What is a Gene? – Definition, Gene Action, Nature of Gene

Gene is the basic physical and functional unit of heredity that are made up of DNA.

In this article I would be answering queries related to Gene. The following concept is to be cleared:

  • What do you mean by Gene and the chemical nature?
  • How does gene act?
  • Modern concept and theories on genes
  • Factors that control gene action
  • and other concepts

Let dive into the world of gene.

Gregor Johann Mendel explained in his monohybrid experiments, the existence of a unit of inheritance which he called a factor or determiner. In modern terminology, the factor or determiner has been referred to as a gene.

This term ‘gene‘ was proposed by Johannsen in 1909.

According to the gene theory postulated by Thomas Hunt Morgan (1926), genes were considered to be the minute particles arranged in a linear order on the chromosomes. According to Castle, genes are the smallest particle of chromatin capable of duplicating themselves.

The gene is the hereditary particles and ultimate unit of heredity, i.e., responsible for the transmission of the bulk of inheritance from parents to the offspring.

A gene is an enormously large particle with a high molecular weight. It is a self-perpetuating body and is regarded as comprising primarily a particular kind of protein, attached in some way to the neighboring gene, at its either end and separated by non-genic material.

The protein of the gene is linked with Deoxyribonucleic acid (DNA). Both these chemical substances are indispensable constituents of genes.

Gene has been defined in various ways e.g. “It is the unit of recombination” or “it is the ultimate unit of mutation”, or “it is the ultimate unit of physiological activity”, or “it is the ultimate unit of self-reproduction”.

But the work of Avery (1944), Chargaff (1947), Wilkins (1962), and Watson and Crick (1953) resulted in a conclusive definition explaining the physicochemical nature of genes.

You can read an article published by NATURE on The Discovery of DNA Double Helix by Watson and Crick.

According to them:

Gene is a certain portion of DNA polymeric macromolecule which is a double helix of polynucleotide chains having four nucleotides in a characteristic sequence. Each nucleotide contains a pentose sugar, phosphoric acid, and nitrogenous purine or pyrimidine base. This DNA molecule is enclosed in a proteinous matrix of chromosomes and passes from one cell to another cell and from one generation to another generation without ordinarily changing its form or constitution”.

In the physical nature which in the biological terms called morphology, we would know about the size, shape, number, position, etc of genes.

Although several efforts have been made to measure the genes, yet it has not been possible practically. It is however believed that they greatly vary in size.

The diameter of one gene is estimated to be about 6 millimicrons. Its molecular weight is calculated to be approximately 10,000.

On the basis of four genes located o a limited length of salivary gland chromosomes, Muller (1974) concluded that they had a mean length of 1250 Å while Pontecorvo estimated the size of a gene in Aspergillus nidulares which was about 4500 Å.

The actual shape of the gene is not known. Pease and Baker (1949) by special techniques are reported to have seen with the help of electron microscope, what they regarded as almost certainly actual genes.

These are small objects inside the chromosomes, about the size of an average virus of variable shapes (leaf-like, spheroidal, and cigar-shaped) and composed of nucleic acid and protein.

Stanley (1952) and Slizynski (1949) studied much about the shape of the gene. According to them, “A gene may be a micro-cylindrical rod-like unit”.

The exact shape of the gene has been described by Watson and Crick (1958) and Wilkins (1962).

According to them; ‘a gene possesses a double-helical structure comprising two long strands spirally coiled with each other and connected by several transverse bands.

The genes being very minute changeable particles, it is difficult to describe their exact number.

Some consider the leptotene chromomeres as visible genes and on this basis, the number of genes in Lilium is supposed to be about 2200 equal to chromomeres.

J. W. Gowen and E. H. Gay studied the gene number and size in Drosophila and concluded that there are about 15,000 loci (sing-locus) in the entire chromosomal set having the same number of genes.

The volume of genes was about 1×10-18 cubic centimeters.

Actually where genes lie on the chromosome was a matter of dispute.

Demerec (1939) studied the position of the gene on the chromosome. He described chromonema as the most important and conspicuous part of the chromosome, which runs all along its length.

The chromonema also possesses the side branches at the right angle to its own axis. The micro-molecular units, the genes, are situated on these branches as well as on the axis.

Genes are quite different from each other in shape, size, structure and also differ physiologically and chemically.

It is a well-established fact that genes may carry one character or many characters, and a group of genes may carry only one character.

Thus, the expression of character is determined by the position of genes and not by their number. It is termed the position effect.

The genes are essentially composed of DNA. This nucleic acid is present in the chromosomes inside the nucleus.

Chemically, DNA consists of following components:

  1. Pentose sugar, i.e, Deoxyribose sugar.
  2. Phosphoric acid (Phosphate)
  3. Nitrogenous bases (Purine and pyrimidine).

A detailed description of DNA and its replication has been given in some other post which is equally applicable to the gene.

The knowledge gained by the earlier part of this century about the gene a got a definite shape of concept which is known as the classical concept of the gene, summarized below :

  1. Genes are only situated in the chromosomes (chromosomal theory of genes) and there are several genes in each chromosome, located in the nucleus.
  2. Each gene occupies a particular position in the chromosome which is called the locus. These loci are arranged in linear order in the chromosome.
  3. A single gene may occur in several mutant forms called alleles which are alternatives for each other at the same locus.
  4. Sudden and rare changes (mutations) in genes, alter them from one form to another.
  5. Genes of chromosomes may be transferred to another homologous chromosome during crossing over in meiotic division due to translocation in non-homologous or chromosomes.
  6. At the end of mitotic division, duplication of a chromosome is accompanied by the self-duplication of genes, situated in it.
  7. Inheritance is the net result of the transmission of genes from one generation to the next.
  8. Expression of the gene is through the production of a chemical substance involving a highly complex mechanism (one gene-one enzyme theory).

Researches on microbes (Viruses and bacteria) revealed that genes as a functional unit, may not be the same thing as the gene as the unit of recombination or the gene as the unit of mutation.

So in 1957, S. Benzer coined the following terms to denote the relationship between DNA molecules and genetic phenomena.

This is the term for the segment of DNA molecule as a functional gene that governs the production of specific protein and ultimately the trait or character. In length, it may contain at least 100 nucleotides but normally (in E. coli) the range of nucleotides is 900-1500 per cistron.

Some cistron may consist of numerous nucleotide pairs and mutation would be possible at several points within a cistron. Recently the term cistron is replaced by Complon.

Recon is the smallest segment of DNA that is interchangeable through genetic recombination at the time of crossing over.

Thus recons are the parts of chromosomes between which crossing over can take place but not within. A recon consists of not more than two pairs of nucleotides of DNA, or it may be only a single base pair.

It is the smallest segment of DNA in a chromosome in which detectable change or mutation can occur. It may be mostly due to the change in a single nucleotide or nucleotide pair.

A muton consists of normally only one pair of nucleotides or rarely two. A cistron will therefore consist of numerous mutons.

Thus the cistron is divisible into smaller units called muton and recon which may be as small as one nucleotide base pair of DNA molecule.

There are many theory regarding the nature of a Gene but two famous one are:

  1. Theory of particulate gene and
  2. One Gene-One Enzyme Theory

Let us know them one by one briefly.

De Vries postulated the ‘one gene-one character’ hypothesis according to which particular trait in an individual is controlled by a particular gene.

This was discarded by the gene theory put forward by T. H. Morgan.

According to particulate theory, genes are like corpuscles on the chromosomes having linear arrangements like beads on a string.

Each of the genes contains different substances and is separated from each other by indifferent substances.

Beadle and E. L. Tatum (1940) proposed one gene-one enzyme theory based on experimental work on Neurospora. According to this theory, genes govern all the processes in living beings.

All biochemical processes in living organisms are under gene control. Every biochemical process is a series of individual step-wise reactions.

Each single reaction is controlled by a single gene which produces only one enzyme for this purpose.

Mutation of a single gene results only in alternation in the ability of the cell to carry out a single primary reaction due to alternation in the production of its enzyme.

The influence of genes resulting in the expression of a genetic character is called gene action. Most genes affect several or many unrelated traits of the organism that carries them.

In other words, genes have manifold or pleiotropic effects. For example, the gene that determines the size of the wings in Drosophila also affects the eye color, the character of the balancer, the position of the dorsal bristle, the shape of the spermatheca, fertility, and length of life, etc.

Each gene has its own actions which may be influenced by environmental conditions. Some genes are more stable and resistant to conditions while others undergo changes producing so-called mutations.

Some important roles controlled by genes are as follows:

The presence of a colorless compound called chromogenic causes coloration in animals. This compound becomes changed by oxidizing enzymes.

Tyrosine is the basic chromogenic material which on enzymatic oxidation forms two types of pigments, the dark eumelanic pigments causing black or brown color and xanthic or phaeomelanic pigments having a red or yellow color.

This mechanism is entirely under gene control.

Hormones secreted from ductless glands influence various bodily characters, size, and abnormal development.

In most cases, the difference in size is due to the main difference in chromosome number, unequal cell growth rate and metabolism, etc.

The development of these glands and the production of hormones are under gene control. They are controlled by a complex of multiple genes.

It has been shown that the size of a mouse (normal and dwarf) results from the influence of a single pair of genes.

Various metabolic defects like alkaptonuria, phenylketonuria, etc. have been found in men. In some people, due to alkaptonuria, the urine turns black on exposure to air.

This defect is inherited as recessive to normal. The urine contains a substance called alkapton (homogentisic acid) which is completely absent in the urine of normal people. This substance causes the urine to blacken.

It was discovered that the blood of normal people contained an enzyme that oxidized alkapton so that it is excreted as carbon dioxide and water and hence they are normal urine. This enzyme was not present in the blood of persons suffering from this defect.

Evidently, the gene for normal causes the enzymes to be produced. The enzyme so produced destroys alkapton and the urine remains normal.

When the gene is totally absent, no enzyme is produced, so that alkapton is not destroyed and the urine turns black.

It has been discussed that genes act by producing enzymes, but all the genes present in a cell do not function at all times.

The question arises, how certain genes act specifically and direct particular functions at particular times and places?

No satisfactory answer has yet been given in this line. The following mechanisms have been suggested to control the action of genes in a cell.

As a result of their experiments on the bacterium (E. Coli), Jacob and Monod (1961) have produced shreds of evidence about the direct relationship between gene and enzymatic activity and also presented a hypothesis to explain the regulation of gene activity.

They have distinguished three kinds of genes which are as follows:

Which produce m-RNA and determine the kind of protein synthesized i.e., by determining amino acid sequence.

These genes act as switches to turn on and off the activity of the structural genes. A single operon usually governs the activity of several closely linked structural genes.

It controls the operator gene. Here genes produce certain proteinous substances called ‘repressors’ which prevent the operator gene from the action.

In other words, these genes inhibit gene activity and their absence causes protein synthesis at an uncontrolled maximum rate.

The group of structural genes (cistron) and the operator gene, together form one unit called the ‘operon’. A single regulator gene may control a whole series of reactions in the cell.

Some studies on E. coli suggest that there is a “feedback mechanism” due to which the biochemical reactions slow down when the end product is in surplus.

It has been shown that the protein histone which forms a part of the chromosome along with DNA suppresses the activity of genes.

Thus, the activity of genes in cells of higher animals and plants might be regulated by the movement of histones, from one part of the chromosome to another, to regulate the production of mRNA.

Hormones and chemical organizers may also influence the movement of histones, thus controlling the activity of genes.

  • Alkaptonuria: A disease caused by a single recessive gene different from the normal.
  • Allele: One of two normally alternate forms of a gene.
  • Chromogen: A heredity determiner in chromosomes.
  • Cistron: A region of a chromosome in which all mutants are non-complementary.
  • Gene: The unit of reproduction and hence of crossing over in the hereditary material.
  • Muton: A sub-division of gene, can affect mutation.
  • Recon: The smallest subdivision of gene, that is interchangeable but not divisible by recombination.
  • Translocation: The exchange of parts of two non-homologous chromosomes.
Who coined the term Gene?

Wilhelm Johannsen who is a Danish botanist, coined the term gene. Along with this, he also described the study of hereditary.

Who postulated the “Gene theory”?

Gregor Johann Mendel postulated the Gene Theory.

Who coined the term Cistron?

Seymour Benzer coined the word Cistron.

Who proposed the Gene-one enzyme theory?

Beadle and E.L. Tatum in 1940 proposed the gene-one enzyme theory.

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