In this article we will discuss about the properties and functions of genetic material in organisms.
The genetic material must have some well-defined properties to fulfill its functions.
These are enumerated below:
1. It must act as a store-house of information and must be able to transmit the information to the biochemical machinery of the cell.
2. It must be able to transfer information as far as possible correctly to each daughter cell as well as to progeny.
3. Simultaneously, the genetic material must also be capable of change within limits (mutation) to make evolution possible. During evolution, these changes which prove beneficial for survival are perpetuated by natural selection.
4. The genetic material must also be able to repair itself in case it is damaged by an external agency, or if any defect originates spontaneously.
In view of the conditions laid down regarding the properties and functions of the genetic material, DNA has proved to be the most appropriate substance among all the biopolymers.
The properties and functions of DNA are briefly discussed below:
1. Information Storage and Transfer:
Although DNA consists of only four types of nucleotides, represented as A (adenylic acid), T (thymidylic acid), G (guanylic acid) and C (cytidylic acid), a segment of a polynucleotide chain of DNA may have an enormous number of combinations of these four nucleotides arranged in sequences characteristic for a given segment.
It is now known that the genetic information is actually preserved or coded in DNA in the sequence of nucleic acid bases. This information is primarily needed for synthesis of proteins which are polymers of amino acids. Each protein has its specific sequence of amino acids and this sequence is determined by the nucleotide sequence of a specific segment of DNA.
It has also been found that a sequence of three consecutive DNA nucleotides corresponds to an amino acid. A segment of DNA in which the information about the sequence of amino acids of a polypeptide chain is coded is called a cistron. When a cell needs the synthesis of a particular protein, the information regarding its amino acid sequence is transferred from DNA to the site of protein synthesis.
The information transfer from DNA to protein takes place through an intermediate which is aptly called a messenger-RNA (m-RNA). This transfer takes place by the process of transcription. In this process, an enzyme, RNA polymerase, copies the information of one of the strands of double-stranded DNA into a complimentary single-stranded RNA molecule, the messenger-RNA. The DNA strand which is so copied is called the template.
The complementarity of a template strand of DNA and m-RNA is shown in Fig. 9.2. It is to be noted that both DNA and RNA strands have a polarity, i.e. they have a 5′-end and a 3′-end. The template DNA strand and the m-RNA strand are antiparallel. RNA synthesis proceeds from 5′-end to 3′-end, while the template DNA strand is copied from the 3′-end to 5′- end.
For the effective binding of the RNA-polymerase molecule to the template DNA strand, the double-helix needs localized denaturation, called melting which means that H-bonds between base- pairs are temporarily disrupted to accommodate the RNA-polymerase.
This is diagrammatically shown in Fig. 9.3:
Transfer of information from DNA to m-RNA is the first step. In the next step, the information carried by m-RNA in its nucleotide sequence is utilized for arranging the amino acids in proper sequence to build a polypeptide molecule. This process is known as translation, because the information coded in a ‘language’ of nucleotides is changed into the ‘language’ of amino acids.
A unit of three consecutive bases of DNA or RNA, known as a codon, stands for one amino acid. The process of translation begins only after m-RNA is associated with ribosomes which are cellular ribonucleoprotein particles, to form an initiation complex. The individual amino acids present in the cytoplasmic pool are picked up and carried to this complex by another kind of RNA, known as transfer-RNA (t-RNA).
A simplified representation of the translation process is shown in Fig. 9.4:
The genetic information stored in DNA is transmitted not only to m-RNA, but also to other types of RNAs, like ribosomal-RNA and transfer-RNA.
2. Information Transfer from Cell to Cell and from Parent to Progeny:
Every biological cell arises from a pre-existing cell and the progeny bears the same genetic makeup as the parent cell. For this to happen, the genetic information contained in the parent requires to be exactly reproduced and a copy to be transmitted to the progeny.
The genetic material, DNA, must, therefore, be able to form an exact copy of itself. This is known as DNA replication. Replication occurs by a semi-conservative mechanism which means that each strand of DNA is used as a template for synthesis of a new strand which is complementary to it resulting in the formation of two double- stranded DNA molecules (Fig. 9.5). DNA replication requires the nucleotide precursors, in the form of triphosphates and a number of proteins and enzymes of which the most important is DNA-polymerase.
In the eukaryotic organisms, the genetic material is organized into well-defined chromosomes which also replicate by the semi-conservative mechanism. Each chromosome contains a single large DNA molecule which replicates.
The chromosome splits longitudinally to form two daughter chromosomes, each of which contains a copy of the parental DNA. Each daughter chromosome is transferred to a daughter cell by the process of mitosis and thus each daughter cell receives the whole set of chromosomes.
In sexually reproducing eukaryotic organisms, each cell contains two sets of chromosomes (diploid). During formation of germ cells (gametes), only one set of chromosomes passes into each gamete (haploid). Union of male and female gametes coming from opposite sexes restores the original chromosome number. The cell division involved in production of gametes is known as meiosis. The process involves new assortment of genetic materials of the two parents.
3. Change in Genetic Information- Mutation:
Genetic information stored in DNA can change only if the nucleotide sequence is changed. Normally, DNA replication is a highly regulated and controlled process and the possibility of a chance error is rare. But even then such errors may occur at a low frequency which results in a change in the nucleotide sequence. The consequence is a mutation.
An organism in which such a change occurs is a mutant. Most mutations prove harmful and leads to death of the organism, i.e. mutations are generally lethal. But, sometimes a mutant may survive under appropriate environmental conditions and may give rise to a stable variant. Such mutants are of practical value for evolution of new forms.
Mutations may arise spontaneously, or they may be artificially induced by a variety of agents, known as mutagens. These may be physical agents, like X-ray, ultraviolet radiation, y-ray etc., or may be chemical agents. The mutagens act in different ways on the DNA, but all of them ultimately result in a change in the base sequence of DNA.
The mechanism of spontaneous mutations is less well understood. One of the probable causes has been thought to be the occurrence of tautomeric shift of adenine and thymine. Such a shift may change their normal base-pairing relationship.
Adenine — which normally pairs with thymine — may undergo a tautomeric shift to form a rare imino form which may pair with cytosine instead of thymine. Similarly, thymine may change into a rare end-form through a tautomeric shift and pair with guanine, instead of its normal pair, adenine.
These altered base-paring relationships are shown in Fig. 9.6:
How these altered base-pairing relationships lead to change in the base-sequence of DNA causing mutation is shown in Fig. 9.7:
4. Repair of Damaged DNA:
From the discussion about the nature and functions of the genetic material made before, it should be obvious that it is absolutely essential that the base-sequence of DNA be rigidly preserved. The mechanism of DNA replication provides for it. But in spite of that, damages to DNA may be inflicted by both internal and external factors. Such damages may destroy the genetic material, or may lead to its malfunction. Organisms must, therefore, possess means to repair damaged DNA.
These are known as DNA repair mechanisms. For example, ultraviolet light causes two adjacent thymine’s to form a dimer in bacterial DNA. Many bacteria can bring back thymine residues to original condition when exposed to visible light. Sometimes, the damaged portion of the DNA may have to be excised and replaced.