Biology

CLASS 12 CHAPTER ONE : REPRODUCTION IN ORGANISMS

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Mutations

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Charles Darwin (1859) in his book ' Origin of Species ' postulated that evolution or formation of newer species of organisms from the pre-existing ones occurs through accumulation of variations in the organisms. Without variations all the individuals of a species shall be alike. None of them would have better characteristics for being favoured in the struggle for existence and natural selection. A change in the environment could kill all of them. Variations increase the adaptability of the individuals and make them better fitted in the struggle for existence and hence natural selection. There are two sources of variations in the populations - recombinations and mutations . (i) Recombinations are also called continuous variations . They are not new traits but are formed by new combination of genes or traits already existing in the population. Recombinations are produced due to three reasons:  (a) Crossing over of nonsister chromatid segments during meiosis.  (b) Random segreg...
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Rho dependent Termination

● This type of termination occur in Prokaryotes for termination of transcription.  ◇ Transcription = Synthesis of RNA from DNA. ● Dna which have such a termination sequence which terminates the transcription only in presence of rho factor are called rho dependent termination. ● This termination sequence is rich in C but devoid of G .This types of sequence is necessary for function rho protein .  ● Rho protein binds with mRNA which is elongated by rna polymerase at rut site (rho utilization site) and then rho protein moves from 5' to 3' end . ● When RNA polymerase reaches rho dependent termination sequence then rho protein shows its helicase activity and unwind DNA-RNA hybrid. ● This will disrupt and terminate transcription bubble and new synthesized mRNA is released.  (Helicase is a enzyme that can unwind the dna strand or we can say breaks H bonds present in between the double helix of DNA )
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POLYMORPHISM IN CNIDARIA(COELENTERATA) - HYDROZOA, SCYPHOZOA, ANTHOZOA

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POLYMORPHISM Definition One of the important characters of Eumetazoa is the division of labour. In the vast majority, i.e., from flat worms to chordates, the division of labour involves the assignment of different functions to different parts or organs of the individual. In coelenterates, low organization and lack of organs do not permit any great degree of specialization for different functions within the limits of an individual. Therefore, in these animals the different vital functions are assigned to different individuals. This form of the division of labour is known as polymorphism. Polymorphism is, thus, the phenomenon of occurrence of an animal in more than one morphological and functional form. The individuals of polymorphic animals are known as the zooids or persons. 2. Grades Some coelenterates possess only two types of zooids, others have three types, and still others several types. These forms are respectively described as dimorphic, trimorphic and polymorphic. There ...
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