Nucleic Acids and Genetic Information | Biochemistry

The synthesis of nucleic acids (DNAs and RNAs) from nucleosides-5′- triphosphates is characterized by the necessity of conserving the genetic infor­mation intact.

The DNA is indeed the carrier of this information which is transmitted from generation to generation; this notion is now well established, but it was long believed that this information was carried by proteins. It would therefore be appropriate to recall the important observations and experiments which led to the correct conclusion that DNA is actually responsible for the hereditary characters.

Proofs of the Role of DNA as Carrier of Genetic Information:

We will only mention a few arguments.

Some of them are relative to notions which cannot be developed here and form the subject of more specific studies in Microbiology and Genetics:

1. In higher organisms, there is a correlation between the quantity of DNA contained in a cell and the quantity of genetic information. Thus, the quantity of DNA contained in the nuclei of diploid, somatic cells (having 2n chromosomes), is twice greater than that contained in the nuclei of haploid, germinal cells called the gametes (having n chromosomes).

2. A correlation can be observed between the genetic relationship of certain species and the analogy of base composition of their deoxyribonucleic acids or even, the degree of homology of the nucleotide sequences of their deoxyribonucleic acids (measured by hybridization, a technique).

3. Agents (physical or chemical) which can cause mutations (changes of hereditary characters) are those capable of altering the deoxyribonucleic acids.

4. In microorganisms, it was observed that the DNA can permit the transfer of one or several hereditary characters:

i. Transformation:

This is the transfer of one character. The addition for example of purified, isolated DNA, originating from streptomycin-resistant bacteria, to a culture of bacteria sensitive to this antibiotic, is followed by the appearance of resistant bacteria, which were therefore transformed and have acquired a new genetic information, transmissible to their progeny.

ii. Transduction:

The transfer of several characters, thanks to a transduc­ing phage DNA carrying several bacterial genes, when it penetrates into in a new bacterial cell.

iii. F-Duction:

It is the transfer of a small number of genes, present together with the sex factor F in a fragment of DNA called episome, which passes from a donor cell to a receptor cell.

iv. Conjugation:

Entering in direct contact, a male bacterial cell can transfer a part or the whole of its chromosome to a female cell. This conjuga­tion can be interrupted and a correlation is observed between the quantity of DNA having penetrated the receptor cell and the number of characters transferred.

5. Phage and Viral Infection:

The infection of a bacterial cell by a phage results from the penetration of the phage DNA only and it can be verified that the proteins of the phage (specifically labeled with ³⁵S for example) do not penetrate. The infection can also be brought about with the help of purified DNA, in cases of phages as well as some viruses.

As complete phage or viral particles will appear in the cell, it must be concluded that the information required for their formation is carried by the DNA.

This conclusion is further supported by the study of the lysogenic pathway where the DNA of the phage is integrated into the bacterial DNA, transmitted from generation to generation with the bacterial genome, and can suddenly start the infectious process (lytic pathway) after remaining for several generations without manifesting itself.

One thus arrived at the conclusion that the DNA is the carrier of genetic information; in other words the genes, responsible for the hereditary charac­ters are DNA fragments and the information is contained (coded) in the nucleotide sequence of these fragments. We must now examine how these genes are expressed, and determine the nature of compounds which are char­acteristic of a given organism.

Protein Nature of the Products of Gene Expression:

Effects of Mutations:

These specific compounds, whose synthesis depends on genetic information, are mostly proteins. For example, in microorganisms there are numerous mutants which differ from the wild type strain because an enzyme is absent (due to a mutation having affected the corresponding gene), with the result that they can no longer perform the reaction in question and the metabolic pathway (synthesis of an amino acid or synthesis of purine nucleotides, for example) is blocked.

In higher organisms particularly in man, numerous examples are presently known where the protein modified after a mutation could be identified (which in fact, enables the management of hereditary metabolic diseases caused by these mutations).

In phenylpyruvic oligophrenia or phenylketonuria, it is phenylalanine-hydroxylase which is absent, blocking the normal pathway of catabolism of this amino acid and thus causing the accumulation of the toxic transformation products (see fig. 7-25).

The absence of another enzyme prevents the formation of melanic pigments from dihydroxy-phenylalanine (see fig. 7-26) and causes albinism, a disease characterized by the non-pigmentation of the skin and hair. Similarly, hemophilia is due to the absence of a protein factor required for the coagulation of blood.

An enzyme is said to be absent when the functional enzyme is lacking, in other words, the corresponding catalytic activity is not detectable in the cell. Often, the polypeptide chain is present, but it is modified after the mutation, with the result that it is biologically inactive; but very often it conserves — at least partly — the antigenic properties of the normal protein, which enables the detection of its presence and its titration by immunological methods (with the help of an immune serum containing the corresponding specific antibody obtained by injecting the animal with the purified normal protein). The modification can be very slight as shown by the example of sickle-cell anemia.

The hemoglobin of individuals affected by this disease is very slightly dif­ferent from normal hemoglobin. Adult human hemoglobin or Hb-A consists of 2α-globin chains having 141 amino acids, and 2β-chains having 146 amino acids (each chain is linked to a prosthetic group called heme).

The analysis of amino acid sequences of polypeptide chains of the Hb-S hemoglobin contained in the red blood cells of patients suffering from sickle-cell anemia (which are sickle- shaped and easily distinguishable from normal red blood cells which are spherical) revealed that the only difference lies at the sixth amino acid (from the N-terminal end) of the β-chain which is a valine, whereas it is a glutamic acid in the case of the normal hemoglobin Hb-A.

Since there are 2β-chains per molecule of hemoglobin, 2 amino acids are therefore substituted out of a total of 574; this may seem to be a very slight difference, but the consequences can be tragic, because the capacity of this modified hemoglobin to bind oxygen is reduced and individuals affected by this disease risk to die very young especial­ly homozygous individuals who can synthesize only abnormal hemoglobin (Hb-S/Hb-S). The prognosis is less serious for heterozygous individuals (Hb-A/Hb-S) who are capable of synthesizing both normal and abnormal hemoglobins.

It is clear that the genes — DNA fragments, carrying the genetic information coded in the form of the nucleotide sequence — contain instructions enabling the cells to polymerize the amino acids in a very precise order and thus synthesize specific proteins. A mutation in a given gene generally results in the synthesis of a modified protein.

This modification may have no influence on the biological properties of the protein (the replacement of a neutral amino acid by another neutral amino acid at a position in the chain which participates neither in the constitution of the catalytic site of the enzyme, nor in the establishment of the three-dimensional conformation of the protein, may not produce any visible effect).

We have seen above, a few cases where a mutation was followed by the disappearance — or at least alteration — of a biological activity essential for the metabolism of the organism.

But a mutation can also have a beneficial effect and confer an advantageous character on the mutated organism; around the year 1850, Darwin made a very significant contribution in advancing a theory of the evolution of species based on natural selection of favourable mutations.

Evolution may therefore be regarded as a series of mutations resulting in modifications of proteins, and in fact one can watch the course of evolution of species by following up the evolution of the amino acid sequence of a given protein (e.g., cytochrome c, or hemoglobin).

Transfers of Information:

The DNA is the carrier of genetic information for all organisms, except RNA-containing phages and viruses (they are mostly plant viruses, but also animal viruses like that of poliomyelitis) which have no DNA and where the RNA is the carrier of genetic information.

It is clear that this information must be conserved during cell division and that is why this division is preceded by a synthesis of DNA which was named DNA duplication or replication, because an exact replica of the genetic information contained in the mother cell must be produced, so that the two daughter cells can inherit this information (i.e. a qualitatively and quantitatively identical information).

This information permits the synthesis of specific proteins in conformity with the rule “one gene → one enzyme” or rather “one gene → one polypeptide chain” since one enzyme can be formed by different polypeptide chains.

But the information carried by the DNA is not used as such for protein synthesis; it must be first transcribed into messenger RNA, thus called because it is in its turn, carrier of the genetic message; it is the messenger RNA which controls the ordered incorporation of amino acids in proteins.

There are therefore different transfers of information’s:

1. DNA → DNA Transfer:

This is the replication or duplication required for the conservation of genetic information;

2. DNA → RNA Transfer:

This is the transcription, permitting the passage of information from the DNA to various ribonucleic acids.

3. RNA → Protein Transfer:

This is the translation allowing the synthesis of specific proteins, thanks to the information contained in the messenger ribonucleic acids.

Besides, in the case of RNA-containing viruses, there are two additional transfer possibilities:

4. RNA → RNA Transfer:

The synthesis of RNA using the viral RNA as template has a double objective: it supplies the genomic RNA thus allowing the course of the infectious cycle (replication of the ribonucleic genome) and it provides the messenger ribonucleic acids coding for the viral proteins required for the infectious process (it is the equivalent of the transcription process).

5. RNA → DNA Transfer:

It is the prerequisite for a ribonucleic genome to be integrated in the deoxyribonucleic genome of the host cell. Furthermore, it appears that for some viruses causing tumors (oncogenic viruses), the ribonucleic genome must be first put in the form of DNA so as to be then transcribed like the genome of the host cell.

We are now ready to take up the study of the biosynthesis of nucleic acids. We have seen how precursors (ribo- and deoxyribonucleosides-5′-triphos­phates) are formed and we know that the synthesis of deoxyribonucleic acids and ribonucleic acids must meet the requirement of a faithful transfer of genetic information.