12th Class Biology Principle Of Inheritance And Variation Genes

Term 'gene' was given by Johannsen (1909) for any particle to which properties of Mendelian factor or determiner can be given. Thomas Hunt Morgan (1910) defined gene as ‘any particle on the chromosome which can be separated from other particles by mutation or recombination is called a gene'. In general, gene is the basic unit of inheritance.

According to the recent information a gene is a segment of DNA which contains the information for one enzyme or one polypeptide chain coded in the language of nitrogenous bases or the nucleotides. The sequence of nucleotides in a DNA molecule representing one gene determines the sequence of amino acids in the polypeptide chain (the genetic code). The sequence of three nucleotides reads for one amino acid (codon).

Gene action

Gene act by producing enzymes. Each gene in an organism produces a specific enzyme, which controls a specific metabolic activity. It means each gene synthesizes a particular protein which acts as enzyme and brings about an appropriate change.

One gene one enzyme theory : This theory was given by Beadle and Tatum (1958), while they were working on red mould or Neurospora (ascomycetes fungus). Which is also called Drosophila of plant kingdom. Wild type Neurospora grows in a minimal medium (containing sucrose, some mineral salts and biotin). The asexual spores i.e. conidia were irradiated with x-rays or UV-rays (mutagenic agent) and these were crossed with wild type. After crossing sexual fruiting body is produced having asci and ascospores. The ascospores produced are of 2 types -

(i) The ascospores, which are able to grow on minimal medium called ‘prototrophs’.

(ii) Which do not grow on minimal medium but grow on supplemented medium called ‘auxotrophs’.

Molecular structure of gene

Gene is chemically DNA but the length of DNA which constitutes a gene, is controversial 3 term i.e. cistron, muton and recon were given by Seymour Benzer to explain the relation between DNA length and gene.

Cistron or functional gene or gene in real sense : Benzer (1955) related gene to arm cistrom or Cistron is that particular length of DNA which is capable of producing a protein molecule or polypeptide chain or enzyme molecule.

Muton or unit of mutation : Muton is that length of DNA which is capable of undergoing mutation. Muton is having one or two pairs of nucleotide.

Recon : Recon is that length of DNA which is capable of undergoing crossing over or capable of recombination. Recon is having one or two pairs of nucleotides.

Complon : It is the unit of complementation. It has been used to replace cistron. Certain enzymes are formed of two or more polypeptide chains. Whose active groups are complimentary to each other.

Operon : Operon is the combination of operator gene and sequence of structure genes which act together as a unit. Therefore it is composed of several genes. The effect of operator gene may be additive or suppresive.

Replicon : It is the unit of replication. Several replicons constitute a chromosome.

Some specific terms

Transposons or Jumping genes : The term ‘transposon’ was first given by Hedges and Jacob (1974) for those DNA segments which can join with other DNA segments completely unrelated and thus causing illegitimate pairing. These DNA segments are transposable and may be present on different place on main DNA. The transposons are thus also called Jumping genes. Hedges and Jacob reported them in bacteria. But actual discovery of these was made by Barbara Mc Clintock (1940) in maize and she named them as controlling elements or mobile genetic elements. For this work, she was awarded nobel prize in (1983).

Retroposons : The term was given by Rogers (1983) for DNA segments which are formed from RNA or which are formed by reverse transcription under the influence of reverse transcriptase enzyme or RNA dependent DNA polymerase enzyme. About 10% of DNA of genome in primates and rodents is of this type.

\[\text{RNA}\xrightarrow{\text{Reverse transcriptase}}\text{DNA (Retroposon)}\]

Split genes or interrupted genes : Certain genes were reported first in mammalian virus and then in eukaryotes by R. Roberts and P. Sharp in (1977) which break up into pieces or which are made of segments called exons and introns. These are called split genes or interrupted genes.

Split gene = Exons + Introns

If mRNA formed from split gene exons are present and not corresponding to introns. So in split genes, exons carry genetic information or informational pieces of split genes are exons.

Pseudogenes or false genes : DNA sequences presents in multicellular organisms, which are useless to the organism and are considered to be defective copies of functional genes (cistrons) are called pseudogenes or false genes. These have been reported in Drosophila, mouse and human beings.