Biology Questions

Generally viruses that infect plants have single stranded RNA. Viruses infecting animals have double stranded RNA or DNA. Viruses which infect bacteria have double stranded DNA.

In cancer cells, there is breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appears to have lost this property. As a result of this, cancerous cells just continue to divide giving rise to masses of cells called tumors. Tumors are of two types: benign and malignant.

AIDS is caused by the Human Immuno deficiency Virus (HIV), a member of a group of viruses called retrovirus, which have an envelope enclosing the RNA genome. Transmission of HIV-infection generally occurs by (a) sexual contact with infected person, (b) by transfusion of contaminated blood and blood products, (c) by sharing infected needles as in the case of intravenous drug abusers and (d) from infected mother to her child through placenta.

The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. In transcription, only a segment of DNA and only one of the strands is copied into RNA. This necessitates defining the boundaries that would demarcate the region and the strand of DNA that would be transcribed.

The two DNA strands are called polynucleotidesbecause they are composed of simpler monomer units called nucleotides. Eachnucleotideis composed of one of four nitrogen-containingnucleobases (cytosine, guanine, adenine or thymine), a sugar called deoxyribose, and a phosphate group.