question_answer 12)

Discuss with your teacher and find out how it distinguish between: (a) Plasmid DNA and chromosomal DNA; (b) RNA and DNA; (c) Exonuclease and endonuclease.  

Answer:

 (a) Plasmid DNA is naked double stranded DNA that forms a circle with no free ends. It is associated with few proteins but contains RNA polymerase enzymes. It is smaller than the host chromosome and can be easily separated. Chromosomal DNA is double stranded linear DNA molecule associated with large proteins. This DNA exists in relaxed and supercoiled forms and provides a template for replication and transcription. It has free ends represented as .   (b) Differences between exonucleases and endonucleases are as follows:

	Exonucleases	Endonucleases
1.	These nucleases cleave base pairs of DNA at their terminal ends.	They cleave DNA at any point except the terminal ends.
2.	They act on single strand of DNA or gaps in double stranded DNA.	They cleave one strand or both strands of double stranded DNA.
3.	They do not cut RNA.	They may cut RNA.

  (c) Difference between DNA and RNA are given in the following table:  

	Exonucleases	Endonucleases
1.	It usually occurs inside nucleus and some cell organelles.	Very little RNA occurs inside nucleus. Most of it is found in the cytoplasm.
2.	DNA is the genetic material.	RNA is not the genetic material except in certain viruses, e.g., Reovirus.
3.	It is double stranded with the exception of some viruses (e.g.,  x 174).	RNA is single stranded with the exception of some viruses (e.g., double stranded in Reovirus).
4.	It is long lived.	Some RNAs are very short lived while others have somewhat longer life.
5.	It contains deoxyribose sugar.	It contains ribose sugar.
6.	Nitrogen base thymine occurs in DNA along with three others ? adenine, cytosine and guanine.	Thymine is replaced by uracil in RNA, The other three - adenine, cytosine and guanine are similar.
7.	Unusual bases are very few or absent.	Many unusual or modified bases are often present.
8.	Hydrogen bonds are formed between complementary nitrogen bases of the opposite strands of DNA (A-T, C-G).	Base pairing through hydrogen bonds occurs only in the coiled parts.
9.	DNA is spirally twisted to produce a regular helix.	The strand may get folded at places to produce a secondary helix or pseudo helix.
10.	It replicates to form new DNA molecules.	It cannot normally replicate itself.
11.	DNA transcribes genetic information to RNA.	RNA translates the transcribed message for forming polypeptides.
12.	Its quantity is fixed for cell.	The quantity of RNA of a cell is variable.
13.	Purine and pyrimidine bases are in equal number.	There is no proportionality between number of purine and pyrimidine bases.
14.	It occurs in the form of prochromosome, chromatin or chromosomes	It occurs in ribosomes or forms association with ribosomes.