Free-living -  Azotobacter, Symbiotic - Rhizobium serve as bio-fertilizers.
They are called bio-fertilizers because they are organisms found in nature that enrich the nutrient quality of the soil.

After infecting the human body the HIV gains entry into the macrophages.
Events that occur in the cells:
i. The virus enters into macrophages where RNA genome of virus replicates to form viral DNA with the help of enzyme reverse transcriptase.
ii. The viral DNA gets incorporated into host cells DNAand directs the infected cell to produce virus particles.
iii. The macrophage continue to produce virus and in this way acts like a HIV factory.
iv. The HIV enters into helper T-lymphocytos (TH ), replicates and produce progeny viruses. The progeny viruses released in the blood attacks other helper T-helper lymphocytes. This is repeated heading to a progressive decrease in the number of helper
v. T_H lymphocytes in the body of infected person. During this period the person suffers from bouts of fever, diarrhoea and weight loss. 
vi. The decrease in the number of the T helper cells leads to infections as the immune system of the person becomes weak. 

CFCs find wide use as refrigerants. CFCs discharged in the lower part of atmosphere move upward and reach stratosphere. In stratosphere, UV rays act on them reacting with the Cl atoms. Cl atoms acts as catalysts and degrades ozone releasing molecular oxygen. Cl atoms are not consumed in the reaction. Hence whatever CFC are added to stratosphere, have permanent and continuing effects on ozone level. 

The sacred groves are highly protected because they have religious and cultural values. They also have a large number of rare and threatened plant species. 

Stop codon - 
Unambiguous codon
Degenerate codon
Universal codon

Tips: -

i. Stop codon -  In genetic code. a stop codon or (termination codon) is a Nucleotide triplet with messenger RNA that signals a termination of the translation.
ii. Unambiguous codon - One codon codes for only one amino acid, hence, it is unambiguous and specific.
iii. Degenerate codon - Some amino acids are coded by more than one codon, hence the code is degenerate.
iv. Universal codon - because a codon in one organism will code for the same amino acid in another organism.