In this article we will discuss about:- 1. Properties of Gene 2. Classification of Genes 3. Changing Concept 4. Fine Structure 5. Descriptions.
Properties of Gene:
Gene has been described by different researchers in various ways.
A gene has various structural and functional properties which are briefly described below:
The alternative form of a gene is known as allele. Generally each gene has two allelic forms. One of these forms in known as wild type and the other as mutant type. Allelic forms are known as dominant and recessive. Some genes have multiple allelic forms, but only two of them are present at a time in a true diploid individual.
Genes are located on the chromosome in a linear fashion like bead on a string. The position which is occupied by a gene on the chromosome is called locus. Studies on linkage, crossing over, sex chromosomes, sex linkage and bacterial transformation and transduction have clearly demonstrated that genes are located on the chromosomes.
Earlier it was believed that genes are the smallest units of inheritance which cannot be divided further. But Benzer demonstrated in 1955 that gene consists of several units of cistron, recon and muton which are the units of function, recombination and mutation within the gene.
Each diploid individual has two copies of each gene and gametic cells have one copy of each gene. Each individual has large number of structural and functional features or characters and each character is controlled by one or more genes.
Thus, each individual has large number of genes. The total number of genes in an individual is always higher than the number of chromosomes. Thus, each chromosome has several genes. The gene number is also fixed per chromosome which may be altered by deletion and duplication.
Genes have a specific sequence on the chromosome. The gene sequence is altered by structural chromosomal changes specially translocations and inversions.
Genes express in various ways. They may show incomplete dominance, complete dominance, over dominance and lack of dominance. When there is lack of dominance, the expression is intermediate between the two parents. The gene which is expressed is known as dominant gene and which is suppressed is known as recessive gene. The phenotypic expression of genes depends on allelic and non-allelic interactions.
7. Change in Form:
The gene may sometimes change from one allelic form to another. The change in the form of gene is brought out by gene mutation and the changed form of gene is called mutant gene, because generally the change occurs from dominant to recessive form. The reverse change is very rare.
8. Exchange of Genes:
The exchange of genes occurs between non-sister chromatids of homologous chromosomes due to crossing over and between non-homologous chromosomes due to translocation.
Gene is a macro molecule which is composed of DNA. In most of the organisms, gene is made up of DNA. However, the genetic material in some bacteriophages is RNA.
Each gene is duplicated at the time of chromosome duplication or replication. It is believed that chromosome duplication takes place because of gene duplication.
The primary function of each gene is to control the expression of a specific character in an organism. However, sometimes two or more genes are involved in the expression of some characters. The characters which are governed by one or few genes are known as oligogenic traits and those characters which are governed by several genes are referred to as polygenic characters.
In some cases, a single gene has manifold effects, means it controls the expression of more than one character. Such genes are known as pleiotropic genes. Each gene controls the production of one enzyme or one polypeptide chain which in turn governs the expression of specific character.
Genes in diploid organisms occur in pairs of alleles. The member of a pair segregates precisely like chromosomes during meiosis. Thus genes show segregation during meiosis.
When a character is governed by two or more genes, they sometimes show interaction. In such interaction one gene has masking effect over the other. The masking gene is known as epistatic gene and the gene which is masked or suppressed is called hypostatic gene. Gene interaction leads to modification of normal dihybrid segregation ratio into various other types of ratios.
Sometimes two or more genes are inherited together, such genes are referred to as linked genes. Some genes are linked with a particular sex, they are called as sex linked gene.
It is quite clear from the above discussion that there are some similarities or parallel features between chromosomes and genes. (Table 13.3).
Classification of Genes:
Genes can be classified in various ways. The classification of genes is generally done on the basis of:
(3) Character controlled,
(4) Effect on survival,
(7) Nucleotide sequence,
(8) Sex linkage,
(9) Operon model, and
(10) Role in mutation.
A brief classification of genes on the basis of above criteria is presented in Table 13.4.
Changing Concept of Gene:
The concept of gene has been the focal point of study from the beginning of twentieth century to establish the basis of heredity. The gene has been examined from two main angles, i.e., (1) genetic view, and (2) biochemical and molecular view.
These aspects are briefly described below:
1. A Genetic View:
The genetic view or perspective of gene is based mainly on the Mendelian inheritance, chromosomal theory of inheritance and linkage studies. Mendel used the term factors for genes and reported that factors were responsible for transmission of characters from parents to their offspring.
Sutton and Boveri (1903) based on the study of mitosis and meiosis in higher plants established parallel behaviour of chromosomes and genes. They reported that both chromosomes and genes segregate and exhibit random assortment, which clearly demonstrated that genes are located on chromosomes. The Sutton-Boveri hypothesis is known as chromosome theory of inheritance.
Morgan based on linkage studies in Drosophila reported that genes are located on the chromosome in a linear fashion. Some genes do not assort independently because of linkage between them. He suggested that recombinants are the result of crossing over.
The crossing over increases if the distance between two genes is more. The number of linkage group is the same as the number of chromosomes. The chromosome theory and linkage studies reveal that genes are located on the chromosomes. This view is sometimes called as bead theory.
The important points about the bead theory are given below:
i. The gene is viewed as a fundamental unit of structure, indivisible by crossing over. Crossing over occurs between genes but not within a gene.
ii. The gene is considered as a basic unit of change or mutation. It changes from one allelic form to another, but there are no smaller components within a gene that can change.
iii. The gene is viewed as a basic unit of function. Parts of a gene, if they exist, cannot function.
The chromosome has been viewed merely as a vector or transporter of genes and exists simply to permit their orderly segregation and to shuffle them in recombination. The bead theory is no more valid for any of the above three points.
Now evidences are available which indicate that:
(i) A gene is divisible,
(ii) Part of a gene can function.
i. The Gene is Divisible:
Earlier it was believed that gene is a basic unit of structure which is indivisible by crossing over. In other words, crossing over occurs between genes but not within a gene. Now, intragenic recombination has been observed in many organisms which indicates that a gene is divisible.
The intragenic recombination has following two main features:
1. It occurs with rare frequency so that a very large test cross progeny is required for its detection. Benzer expected to detect a recombination frequency as low as 10-6, the lowest he actually found was 10-4(0.01 x 2 = 0.02%).
2. The alleles in which intragenic recombination occurs are separated by small distances within a gene and are functionally related.
Examples of intragenic recombination include bar eye, star asteroid eye and lozenge eye in Drosophila. The bar locus is briefly described below. Lozenge eye and star asteroid have been discussed under pseudo alleles.
Bar Eye in Drosophila:
The first case of intragenic recombination was recorded in Drosophila for bar locus which controls size of eye. The bar locus contains more than one unit of function. The dominant bar gene in Drosophila produces slit like eye instead of normal oval eye. Bar phenotype is caused by tandem duplication of 16A region in X chromosome, which results due to unequal crossing over.
The flies with different dose of 16A region have different types of eye as follows:
The homozygous bar eye (B/B) produced both wild and ultra-bar types though at a low frequency which indicated intragenic recombination in the bar locus but the frequency was much higher than that expected due to spontaneous mutations.
ii. Part of a Gene Can Function:
It was considered earlier that gene is the basic unit of function and parts of gene, if exist, cannot function. But this concept has been outdated now. Based on studies on rll locus of T4 phage, Benzer (1955) concluded that there are three sub divisions of a gene, viz., recon, muton and cistron.
These are briefly described below:
Recons are the regions (units) within a gene between which recombination’s can occur, but the recombination cannot occur within a recon. There is a minimum recombination distance within a gene which separates recons. The map of a gene is completely linear sequence of recons.
It is the smallest element within a gene, which can give rise to a mutant phenotype or mutation. This indicates that part of a gene can mutate or change. This disproved the bead theory according to which the entire gene was to mutate or change.
It is the largest element within a gene which is the unit of function. This also knocked down the bead theory according to which entire gene was the unit of function. The name cistron has been derived from the test which is performed to know whether two mutants are within the same cistron on in different cistrons. It is called cis-trans test which is described below.
d. Cis-Trans Test:
When two mutations in trans position produce mutant phenotype, they are in the same cistron. Complementation in trans position (appearance of wild type) indicates that the mutant sites are in different cistrons. There is no complementation between mutations within a ciston.
It is now known that some genes consist of only one cistron; some consist of two or even more. For example, the mutant miniature (m) and dusky (dy) both decrease wing size in Drosophila and map in the same part of X chromosome. But when brought together in dy +/+m heterozygote, the phenotype is normal which indicates that the locus concerned with wing size is composed of at least two cistrons.
2. A Biochemical View:
It is now generally believed that a gene is a sequence of nucleotides in DNA which controls a single polypeptide chain. The different mutations of a gene may be due to change in single nucleotide at more than one location in the gene. Crossing over can take place between the altered nucleotides within a gene.
Since the mutant nucleotides are placed so close together, crossing over is expected within very low frequency. When several different genes which affect the same trait are present so close that crossing over is rare between them, the term complex locus is applied to them. Within the nucleotide sequence of DNA, which represents a gene, multiple alleles are due to mutations at different points within the gene.
Fine Structure of Gene:
Benzer, in 1955, divided the gene into recon, muton and cistron which are the units of recombination, mutation and function within a gene. Several units of this type exist in a gene. In other words, each gene consists of several units of function, mutation and recombination. The fine structure of gene deals with mapping of individual gene locus.
This is parallel to the mapping of chromosomes. In chromosome mapping, various genes are assigned on a chromosome, whereas in case of a gene several alleles are assigned to the same locus. The individual gene maps are prepared with the help of intragenic recombination.
Since the frequency of intragenic recombination is extremely low, very large population has to be grown to obtain such rare combination. Prokaryotes are suitable material for growing large population. In Drosophila, 14 alleles of lozenge gene map at four mutational sites which belong to the same locus (Green, 1961). Similarly, for rosy eye in Drosophila, different alleles map at 10 mutational sites of the same locus.
Descriptions about Each Genes:
There are some genes which are different from normal genes either in terms of their nucleotide sequences or functions. Some examples of such genes are split gene, jumping gene, overlapping gene and pseudo gene.
A brief description of each of these genes is presented below:
1. Split Genes:
Usually a gene has a continuous sequence of nucleotides. In other words, there is no interruption in the nucleotide sequence of a gene. Such nucleotide sequence codes for a particular single polypeptide chain. However, it was observed that the sequence of nucleotides was not continuous in case of some genes; the sequences of nucleotides were interrupted by intervening sequences.
Such genes with interrupted sequence of nucleotides are referred to as split genes or interrupted genes. Thus, split genes have two types of sequences, viz., normal sequences and interrupted sequences.
i. Normal Sequence:
This represents the sequence of nucleotides which are included in the mRNA which is translated from DNA of split gene (Fig. 13.2). These sequences code for a particular polypeptide chain and are known as exons.
ii. Interrupted Sequence:
The intervening or interrupted sequences of split gene are known as introns. These sequences do not code for any peptide chain. Moreover, interrupted sequences are not included into mRNA which is transcribed from DNA of split genes.
The interrupted sequences are removed from the mRNA during processing of the same (Fig. 13.2). In other words, the intervening sequences are discarded in mRNA as they are non-coding sequences. The coding sequences or exons are joined by ligage enzyme.
The first case of split gene was reported for ovalbumin gene of chickens. The ovalbumin gene has been reported to consist of seven intervening sequences (Fig. 13.2). Later on interrupted sequences (split genes) were reported for beta globin genes of mice and rabbits, tRNA genes of yeast and ribosomal genes of Drosophila.
The intervening sequences are determined with the help of R loop technique. This technique consists of hybridization between mRNA and DNA of the same gene under ideal conditions, i.e., at high temperature and high concentration of form amide. The mRNA pairs with single strand of DNA.
The non-coding sequences or intervening sequences of DNA make loop in such pairing. The number of loops indicates the number of interrupted sequences and the size of loop indicates length of the intervening sequence. These loops can be viewed under electron microscope.
The ovalbumin gene has seven interrupted sequences (introns) and eight coding sequences (exons). The beta globin gene has been reported to have two intervening sequences, one 550 nucleotides long and the other 125 nucleotides long.
The intervening sequences are excised during processing to form mature mRNA molecule. Thus, about half of the ovalbumin gene is discarded during processing. Earlier it was believed that there is co-linearity (correspondence) between the nucleotide sequence and the sequence of amino acids which it specifies.
The discovery of split genes has disproved the concept of co-linearity of genes. Now co-linearity between genes and their products is considered as a chance rather than a rule. Split genes have been reported mostly in eukaryotes.
2. Jumping Genes:
Generally, a gene occupies a specific position on the chromosome called locus. However, in some cases a gene keeps on changing its position within the chromosome and also between the chromosomes of the same genome. Such genes are known as jumping genes or transposons or transposable elements.
The first case of jumping gene was reported by Barbara McClintock in maize as early as in 1950. However, her work did not get recognition for a long time like that of Mendel. Because she was much ahead of time and this was an unusual finding, people did not appreciate it for a long time. This concept was recognized in early seventies and McClintock was awarded Nobel prize for this work in 1983.
Later on transposable elements were reported in the chromosome of E. coli and other prokaryotes. In E. coli, some DNA segments were found moving from one location to other location. Such DNA segments are detected by their presence at such a position in the nucleotide sequence, where they were not present earlier. The transposable elements are of two types, viz., insertion sequence and transposons.
i. Insertion Sequence:
There are different types of insertion sequences each with specific properties. Such sequences do not specify for protein and are of very short length. Such sequences have been reported in some bacteria, bacteriophages and plasmids.
These are coding sequences which code for one or more proteins. They are usually very long sequences of nucleotides including several thousand base pairs. Transposable elements are considered to be associated with chromosomal changes such as inversion and deletion.
They are hot spots for such changes and are useful tools for the study of mutagenesis. In eukaryotes, moving DNA segments have been reported in maize, yeast and Drosophila.
3. Overlapping Genes:
Earlier it was believed that a nucleotide sequence codes only for one protein. Recent investigations with prokaryotes especially viruses have proved beyond doubt that some nucleotide sequences (genes) can code for two or even more proteins.
The genes which code for more than one protein are known as overlapping genes. In case of overlapping genes, the complete nucleotide sequence codes for one protein and a part of such nucleotide sequence can code for another protein.
Overlapping genes are found in tumor producing viruses such as ɸ X 174, SV 40 and G4. In virus ɸX 174 gene A overlaps gene B. In virus SV 40, the same nucleotide sequence codes for the protein VP 3 and also for the carboxyl-terminal end of the protein VP2. In virus G4, the gene A overlaps gene B and gene E overlaps gene D.
The gene of this virus also contains some portions of nucleotide sequences which are common for gene A and gene C.
There are some DNA sequences, especially in eukaryotes, which are non-functional or defective copies of normal genes. These sequences do not have any function. Such DNA sequences or genes are known as pseudogenes. Pseudogenes have been reported in humans, mouse and Drosophila.
The main features of pseudogenes are given below:
1. Pseudogenes are non-functional or defective copies of some normal genes. These genes are found in large numbers.
2. These genes being defective cannot be translated.
3. These genes do not code for protein synthesis, means they do not have any significance.
4. The well-known examples of pseudogenes are alpha and beta globin pseudogenes of mouse.