A mutation is a change in genetic information.
genetic information is encoded by the order of the nucleotide bases
of DNA, adenine (A), thymine (T), guanine (G), and cytosine (C), a
mutation represents some sort of change in that order. Mutations may
occur in both somatic and sex cells.
mutations that occur in sex cells can be passed from parent to offspring.
Types of Mutations
or SNP (Single Nucleotide Polymorphism), involves the
substitution of a wrong base during the
replication process. As the enzyme
DNA polymerase chugs down one side of a DNA
molecule, forming base pairs to build a new complementary strand, it occasionally adds the wrong base.
However, DNA polymerase makes very few errors and it corrects most
of those quickly. In addition, there are other enzymes that follow
along and "proofread" the nucleotides to be sure that the new nucleotides
are actually complementary to the template strand. Any misfits are
booted out and replaced with the proper base. Thanks to this
DNA is consistently replicated with
less than one mistake per billion nucleotides!
Nonetheless, this type of
mutation does occur and is responsible for the many subtle and not so
subtle variations found within and among species. There are three possible
outcomes of an overlooked change of one nucleotide during replication.
Surprisingly, the "mistake" may have no affect on the organism. For
one thing, the
genetic code is such that
many amino acids are represented by more than one
gene expression.) Each
individual codon is a three nucleotide sequence. In many instances,
there are several sequences that code for the same amino acid, their
sequence frequently differing by only one nucleotide.
For example, the triplet codons for the amino acid isoleucine are AUU, AUC, AUA, and AUG. Substitution of the the last nucleotide in the DNA
sequence coding for this amino acid would result in no change in the
resulting protein because isoleucine would be inserted in the protein
chain in each case.
On the other hand, an error which changed the
first base of the codon to either a U, C, or G would cause the wrong amino
acid to be inserted in place of isoleucine. For example, a
substitution of a G for the first A in the codon would result in insertion
of the amino acid valine instead of isoleucine.
The insertion of the wrong amino
acid in a functional region of a protein may cause the protein to be so
severely misshapen that it cannot function--even to the point of causing
the death of the organism. For example, swapping an A for a T in the
gene for hemoglobin results in the insertion of valine instead of
glutamine in the protein molecule causing the disease sickle cell anemia.
(eight out of the 146 amino acid units of normal hemoglobin)
hemoglobin (the same section as above as found in Sickle-cell
Good red blood
The function of normal human red
blood cells, which are disk-shaped, is to transport oxygen
from the lungs to the other organs of the body. Each red
blood cell contains millions of molecules of hemoglobin that
carries the oxygen.
A slight change in the order of the amino acids in the
hemoglobin molecule (valine substituted for glutamine), which
has only 146 amino acids, causes sickle-cell disease.
Abnormal hemoglobin molecules stick together and crystallize
deforming the red blood cells. The deformed blood cells then
clog tiny blood vessels impeding the flow of blood.
Sickle-cell anemia kills about 100,000 people per year in the
On the other hand, if the
substitution affects a less critical region of the protein or occurs in a noncoding (junk) region of DNA, there may be no
discernable effect at all. And finally, there are the rare
substitutions that are actually beneficial causing the protein to function
in such a way as to give the organism a survival advantage.
SNPs are the type of mutations
reported in mitochondrial DNA testing for genealogy. Likewise they
are used in some tests for Native American Indian heritage.
For more about SNPs in the human
genome, I highly recommend the
SNP Fact Sheet from
the Human Genome Project.
Insertions or Deletions
Another type of mutation involves either the
insertion or deletion of one or more
(some number that is not a multiple of three) nucleotides into a DNA
sequence. This type of mutation is known as a
frameshift mutation. For an illustration of how devastating
this type of mutation can be if it occurs in the coding region of a gene,
delete the w from the sentence below.
jumped over the moon.
The coj umpedo vert hem oon.
insertion of nucleotides in multiples of three, if not corrected during
the culling of introns from messenger RNA, will cause the insertion of an
extra amino acid for each three additional nucleotides.
Trinucleotide repeats are a sequence of three nucleotides that
repeat in tandem and vary in the the number of repeats.
Trinucleotide repeat mutations are known to cause at least eight genetic
disorders affecting the nervous or neuromuscular system. For more
about this topic see
YAP an alu insertion
particularly useful in population studies is the YAP, which stands for "Y
chromosome alu polymorphism." Alu is a sequence of approximately 300 base
pairs which has inserted itself into a particular region of the DNA. There
have been some half a million alu insertions in human DNA; YAP is one of
the more recent.
Unstable indels and SNPs
are relatively rare and, in the case of the latter, so infrequent that it
is reasonable to assume they have occurred at any particular position in
the genome only once in the course of human evolution. SNPs and stable
alus have been termed "unique event polymorphisms" (UEPs).
Some mutations involve all or significant parts of a whole chromosome.
An inversion flips over a segment of the
chromosome so that the nucleotide sequence of the DNA is backward.
Surprisingly, the brilliant little enzymes that coordinate protein
synthesis are often, but not always, able to figure this out and use the
sequence to direct the proper arrangement of the amino acids anyway.
Less easy to compensate for is the actual breakage
of a chromosome and consequent loss of a
fragment. Sometimes these fragments wander off and attach
themselves to another chromosome where they may or may not retain their
ability to function.
Mutations can happen in the junk
regions of DNA as well as in the regions that code for proteins. Because junk DNA does not code for protein,
these mutations usually have no effect on the individual. They,
however, are the mutations that are of value to the
nonfunctional stretches are short, moderately repetitive base pair
sequences. There are
two types of these repetitive sequences. VNTRs
(variable number tandem repeats) are repeated sequences that
typically range from 10 to 80 bps. These occur fairly frequently in
the human genome but there are relatively few different types.
Short tandem repeat (STR) sequences
(sometimes called microsatellites) are much
shorter (2-10 bps) and may be repeated as many as 100 times at a given
location on a chromosome. The human genome contains hundreds of
thousands of these STRs all evenly distributed on all the chromosomes.
It is the microsatellites on the Y chromosome that are of particular
interest to the genealogists seeking to identify their patrilineal line of