What is the genetic code and how is it determined?
Genetic code, the sequence of nucleotides in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) that determines the amino acid sequence of proteins. Though the linear sequence of nucleotides in DNA contains the information for protein sequences, proteins are not made directly from DNA.
The genome of an organism is inscribed in DNA, or in some viruses RNA. The portion of the genome that codes for a protein or an RNA is referred to as a gene. Those genes that code for proteins are composed of tri-nucleotide units called codons, each coding for a single amino acid.
- All the cells in a person's body have the same DNA and the same genes. However, the difference between cells in different tissues and organs is that the "expression" of the genes differs between cells.
- Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T).
- Concept 22 DNA words are three letters long. The genetic code had to be a "language" — using the DNA alphabet of A, T, C, and G — that produced enough DNA "words" to specify each of the 20 known amino acids.
Key Features of the Genetic Code
- 61 codons code for 20 amino acids.
- Three codons do not code for any amino acids.
- Each codon codes for only one amino acid, hence, it is unambiguous and specific.
- Some amino acids are coded for by more than one codon, hence the genetic code is degenerate.
- The steps in translation are:
- The ribosome binds to mRNA at a specific area.
- The ribosome starts matching tRNA anticodon sequences to the mRNA codon sequence.
- Each time a new tRNA comes into the ribosome, the amino acid that it was carrying gets added to the elongating polypeptide chain.
- The nucleotide triplet that encodes an amino acid is called a codon. Each group of three nucleotides encodes one amino acid. Since there are 64 combinations of 4 nucleotides taken three at a time and only 20 amino acids, the code is degenerate (more than one codon per amino acid, in most cases).
- A protein is a chain of amino acids connected together. You can think of this like a beaded necklace. The beads (amino acids) are connected together by a string (bond), which forms a long chain (protein). Therefore, a protein is "intact" or "whole."
The genetic code is the set of rules used by living cells to translate information encoded within genetic material (DNA or mRNA sequences) into proteins. The code defines how sequences of nucleotide triplets, called codons, specify which amino acid will be added next during protein synthesis.
- genetic code. …a unit known as the codon, which codes for an amino acid. For example, the sequence AUG is a codon that specifies the amino acid methionine. There are 64 possible codons, three of which do not code for amino acids but indicate the end of a protein.
- A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis. DNA and RNA molecules are written in a language of four nucleotides; meanwhile, the language of proteins includes 20 amino acids.
- The genetic code is degenerate. Some amino acids are encoded by more than one codon, inasmuch as there are 64 possible base triplets and only 20 amino acids. In fact, 61 of the 64 possible triplets specify particular amino acids and 3 triplets (called stop codons) designate the termination of translation.
Updated: 20th September 2018