Biology Status

Diseases caused by Viroid: Viroids are plant pathogens. They infect higher plants and cause diseases like potato spindle tuber disease, and chrysanthemum stunt disease. These infectious RNA particles are responsible for crop failures and subsequently, the loss of millions of money in agriculture annually. Potato, cucumber, tomato, chrysanthemums, avocado, and coconut palms are plants that are commonly subjected to viroid infections. Viroids causes over 20 different plant diseases. The only human disease known to be caused by a viroid is hepatitis D. The list of diseases caused by Viroids are given below:   Transmission: Viroid infections in plants are transmitted by ➢ cross-contamination ➢ mechanical damage to plants as a result of horticultural or agricultural practices. ➢ Some seed or pollen transmitted ➢ aphids ➢ leaf contact (transferred from plant to plant) Viroid infections (hepatitis D) in humans are transmitted by ➢ blood or serum transfusions. Viroid disease control: Viroid i

  Multiplication/Replication Viroids multiply inside the nucleus (Pospiviroidae) or chloroplasts (Avsunviroidae) of infected plant cell by ‘Rolling Circle’ mechanism, using the RNA polymerase enzyme of host. cell. Viroids require RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid as template. The resulting long RNA molecule is produced which cut into pieces (monomers) and then ligated either the monomers (autocatalytically or by cellular factors depending on a viroid) to form an individual circular viroid. There are two variations of this rolling circle mechanism:  (A) In the first variation, i) the circular plus (+) strand is copied by viroid RNA-dependent RNA polymerase to form a concatemer minus (-) strand (step 2). ii) Site-specific cleavage (arrows) of this strand produces a monomer that is circularized by a host RNA ligase (step 3) and the

Structure Viroids are composed of small, circular, single stranded RNA molecules which have some double-stranded regions. The closed single-stranded RNA circle has extensive intra-strand base pairing and interspersed unpaired loops.   They consist of a short stretch of highly complementary circular single-stranded RNA without a protein coat.  The molecular weight of Viroid ranged between 85,000 and 1,30,000 daltons.  Viroids are 240-380 nucleotides long and all of them have dump-bell-shaped structures.  The range in size is approximately 20 nm.  Viroids are catalytic RNAs (ribozymes) that cleave RNA to produce fragments containing a 5'-hydroxyl and a 2', 3'-cyclic phosphate. Structure of PSTV:   H.J. Cross (1979) sequenced the nucleotide sequence of the potato spindle tuber virus (PSTV). The structure of PSTVd is as follows It consists of 359 ribonucleotides and is characterized by numerous intermolecular base pairing. The single-stranded circular RNA makes pairing betwee

Characteristic features of Viroids: More than 40 viroid species and many variants have been characterized. The characteristics features of Viroids are as follows: 1) It is an infectious RNA particle. 2) Viroids are made up of small single-stranded circular RNA having 240-380 nucleotides. 3) Viroids do not contain a protein coat/capsid or an envelope. 4) It has no coding capacity because the RNA of a viroid does not have an initiation code (AUG) for protein synthesis. 5) Circular RNA molecules are capable of autonomous replication. 6) Since viroids are RNA particles, they can be inactivated by ribonuclease digestion but are resistant to proteinase K and trypsin digestion. 7) Viroid has a smaller size than viruses. Its size is approximately 20 nm. 8) Viroids require host cell DNA-dependent RNA polymerase enzyme to replicate. Use host-encoded polymerase for replication. 9) Viroids infect only higher plants (exception: hepatitis D virus in humans is similar to viroid). 10) It is mainly fo

The word VIROID is composed of two words ‘VIR (means virus)’ and ‘OID (means like)’. Thus, Viroid means virus-like organism/particle. Viroids were first discovered and named by the plant pathologist Theodor O. Diener in 1971 to describe the causal agent of the potato spindle tuber disease. The first viroid identified was Potato Spindle Tuber Viroid (PsTVd). A viroid is an infectious RNA particle formed from a single-stranded circular RNA. It infects only plants (except Hepatitis D in humans). Viroid may cause disease by triggering RNA slicing. Some are found in the infected host cell nucleus, others found in the chloroplast. More than 40 viroid species and many variants have been characterized. Definition: Viroids are infectious, non-protein-coating, highly structured small circular naked RNA molecules able to replicate autonomously and induce diseases in higher plants. The term viroid was coined by T.O. Diener (1971) to describe the causal agent of the potato spindle tuber disease. In

Miller and Urey’s Experiment: To prove Oparin and Haldane's biochemical theory of the origin of life Miller and Urey in 1952 conducted an experiment. They designed an apparatus containing a glass tube, flask, and gas chamber. They kept NH3, CH4, H2, CO in the gas chamber, and water is boiled in the flask to produce water vapor. The energy was supplied by heating and by electric sparks. The condenser was used to cool the reaction. They run the experiment for a week. Finally, they analyzed the products formed within the tube. They found different organic compounds like fatty acid, amino acid, sugar, glycerol etc in the tube. Then they proved Oparin and Haldane's theory of the origin of life through a series of chemical reactions among the inorganic gases under the influence of temperature, electric discharges, and cooling.   Drawbacks of Urey Miller Experiment • By examining rocks- dated to be 3.7 billion years old, geologists determined that earth had an oxygenic atmosphere. •

F) Formation of colloids, co-acervates (protobionts or pre cells) or origin of primitive cell: The complex organic compounds of primordial soup in ocean aggregated together forming colloidal mass which was surrounded or bounded by water layer was called coacervates. They can grow by absorbing nutrients. Oparin called these aggregates as coacervates while Sidney fox called them Protenoids or Microspheres. The nucleic acid controlled the activities of coacervate and the fats formed a layer around it and showed a great affinity to water. This structure is called Eobiont. Some of the proteins within coacervates acted as enzymes and began metabolic activities. G) Origin of the first cell (Prokaryote): Coacervates may produce the first cell-like structure. The first cell-like structure with the power of division is called Eobiont or Pre-cell. Oparin considered that the coacervates with nucleoprotein gave rise to primitive cells or Eobionts. Oparin also called it as protobiont. The eobion