Genes Regulation

Term 'gene' was given by Johannsen (1909) for any particle to which properties of Mendelian factor or determiner can be given. T.H Morgan (1925) 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).

(1) 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.

(i) One gene one enzyme: 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 -

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

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

(2) 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.

(i) Cistron or functional gene or gene in real sense: Cistron is that particular length of DNA which is capable of producing a protein molecule or polypeptide chain or enzyme molecule.

(ii) Muton or unit of mutation: Muton is that length of DNA which is capable of undergoing mutation. Muton is having one or part of nucleotide.

(iii) 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.

(iv) 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.

(v) 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.

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

(3) Some specific terms

(i) 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 in maize or mobile genetic elements in maize. For this work, she was awarded Nobel Prize in (1983).

(ii) 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.

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

 Note: q  About 10% of DNA of genome in primates and rodents is of this type.

(iii) 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

In 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.

(iv) 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.

Important Tips

• Rarely a functional centromere is absent and the whole surface of chromosome functions as such. Such a chromosome is called holocentric.
• Inheritance is based on particles (genes).
• Genetically identical progeny is produced when the individual produces identical gametes.
• Gene flow is spread of genes from one breeding population to another by migration.
• The genes, which enhance the effect of other gene, is also known as extender.
• Single copy genes: Represented only once in the whole genome.
• Multigenes: A group of nearly similar genes.
• Sutton and Winiweter (1900) expressed that number of chromosome is reduced to half in meiosis and doubled in fertilization.
• Flemming clarified the chromosomal events involving mitosis and transfer of it from parent to progeny.
• A human diploid cell has about 100000 genes on its 46 chromosomes, out of which only 5-15% (average 10%) genes are expressed at a time.
• \[{{H}_{1}},\text{ }{{H}_{2a}},\text{ }{{H}_{2b}}\] protein of nucleosome rich in lysine amino acid and \[{{H}_{3}},\text{ }{{H}_{4}}\] rich in arginine
• Sometimes two satellites are present in a chromosome these chromosome are called tandem SAT-Chromosomes.
• SAT Chromosomes are used as marker chromosomes.
• Deletion is common to acentric chromosomes.
• Lampbrush chromosomes are larger than polytene chromosomes.
• Lampbrush loops and polytene puff are analogous.
• Plasma genes occur in plastids, mitochondria, plasmid, sigma particle & kappa particle.
• Hyper chromism is presence of some chromosome more than one.
• The former gene which have been mutated to such as extent that they cannot be transcribed further in m RNA are called Pseudogenes.
• Chromosomal theory of inheritance in the present form was modified by B. Bridges.
• Genes modify the effect of other gene called modifiers.
• Super numerary chromosomes formed due to non-disjunction at the time of meiosis and called planosome.
• 3-11 nucleotide sequence of ribosome recognisition site on mRNA is called SD sequence or shine Dalgrano site.
• The term gene refers to a portion of DNA Gene is formed of polynucleotide. Which can synthesis a single protein.
• Number of genes on a chromosome is infinite.
• In a chromosomes the protein content is trace.
• Spring al. In 1975 reported lampbrush chromosome in Acetabularia.
• The drug mercaptolethanol when applied early in mitosis, interferes with the centriole apparatus, it therefore affects mitosis by disrupting the spindle formation.
• Genetic drift is the random change in gene frequencies.
• An allele is said to be dominant if it is expressed only in both homozygous & heterozygous condition.
• Holandric genes are genes located on non- homologous segment of Y chromosome.
• Cytochimera means cell having different chromosomes other than vegetative cell.
• Translocation is a type of chromosomal aberration where a part of one chromosome is exchanged between non homologous counterpart.
• The genetic basis of evolution (particular adaptation) was demonstrated in bacteria by Lederberg and E. M. Lederberg.
• The factors controlling change in gene frequencies are natural selection, mutation, migration, and genetic drift.
• Gene flow is described as the transfer of gene between population, which differ genetically from one another but can interbreed.
• Carmine is a dye extracted from the cochineal insect (Coccus cacti).
• Haematoxylin is a dye extracted from the heartwood of a tropical tree etc. Haematoxylin campechianum both are stain the chromosome and nucleus.

Genetic Code

Introduction: Defined as structure of nitrogen bases (nucleotides) in mRNA molecule which contain the information for the synthesis of protein molecule. It is discovered by frame shift mutation by Crick.

Codon is the sequence of nitrogen bases (nucleotides) in mRNA, which codes for a single amino acid. Nirenberg and Mathaei (1961) experimentally proved that a single amino acid is determined by a sequence of three nitrogen bases which is known as triplet code. Khorana has got Nobel prize on genetic code.

The Genetic Code Dictionary

Salient Features

(i) Triplet: A single amino acid is specified by a sequence of three nucleotides in mRNA i.e. called codon. Due to triplet nature, it consist 64 codon.

(ii) Universal: A codon specifies the same amino acid in all organisms from viruses to human beings.

(iii) Commaless: There is no pause, so it reads continously.

(iv) Non-overlapping: No overlapping between adjacent nucleotide.

(v) Initiation codon : The synthesis of polypeptide chain initiated by initiation codon, which located beginning the cistron i.e., AUG or GUG, which codes to methionine and valine amino acid respectively.

(vi) Termination codon: Termination is done by codon. These are UAA, UGA or UAG which does not code to any amino acid. These are also called nonsense codon.

(vii) Degeneracy: A single amino acid may be specified by many codon i.e., called degeneracy. Degeneracy is due to the last base in codon, which is known as wobble base. Thus first two codon are more important to determining the amino acid and third one is differ without affecting the coding i.e., known wobble hypothesis, which establishes a economy of tRNA molecule and put forwarded by Crick. Degeneracy of genetic code was discovered by Berrfield and Nirenberg.

Important Tips

• George Gamow (1954) first propose triplet code and given the term genetic code also proposed diamond code model.
• Different genetic code - in yeast mitochondria, UGA represent tryptophan while generally it is stop single. In certain ciliates UAA and UAG represent Guanine, Mitochondria DNA has genes for 22 tRNA (instead of 35 in universal code).
• Khorana synthesized gene of tyrosine suppressor tRNA gene of E.Coli in 1979, which contained 207 nucleotide pairs functional.
• First artificially gene synthesized Alanine tRNA with 77 base pairs by Hargovind Khorana non-functional.
• Protein language is composed of 20 alphabets.