Deciphering of genetic code

DECIPEHERING OF GENETIC CODE

What is the genetic code?

The genetic code is a series of symbols that if they are separately do not represent anything, but when combined they can generate a compressible language so that it is understood. All living things biologically speaking function in the same way (with the exception of some bacteria).

All cells from the smallest animal to the largest are all the same. And this is due to the genetic code, which allows the information of each gene to be transmitted to the proteins, which are the ones that have the information.

The genetic code will allow us to learn the nucleotide sequence of DNA and RNA and how amino acids are formed.

Process for the production of proteins.

Genes contain instructions, for them to be able to generate proteins, these two steps must be carried out:

• Transcription is the process through which part of the genetic information is copied from its original (DNA) into mRNA.

• Translation is the process through which proteins are synthesised by using the information contained in the mRNA molecule formed during transcription.

How is the genetic code deciphered?

The genetic code is based on nucleotide triplets called codons, which specify individual amino acids in a polypeptide (or end "stop" signals). The codons of an mRNA are "read" one by one into RNA and protein structures called ribosomes; You start at the 5' end of the gene and work your way to the 3' end.

Cells decode mRNA when they read its nucleotides in groups of three, called codons. Each codon indicates a particular amino acid or, in some cases, provides a termination signal that ends translation. Furthermore, the AUG codon has a special role: it serves as the start codon at which translation begins, so this means that when the ribosome reads AUG it's a signal to start reading and code the sequence. The complete set of correspondences between codons and amino acids (or termination signals) is known as the genetic code.

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To decipher the genetic code, one needs to find out how the nucleotide sequences of a DNA or RNA molecule can encode the amino acid sequence of a polypeptide.

Proteins are normally 20 amino acids and only 4 nucleotide bases in RNA or DNA. Therefore, the genetic code must be made up of something more complex than a one-to-one relationship between nucleotides and amino acids.

Table of the 20 amino acids:

Codon types (start, stop and "normal")

 

The translation of the genetic code always begins at the start codon, which has the sequence AUG and encodes the amino acid methionine (Met). Therefore, all polypeptides usually start with methionine, although the initial methionine can be cleaved in later stages of processing. A start codon is needed to start translation, but the AUG codon can also appear later in the coding sequence of an mRNA and will simply specify the amino acid methionine.

Once the translation has started at the start codon, the next codons of the mRNA will be read one by one in the 5 'to 3' direction. As each codon is read, the corresponding amino acid is added to the carboxyl terminus of the polypeptide.

Most of the codons in the genetic code specify amino acids and are read during this phase of translation:

The codon table is logically organized.

 

 

To see how the codon table works, let's do an example. Suppose we are interested in the CAG codon and we want to know what amino acid it codes for.

 

• First we will look for the letter C (in this case) in the left column in which the 4 letters appear, when we locate the letter C we will have to look at that row. This column indicates the first letter of the codon.

 

• Then we will look at the top row of the table, in which the 4 letters are found again, in this case we will look for the letter A and we will focus on the column that indicates that letter. This column indicates the second letter of the codon.

 

• When we are focused on that column, we look at the row of the first letter finding a single box.

And when we have finally located the box, we will look in the right column of the table, in which the 4 letters will appear per row, we will focus on the box that has come out, and in it we will look for the last letter of our codon.

When we have located the three letters together, a word will appear next to it that will tell us what type of protein it is.

 

Translation continues until a stop codon is reached. There are three stop codons in the genetic code: UAA, UAG, and UGA. Unlike the start codon, the stop codons do not code for amino acids. Instead, they function as "stop" signals: they indicate that the polypeptide is complete and cause it to be released from the ribosome. There may be more nucleotides after the stop codon in the mRNA, but they will not be translated as part of the polypeptide.

The universal genetic code is made up of several codons or trio bases. There are a total of 64 codons

There are two punctuation marks in the genetic code called the START and STOP codons that signal the end of protein synthesis in all organisms.

 

Reading frames

 

The start codon is critical because it determines where the translation of the mRNA will begin. and the position of the start codon is also important since it determines the reading frame, that is, the way in which the mRNA sequence is read since it is divided into groups of three nucleotides within the ribosome.

As shown in the diagram below, the same nucleotide sequence can encode completely different polypeptides depending on the frame in which it is read.

The start codon determines the chosen reading frame and thus ensures that the correct polypeptide is produced.

 

 

So when we read a code, it makes sense because we read it in the correct frame (we divide it correctly into groups of three letters): such as “TOMORROW WE GO TO THE MOUNTAIN”. If we change the reading frame and group the letters into groups of three and begin to read a position later, we obtain: "TOM WEG ORR OWT OTH UNT AIN EMO". Scrolling in the frame causes the message to no longer make sense.

Then it should be noted that the nucleotides of a gene are not physically organized in groups of three. Rather, what constitutes a codon is simply a matter of where the ribosome begins to read and what nucleotide sequence lies after the start codon. Mutations that insert or remove just one nucleotide can alter the reading frame, resulting in the production of a "not understood" protein similar to the scrambled sentence in the previous example.

 

One amino acid, many codons

The genetic code consists of 64 codons. And there are only 20 amino acids, some of them are stop codons, but most are not. We can say that the genetic code is a degenerate code, since some amino acids are specified by more than one codon. 

For example proline is represented by four codons (CCU, CCC, CCA, and CCG). If any of them appear in an mRNA, it will cause proline to be added to the polypeptide chain.

Most of the amino acids in the genetic code are encoded by at least two codons. In fact, methionine and tryptophan are the only amino acids that are encoded by just one codon. There is no ambiguity (uncertainty) in the genetic code. A particular codon will always predictably translate into a particular amino acid or termination signal.

 

 

STOP codons

There are 3 STOP codons in the genetic code - UAG, UAA, and UGA. These codons signal the end of the polypeptide chain during translation. These codons are also known as nonsense codons or end codons as they do not code for an amino acid.

The three STOP codons have been named as amber (UAG), opal or umber (UGA) and ocher (UAA). "Amber" or UAG was discovered by Charles Steinberg and Richard Epstein and they named it amber after the German meaning of their friend Harris Bernstein's last name. The two STOP codons remaining were then named "ocher" and "opal" to maintain the "color name" theme.

During protein synthesis, STOP codons cause the release of the new ribosome polypeptide chain. This occurs because there are no tRNAs with the anticodons complementary to the STOP codons.

 

The genetic code is (almost) universal

With a few exceptions, all living things on Earth use the same genetic code. This means that the codons that specify the 20 amino acids in your cells are the same ones used by bacteria that inhabit hydrothermal vents at the bottom of the Pacific Ocean. Even in organisms that do not use the "standard" code, the differences are relatively small, such as a change in the amino acid encoded by a particular codon.

A genetic code shared by such diverse organisms provides important evidence for a common origin of life on Earth. That is, the many species on Earth today probably evolved from an organism in which the genetic code was already present. Because the code is essential for cell function, it should tend to remain unchanged in the species through generations, since individuals with major changes would be unable to survive. This type of evolutionary process can explain the remarkable similarity of the genetic code in the organisms present today.