Re: What mutations are possible when radiation is possessed upon plants? Why?

Hi Nicole,

There are a few different kinds of mutations possible when plants are 
exposed to radiation, depending on what kind of radiation.

UV radiation can cause adjacent pyrimidine bases to become covalently 
attached to one another, creating a pyrimidine dimer.  When that section of 
the DNA is replicated, the DNA polymerase cannot read the dimer.  Sometimes 
the dimer is repaired, and the polymerase can continue on its way.  If it 
is not repaired, replication may be blocked there, creating a gap in the 
new strand of DNA.  Alternatively, the DNA polymerase complex may bypass 
the damage by using error prone DNA replication.  When this happens, the 
polymerase puts in adenine bases, regardless of the original DNA sequence.  
This may create point mutations.

Higher energy radiation, such as X-ray or gamma radiation, has enough 
energy to break the DNA into pieces.  This is like cutting a piece of 
string into two pieces.  In this case, there are several possible outcomes. 
 The cell will try to fasten the DNA ends back together.  If there was only 
one break and the cell can find both ends, the cell can fasten them back 
together, restoring the DNA to its original state.  However, if the DNA was 
broken into several pieces, things become more complicated.  Imagine 
someone cutting a piece of string into several pieces, then handing the 
pieces to you to put back together.  You can tie them all together, but you 
don't have a good way of knowing that the pieces are in their original 
order.  In other words, the order of the pieces may have been rearranged.  
The cell has the same problem, and so the repair of the broken DNA may 
result in a rearrangement.  Another possibility is that the cell does not 
find all the pieces, so when it puts the DNA strand back together, there is 
a missing piece.  This is called a deletion.

Point mutations, chromosomal rearrangements, and deletions can all be 
harmful if they disrupt important genes.  On the other hand, if they do not 
disrupt the function of any genes, these mutations can be totally harmless.

Keep in mind that a plant cell really has three separate genomes.  There is 
a genome in the nucleus, a genome in the chloroplast, and a genome in the 
mitochondria.  All three of these are subject to any of the mutations 
described above.

Alex