The genetic code is universal and is the same in all organisms.
This means DNA from a jellyfish can be transferred to a rabbit, for example.
However, with all the billions of base pairs within DNA, mistakes can occur.
These mistakes are mutations. Often, changes in DNA can be repaired by
enzymes, but sometimes, they persist through generations. Here, we see a table
showing the genetic code. The genetic code is made up of triplets of bases:
sequences of 3 bases transcribed from the DNA molecule to the messenger RNA
molecule. The bases are: adenine, guanine, uracil, and cytosine. The codons code for
an amino acid such as ACU for Threonine, and the order of amino acids determines
what protein will be made. This is the process of translation that is part of
the central dogma, but when mistakes and replication and transcription are made,
the bases in the codon will be different, and thus the protein that is made from
the codon will be different. Mutations are changes in the normal base sequence
of a DNA molecule. So, here's an analogy of the genetic code using three-letter
words to stand for codons. Each letter represents a base, and each word
represents a codon. The sentence "The cat ate the rat" stands for a protein. Let's
look at a change in one of the letters of the second codon. We know, in English,
that the letter C in the letter K can have the same sound, so when we read the
sentence, the sound and sense of it have not changed. This is an example of a
silent mutation because of a different base. In this analogy, the K replacing the
C does not change the sense of the sentence, and the protein remains the
same. But what if we change the base, that is
one of the letters in a word, to a different one? If we change the C and cat
to an H, we have the sentence "the Hat ate the rat," which changes the meaning of the
sentence. This is a substitution mutation. We have substituted one letter which
resulted in a very different sentence, and thereby change the protein the
sentence stands for. Now, what happens if we delete a letter? The change is more
profound, as every single three-letter word after the deletion is changed, and
the resulting protein, or sentence, cannot be read. Finally, what would happen if we
inserted a letter? The result is similar to the deletion example because every
codon after the insertion is changed, and thus the sentence does not make sense.
Silent mutations and substitutions are also called "point mutations," because they
change the nucleotide sequence only at one point. Insertions and deletions are
"frameshift mutations" because they shift every codon that follows the mutation.
Here we see two photos of human blood: on the left we see normal red cells, and on
the right we see sickled cells, characteristic of persons who have at
least one sickle cell allele. The sickle cell allele is due to a point mutation,
so let's look at the difference in the DNA code between the normal hemoglobin
and sickle cell hemoglobin. Recall C stands for cytosine, T stands for thymine,
and a stands for adenine. Part of the normal hemoglobin code in DNA contains
the bases CTC, but in the mutated sickle cell hemoglobin code, the thymine has
been replaced by adenine. So we have CAC. The RNA code codons are transcribed
from the DNA molecule. In normal hemoglobin, the RNA codon, which is
complementary to the DNA, is GAG. But the RNA codon complimentary in the
sickle cell hemoglobin is GUG. GAG codes for glutamic acid, abbreviated here
as GLU. However, in the sickle cell allele, the codon is GUG, which codes for
valine, abbreviated here as VAL. This one point mutation changes the shape of the
hemoglobin molecule and causes it to form stiff rods under certain conditions.
The formation of these stiff rods causes the symptoms of sickle cell trait in
persons who have one mutated allele, and sickle cell anemia in persons with two
mutated alleles. Sickle cell is one example of a mutation that has changed
the evolution of mankind.