Mutations -- Part III

Chromosomal Mutations.

Point mutations affect single genes but chromosomal mutations can affect many genes simultaneously. Chromosomal mutations include:



A chromosomal deletion occurs when a chromosome is fragmented in one way or another. The portion containing the centromere (centric fragment) behaves in the same way other chromosomes do during meiosis or mitosis. The acentric (part that lacks the centromere) fragment moves very little and so its fate is unpredictable.

Usually one daughter cell gains a DNA fragment, but the other cell forms without the corresponding segment, with an obvious deletion to one of its chromosomes.



The loose fragment may form a ring or it might be inserted into another chromosome -- chromosomal addition.



If two breaks occur in the same chromosome the three fragments which result may be rejoined by repair enzymes (ligases). Although efficient, the repair enzymes have no way of telling how to rejoin the fragments. One possibility is a chromosomal inversion. The middle fragment is inserted the wrong way around.


If two nonhomologous chromosomes break, followed by fusion of each fragment to the opposite chromosome the result is a translocation mutation.


Transpositions duplicate genes by inserting copies of DNA segments into new positions in the genome. Unlike the other types of mutations transpositions result from a biological process not a random chemical or physical one.

Transpositions are the result of "jumping genes", mobile genetic elements called transposons. A transposon is at the minimum composed of a gene coding for a special enzyme and two short flanking segments of DNA called inverted repeats.

Because the gene product of the transposon is an enzyme called transposase it can insert a copy of the transposon almost anywhere in the genome. This ability suggest that it could be a cancer causing agent or it could make a mess of gene regulation by jumping into the middle of an important regulatory region in the genome.

DNA Repair Mechanisms

Check out this site on DNA repair mechanisms.

Cells which lack efficient repair mechanisms will eventually succumb to cancer.

Mutations are one method of studying how the steps in cell metabolism take place. The Noble Prize was awarded to Dr. Michael Smith for using mutations to discover how genes and proteins carry out their biological functions. Learn more about this technique called Site-directed Mutagenesis.

Page modified March 7, 2003