For years, the exact means by which the DNA coded for structures, chemicals and other behaviors in living things was a mystery. Scientists now know which mRNA codons match each amino acid. This means that if you know the sequence of DNA or mRNA, you can figure out the sequence of amino acids that make a protein.
There are 20 kinds of amino acids, each of which is coded for by a three nucleotide mRNA codon (such as CCG or UAG). With four letters, A. U, C, & G, a possible 64 mRNA codons can be formed; so most amino acids can have several codons (such as AAA, AAC, AAU and AAG).
The image below is a mRNA table and a mRNA codon wheel for determining which mRNA codons will code for each amino acid. You should download the mRNA Codon Wheel/Codon Table Charts to have as a reference for upcoming assignments. You can use either to get the correct amino acids.
A mutation is any random change in the DNA. The sequence of amino acids is critical to the performance of a protein; even a single amino acid in the wrong place can cause an enzyme to be non-functional or cause a disease such as Sickle Cell Anemia.
What causes mutations?
- Errors in copying code during DNA replication, transcription or translation.
- Mutagens – anything that causes a mutation. Some well known environmental examples are ultraviolet radition, tars from tobacco, and asbestos.
Facts about mutations:
- Mutations are random.
- Most mutations are minor.
- Many mutations are harmful.
- Some mutations are lethal.
- Very few mutations are helpful.
There are a two main types of mutations that can occur in DNA: genes (nitrogenous bases) and entire chromosomes. Let’s take a closer look at these two groups of mutations:
Gene mutations involve mutations in base pairs of the DNA. If the mutation were in the sex cells of an individual, then they could pass it on to offspring. Point mutations involve the change of one nitrogen base in DNA. They are caused by substitution of one base pair for another in the sequence.
Point mutations can be further classified as missense, nonsense and silent mutations based on how the “mutated” protein behaves:
- Missense – base pair change results in a different amino acid; this may or may not affect the protein that results (example: sickle cell anemia)
- Nonsense – base pair change results in a premature stop codon; protein is nonfunctional (Cystic fibrosis)
- Silent – base pair change results in the same amino acid; no change in the protein
Stop and Think: Which type of point mutation is shown above? Missense, Nonsense or Silent?
Frameshift mutations result from the addition or deletion of a nitrogen base. This type of mutation changes the reading frame (groups of 3 nucleotides, or codons), resulting in a completely different protein than intended.
For instance, take the following sentence composed of three letter words (just like codons):
You and bio are rad.
If one of the letters is deleted, will this sentence still make sense? Let’s remove the “u.”
Yoa ndb ioa rer ad.
You can see that a frameshift mutation results in a nonsense mutation most of the time, coding for all the incorrect amino acids.
Let’s look at a real example:
A deletion of the C – G in the DNA here caused the first letters in the next codon to be included in this codon…all codons from here on have their first letter shifted to the previous codon and now code for different amino acids.
|Type of Chromosome Mutation||Description||Picture|
|Deletion||a piece of a chromosome breaks off and is lost|
|Insertion||a piece of one chromosome breaks off and attachs within another chromosome|
|Inversion||a piece of a chromosome breaks off and reattaches itself in reverse order.|
|Translocation||a broken piece attaches to a nonhomologous chromosome|
|Nondisjunction||a pair of chromosomes fail to separate during Anaphase I of meiotic cell division; result is too many or too few of one chromosome||
This karyotype shows three of the 21st chromosome; this results in the condition known as Down Syndrome.