Genetic code: Definition, Types, Properties, and Codon Degeneracy
The Genetic code, which refers to the unique configuration of DNA nucleotides that are translated into three-letter phrases or codons, regulates how amino acids are synthesized into proteins. In other words, the genetic code is a set of instructions that direct living cells in producing proteins from data stored in genetic material (DNA or RNA sequences) (amino acid sequences).
The genetic code, which is the arrangement of nucleotides in DNA and RNA, determines the amino acid sequence of proteins. Although the information for protein sequences is contained in DNA’s linear arrangement of nucleotides, DNA does not directly make proteins. Instead, a messenger RNA (mRNA) molecule made from the DNA controls the creation of the protein. RNA is made up of the four nucleotides adenine (A), guanine (G), cytosine (C), and uracil (U).
The codon, which codes for an amino acid, is a unit of three adjacent nucleotides. The amino acid methionine, for example, is specified by the codon AUG. Out of the 64 possible codons, three signify the end of a protein but do not code for amino acids. The remaining 61 codons define the 20 amino acids required to create proteins. Each mRNA starts with the AUG codon, which not only specifies the amino acid methionine but also marks the start of a protein. Methionine and tryptophan are the only two amino acids whose coding is restricted to a single codon (AUG and UGG, respectively). The remaining 18 amino acids are coded by 2 to 6 codons. Since many codons are used to encode the majority of the 20 amino acids, the coding is referred to as degenerate.
The mitochondrial DNA of some creatures and some eukaryotes are somewhat different from the genetic code, which was previously thought to be the same in all forms of life. The genetic code was deciphered in the early 1960s by American biochemists Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana.
The three nucleotides in each codon are different from one another. The codon is read as a continuous string of bases, taken three at a time, from a predetermined starting location; there is no overlap. The crucial beginning point is the reading frame.
Of the 64 codons that make up the genetic code, 3 do not encode any amino acids. These are also known as stop codons, non-sense codons, and termination codons. There are three stop codons: UAA, UAG, and UGA. These don’t encode any amino acids. The ribosome stops and detaches from the mRNA.
The majority of proteins’ initiator codon is AUG. Methionine is the only amino acid present, and occasionally the initiator codon is GUG.
The following are the key points regarding genetic code:
1. Triplets of bases known as codons are used to “read” the genetic code. To put it another way, a codon is made up of a group of three nucleotide bases.
2. In a triplet code, three RNA bases represent one amino acid.
3. There are 64 codons, which translate into 20 amino acids and signals for the beginning and end of transcription.
4. Codons are used by the code to create the amino acids that become proteins.
5. Each triplet [codon] designates either a start or stop signal for protein synthesis or specific amino acid in a protein structure.
6. The code provides the link between the sequence of amino acids in proteins and the sequence of bases in nucleic acids (DNA and complementary RNA).
7. The code describes the process through which genetic data is kept in living things.
Genetic Code Types
There are two different types of genetic code. RNA codons or DNA codons are two ways the genetic code might be expressed.
Messenger RNA (mRNA) contains RNA codons, which are the codons that are really “read” when polypeptides are produced (the process called translation). However, each mRNA molecule obtains its nucleotide sequence by transcription from the corresponding gene [DNA]. Because DNA sequencing has become so quick and because most genes are now discovered at the DNA level before they are discovered as mRNA or as a protein product, having a table of codons expressed as DNA is very helpful.
These codons are read from the sense (5′ to 3′) strand of DNA. They have the same reading as RNA codons, with the exception that thymine (T) is present instead of uracil (U). But the antisense strand of DNA (3′ to 5′) serves as the template for the actual mRNA synthesis.
Types of Codon:
There are 64 triplets of nucleotides in the genetic code. The three of them are known as codons. Except for three, each codon specifies one of the 20 amino acids required for protein synthesis. The code becomes rather redundant as a result. Numerous codons are used to encode the majority of amino acids. Two similar processes are carried out by one AUG codon. It marks the beginning of translation and specifies how the amino acid methionine (Met) will be incorporated into the expanding polypeptide chain.
The codons are of two types.
1. Sense Codon: Sense codons are those codons that encode amino acids. In the genetic code, 61 sense codons code for 20 amino acids.
2. Signal Codons: Signal codons are those codons involved in protein synthesis that encode signals. The signal is codified by four different codons. They are UAA, UAG, AUG, and UGA.
Signal codons are of two types
Start Codons: The start codon is the codon that initiates the translation process. As a result of its role in starting the synthesis of polypeptide chains, it is also known as an initiation codon. AUG is an illustration of such a codon. The amino acid methionine can also be encoded by this codon. The start signal may occasionally be coded using valine (GUG). Methionine is the first amino acid used by eukaryotes, whereas N-formyl methionine is used by prokaryotes.
Stop Codons: Stop codons are those codons that serve as a signal to end a polypeptide chain. Because they provide a signal for the termination and release of a polypeptide chain, these codons are also known as termination codons. Stop codons include UAA, UAG, and UGA as examples. Stop signal codons were formerly known as nonsense codons since they do not code for any amino acids.
The proteins known as release factors can read the signals of stop or termination codons. The tRNA molecules are unable to read stop signals. RF1, RF2, and RF3 are the release factors found in prokaryotes. While RF2 detects UAA and UGA, factor RFI only recognizes stop codons UAA and UAG. To stimulate RFI and RF2, RF3 serves a purpose. A single release factor (RF) in eukaryotes can identify all three stop codons.
Properties of Genetic Code:
Genetic code has some important properties.
1. The Code is Triplet: There are three sets of genetic codes. Codons are the nucleotide triplets that make up this structure. In addition to 20 amino acids and start and stop signals used in polypeptide chain synthesis, the triplet code’s 64 codons are enough to code for all of these. One amino acid is coded for by three RNA nucleotides in a triplet.
2. The Code is Universal: The genetic code is almost universal. In the vast majority of genes in animals, plants, and microbes, the same codons are paired with the same amino acids as well as the same START and STOP signals. However, certain exceptions have been discovered. The majority of these involve replacing one or two of the three STOP codons with an amino acid. There have been several documented exceptions for the mitochondrial genome and in unicellular eukaryotes for the synthesis of uncommon proteins such as selenocysteine and pyrolysine.
3. The Code is Commaless: The genetic code is thought to be uniform. In other words, there are no boundaries between codons and the codons are continuous. The amino acid sequence when a single base is deleted in a commaless code changes completely, as shown below. The code lacks commas. There is no need to pause between two adjacent codons. No letters are wasted since after the first amino acid is coded, the next two amino acids are automatically coded by the next three letters. There is not a void. There are “start” and “stop” signals in the genetic code. The translation process is initiated by a single start codon (AUG, the initiation codon), but it is terminated by three stop codons (UAA, UGA, and UAG), which are all present.
4. The Code is Non-Overlapping: One amino acid can be encoded using three nucleotides or bases. Six bases will encode two amino acids in a non-overlapping code. Each letter in a non-overlapping code is only ever read once. Six nucleotides or bases can code for four amino acids in overlapping coding since each base is read three times.
5. The Code is Non-ambiguous: In the genetic code, there are 64 codons. For 20 different amino acids, 61 of these codons are used. The codons, however, only encode one amino acid each. Therefore, only one amino acid can be encoded by each codon. The genetic code is unambiguous, as evidenced by this. One codon ought to encode more than one amino acid in the case of ambiguous code. No ambiguity exists in the genetic code. Unambiguous is the genetic code. Since each code only codes one amino acid, each one has a singular meaning. As an illustration, AUG exclusively codes for methionine, one of the essential amino acids. No other amino acids can ever be coded. Valine GUG codes. GUG can also code for valine if AUG is not present, though.
6. The Code Has Polarity: The polarity of the code designates the clear direction in which the message should be read. Because the base sequences in the code have changed, reading a codon the other way will specify a different amino acid. The following codons define different amino acids when read from the left to the right or right to the left. Because the following codon, which codes for a different amino acid, is interpreted as UUG from left to right and as GUU from right to left, respectively.
7. The genetic code is degenerate: The majority of amino acids have several codons; for instance, each of the amino acids arginine, leucine, and serine has six distinct codons. These codons for the leucine amino acid are CUA, CUC, CUG, CUU, UUA, and UUG. Similarly, the codes for glycine are GGG, GGA, GGC, and GGU.
Degeneracy of Genetic Code:
According to Crick’s research, the genetic code is degenerate. Simply put, it means that at least some amino acids must be specified by two or more different triplets and that each of the 64 triplets must have some meaning inside the code. Most frameshift mutations are predicted to result in nonsensical words if just 20 triplets are employed (the remaining 44 are gibberish because they do not code for any amino acids), which ostensibly halts the process of creating proteins. If so, the inhibition of frameshift mutations would infrequently, if ever, be effective. However, if each triplet had its specific amino acid, the protein would only have the erroneous amino acids as a result of the modified words. Since many or all amino acids must have several names in the base-pair code, Crick reasoned; this idea was eventually verified biochemically.
There are no overlaps in the genetic code. An amino acid is encoded by three bases. known as codons. From a predetermined starting place, the code is read through to its conclusion. We are aware of this because the codon alignment for the remainder of the sequence is changed by a single frameshift mutation anywhere in the coding region. Because some amino acids are specified by more than one codon, the code degenerates.
Only 20 naturally occurring amino acids are needed to make proteins, and several of these amino acids are designated by multiple codons, a situation is known as codon degeneracy. When two or more codons specify the same amino acid, there are two ways to do it:
1. Different synonymous codons require different tRNAs to accept the same amino acid. These tRNAs are known as “isoacceptor tRNAs,” and their anticodons vary. Leucine, for instance, is carried by two different tRNAs, tRNA1 leu and tRNA2 leu, both of which have the anticodon 3′ GAC5′.
2. Two or more synonymous codons pair with a single kind of tRNA. For instance, tRNA. Yeast’s tRNAaIa, which accepts alanine, carries the anticodon 3′ CGI5′, which can mate with the mRNA codons 5′ GCU3′, 5′ GCC3′, and 5′ GCA3′. To explain the coupling of a single type anticodon with synonymous codons, Crick put forth the “wobble hypothesis” in 1966. The “wobble position” is defined by the Wobble hypothesis as the base position at the 5′-end of the anticodon. The two bases of the codon complement the two bases of the anticodon from the 3′-end (in mRNA). Different bases can be paired with the base in the wobbling position. For instance, a single type of tRNAgly can pair with the codons 5’GGU3′, 5’GGC3′, and 5’GGA3′ to designate the amino acid glycine. This anticodon is 3′ CCI5′. In this way, inosine (I) in the codon’s wobble position can combine with U, C, and A. Similarly, G in the wobbling position can couple with C and/or U, whereas U can pair with A and G.
Since the majority of genes in plants and microbes have similar START and STOP signals and similar codons paired with similar amino acids, the genetic code is universal. With a few exceptions, such as stop codon. Despite GUG being intended for valine, the beginning codons AUG and GUG may code for methionine. The non-ambiguousness property is broken by this. In light of this, it can be claimed that not many codes deviate significantly from the nonambiguous or universal code.
Important Questions To Remember
Following is a list of 20 amino acids and their short names.
Ala=Alanine Asp=Acid aspartate acid Glu=glutamic Iso=Isoleucine Meth=Methionine Ser=Serine Tyr=Tyrosine Arg=Arginine Cys=Cysteine Gly=Glycine Leu=Leucine Phe=Phenylalanine Thr=Threonine Glu=Glutamine Val=Valine Asn=Asparagine His=Histidine Lys=Lysine Pro=Proline Trypto=Tryptophan
Question: What are genetic codes and their types?
Ans: The genetic code, which refers to the unique configuration of DNA nucleotides that are translated into three-letter phrases or codons, regulates how amino acids are synthesized into proteins. In other words, the genetic code is a set of instructions that direct living cells in producing proteins from data stored in genetic material (DNA or RNA sequences) (amino acid sequences). There are two different types of genetic code. RNA codons or DNA codons
Question: What is genetic code and its features?
Ans: 1. The Code is Triplet
2. The Code is Universal
3. The Code is Commaless
4. The Code is Non-Overlapping
5. The genetic code is degenerate
Question: Who gave the genetic code?
Ans: The genetic code was deciphered in the early 1960s by American biochemists Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana.
Question: How many genetic codes are there?
Ans: There are 64 codons, which are translated into 20 amino acids and act as transcription’s start and stop signals.