ZOOLOGY BOTANY CHEMISTRY

We have seen earlier that the continuance of the information flow to progeny and somatic cells is maintained by the DNA. This task is accomplished by the meticulous plan of DNA replication. Since DNA makes its own copies using the pre-existing DNA, the synthesis of DNA is referred as replication. Since only one strand of the double stranded DNA is newly synthesized, therefore, its replication is called as semi-conservative. However, in terms of contents, the resulting two helices are exactly identical with the original helix. The two strands of DNA double helix are united by hydrogen bonds between purine and pyrimidine base pairs. The initiation of the replication is marked by a cut or nick made by endonuclease on one of the strands of DNA. For replication, DNA binding proteins and DNA gyrase open up the DNA helix. Gyrase binds on a single strand and initiates uncoiling of coiled strands. The unwinding of strands results in the development of strain in the remaining helix and is reliev...

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...

Avery, Macleod and Mc Carty (1944) - proved that DNA is genetic material and gave Biochemical nature of gene (DNA). Arber, Smith and Nathani- Discovered restriction endonucleases in bacteria which are widely used in genetic engineering. Altmann - introduced the term "Nucleic acid" in 1989. A. Folling  the Norwegian physician, discovered phenylketonuria in 1934. Alfred D. Hershey and Martha Chase - proved DNA as genetic material in 1952. Benzer - Fine structure of gene (cistron, recon and muton) Briggs and King - Performed transplantation experiments of frog and toad eggs and proved that developmental changes are determined by molecular changes in the nucleus. When the nucleus from young embryonic cell is transplanted into non nucleated egg the tadpole develops. Bateson - Father of animal genetics, gave the terms allele, F1, F2, homozygous, heterozygous and genetics; applied Mendel's law on animals. Alongwith Punnett, he gave coupling and repulsion theory for genes. Br...

CANAL SYSTEM - PORIFERA A. Definition. A system of connected cavities found in a sponge body is known as the canal system. B. Types . There are three main types of canal system. These, in order of increasing complexity, are asconoid, syconoid and leuconoid. 1) Asconoid Canal System This is the simplest type of canal system. It is found in Leucosolenia . Its important features are enumerated below- (i) The sponge wall is thin and unfolded. (ii) The mesenchyme is feebly developed. (iii) The ostia lead directly into the spongocoel. An ostium is intracellular, i.e., it is a passage through a tubular cell called the porocyte . (iv) The spongocoel is large and lined all over by choanocytes. It opens out by a single terminal osculum. (v) The route followed by the water current includes ostia, spongocoel, and osculum. The asconoid canal system occurs in only a few sponges. The sponges with such a canal system are small in size and have a radially symmetrical, vase-like body. 2) Syconoid Canal...

1. Early Development Development of the zygote begins within the worm and by the time the proglottid is shed, has already changed into an embryo. The zygote (Fig. 7.11 A, 7.11 B) undergoes complete but unequal cleavage. It produces a mass of blastomeres of 3 types : 2 or 3 macromeres , 3 to 5 mesomeres and numerous micromeres (Fig. 7.11 C). The micromeres form the embryo with 3 pairs of claw-like hooks at the posteiror end. The 6-hooked embryo is called hexacanth . The mesomeres and macromeres form (Fig. 7.11 G) the inner and outer embryonic membranes respectively round the hexacanth. The inner (Fig. 7.11 D, E) membrane later forms a thick, chitinous, radially striated secondary shell or embryophore (Fig. 7.11 F). The hexacanth with embryophore, outer embryonic membrane and shell or capsule wall is known as the onchosphere (Fig. 7.11 G). (Gr. onchos = hook). It is about 40 μm in diameter. It is actually the onchospheres which are set free when the proglottides disintegrate in ...

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 ...