Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Effects of X-rays on cells and chromosomes

  1. Oct 28, 2008 #1
    Could someone PLEASE explain in simple language "procedures whereby scientists modify the germ plasm of plants and animals by the use of X rays?" And, how might that effect chromosomes?

    If you do not know, please advise where I can find this information. I am not a student and this is related to a historical section of a 2,100 page book... I really would appreciate understanding this!
    Last edited: Oct 28, 2008
  2. jcsd
  3. Oct 29, 2008 #2

    jim mcnamara

    User Avatar

    Staff: Mentor

    1. Light has a property called wavelength. Humans can see only a very small portion of all possible wavelengths of light. For example we cannot see microwaves or ultra-violet light.

    2. We have a lot of names for those light (particles) that come in different wavelengths.
    Here are some names in order of wavelength, from shortest to longest:
    gamma ray, xray, ultraviolet, visible, infared, microwave, radio.

    3. These group names are seem arbitrary because many were coined by scientists before there was a good understanding of the behavior of light particles (photons)

    4. Wavelength embues the photons with other proprties, among them is an energy level.
    Radio has low-energy photons, gamma rays have extremely energetic photons. xrays are also very energetic.

    5. When energetic photons hit living things, if the photon hits a molecule, say a DNA molecule or a protein molecule, it damages the molecule, often breaking it apart.

    6. germ plasm is an old word that means the genetic material of an organism. Nowadays we mostly refer to DNA. Damaged DNA in a seed or in germ plasm results in abnormal offspring.

    7. Years ago biologists exposed seeds to xrays and noticed that resulting plants were frequently deformed. A high dose of xrays killed the seeds. Very low doses of xrays produced fewer deleterious effects. They also experimented with animals, bacteria, fungi - you name it.

    Therefore, 'procedures whereby scientists modify the germ plasm of plants and animals by the use of X rays' simply means exposing pollen, seeds & ovaries of plants or reproductive organs of animals to xrays. The xrays damage the DNA (a little or a lot) and change the genetic makeup of offspring. Almost always for the worse.

    Think of those 1950's post-atomic-war-apocalyptic movies with "mutants" running around. Their parents were exposed to ionizing radiation (damaging high energy photons)
    so the kids were "mutants".

    We now know that exposure to xrays increases the probability of cancers and leukemias
    in humans or animals, so we keep xray exposure to a minimum.
  4. Oct 29, 2008 #3
    In the most basic of terms, X-rays denature macromolecules by breaking bonds between molecules and atoms, because they are so energetic. In biology, STRUCTURE DETERMINES FUNCTION (I can't stress that enough). DNA is important in that it contains the code necessary for a cell to carry out biological processes (basically, DNA is copied, ribosomes convert that code into amino acids, which form proteins). When DNA has bonds broken between those atoms in macromolecules, you get an incomplete structure, which will carry out a different function and the organism will either die or be terribly mutated.
  5. Oct 29, 2008 #4
    Thank you Jim, for your step-by-step explanation. Also thank you Desh, for your contributions toward my understanding.

    "These mutant traits appearing in the first generation resulted from certain changes which had been wrought in the configuration and in the chemical constituents of the inheritance factors of the [aborigine] germ plasm [DNA]..."

    It sounds as if a process similar to high energetic photon exposure effected the DNA creating mutant traits. However, there are indications that these were favorable mutations, at least in most cases.

    It is rather puzzling but now that I have a better grasp of what this may mean, I know what direction to take my research.

    Thank you again for taking the time to answer!

    Mary L.
  6. Oct 29, 2008 #5
    Sort of. By "mutant DNA," once again, you're referring to DNA that's been denatured.

    What holds DNA together is a series of hydrogen bonds between your As, Ts, Cs, and Gs, along with a phosphodiester backbone, if memory serves me right. What happens is, since those photons have so much energy, they break up both the hydrogen bonds and the phosphodiester linkages in the backbone. I'm not sure how much bio you've taken, but in case you haven't, DNA essentially directs all of a cell's activities, as well as what contains the necessary materials to create amino acids, and hence proteins (as well as other macromolecules, but let's stick to proteins for now as it's the easiest example). When DNA is denatured, the protein that results is incomplete, which means it either can't carry out what it was supposed to do to entirety or doesn't do what it's supposed to do at all in the first place. This causes abnormal behavior, and hence what you've coined as "mutant."
  7. Oct 30, 2008 #6
    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?

    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?

    My brain hurts from too much stretching :-) Guess I'll go to sleep now.

    Thank you again for helping me to understand this (to me) complicated subject.

    Mary L.
  8. Oct 30, 2008 #7


    User Avatar
    Science Advisor
    2017 Award

    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.


    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:


    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:


    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, lets consider the effect of a deletion on our sequence:


    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:


    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.

    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.
  9. Oct 30, 2008 #8
    Yggg... amazing info for my little brain :-) Thank you! But, now I have more questions than before, if you are willing.

    1. Do all living organisms have the same four bases of DNA (C-T-A-G)? Organisms like bacteria, fungus, plants, animals as well as humans? Is the DNA of bacteria as complex as for a human?

    2. Do all living organisms have the same codon functions or is that relative to the complexity of the organism?

    3. This is weird because I was sure someone used this example but I just reread everything and there is nothing about this... ultrasonic dental cleaning: While this is not energetic light, I think it is energetic sound. And, if I am correct, application of ultrasonic waves kills bacteria. Is it safe to say that ultrasonic waves do not destroy human cells but are able to destroy bacteria? Is there a simple explanation for this (such as resonance)?

    Thank you! This is a lot to digest.

    Mary L.
  10. Oct 30, 2008 #9


    User Avatar
    Science Advisor
    2017 Award

    All organisms on Earth use the same four DNA bases. The genomes of higher organisms (like humans) generally contain more DNA than bacteria. Also, the DNA in higher organisms is packaged by specific proteins, which adds some complexity to turning genes on and off in higher organisms.

    All organisms on Earth have a triplet genetic code (i.e. three nucleotides code for one amino acid). Most organisms share the same genetic code (i.e. the same mapping from codon to amino acid). For example, you can take a gene from a human, put it into E. coli (a type of bacteria studied by a lot of biologists) and the bacteria will make the correct human protein. The same works in reverse as well; human cells can express bacterial genes.

    Here, I think the sound waves are used to physically dislodge the bacteria from teeth, not to kill them. I would suspect that bacteria might be more resistant to sound waves than humans because their outer cell membranes have more structural support than ours.
  11. Oct 30, 2008 #10
    This is amazing for the layman. To think that everything living on earth has the same genetic code is a huge idea to wrap my mind around.

    As I understand it, all bacteria is not harmful to humans, i.e. the bacteria in the digestive tract. Are humans born with that bacteria? Does that bacteria enter the fetus through the mother's system?

    Hmmm... I am not sure where I am going with this now. I am wondering -- you have explained how proteins are made but what about hormones such as insulin used by the pancreas? Diabetes Type II is on the rise and yes, we can take pharmaceutical substitutes but is there work being done to reprogram the human body to, i.e. produce the proper amount of insulin?

    And, if I may, what is the greatest, in your opinion, work being developed as a result of the "Genome Project"

    Thanks, you make me wish I was much younger!! :-)

    Mary L.
  12. Oct 30, 2008 #11


    User Avatar
    Science Advisor
    2017 Award

    Yes, it is quite amazing isn't it.

    I'm not sure whether we would be born with them.

    Insulin is a protein and it is produced in the same way as most other proteins. In type II diabetes, the problem isn't in the production of insulin. Type II diabetes occurs because the cells in the body become insensitive to insulin.

    Type I diabetes (aka juvenile-onset diabetes), however, is caused by the loss of the specific cells that produce insulin (pancreatic beta islet cells). Here, scientists are working on ways to use stem cells to regenerate pancreatic islet cells in type I diabetes patients.

    There is a lot of great work that has been enabled by the human genome project and other genome sequencing projects. As a biologist, I think that one exciting result of these genome sequencing projects has been the ability to take a bird's eye view of evolution and compare protein sequences from a vast number of varied and diverse organisms. Looking and comparing at the sequences from vast numbers of proteins has allowed us to look at the see new and exciting connections. By taking this bird's eye view, scientists have been able to generate and test many hypotheses.
  13. Oct 31, 2008 #12
    Thank you again, to all who replied. I need time to absorb this information. I also discuss these matters with a friend (by mail-no computer) and may have more and better questions in the future. I am very thankful I came across this forum!

    Mary L.
  14. Oct 31, 2008 #13

    jim mcnamara

    User Avatar

    Staff: Mentor

    Another major outcome of the Human Genome Project is the deveopment of lab procedures, software and lab "machinery" to anlyze and sequence any DNA.

    A lot of plants and animals have been sequenced or are being sequenced. The outcome of sequencing and other DNA studies has meant that we learned relationships between groups of plants much much more accurately. Before, we used flower morphology (structure). Now, DNA.

    We used to believe that the Lotus plant was related to water lilies. Now we know because of DNA studies, that the sycamore (plane) tree is the closest living relative of the lotus, for example.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook