Concept of Genes

1. CONCEPT OF A GENE
By Dr. Sadguru Prakash
Department of Zoology, M.L.K.P.G. College, Balrampur, U.P.
The genetic blueprint contained in the nucleotide sequence can determine the phenotype
of an individual. The hereditary units, which are transmitted from one generation to the next
generation are called genes.
A gene is a fundamental biological unit (Functional and Structural unit) like atom which
is the fundamental physical unit. Mendel was the first scientist who proposed genes as
particulate units and called them hereditary elements or factors. Wilhelm Johanssen (1909)
gave the term ‘Gene’ to replace the Mendelian factor.
Concept of gene has undergone a considerable change since Mendel’s time. Gene
Concept was given by Sutton.
1. Gene is an inherited factor that control specific phenotypic trait (G.J. Mendel, 1866).
2. Genes are located on the chromosome and arranged in single linear order. Each Gene
occupies a specific position on chromosome known as Locus (T.H. Morgan, 1909).
3. Gene occurs in functional states called Alleles. Generally each gene has 2 alleles but
some genes have more than 2 alleles known as Multiple alleles.
Thus the gene is a discrete particle containing genetic information, inherited in a
mendelian fashion, occupies a definite locus on the chromosome and responsible for the
expression of a specific phenotypic character.
4.The idea that genes are responsible for the synthesis of proteins i.e. “one gene-one
metabolic block theory” was first proposed by a British physician, Sir Archibald
Garrod (1902).
5. Modern Concept of Gene: A gene can be described as a polynucleotide chain, which
is a segment of DNA. It is a functional unit controlling a particular trait such as eye colour.
George W. Beadle and Edward L. Tatum (1940) concluded by various experiments that gene
is a segment of DNA that codes for one enzyme. They proposed “One gene: one enzyme
hypothesis”. But as some genes code for proteins that are not enzymes, the definition of gene
was changed to “one gene-one protein hypothesis”.
Since many enzymes contain two or more different polypeptides, each encoded by a
separate gene (and also many polypeptides are not enzymes at all, but serve non-catalytic
functions) the one gene-one enzyme concept subsequently was modified to “one gene-one
polypeptide theory” or (U.M. Ingram,1957).
Protein Hypothesis: The concept of gene has undergone further changes as the new
facts came to light. Since proteins are polypeptide chains of amino acids translated by mRNA,
gene was defined as one gene-one polypeptide relationship. Some proteins have two or more
different kinds of polypeptide chains, each with a different amino acid sequence. They are
products of different genes. For example, hemoglobin has two kinds of chains a andβ chains,
which differ in amino acid sequence and length. They are encoded by different genes. Thus,
gene is defined as one gene one polypeptide relationship.
6. Gene is the unit of genetic information, i.e., the sequence of DNA that usually
corresponding to a single mRNA which will be translated into a polypeptide.

2. 7. A gene is a region / segment of DNA consisting of polynucleotides that specify the
sequence of amino acid in the polypeptide chain (The Genetic Code) or protein /
enzyme which controls hereditary, physical as well as physiological characters.
8. DNA, a genetic material is made up of genes and carries the information of trait /
hereditary characters of the individual.
9. Gene is a unit of recombination (C. Yanofsky & Co-workers, 1960)
10. Some genes may undergo sudden change in expression called as Mutant Gene
(Mutation).
11. Gene can duplicate themselves very accurately (Replication)
12. Genes may be transferred to its homologous (Cross-over) or non-homologous
counterpart (Translocation).
Thus the gene is the unit of genetic information that specifies the synthesis of one
polypeptide.
Number of Genes on a Single Chromosome:
Total number of genes on a single chromosome is different in different organisms.
Bacteriophage virus R17 consists of only three genes, SV40 consists of 5-10 genes. E.coli
bacteria have more than 3000 genes on single 1 mm long chromosome.
Size of a Gene:
In E. coli there are more than four million pairs of nucleotides (4638858 base pairs). It
has been estimated that there are about 3000 genes in E. coli.
The minimum size of a gene that encodes a protein can be directly estimated. Each
amino acid of a polypeptide chain is encoded by a sequence of three consecutive nucleotides
in a single strand of DNA. Therefore, by measuring the size of the polypeptide chain, the size
of a gene can be directly measured.
The average polypeptide chain has about 450 amino acids, which are encoded by 1350
nucleotides. Therefore, in E. coli the number of genes will be around 3000 (4000000
/1350=3000). Human genome contains about 30000 genes, (Source: International Human
genome sequencing consortium led in the United States by National Human Genome Research
Institute (NHGRI) have estimated the number of human protein coding genes to be less than
30000. Simple round worm C. elegans has about 20000 genes).
A single copy of chromosome is composed of more than 3 billion base pairs. Coding
regions of these genes take up only 3% of the genome.

3. FINE OR MOLECULAR STRUCTURE OF GENE
A gene is present only in one strand of DNA, which is a double stranded helix. A gene
consists of several different regions. The main region is the coding sequence which carries
information regarding amino acid sequence of polypeptides. The region on the left side of
coding sequence (upstream or minus region or 5/
end of mRNA) and on the right side
(downstream or plus region or 3/
end of mRNA) consists of fairly fixed regulatory sequences.
Regulatory sequences consist of promoters which are different in prokaryotes and eukaryotes.
Seymour Benzer (1957) proposed the fine or molecular structure of genes and coined
the new terms, cistron, recon and muton to denote the relationship between DNA molecule and
genetic phenomenon. According to this, cistron, recon and muton genes 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.
1.Cistron (unit of function): One gene-one enzyme hypothesis of Beadle and Tatum
was redefined by several workers. A single mRNA is transcribed by a single gene. Therefore,
One-gene-one mRNA hypothesis was put forth. Exceptionally, a single mRNA is also
transcribed by more than one gene and it is said to be polycestronic. Therefore, the concept has
been given as one-gene-one protein hypothesis. The proteins are the polypeptide chain of
amino acids translated by mRNA. Therefore, it has been correctly used as one-gene-one
polypeptide hypothesis. Moreover, genes are present within the chromosome and their cis-trans
effect governs the chromosome function. Therefore, S. Benzer termed the functional gene as
cistron. The cis and trans arrangement of alleles may be written as below:
+ + a +
a b + b
Cis (wild) Trans (Mutant)
Thus, cistron is the largest unit of DNA (gene) that actually contains the genetic
information. It contains information for the synthesis of a polypeptide chain. Crossing over
within cistron is possible.
2.Recon (unit of recombination): Earlier, it was thought that crossing over occurs
between two genes. In 1962, S. Benzer demonstrated that the crossing over or recombination
occurs within a functional gene or cistron. In a cistron the recombinational units may be more
than one. Thus, the smallest unit capable of undergoing crossing over or genetic recombination
is known as recon.
3.Muton (unit of mutation): It is smallest unit of chromosome that can undergo
mutational changes. Hence, muton may be defined as “the smallest unit of DNA which may be
changed in the nucleotide.” Thus, changes at nucleotide level is possible . The smallest unit of
muton is the nucleotide. Therefore, cistron is largest unit in size followed by recon and muton.

4. Fig. The genes as a unit of Function i.e. Cistron (A), recombination i.e. Recon
(B)and Mutation i.e. (C)
Thus, a DNA consists of several cistron, a cistron contains many recon and a recon have
a number of mutans. However, if the size of recon is equal to muton, there would be no
possibility in recon for consisting of several mutons.
The gene was now defined as ‘the unit of function i.e., the sequence of DNA that
specifies one polypeptide’. In this way we can say that the gene was equivalent to the cistron.
According to modern view, gene is a ‘unit of information’ that codes for one
polypeptide. We also know that the gene is flanked by regulatory units that are necessary for
the proper functioning of the gene. The regulatory units and organization of genes differs in
prokaryotes and eukaryotes.
Structure of Prokaryotic Gene:
In prokaryotes, the genes are organized as ‘Operons’. An operon is a cluster of
functional genes under the control of one promoter and one operator. Each gene produces a
different product.
Operon = Promoter + Operator + Gene1 + Gene 2 + ……

5. In prokaryotes, structural proteins with related functions such as the genes which
encode the enzymes that catalyze the many steps in a single biochemical pathway are usually
encoded together within the genome in a block called an operon and are transcribed together
under the control of a single promoter. This forms a polycistronic transcript (Figure). The
promoter then has simultaneous control over the regulation of the transcription of these
structural genes because they will either all be needed at the same time, or none will be needed.
Thus operon is the combination of promoter gene, operator gene and sequence of structural
gene which act together as a unit.
Structure of Eukaryotic Gene:
In Eukaryotes, genes are not grouped into operons but are split into interon and exon.
Genes are made up of promoter regions and alternating regions of introns (noncoding
sequences or i.e. not code mRNA) and exons (coding sequences i.e. code for mRNA)

6. An intron is any nucleotide sequence within a gene which is represented in the primary
transcript of the gene, but not present in the final processed form. The overall length of a gene
is determined largely by its introns. Introns range in size from about 50 nucleotides to >
100,000 nucleotides. Exons are usually short, typically on the order of 150 nucleotides.
After RNA synthesis, interon (interfering sequences) are removed and exons are joined
to form a functional RNA.
The production of a functional protein involves the transcription of the gene from DNA
into RNA, the removal of introns and splicing together of exons, the translation of the spliced
RNA sequences into a chain of amino acids, and the posttranslational modification of the
protein molecule.
Types of Genes:
1. Simple Genes: Simple genes have a coding sequence of bases in one DNA strand.
Upstream (5/
) the coding region, the promoter is present. Downstream (3/
), the termination
region is present.
2. Split Genes: In most of eukaryotes, many non-coding sequences are present between
coding sequences. The coding sequences of DNA of the genes are called exons. In between
exons are present non-coding sequences called introns. Exons alternate with introns. Normally

7. introns do not possess any genetic information and are not translated. Such genes are called
split genes or interrupted genes.
Fig. Splicing
The mRNA transcribed from this DNA is called precursor mRNA (pre-mRNA) and
contains exons as well as introns. The introns are removed by excision and discarded. This
process is known as splicing. The remaining segments or exons are joined together to form the
mature mRNA which takes part in translation. The mature mRNA is much smaller than the
pre-mRNA for example α -globn has two introns, ovalbumin has seven introns and α-collagen
has 52 introns.
3. Overlapping Genes: Most genes consist of DNA sequences that code for one
protein. But there are some sequences that code for more than one protein. Fredrick Sanger
discovered this phenomenon in bacteriophage φ x 174. Overlapping genes are common in many
viruses. Here the small length of viral DNA is exploited for synthesizing different proteins.
This is achieved in different ways. In some cases, one gene generates two proteins by
having different starting points. Similarly, the same gene generates two proteins by terminating
the expression at different points.
In other cases, a sequence of DNA makes no distinction between exons and introns.
This sequence of DNA, which uses only exons for expression, also uses adjoining introns at
other times for expression. The differential splicing of a single stretch of mRNA leads to
overlapping and therefore different proteins. In this way, multiple proteins can be generated
from a single stretch of DNA.
4. Jumping Genes or Transposons: Earlier it was thought that genes are static and
have definite and fixed locus. However, recently it has been discovered that segments of DNA
can jump to new locations in the same or different chromosome. First of all it was discovered
by Barbara Mc Clintock in Indian maize corn. It has cobs with kernels of different colours.
The light coloured kernels were caused by segments of DNA that move into genes coding for
pigmented kernels, thereby inactivating pigmented kernels.
These mobile genes are called transposable elements or transposons. They can jump
within the genome, thus affecting the gene expression. Transposable elements are components
of moderately repetitive class of DNA.
A transposon has well defined ends. It consists of a long central portion. On either end
each transposon has specific sequence of bases which are inverted repeats or palindromes on
opposite strands. These terminal repeats help in identifying transposons. The site where a
transposon is inserted is called target site or recipient site.

8. Transposable elements can lead to change in the expression of genes. They can also
cause mutations. In bacteria, they are present on plasmids.
5. Variable Genes: Certain polypeptides are coded not by one gene but they are coded
by more than one gene present on the same or different chromosomes.
Open Reading Frame: A gene is a segment of genome which is transcribed into RNA. If the
RNA is a transcript of a protein coding gene then it is called messenger RNA or mRNA. This
is translated into protein. If the RNA is non-coding as ribosomal RNA (rRNA) or transfer RNA
(tRNA) it is not translated.
The part of the protein coding gene which is translated into protein is called open reading
frame. It has triplet nucleotide codons. Open reading frame starts with an initiation codon and
ends with a termination codon. The region of DNA before a gene is called up-ream region
denoted with a minus (-) sign while region after the gene is called downstream denoted with a
plus (+) sign. Many genes are split between exons and introns. The introns are removed by
splicing to produce a functional RNA before translation.