Well, a sequence of three nucleotides constitutes a codon or we can say trinucleotide sequence makes a codon on mRNA. The basic nucleobases of RNA nucleotides are:. Therefore, how many bases are in a codon? Because a codon consists of three nucleotides, a codon would, then, have three bases. So a typical example of a genetic codon would be a triplet code, e. What does codon code for? Because the collection of codons on mRNA forms the genetic code, a codon is the smallest unit of genetic code.
In simple terms, each codon actually encrypts a specific amino acid. It may also code for a signal to stop or start the process of protein synthesis in a cell. See Figure 1. Synonym: coding triplet.
Genetic codes are the basic connection between the sequence of the amino acids in proteins and the nucleotide sequence of RNA or DNA. Thorough investigations have established few basic properties of the genetic codes. Want to hear more about the genetic code? What is the function of the codons? To understand this, we should know about translation.
A basic understanding of this cellular process will explain the connection between a codon and an amino acid. Each DNA has a number of genes that carry out essential functions of life by generating the requisite protein molecules. Thus, the expression of genes entails the production of the encoded protein through it. The process of protein synthesis is essentially carried out in two steps. They are transcription and translation.
DNA functions as a template for complementary base-pairing to transfer information of protein expression to mRNA. An mRNA is a single-stranded nucleic acid. The genetic information it carries is acquired from the DNA molecule via transcription. The genetic code includes codons that will be translated into proteins. Thus, codons and amino acids are closely related to each other and are essential for all life processes.
Anticodons are nucleotide sequences that are complementary to the base sequences on the mRNA. The presence of anticodons on tRNA ensures that suitable amino acid is introduced into the protein structure. Watch this video showing the process of protein synthesis. All the genetic information is encrypted in the DNA molecule.
The genetic information is, then, transferred to mRNA as codons. The codons are eventually expressed as protein. Thus, the basic function of the codon is to encode the amino acid which eventually forms the proteins.
Would you like to know more about the role of mRNA in protein synthesis? There are basically two types of codons: the signal codons and the non-signal codons. The signal codons are the codons that provide the signal during the translation process; these signal codons can be further classified as start codons i. AUG and stop codons i. The non-signal codons are the codons that are primarily for translation, typically after translating the start codon. Another way to classify codons is if they code for an amino acid or not.
Those that code for a particular amino acid is called a sense codon whereas those that do not code for an amino acid are called a non-sense codon e. Going further, we now need to understand how many codons there are in the genetic code. There are a total of 64 codons, i. And so, how many different codons code for amino acids? Out of the 64 codons, 61 codons encode for the 20 amino acids, and the rest codes for codon signals.
The 61 codons code for only twenty amino acids and not 61! That means there are amino acids that are specified by more than one codon. How is that possible? The tRNA should be able to detect a reading frame. A Reading Frame would consist of a sequence of nucleotide triplets sense codons for translation. The tRNA would recognize a reading frame for translation if a start codon precedes the sense codons. Codons provide the key that allows these two languages to be translated into each other.
Each codon corresponds to a single amino acid or stop signal , and the full set of codons is called the genetic code. The genetic code includes 64 possible permutations, or combinations, of three-letter nucleotide sequences that can be made from the four nucleotides. Of the 64 codons, 61 represent amino acids, and three are stop signals. The genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon.
Other proteins play roles in DNA replication and transcription. Yet other proteins provide structural support for the cell, create channels through the cell membrane, or carry out one of many other important cellular support functions. This page appears in the following eBook. Aa Aa Aa. The ribosome assembles the polypeptide chain.
What is the genetic code? More on translation. How did scientists discover how ribosomes work? What are ribosomes made of? Is prokaryotic translation different from eukaryotic translation? Figure 1: In mRNA, three-nucleotide units called codons dictate a particular amino acid. For example, AUG codes for the amino acid methionine beige. The codon AUG codes for the amino acid methionine beige sphere. The codon GUC codes for the amino acid valine dark blue sphere.
The codon AGU codes for the amino acid serine orange sphere. The codon CCA codes for the amino acid proline light blue sphere. The codon UAA is a stop signal that terminates the translation process. The idea of codons was first proposed by Francis Crick and his colleagues in During that same year, Marshall Nirenberg and Heinrich Matthaei began deciphering the genetic code, and they determined that the codon UUU specifically represented the amino acid phenylalanine.
Following this discovery, Nirenberg, Philip Leder, and Har Gobind Khorana eventually identified the rest of the genetic code and fully described which codons corresponded to which amino acids.
Reading the genetic code. Redundancy in the genetic code means that most amino acids are specified by more than one mRNA codon. Methionine is specified by the codon AUG, which is also known as the start codon.
Consequently, methionine is the first amino acid to dock in the ribosome during the synthesis of proteins. Tryptophan is unique because it is the only amino acid specified by a single codon. The remaining 19 amino acids are specified by between two and six codons each.
Figure 2 shows the 64 codon combinations and the amino acids or stop signals they specify. Figure 2: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. Figure Detail. What role do ribosomes play in translation? As previously mentioned, ribosomes are the specialized cellular structures in which translation takes place. This means that ribosomes are the sites at which the genetic code is actually read by a cell. Figure 3: A tRNA molecule combines an anticodon sequence with an amino acid.
These nucleotides represent the anticodon sequence. The nucleotides are composed of a ribose sugar, which is represented by grey cylinders, attached to a nucleotide base, which is represented by a colored, vertical rectangle extending down from the ribose sugar. The color of the rectangle represents the chemical identity of the base: here, the anticodon sequence is composed of a yellow, green, and orange nucleotide.
At the top of the T-shaped molecule, an orange sphere, representing an amino acid, is attached to the amino acid attachment site at one end of the red tube.
During translation, ribosomes move along an mRNA strand, and with the help of proteins called initiation factors, elongation factors, and release factors, they assemble the sequence of amino acids indicated by the mRNA, thereby forming a protein. In order for this assembly to occur, however, the ribosomes must be surrounded by small but critical molecules called transfer RNA tRNA.
Each tRNA molecule consists of two distinct ends, one of which binds to a specific amino acid, and the other which binds to a specific codon in the mRNA sequence because it carries a series of nucleotides called an anticodon Figure 3. In this way, tRNA functions as an adapter between the genetic message and the protein product.
The exact role of tRNA is explained in more depth in the following sections. What are the steps in translation? Like transcription, translation can also be broken into three distinct phases: initiation, elongation, and termination.
All three phases of translation involve the ribosome, which directs the translation process. Multiple ribosomes can translate a single mRNA molecule at the same time, but all of these ribosomes must begin at the first codon and move along the mRNA strand one codon at a time until reaching the stop codon. This group of ribosomes, also known as a polysome , allows for the simultaneous production of multiple strings of amino acids, called polypeptides , from one mRNA.
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