In both eukaryotic and prokaryotic cells, the molecule that serves as the ultimate agent of chemical control is deoxyribonucleic acid (DNA), while the inheritable material of viruses may be either DNA or ribonucleic acid (RNA). Knowing the Watson-Crick structure for DNA makes possible the definition of a gene both chemically and functionally.
A gene is a portion of a DNA molecule composed of a specific series of nitrogenous bases that chemically codes for the production of a specific protein or RNA molecule, or serves as an operator in controlling the transcription of RNA within an operon unit.
The chemical code for the placement of an amino acid is a specific triplet nucleotide sequence. Since protein molecules contain an average of 300 amino acids, the average gene is made up of about 900 nucleotide pairs.
The DNA of E. coli is one of the most thoroughly investigated nucleoids and contains about 5 X 106 base pairs. This amounts to approximately 5000 genes, many of which have been identified in their proper sequence.
An organism’s DNA constitutes a catalog of genes known as the genotype of the organism. The expression of these genes will result in a certain collection of characteristics referred to as the phenotype.
While the phenotype of an organism consists of its observable characteristics, the genotype is not visible, since it is the DNA chemical code (formula) of an organism. There is not always a total expression of the genotype. Particular genes may not express themselves for a variety of reasons.
In some cases, the physical environment will determine if certain genes will have a chance to express themselves. For example, if lactose is not supplied to a bacterial population that can metabolize this sugar, the phenotype will not be seen because the presence of lactose is required to induce the formation of the enzymes needed for its breakdown.
DNA is very stable, thus it is an excellent molecule to serve as the transmitter of chemical codes through generations. The stability of DNA and its resistance to change ensures the continuation of a species even though alterations regularly occur in gene structure.
Any permanent change in the nitrogenous base sequence of DNA is called a mutation. A single gene may mutate too many different forms. Those forms of a gene that affect the same characteristic but produce different expressions of that characteristic are called alleles.
For example, in humans there are alleles for eye colors such as blue and brown. In bacteria, there are alleles for enzyme production. In some bacteria the genet that controls the operation of the same operon functions on an inducible basis (“off” and “on”), while others have a different allele of this gene which enables the same operon to function on a constitutive (always “on”) basis.
Various mutations may be produced in DNA. A single nitrogenous base may be lost and replaced by a different base. This is known as a point mutation and may cause a single amino acid change in a protein. The sugar-phosphate backbone of the DNA may beak, resulting in a change or loss of a sizable portion of the molecule.
This kind of event in DNA changes more than just a nucleotide base. If the damaged section is lost and not repaired, the mutation is called a deletion mutation.
If the damage is required by the insertion of the same piece of DNA in reverse order, it is called an inversion mutation. When a totally new base sequence is synthesized to fill the gap, it is called an insertion mutation.
Mutations may have any one of a number of effects on the cell in which they occur. In some cases, the change is not harmful. After a single base change (point mutation), the protein synthesized by the gene may still be functional and no phenotypic change may be seen.
This may occur because of the nature of the nucleotide code system since, in some instances, the same amino acid can be coded for by more than one triplet codon sequence.
Therefore, even if a point mutation takes place, the new triplet codon formed can still call for the positioning of the same amino acid in the protein being synthesized.
The protein may also remain active if another very similar amino acid is .coded for placement and serves the same function in the completed molecule. Other more extensive mutations may result in the severe reduction of enzyme activity.
This reduction may be due to a distortion of the active site on the enzyme surface. Poorly constructed enzymes may also be more susceptible to environmental changes.
Minor fluctuations in temperature, ion concentration, or oxidizing agents my alter the enzyme’s three-dimensional shape and cause inactivation. In some cases, the mutation may result in the complete loss of gene activity and death of the cell if an essential gene is inactivated.
Mutations may occur either spontaneously or be caused by agents such as radiation and chemicals. Anything that causes permanent changes in the DNA of a cell is called a mutagenic agent.
Mutagenic agents include x-rays, mustard gas, and nitrous acid. Naturally-occurring spontaneous mutations are at a relative low rate in microbes. About one in one mission (1 x 10-6) bases may undergo a natural reorganization of chemical bonds within the DNA.
For example, thymine may quickly and temporarily shift its internal bonds into a different arrangement. If this rearrangement takes place when thymine is base-paired with adenine (A-T), an error in base pairing will occur at the moment of DNA duplication.
Because of this spontaneous bond rearrangement, normal base pairing (A-T) cannot take place, and the guanine-containing nucleotide is hydrogen bonded into the sequence by mistake (G-T).
Point’s mutations such as this may also be caused by mutagenic agents. Ultraviolet radiation induces mutations by causing the formation of bonds between thymine nitrogenous bases located next to one another on the same strand of the DNA double helix. These lined bases are known as thymine dimers.
This change in bonding results in a fraineshift mutation since the dimer will be skipped during replication and the transcription portion of protein synthesis. Reading-frame shift mutations might be compared to “skipping” the two letters p and e in the word “independence.” It would then read “independence” and make no sense.
This type of mutation and others may also be caused by chemicals such as nitrous acid, acridine dyes, and alkylating agents. They induce changes that result in mispairing, reading-frame shifts, and deletions.
However, many bacteria have the ability to repair some of these changes. The repair process known as photo reactivation requires the presence of visible light immediately after dimer formation. The visible light energy is able to break the dimer and reestablish the original base sequence before a permanent change is fashioned into the DNA.
Another, more complex, repair system, called dark repair, may also correct ultraviolet damage. In this series of reactions, visible light is not required. The damaged section of DNA is “clipped out” of the strand enzymatically, the “hole” is enlarged to ensure a better “patch job,” and a new segment of DNA is synthesized.
Neither of these repair mechanisms is foolproof and many types of mutagenic agents may produce a variety of alterations in DNA structure and function that are easily recognized as phenotypic changes.