The genome contains genetic information that is transmitted from parents to offspring, from one generation to the next. The genome is the blueprint for the building and maintaining of organisms. In human beings, the genome comprises of 46 molecules of DNA known as chromosomes. Of the 46, 23 come from the father and 23 from the mother. Together, these 46 chromosomes contain about 20,500 genes that are the functional units of the chromosome.
Each gene more or less, encodes information that is required for one function, which may be responsible for the physical, physiological, or biochemical characteristics of the offspring.
Recent research has provided further information on gene structure, paving the way for a more specific definition of the gene and associated regions in the chromosome.
Accordingly, a gene:
- is a segment of deoxyribonucleic acids
- has a sequence that is unique in the entire genome
- encodes information required to produce one protein, rRNA or tRNA
- undergoes crossing over and mutations to become recombinant
- sometimes has information that is not in one segment, but in parts spread over a region
- may (and frequently does) influence other traits
- may only be partially responsible for one trait while other genes also exert their influence on
that trait
In light of the above definition of a gene, and further research on gene structure, it became
necessary to divide the gene into further divisions called cistron, recon and muton.
- G W Beadle and E L Tatum (1948) proposed One gene – one enzyme hypothesis
- The fine structure of gene was proposed by Seymour Benzere(1962)
- Yanofsky (1965) proposed One gene (cistron) – one polypeptide hypothesis
- S. Benzer (1957) coined different terms for different nature of gene and genetic material
in relation to the chromosome on the basis of genetic phenomena to which they involve.- i) Genes as unit of transmission or cistron
- ii) Genes as unit of recombination or recon
- iii) Gene as unit of mutation or muton
- Benzer, in view of his work, coined the terms cistron (unit of function), recon (unit of
recombination) and muton (unit of mutation).
GENE
- Gene is the Unit of Function (one gene specifies one character), Recombination, and Mutation.
- Genes are discrete particles inherited in mendelian fashion that occupies a definite locus in the chromosome and responsible for expression of specific phenotypic character.
- Number of genes in each organism is more than the number of chromosomes; hence several genes are located on each chromosome.
- The genes are arranged in a single linear order like beads on a string.
- Each gene occupies specific position called locus.
- If the position of gene changes, character changes.
- Gene was coined by W Johansen in 1909.
- E R Garrod (1908) proposed One gene – one product hypothesis Gene theory was proposed by T H Morgan in 1911.
- L Pauling and Ingram(1949) established the role of genes in protein synthesis.
- Genes can be transmitted from parent to off springs.
- Genes may exist in several alternate formed called alleles.
- Genes are capable of combined together or can be replicated during a cell division.
- Genes may under for sudden changes in position and composition called mutation.
- Genes are capable of self duplication producing their own exact copies,
Essential features of a gene
- Can duplicate themselves very accurately (Replication).
- Synthesizes a particular Protein.
- Determines the sequence of amino acid in the polypeptide chain
- Determines the physical as well as physiological characters.
- Situated in the chromosome.
- Occupies a specific position known as Locus.
- Arranged in single linear order.
- Occur in functional states called Alleles.
- Some have more than 2 alleles known as Multiple Alleles.
- Some may undergo sudden and permanent change in expression called as Mutant Gene (Mutation).
- May be transferred to its homologous (Cross-over) or non-homologous counterpart (Translocation).
Cis and trans position:
- Cis position: Genes in the cis position are on the same chromosome of a pair of homologous chromosomes.
- Trans position: Genes in the trans position are on the different chromosomes of a pair of homologous chromosomes.
Gene-Cistron Relationship
Prokaryotes: Genes and Cistrons are equivalent.
Eukaryotes: Cistron is equivalent to the exons.
CISTRON
It is the largest element in a gene which encodes a polypeptide during protein synthesis. The
term “cistron” refers to a test called cis-trans test, which is similar to a complementation test.
The term “cistron” is a unit of function.
It is the unit of Function. It is the Gene in real sense capable of synthesizing a Polypeptide
chain of an Enzyme.
There Are Several Recons and Mutons.
RECON
It is the smallest unit of DNA capable of undergoing Crossing Over and Recombination.
They are locations within a gene which participate in recombination. There is a minimum
distance between recons within a gene, and recombination cannot occur within a recon. The
term “recon” is a unit of recombination.
The smallest segment of DNA capable of being separated and exchange with other
chromosome is called recon.
A recon consists of not more than two pairs of nucleotides.
MUTON
It is the smallest unit of DNA which can undergo Mutation.
They are elements within a gene that can undergo a mutation and lead to the production of
mutant phenotype. The term “muton” is a unit of mutation. Each recon may have several
mutons within them, and many mutons within a recon may stay linked because of
recombination.
Thus, a gene may have one more than one cistrons, a cistron may have several recons, and a
recon may have several mutons.
COMPLON: It is the unit of complementation.
EXONS AND INTRONS
Exons are segments of a gene that encode mature mRNA for a specific polypeptide
chain.
Introns are segments of a gene that do not encode mature mRNA. Introns are found in
most genes in eukaryotes and in some gene of bacteriophage and archae.
Read More: Biology Notes
Share your Friends