Decoding the Elements of Your Genetic Code (2024)

Updated on November 05, 2019

Dissecting the Genetic Code

Codons

RNA codons designate specific amino acids. The order of the bases in the codon sequence determines the amino acid that is to be produced. Any of the four nucleotides in RNA may occupy one of three possible codon positions. Therefore, there are 64 possible codon combinations. Sixty-one codons specify amino acids and three (UAA, UAG, UGA) serve as stop signals to designate the end of protein synthesis. The codon AUG codes for the amino acid methionine and serves as a start signal for the beginning of translation.

Multiple codons may also specify the same amino acid. For example, the codons UCU, UCC, UCA, UCG, AGU, and AGC all specify the amino acid serine. The RNA codon table above lists codon combinations and their designated amino acids. Reading the table, if uracil (U) is in the first codon position, adenine (A) in the second, and cytosine (C) in the third, the codon UAC specifies the amino acid tyrosine.

Amino Acids

The abbreviations and names of all 20 amino acids are listed below.

Ala: Alanine Arg:ArginineAsn:AsparagineAsp: Aspartic acid

Cys: CysteineGlu:Glutamic acidGln:GlutamineGly:Glycine

His:HistidineIle:IsoleucineLeu:LeucineLys:Lysine

Met:MethioninePhe: Phenylalanine Pro:ProlineSer:Serine

Thr:ThreonineTrp:TryptophanTyr:TyrosineVal: Valine

Protein Production

Proteins are produced through the processes of DNA transcription and translation. The information in DNA is not directly converted into proteins, but must first be copied into RNA. DNA transcription is the process in protein synthesis that involves the transcribing of genetic information from DNA to RNA. Certain proteins called transcription factors unwind the DNA strand and allow the enzyme RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). When RNA polymerase transcribes the DNA, guanine pairs with cytosine and adenine pairs with uracil.

Since transcription occurs in the nucleusof a cell, the mRNA molecule must cross the nuclear membrane to reach the cytoplasm. Once in the cytoplasm, mRNA along with ribosomes and another RNA molecule called transfer RNA, work together to translate the transcribed message into chains of amino acids. During translation, each RNA codon is read and the appropriate amino acid is added to the growing polypeptide chain by transfer RNA. The mRNA molecule will continue to be translated until a termination or stop codon is reached. Once transcription has ended, the amino acid chain is modified before becoming a fully functioning protein.

How Mutations Effect Codons

A gene mutationis an alteration in the sequence of nucleotides in DNA. This change can affect a single nucleotide pair or larger segments of achromosomes. Altering nucleotide sequences most often results in non-functioning proteins. This is because changes in the nucleotide sequences change the codons. If the codons are changed, the amino acids and thus the proteins that are synthesized will not be the ones coded for in the original gene sequence.

Gene mutations can be generally categorized into two types: point mutations and base-pair insertions or deletions. Point mutations alter a single nucleotide. Base-pair insertions or deletions result when nucleotide bases are inserted into or deleted from the original gene sequence. Gene mutations are most commonly the result of two types of occurrences. First, environmental factors such as chemicals, radiation, and ultraviolet light from the sun can cause mutations. Secondly, mutations may also be caused by errors made during the division of the cell (mitosisandmeiosis).

Key Takeaways: Genetic Code

• The genetic code is a sequence of nucleotide bases in DNA and RNA that code for the production of specific amino acids. Amino acids are linked together to form proteins.
• The code is read in triplet sets of nucleotide bases, called codons, that designate specific amino acids. For example, the codon UAC (uracil, adenine, and cytosine) specifies the amino acid tyrosine.
• Some codonsrepresent start(AUG) and stop (UAG) signals for RNA transcription and proteinproduction.
• Gene mutations can alter codon sequences and negatively impact protein synthesis.

Sources

• Griffiths, Anthony JF, et al. "Genetic Code." An Introduction to Genetic Analysis. 7th Edition., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK21950/.
• "Introduction to Genomics."NHGRI, www.genome.gov/About-Genomics/Introduction-to-Genomics.