A codon chart or table is used to which amino acid corresponds to DNA or RNA. A codon chart can help to put together a polypeptide chain, but you will need to know the codons first.
Nucleotides are what composes our DNA. It is a language that defines all the things that make us who we are genetically. Codons are the translators that link the language of DNA with the language of proteins (amino acids).
Codons. Credit: Wikicommons/Edited by Seth Miller User:arapacana, Original file designed and produced by: Kosi Gramatikoff User:Kosigrim, courtesy of Abgent, also available in print (commercial offset one-page: original version of the image) by Abgent, image licensed under CC0
Nucleotides are the building blocks of DNA that are composed of three parts: a sugar, a phosphate, and a nitrogenous base. There are four bases in our DNA: adenine (A), guanine (G), cytosine (C), and thiamine (T). These four bases are used to encode the different genes, proteins, regulators, and everything else that our DNA is used for.
Since our DNA is a double helix, it means that there are two strands composed of many nucleotides. These two strands are attached to each other via these nucleotides as each of the base pairs with another base. Base A pairs with T and base G pairs with C and vice versa. When DNA becomes RNA, thiamine is replaced with uracil (U). In RNA, U pairs with A in lieu of T.
Among the many different RNA that are produced in the body, some of the ones produced for the purpose of protein synthesis contain a three-digit line of code that represents a particular aspect of the protein. These three-digit codes are referred to as codons and are a crucial part of ensuring that a protein is synthesized properly.
Codons And Protein Synthesis
The codons are three digits that are composed of any combination of the four RNA bases. The DNA codon is the same except there is T instead of U. Looking at the table, there are 64 total codons and they each represent a particular amino acid or function for the purposes of protein synthesis.
In protein synthesis, a messenger RNA (mRNA) arrives at the ribosome, which is a structure made of proteins and RNA, and begins the process of creating a protein. The mRNA contains the sequence for an unedited protein that the ribosome will “read” and create a protein.
Generally, the location of the AUG, methionine, codon indicates the start of protein synthesis. When the ribosome finds the start codon, it brings in a transfer RNA (tRNA). This tRNA contains the anticodon along with an amino acid attached to them. The anticodon is simply the complementary sequence to the codons of the mRNA. So AUG is complemented by UAC.
“DNA is like a computer program, but far, far more advanced than any software ever created.” — Bill Gates
For protein synthesis to work, the codon on the mRNA must match the anticodon on the tRNA. Once matched, the ribosome will bring in the next tRNA, since it can hold two at a time, and match it with the next codon. If it is a match, the amino acid on this new tRNA will be bound to the existing amino acid and the ribosome will shift to the next codon.
This process continues until the ribosome encounters the STOP codon on the mRNA, which can be UAA, UAG, or UGA. Any one of these three will halt protein synthesis. At this point, the synthesized protein is released and sent to be finished processing.
Since there are three STOP codons, that means that 61 of the remaining codons encode for the amino acids. Because there are only 20 amino acids for humans, there are a lot of redundancies within the codons. For instance, UUA and UUG both encode for leucine. CCU, CCC, and CCA all encode for proline.
These are the 20 amino acids that we have. They can be grouped into four groups and are all represented by particular codon or set of codons. Credit: Dancojocari/Wikipedia, image licensed under CC-BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0/deed.en
Redundancies and Mutations
The codons exist in such high numbers as a preventative measure against mutations and problems that they might face which could disrupt protein synthesis. Mutations in our bodies can be problematic because they can lead to a variety of cancers and genetic disorders. Some mutations can even put us at risk for many diseases and non-genetic disorders.
The redundancies of the codons help to ensure that these mutations are minimized, although there are always risks.
Synonymous Substitutions
Synonymous substitutions are often referred to as silent mutations because they are mutations in the DNA sequence, that represents an amino acid for a particular protein, that does not change the amino acid to something else. For instance, in DNA CTT would be leucine. A synonymous mutation would change it to CTC, which still codes for leucine so there is no problem.
Now while these synonymous mutations are generally harmless for those amino acids with multiple codons, it has been shown that some species prefer to have a particular codon sequence over another for a particular amino acid.
The redundancies of the codons allow these mutations to be negligible and cause no harm as long as the codon sequence still encodes for the same amino acid that the original did. However, not all codons represent a redundant form. Methionine, represented by only ATG, can easily become mutated to something else by a “silent” mutation because there are no other codons that encode for it. So, ATG changed to ATA would change methionine to isoleucine, a completely different amino acid.
Nonsynonymous Substitutions
Like the possible changes to methionine, not all mutations are silent and harmless. Those that do cause harm are generally referred to as nonsynonymous substitutions (or mutations). These cover a variety of problems that can occur to change the amino acids in a protein.
For instance, if CTT (leucine) is changed to CCT (proline) then a radical and noticeable change occurs in the protein. This is often referred to as a missense mutation.
Another form of substitution would be nonsense mutations that result in changes of an amino acid codon to a STOP codon, which would prematurely end the protein synthesis process.
These are some of the mutations that can alter the codon sequences and have potentially harmful effects on the proteins they synthesize. Others can be deleterious mutations that delete a particular nucleotide from the string of codons for a protein. Since the ribosome starts “reading” the mRNA at a certain point, deletions would alter where it starts and create an altered protein product. Similarly, insertion mutations that insert a nucleotide into the string of codons would change where the ribosome starts “reading” the mRNA.