Mary L. said:
Thanks Desh, I haven't had any biology classes. If I have this correct... within a cell nucleus are 46 chromosomes with roughly 100,000 genes that contain about three billion bits of DNA. This is where I get confused - In all that DNA material we have 64 codons; 20 being used or having functions, 3 act as stop and start commands, and 41 do not seem to be coding anything.
If extremely energetic photons changed the DNA and "certain changes which had been wrought in the configuration [condons?] and in the chemical constituents [bonds and amino acids?] of the inheritance factors [genes?]" resulted in something positive, what might that possibly be? Any guesses?
Here's a general overview of how genetic information is read. Below is a small sequence of DNA from a protein coding region of the genome.
CTG ACT CCT GAG GAG AAG TCT
Somehow, a language consisting of only 4 characters (the four bases of DNA: cytosine, adenine, thymine, and guanine) needs to be translated into a language consisting of 20 characters (the twenty protein amino acids). Obviously, one DNA base cannot code for one protein amino acid, and two DNA bases do not give enough combinations to cover the 20 protein amino acids. Therefore, nature has evolved a code (the genetic code) by which three DNA bases code for one protein amino acid. Of course, since there are 64 possible three base combinations (codons), the genetic code is degenerate; each amino acids can be specified for by multiple codons. The correspondence between codons and amino acids can be found here (
http://tigger.uic.edu/classes/phys/phys461/phys450/ANJUM02/codon_table.jpg note: for our purposes U and T are the same).
Using the genetic code table, we can now translate the above DNA sequence to a protein sequence:
Leu Thr Pro Glu Glu Lys Ser
The sequence of amino acids in a protein gives it certain chemical and physical properties. One principle in biochemistry is that the sequence of amino acids in a proteins fully determines the there-dimensional structure of a protein and hence its function in a cell.
Mutation (changing one DNA base to another DNA base) can often cause changes in the amino acid sequences of proteins. However, this is not always the case. First, the cell has ways of fixing DNA damage, so not all DNA damage causes permanent changes to the DNA. Second, even if the mutation evades the DNA repair machinery, not all of the DNA in a cell codes for protein. Changes in non-protein coding sequences can have huge effects on the cell, but we still do not know what the vast majority of the DNA in cells does and putting changes in some of those regions doesn't seem to have any noticeable effect. Third, even if the mutation hits a protein-coding sequence, a change to the DNA bases might not cause a change in protein seqeunce because the genetic code is degenerate. As an example, consider the mutation to our original sequence:
CTG ACT CCT GA
A GAG AAG TCT
Here the change is from GAG to GAA both of which code for the amino acid glycine. Mutations such as these are known as silent mutations.
Some single base pair substitutions, however, can cause huge changes in the function of a protein. For example, consider the mutation:
CTG ACT CCT G
TG GAG AAG TCT
This mutation changes GAG, which codes for glycine, to GTG, which codes for valine. Thus, this mutation causes a change in the protein that is produced. This mutation that I've shown you is the mutation in the gene for hemoglobin (the protein that makes our blood red) that causes sickle-cell anemia (
http://en.wikipedia.org/wiki/Sickle-cell_disease note: the sequence is only a small portion of the hemoglobin gene). This is an example of a single base pair change that leads to the change in the shape of an entire cell!
As it turns out, the mutation can have positive effects too. While having two copies of the sickle-cell gene turns out to be very bad, having one copy is thought to make one more resistant to malaria.
With x-ray damage, the problem is worse than single base pair changes. X-rays are energetic enough to snap DNA strands in half, causing damage called double-strand breaks (DSBs). Normally, the cell is clever enough to fix these DSBs without trouble, but sometimes, sequences get lost when the cell re-joins the two broken strands. Now, let's consider the effect of a deletion on our sequence:
CTG AC_ CCT GAG GAG AAG TCT
The sixth base has been deleted. Now, if you try to translate the sequence, you notice something. The deletion not only affects the codon containing the mutation, but everything after the deletion as well! The mutation causes a "frameshift." The sequence would now be translated to the following protein sequence:
CTG ACC CTG AGG AGA AGT CT
Leu Thr Leu Arg Arg Ser
The protein sequence is now completely different than the original protein sequence (Leu Thr Pro Glu Glu Lys Ser), and the resulting mutant protein will have completely different properties (most likely it won't be functional).
X-rays can also cause rearrangements in the physical structure of chromosomes (called chromosome translocations). Sometimes, these can put two regions of DNA together to create a protein with new properties.
And, this may show my lack of education on the subject, but aren't we electro-chemical beings? Don't we tend to think of the electrical component as limited to brain and nerve functions? Is it possible that energetic wavelength / frequencies might have activated a codon, or is that a chemical function?
Electrical signals (in neurons) and even light can change what genes a cell produces. However, these signals need to be interpreted by proteins first and then sent to the DNA reading machinery through chemical signals.
