Nucleic acids

Nucleic acids

Nucleic acids (NA) were discovered by Freidrich Miescher in 1869.

In nature there are only two types of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) and they are present in all cells.

Its biological function was not fully confirmed until Avery and his collaborators demonstrated in 1944 that DNA was the molecule that carried genetic information . L

Nucleic acids have at least two functions: to pass on hereditary characteristics from one generation to the next, and to direct the synthesis of specific proteins.

Both the RNA molecule and the DNA molecule have a helical shaped structure.

Chemically, these acids are formed, as we said, by units called nucleotides: each nucleotide, in turn, is formed by three types of compounds:

1. A pentose or five-carbon sugar: two types of pentoses are known to form part of nucleotides, ribose and deoxyribose, the latter differs from the former because it lacks an oxygen and hence its name. DNA has only deoxyribose and RNA has only ribose, and their names, deoxyribonucleic acid and ribonucleic acid, respectively, have been derived from the pentose they carry.

2. A nitrogenous base: which are ringed compounds that contain nitrogen. Five of them can be identified: adenine, guanine, cytosine, uracil, and thymine

3. A phosphate radical: it is derived from phosphoric acid (H ₃ PO ₄ ^- ).

NA are linear polymers in which the repeating unit, called a nucleotide (figure below), is made up of: (1) a pentose (ribose or deoxyribose), (2) phosphoric acid, and (3) a nitrogenous base ( purine or pyrimidine).

The union of the pentose with a base constitutes a nucleoside (colored zone of the same figure). The union by means of an ester bond between the nucleoside and the phosphoric acid gives rise to the nucleotide.

The sequence of the nucleotides determines the code for each particular nucleic acid. In turn, this code tells the cell how to reproduce a duplicate of itself or the proteins it needs for survival.

DNA and RNA differ because:

- the molecular weight of DNA is generally greater than that of RNA

- RNA sugar is ribose and DNA sugar is deoxyribose

- RNA contains the nitrogenous base uracil, while DNA contains thymine

The spatial configuration of DNA is that of a double helical, while RNA is a linear polynucleotide, which can occasionally present intrastrand pairings.

Deoxyribonucleic acid (DNA)

Deoxyribonucleic Acid or DNA (in English DNA) contains the genetic information of all living things.

Each living species has its own DNA, and in humans it is this strand that determines individual characteristics, from eye color and musical talent to susceptibility to certain diseases.

It's like the barcode of all living organisms on earth, which is made up of segments called genes .

The combination of genes is specific for each organism and allows us to individualize ourselves. These genes come from the inheritance of our parents and for this reason DNA tests are used to determine the relationship of a person.

In addition, DNA is used to identify suspects in crimes (as long as a sample is available to link them).

Currently, the composition of the human genome has been determined, which makes it possible to identify and make therapies for genetically transmitted diseases such as: dwarfism, albinism, hemophilia, color blindness, deafness, cystic fibrosis, etc.

Mutagenic agents and the different alterations they can produce in DNA

Mutations can arise spontaneously (natural mutations) or be artificially induced (induced mutations) by radiation and certain chemical substances that we call mutagens. These agents significantly increase the normal frequency of mutation. Thus, we distinguish:

1) Radiation , which, depending on its effects, can be:

a) Non-ionizing, such as ultraviolet (UV) rays that are highly absorbed by DNA and favor the formation of covalent bonds between adjacent pyrimidines (thymine dimers, for example) and the appearance of tautomeric forms that cause gene mutations.

b) Ionizing, such as X-rays and gamma rays, which are much more energetic than UV; They can originate tautomeric forms, break the rings of the nitrogenous bases or the phosphodiester bonds with the corresponding breakage of the DNA and, consequently, of the chromosomes.

2) Chemical substances that react with DNA and can cause the following alterations:

a) Modification of nitrogenous bases. Thus, HNO ₂ deaminates them, hydroxylamine adds hydroxyl groups, mustard gas adds methyl, ethyl,...

b) Substitution of a base by another analogous one. This causes pairings between bases other than the complementary ones.

c) Intercalation of molecules. These are linked base pair-like molecules, capable of nesting between the base pairs of DNA. When duplication occurs, insertions or deletions of a base pair can occur with a corresponding shift in the reading frame.

Ribonucleic Acid (RNA) : The “helper” of DNA

Nucleic acid made up of nucleotides in which the sugar is ribose, and the nitrogenous bases are adenine, uracil, cytosine, and guanine. It acts as an intermediary and complement to the genetic instructions encoded in DNA.

Genetic information is somehow written on the DNA molecule, which is why it is known as “genetic material”. For this reason, together with ribonucleic acid (RNA), they are essential for living beings.

RNA acts as an assistant to DNA in using this information. That is why in a eukaryotic cell (which contains a nuclear membrane) DNA is found only in the nucleus, either forming the genes, on the other hand, RNA can be found both in the nucleus and in the cytoplasm.

Transcription or synthesis to RNA

Basically, the relationship between DNA, RNA and proteins develops as a flow of cellular activity. This flow, which today constitutes the central dogma of molecular biology, could be graphed as follows:

DNA --------> RNA ----------------> PROTEINS
replication --> transcription --> translation

Descriptively, we will say that DNA directs its own replication and its transcription or synthesis to RNA ( anabolic reaction ) , which in turn directs its translation ( anabolic reaction ) to proteins.

From the above it follows that transcription (or transcription) is the process through which RNA is formed from DNA information in order to synthesize proteins (translation).

For greater understanding, the process of RNA synthesis, or transcription, consists of making a complementary copy of a piece of DNA. RNA differs structurally from DNA in the sugar, which is ribose, and in a base, uracil, which replaces thymine. Furthermore, RNA is a single chain.

DNA, therefore, would be the " master copy " of the genetic information, which remains in "reserve" within the nucleus.

The RNA, instead, would be the " working copy " of the genetic information. This RNA that carries the instructions (translation) for protein synthesis is called messenger RNA (mRNA) .

Replication and transcription differ in one very important aspect, during replication the entire DNA chromosome is copied, but transcription is selective, it can be regulated.

mRNA

Messenger RNA: RNA molecule that represents a negative copy of the amino acid sequences of a gene. The non-coding sequences (introns) have already been removed. mRNA is a complete reflection of DNA bases, it is very heterogeneous with respect to size, since proteins vary greatly in their molecular weights. It is capable of associating with ribosomes for protein synthesis and has a high turnover rate.

Messenger RNA is a single strand, very similar to DNA, but it differs in that the sugar that makes it up is slightly different (it's called Ribose, while the one that makes up DNA is Deoxy Ribose). One of the nitrogenous bases differs in RNA and is called Uracil, replacing the thymine.

RNA types

Transcription products are not just mRNA. There are several different types of RNA, related to protein synthesis. Thus, there is messenger RNA (mRNA), ribosomal RNA (rRNA), translator RNA (tRNA) and nuclear heterogeneous RNA (HnRNA).

Within the DNA there are genes that code for tRNA and rRNA.

hRNA

Nuclear heterogeneous RNA = primary mRNA: located in the nucleus and of variable size. Precursor of messenger RNA, it becomes it after the elimination of introns, the sequences that do not code for genes.

mRNA

With few exceptions, the mRNA has a sequence of about 200 adenines (poly A tail) attached to its 3' end that is not encoded by DNA.

codons and amino acids

The information for the synthesis of amino acids is encoded in the form of triplets, each three bases constitute a codon that determines an amino acid. The rules of correspondence between codons and amino acids constitute the genetic code.

Protein synthesis or translation takes place on ribosomes in the cytoplasm. The amino acids are transported by the transfer RNA , specific for each one of them, and are taken to the messenger RNA , where its codon and the anticodon of the transfer RNA are paired, by base complementarity, and in this way they are put them in their proper position.

Once the synthesis of a protein is finished, the messenger RNA is free and can be read again. In fact, it is very common that before one protein ends another is already beginning, with which the same messenger RNA molecule is being used by several ribosomes simultaneously.

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