Gene concept illustrates the information regarding gene.

1. GENE CONCEPT

2. Genes
• Gene is the smallest and individually
functional part of the genetic material.
• Gene is a basic unit of inheritance, a sequence
of DNA nucleotides which codes for
afunctional product of RNA or a polypeptide.
• The term gene was coined by Johannsen in
1909 for Mendelian factors or hereditary
determiners.

3. 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:
1. Forms: The alternative form of a gene is known as allele.
Generally each gene has two allelic forms. One of these forms
is 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.
2. Location: 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.

4. Properties of Gene
3. Status: 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.
4. Number: 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.

5. Properties of Gene
5. Sequence: Genes have a specific sequence on the chromosome. The
gene sequence is altered by structural chromosomal changes
specially translocations and inversions.
6. Expression: Genes express in various ways. They may show
incomplete dominance, complete dominance, over dominance and
lack if 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 gene
depends on allelic and non-allelic interactions.
7. Change in form: The gene may sometimes change from 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.

6. Properties of Gene
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 translocations.
9. Composition: 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.
10. Duplication: Each gene is duplicated at the time of chromosome
duplication or replication. It is believed that chromosome
duplication takes place because of gene duplication.
11. Segregation: Genes in diploid organisms occur in pair of alleles.
The member of a pair segregates precisely like chromosomes
during meiosis. Thus genes show segregation during meiosis.
12. Linkage: 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.

7. Properties of Gene
13. Function: 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.
14. Interaction: 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.

8. Similarities and dissimilarities between
chromosomes and genes
Sr. No. Chromosomes Genes
Similarities
1. Each diploid cell has two sets of a
chromosome.
Each diploid cell has two
copies of a gene.
2. A gamete contains half number of
chromosome than somatic cell.
A gamete also contains half
number of genes than
somatic cell.
3. Each number of homologous pair of
chromosome assorts independently
during meiosis (anaphase I).
Two or more genes assort
independently.
4. Chromosomes show segregation
during meiosis.
Genes also segregate during
meiosis.
5. Chromosomes duplicate during
meiosis.
Genes also duplicate during
meiosis.
6. Chromosomes mutate. Genes also mutate.

9. Similarities and dissimilarities between
chromosomes and genes
Sr. No. Chromosomes Genes
Dissimilarities
1. Chromosomes are visible under
microscope.
Genes are not visible under
microscope.
2. Chromosomes are composed of
DNA, histones and RNA.
Genes are composed of either
DNA or RNA.

10. Classification of Genes
Classification of genes A brief description
1. Based on Dominance
Dominant genes Genes that express in the F1.
Recessive genes Genes whose effects is suppressed in F1.
2. Based on Interaction
Epistatic gene A gene that has masking effect on the other gene
controlling the same trait.
Hypostatic gene A gene whose expression is masked by another
gene governing the same trait.
3. Based on character controlled
Major gene A gene that governs qualitative trait. Such genes
have distinct phenotypic effects.
Minor gene A gene which is involved in the expression of
quantitative trait. Effect of such genes cannot be
easily detected.

11. Classification of Genes
Classification of genes A brief description
4. Based on Effect on Survival
Lethal gene A gene which leads to death of its carrier when in
homozygous condition. It may be dominant or
recessive.
Semilethal gene A gene that causes mortality of more than 50% of its
carriers.
Sub-vital gene A gene that causes mortality of less than 50% of its
carriers.
Vital gene A gene that does not have lethal effect on its carriers.
5. Based on Location
Nuclear genes Genes that are found in nuclear genome in the
chromosomes.
Plasma genes Genes that are found in the cytoplasm in
mitochondria and chloroplasts. Also called
cytoplasmic or extranuclear genes.

12. Classification of Genes
Classification of genes A brief description
6. Based on Position
Normal genes Genes that have a fixed position on the chromosomes.
Most of the genes belong to this category.
Jumping genes Genes which keep on changing their position on the
chromosome of a genome. Such genes have been
reported in maize
7. Based on Nucleotide Sequence
Normal genes Genes having continuous sequence of nucleotides
which code for a single polypeptide chain.
Split gene A gene having discontinuous sequence of
nucleotides. Such genes have been reported in some
eukaryotes. The intervening sequences do not code
for amino acids.
Pseudo genes Genes having defective nucleotides which are non-
functional. These genes are defective copies of some
normal genes.

13. Classification of Genes
Classification of genes A brief description
8. Based on Sex Linkage
Sex linked genes Genes which are located on sex or X-chromosomes.
Sex limited genes Genes which express in one sex only.
Sex influenced genes Genes whose expression depends on the sex of
individual e.g., gene for baldness in humans.
9. Based on Operon Model
Regulator gene A gene found in lac operon of E. coli which directs
synthesis of a repressor.
Operator gene In lac operon, a gene which controls the function of
structural genes.
Promotor gene A gene in lac operon of E. coli which initiates mRNA
synthesis.
Structural genes The genes in lac operon of E. coli which control the
synthesis of protein through mRNA.

14. Classification of Genes
Classification of genes A brief description
10. Based on Sex Linkage
Mutable genes Genes which exhibit higher mutation rate than others
e.g., white eye gene in Drosophila.
Mutator genes Genes which enhance the natural mutation rate of
other genes in the same genome e.g., dotted gene in
maize.
Antimutator genes Genes which decrease the frequency of natural
mutations of other genes in the same genome. Such
genes are found in bacteria and bacteriophages.

15. Fine structure of Gene
Classical Vs Modern concept of Gene:
Classical concept of gene Modern concept of gene
1 According to this concept, the
term factors for genes and
reported that factors were
responsible for transmission of
characters.
According to this concept, a
gene is a sequence of
nucleotides in DNA which
controls a single polypeptide
chain.
2 The gene is considered as a basic
unit of change or mutation, it
changes form to another, but there
are no smaller components within
a gene that can change.
The different mutations of a
gene may be due to change in
single nucleotide at more than
one location in the gene.
3
`
The gene is viewed as a
fundamental unit of structures,
indivisible by crossing over.
Crossing over occurs between
genes but not within a gene.
Crossing over can take place
between the altered nucleotides
within a gene.

16.  It was considered earlier that gene is the basic unit of function
and parts of gene, if exist, cannot function.
 But based on studies on rll locus of T4 phase, Benzer (1955)
concluded that there are three sub divisions of a gene, viz, recon,
mutton and clstron.
Recon:
 Recons are the regrons (units) within a gene between which
recombinations can occur, but the recombination cannot occur
within a recons.
Muton:
 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 mutant or change.
Cistton :
 It is the largest element within a gene. Which is the unit of
function.

17. Gene Regulation in Prokaryotes
In prokaryotes, the operon model of gene regulation is widely accepted.
This model of gene regulation was proposed by Jacob and Monod in
1961 for which they were awarded nobel prize in 1965. The operon refers
to a group of closely linked genes which act together and code for the
various enzymes of a particular biochemical pathway. In other words,
operon is a unit of bacterial gene expression and regulation, including
structural genes and control elements in DNA recognized by regulator
gene product(s). Thus operon is a model which explains the on-off
mechanism of protein synthesis in a systematic manner. The main points
of operon model of gene regulation are presented below.
(i) Developed By: In prokaryotes, the operon model of gene regulation
was proposed by Jacob and Monod in 1961 for which they were
awarded nobel prize in 1965.
(ii) Organism Used: The operon model was developed working with
lactose region [lac region] of human intestine bacteria E.coli. The
gene regulation was studied for degradation of the sugar lactose.

18. Gene Regulation in Prokaryotes
(iii) Genes involved: In the operon model of gene regulation, four
types of genes viz, (i) structural genes (ii) operator gene (iii)
promoter gene (iv) regulator gene are involved. In addition,
repressor, co-repressor and inducer molecules are also involved.
(iv) Enzymes Involved: Four types of enzymes are involved in
gene regulation of prokaryotes. These are beta-galactosidase,
galactosidase permease, transacetylase and RNA polymerase. The
beta-galactosidase catalyses the break down of lactose into glucose
and galactose. The galactosidase permease permits entry of lactose
from the medium into the bacterial cell. The enzyme transacetylase
transfers an acetyl group from acetyl co-enzyme A to beta-
galactosidase. The enzyme mRNA polymerase controls on-off of
the transcription.
(v) Type of control: In lac operon of E. coli, there are two types of
control of gene regulation, viz., (i) negative control and (ii) positive
control.

19. Gene Regulation in Prokaryotes
(vi) Types of operon: In prokaryotes, operons are of two types,
viz, inducible and repressible. The example of an inducible operon
is the lactose operon, which contains genes that encode enzymes
responsible for lactose metabolism. An example of repressible
operon is the Trp operon, which encodes enzymes responsible for
the synthesis of the amino acid tryptophan (trp for short).

20. Gene Regulation in Prokaryotes
Gene regulation: Gene regulation refers to the control of the rate or
manner in which a gene is expressed. The process by which the cell
determines (through interactions among DNA, RNA, proteins and other
substances) when and where genes will be activated and how much gene
product will be produced.
Or
The study of on-off mechanism of protein synthesis, also known as
regulation of gene action, regulation of gene expression and regulation of
protein synthesis.
Operon Model: A group of closely linked genes which act together and
code for the various enzymes of a particular biochemical pathway.
Structural genes: In the lac operon of E.coli, the genes which control the
synthesis of protein through mRNA.
Operator gene: In the lac operon of E.coli, a gene which controls the
functional of structural genes.
Promoter gene: In the lac operon of E.coli, a gene found at the end and
directs the synthesis of repressor protein molecule.

21. Gene Regulation in Prokaryotes
Negative regulation: Inhibition of transcription by repressor
through inactivation of promoter such as lac operon of E. coli.
Positive regulation: Enhancement of transcription by an effector
molecule through inactivation of promoter such as lac operon of E.
coli.
Repressor: In the lac operon of E. coli, a protein molecule which
prevents transcription. The process of inhibition of transcription is
called repression.
Co-repressor: A combination of repressor and metabolite which
prevents protein synthesis. Such process is termed as co-repression.
Inducer: The substance which allows initiation of transcription
(lactose in lac operon). Such process is known as induction.
Effector: The molecule which acts as an inducer or co-repressor in
the operon model of E. coli.

22. Gene Regulation in Prokaryotes
Constitutive enzyme: An enzyme whose production is enhanced
by adding the substrate in the culture medium. Such system is
called inducible system.
Repressible enzyme : An enzyme whose production can be
inhibited by adding an end product. Such system is called
repressible system.
Constitutive Transcription : When transcription is carried out
continuously.
Allosteric Molecules: Molecules that change shape when they bind
to another molecule.

23. Inducible Operon
Inducible enzyme An enzyme whose production is enhanced by
adding the substrate in the culture medium is called inducible enzyme
and such system is called inducible system. The example of an
inducible operon is the lactose operon, which contain genes that
encode enzymes responsible for lactose metabolism. In bacteria,
operon refers to a group of closely linked genes which act together
and code for the various enzymes of a particular biochemical
pathway. The model of lac operon of E.coli looks like this:

24. (1) Structural Genes. There are three structural genes of the lac operon i.e. lac Z,
lac Y and lac A. The main function of structural genes is to control of protein
synthesis through messenger RNA. Function of these genes is as follows.
(i) Lac Z: It encodes the enzyme beta-galactosidase, which catalyses the break
down of lactose into glucose and galactose.
(ii) Lac Y: It encodes the enzyme galactosidase permease, which permits entry of
lactose from the medium into the bacterial cell.
(iii) Lac A: It encodes the enzyme transacetylase, which transfers an acetyl group
from acetyl co-enzyme A to beta .
(2) Promoter Gene. The above three structural genes are under the control of the
promoter gene [designated P]. In the promoter, RNA polymerase binds to the DNA
and prepares to initiate transcription. The main function of promoter gene is to
initiate mRNA transcription.
(3) Operator Gene. The other regulatory element in an operon is the operator
(designated O). This is the element that determines whether or not the genes of the
operon are transcribed. The main function of operator gene is to control function of
structural genes.
(4) Regulator Gene. This is designated as I. It is expressed all the time, or
constitutively and plays an important role in operon function. This is the lac I gene,
which encodes a protein called the lac repressor.

25. (4) Regulator Gene. The lac repressor has two functional domains or regions: one that
binds to the DNA of the operator region and one that binds to lactose. When the repressor
binds to the operator, it prevents RNA polymerase advancing along the operon and
transcription does not occur. The regulation of the operon depends on regulating whether
or not the repressor binds to the operator. The function of regulator gene is to direct
synthesis of repressor, a protein molecule. Its function differs in the presence and absence
of lactose as discussed below.
When Lactose is Absent
When the Lactose is absent in the environment, events take place in this way. The lac I
gene is transcribed [constitutively i.e. continuously] and the mRNA is translated,
producing the lac repressor. The repressor binds to the operator and blocks RNA
polymerase. When RNA polymerase is blocked, there is no transcription. Thus the
enzymes for lactose metabolism are not synthesized, because there is no lactose to
metabolize. Thus when lactose is absent, lactose-metabolizing enzymes are not produced.
When Lactose is Present
When Lactose is present in the environment, the events occur in a different way. A small
amount of the lactose enters into the cell and affects regulation of the operon.
The lac repressor is till synthesized. The repressor can bind to lactose. After binding to
lactose, the repressor undergto the oes a conformational change (change of shape).
Molecules that change shape when they bind to another molecule are called allosteric
molecules. With this change, the lac repressor is unable to bind to the operator region.
Hence RNA polymerase