first, way back in about 1130!) that scientists build new ideas in science by “standing on the shoulders of giants.”
Each experiment or discovery uses the prior knowledge as a foundation to move further. Mendel already knew of the work of previous researchers who had studied inheritance in peas. Avery knew the work of Griffith with his “transforming principle” in bacteria, showing that proteins were not the information carriers, and Hershey and Chase took the next step with viruses, showing the same thing. Rosalind Franklin’s work with X-ray crystallography of DNA molecules, along with Chargaff’s rule, gave the clues needed for Watson and Crick to figure out the double helix structure of DNA.
From Gene to Protein
Compare DNA to a cookbook. The book is kept in a library and cannot be checked out (removed). Start with the letters and words in the cookbook compared to the bases and codons in DNA; end with the amino acid chain being folded into a protein, and a cake being baked.
You need to make a copy of the recipe since you cannot remove the cookbook from the library; this is like the mRNA which is transcribed from a portion of the DNA molecule. You carry the recipe home with you, where you read it and assemble all the ingredients in the proper order; this compares with the ribosome reading the mRNA and translating it by assembling the amino acids in the correct sequence. Finally you put your cake in the oven to bake; the amino acid chain folds into its proper structure.
Regulating Gene Expression
What would happen if all the genes in a cell were always active?
It would take a huge amount of materials and energy in order to make all the proteins. Most of them would not be needed, and would take up space in the cell, or else they would need to be broken down again so the raw materials could be reused. Also, all cells contain all instructions, but most cells don’t need but a certain selection of the instructions. For instance a cell near your ear does not need to make the molecules that help your stomach with digestion. It would be terribly wasteful in terms of space, energy, and materials. That is why most genes are “off” most of the time.
Altering the Genetic Message
Mutations can occur in somatic (body) cells or in germ cells. What problems or opportunities does each kind of cell face?
If the mutation is in a somatic cell, it may cause no change, or it could cause some minor problems, or it could cause the cell to begin to multiply with controls; this is called cancer. If the mutation occurs in a germ cell, it causes a possible change in an allele for some trait. Of course, millions of eggs and sperm of every species of organism end up unfertilized, so there is only a small chance that the germ cell will actually end up fertilized, and that the mutation is in a position where it affects an actual allele, and that it is a dominant (or codominant) allele. It may then code for a protein that is worse, or better, than the original, or it could be essentially the same.