Operator gene (O) : It gives passage to RNA polymerase when the structural genes are to express themselves. Normally, it is covered by a repressor. Operator gene of lac operon is small, made of 27 base pairs.
Promoter gene (P) : It is recognition centre / initiation point for RNA polymerase of the operon.
Regulator gene (i Gene) : It produces a repressor that binds to operator gene for keeping it nonfunctional (preventing RNA polymerase to pass from promoter to structural genes).
Repressor : It is a small protein formed by regulator gene. Which binds to operator gene and blocks passage of RNA polymerase towards structural enzymes. Repressor has two allosteric sites, one for attaching to operator gene and second for binding to inducer. Repressor of lac operon has a molecular weight of 160,000 and 4 subunit of 40,000 each.
Inducer : It is a chemical which attaches to repressor, changes the shape of operator binding site so that repressor no more remain attached to operator.
Differences between induction and repression
| Incomplete dominance | Codominance | ||||
| Effect of one of the two alleles is more conspicuous. | The effect of both the alleles is equally conspicuous. | ||||
| It produces a fine mixture of the expression of two alleles. | There is no mixing of the effect of the two alleles. | ||||
| The effect in hybrid is intermediate of the expression of the two alleles. | more...
Introduction : "When genes are closely present link together in a group and transmitted as a single unit this phenomenon is called linkage".
Theories of linkage
Sutton's hypothesis of linkage (1903) : The number of groups of genes are equivalent to the number of chromosomes.
Morgan's hypothesis of linkage (1910) : It was given by T. H. Morgan. According to him the genes of homologous parents enter in the same gamete and tend to remain together, which is opposite in heterozygous parents. Linked group are located on the same chromosome and distance between linked group of gene limits the grade of linkage.
Coupling and repulsion hypothesis : Proposed by Bateson and Punnet (1906) that dominant alleles tend to remain together as well with recessive alleles, called gametic coupling. If dominant and recessive alleles are present in different parents they tend to remain separate and called repulsion. When BBLL and bbll are crossed, the \[{{F}_{1}}\] is BbLl and the test cross of it will show progeny in 7 : 1 : 1 : 7 ratio i.e., BbLl : Bbll : bbLl : bbll (coupling) when BBll is crossed with bbLL the \[{{F}_{1}}\] is BbLl or the test cross progeny will show 1 : 7 : 7 : 1 ratio i.e., BbLl : Bbll : bbLl : bbll (repulsion). Coupled and repulsed genes are known as linked genes. Linkage has coupling phase and repulsion phase. In coupling phase both the linked genes have their dominant alleles in one chromosome and recessive alleles in other chromosomes. The heterozygotes with such constitution is called cis heterozygote. Cis-arrangement is a original arrangement. Which form two types of gametes as (AB) and (ab). In Human X–chromosomes carry 102 genes and Y chromosome carries 10 genes only.
In repulsion phase the normal alleles as well as mutant alleles lie in opposite chromosomes of the homologous pair, such heterozygote is called as trans heterozygote. It is not original arrangement, caused due to crossing over, which form two types of gametes as (Ab) and (aB).
Chromosomal hypothesis of linkage : It was given by Morgan and Castle. According to them linked genes are bound by chromosomal material and are transmitted as a whole.
Types of linkage
Depending upon the absence or presence of nonparental or new combination of linked genes, linkage has been found to be complete or incomplete.
Complete linkage (Morgan, 1919) : Such cases in which linked genes are transmitted together to the offsprings only in their original or parental combination for two or more or several generations exhibit complete linkage. In such cases the linked genes do not separate to form the new or non-parental combinations. This phenomenon is very rare. Some characteristics in males of Drosophila are found to exhibit complete linkage.
Incomplete linkage : In majority of cases, the homologous chromosomes undergo breakage and reunion during gametogenesis. During reunion the broken pieces of the chromatids are exchanged, producing some more...
Gregor Johann Mendel first "geneticist", also known as father of genetics was born on July 22, in 1822 in Silisian, a village in Heizendorf (Austria). In 1843, he joined Augustinian monastry at Brunn (then in Austria, now Brno Czechoslovakia). In 1856, Mendel got interested in breeding of Garden pea (Pisum sativum). He selected pure breeding varieties or pure lines of pea. Breeding experiments were performed between 1859 – 1864. The results were read out in two meetings of Natural History Society of Brunn in 1865 and published in 1866 in "Proceedings of Brunn Natural History Society" under the topic "Experiments in Plant Hybridisation". Mendel died in 1884 without getting any recognition during his lifetime.
Rediscovery of Mendel's work : In 1900, Hugo de Vries of Holland, Carl Correns of Germany and Erich von Tshermak of Austria came to the same findings as were got by Mendel. Hugo de Vries found the paper of Mendel and got it reprinted in ‘Flora’ in 1901. Correns converted two of the generalisations of Mendel into two laws of heredity. These are law of segregation and law of independent assortment.
Reasons for Mendel’s success
Method of working : He maintained the statistical records of all the experiments and analysed them. He selected genetically pure (pure breed line) and purity was tested by self-crossing the progeny for several generations.
Selection of material : Mendel selected garden pea as his experimental material because it has the following advantages.
It was an annual plant. Its short life–cycle made it possible to study several generations within a short period and has perfect bisexual flowers containing both male and female parts. The flowers are predominantly self-pollinating because of self-fertilization, plants are homozygous. It is, therefore, easy to get pure lines for several generations and also easy to cross because pollens from one plant can be introduced to the stigma of another plant by removing anthers (emasculation) and bagging. In addition to that there was one reason more for his success. He studied seven pairs of characters which were present on four different pairs of chromosomes.
Selection of traits : Mendel selected seven pairs of contrasting characters as listed in the table. Luckily all were related as dominant and recessive.
Seven pairs of contrasting characters in pea plant
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(ii) Codominance (1:2:1 ratio) : In codominance, both the genes of an allelomorphic pair express themselves equally in \[{{F}_{1}}\] hybrids. 1:2:1 ratio both genotypically as well as phenotypically in \[{{F}_{2}}\] generation.
Example : Codominance of coat colour in cattle, Codominance in andalusian fowl and Codominance of blood alleles in man.
Differences between incomplete dominance and codominance
Term was given by Zacharis, which is found in the cells of all living organisms except plant viruses, where RNA forms the genetic material and DNA is absent. In bacteriophages and viruses there is a single molecule of DNA, which remains coiled and is enclosed in the protein coat. In bacteria, mitochondria, plastids and other prokaryotes, DNA is circular and lies naked in the cytoplasm but in eukaryotes it is found in nucleus and known as carrier of genetic information and capable of self replication. Isolation and purification of specific DNA segment from a living organism achieved by Nirenberg. H.Harries is associated with DNA-RNA hybridization technique.
Chemical composition
The chemical analysis has shown that DNA is composed of three different types of compound.
(i) Sugar molecule : Levene identified a five carbon sugar, ribose in nucleic acid in 1910. It is represented by a pentose sugar the deoxyribose or 2-deoxyribose which derived from ribose due to the deletion of oxygen from the second carbon.
(ii) Phosphoric acid : \[{{H}_{3}}P{{O}_{4}}\] that makes DNA acidic in nature.
(iii) Nitrogeneous base : Kossel demonstrated the presence of two pyrimidines (cytosine and thymine) and two purines (adenine and guanine) in DNA and he was awarded Nobel Prize in 1910. These are nitrogen containing ring compound. Which classified into two groups:
(a) Purines : Two ring compound namely as Adenine and Guanine.
(b) Pyrimidine : One ring compound included Cytosine and Thymine. In RNA Uracil is present instead of Thymine.
Nucleosides : Nucleosides are formed by a purine or pyrimidine nitrogenous base and pentose sugar. DNA nucleosides are known as deoxyribosenucleosides.
Nucleotides : In a nucleotide, purine or pyrimidine nitrogenous base is joined by deoxyribose pentose sugar (D), which is further linked with phosphate (P) group to form nucleotides.
