Codons – DNA: RNA, Proteins Construction Sequence Reading Frame and Degeneration

DNA is written in a language that uses 3-letter words.

Each word of 3 letters is called a codon. A complete sentence in this DNA language is a gene. This lesson defines the codons and analyzes the different sequences that give rise to the amino acids.

The genetic code is contained in genetic material, also known as DNA or mRNA sequences.

This code is translated into proteins by living cells. Ribosomes decode mRNA sequences to build proteins. Ribosomes can read coding, which is written using 3-letter words called codons.

The genetic code is very similar among all organisms. In the transfer RNA or tRNA, the molecules carry amino acids to the ribosomes and read the mRNA three nucleotides or one codon at a time.

tRNAs can read codons because they have an anticodon as part of their sequence. As in mRNA, the anticodon of the tRNA encodes a specific amino acid, just in the reverse order of the mRNA.

RNA codons

Traditionally, the genetic code was represented by RNA codons since it is the messenger RNA (mRNA) that directs the translation. The codons in the mRNA are decoded by transfer RNA (tRNA) during protein synthesis.


Thanks to advances in genomics and computational technology, genes are discovered mainly at the DNA level before conversion to mRNA and proteins. It has become more popular to use DNA codons.

The codons of DNA are identical to those of RNA, except for the base of thymine (T), which replaces uracil (U) in the codons of RNA.

Multiple codons encode most amino acids. Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, and Tyr have two codons; Ala, Gly, Pro, Thr, and Val have four codons; and Arg, Leu, and Ser have six codons. Met and Trp, only two amino acids are encoded by a single codon each.

Codons and building proteins

  • DNA is the code used to produce proteins.
  • First, the DNA replicates, so there are two identical copies of the DNA. Then, the DNA is transcribed into RNA, which is then translated or read by tRNA to produce the proteins.
  • This process of replication, transcription, and translation is called the “central dogma.”
  • Like DNA, RNA is composed of a four-letter alphabet. However, thymine (T) in the DNA is replaced by uracil (U) in the RNA.
  • The message encoded in RNA is read in three-letter words called codons.
  • Codons encode specific amino acids, which are the building blocks of proteins. Therefore, by knowing the sequence of bases in a gene, it is possible to predict the codons and, ultimately, the amino acid sequence of the protein that produces the gene.
  • There are twenty possible amino acids. However, most amino acids can be encoded by more than one codon.
  • The start of a coding sequence is indicated by a start codon, a unique line; the start codon also encodes a methionine.
  • Three codons mark the end of the amino acid sequence. These are called stop codons.

Sequence reading frame

Because codons have three letters, the genetic code can be interpreted differently. These three different forms of interpretation are called reading frames.

For example, the CGAGCCTCC gene, if read from the first position, or the first frame, contains the codons CGA, GCC, and TCC. If it is read from the second position or the second table, it has the codons GAG and CCT.

It contains the codons AGC and CTC if read from the third position or the third frame. Note that every second and third reading frame has only two complete codons because the code is read in three-letter codons.

As a result of the different reading frames, each DNA sequence or gene can be read in three different ways. Each bracket will produce a different amino acid sequence when translated.

Only one table is the correct table and will produce a viable protein. The other two frames do not. Fortunately, the actual frame in which a protein sequence is translated is defined by a start codon in our cells.

What is codon degeneration, and what is its importance?

Codon degeneration is a single codon that encodes more than one amino acid. Traditionally, 20 amino acids encode a large variety of proteins in a living organism.

The nucleotides are 4; a codon is made of 3 nucleotides. Through simple mathematics, it can be determined that the total number of different points will be 64. One is a start codon, 3 are stop codons, and a rest code for all 20 amino acids.

This code is universal except in some cases, such as mitochondria. It has been suggested that degeneration makes DNA more tolerant of point mutations.

A point mutation in a codon doesn’t need to lead to a change in the conformation of the peptide. A synonym amino acid could replace it. Therefore, no change in the final protein is observed.