What is the Difference Between hnRNA and mRNA?

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RNA synthesis takes place in the nucleus in eukaryotes. However, when RNA is formed, it is transferred from the nucleus to the cytoplasm. The physiological environment in the nucleus is quite different than than of the cytoplasm. This physiological difference may harm newly formed RNA in the cytoplasm. In order to prevent RNA from any damage, it is processed first and then transferred to the cytoplasm.

A newly formed RNA before processing is called heterogenous RNA (hnRNA) and RNA after processing is called mRNA (messenger RNA). Processing involves capping and tailing of RNA. Capping is done by adding 5-methyl guanosine, and tailing is done by adding polyA to the 3'-OH using poly A polymerase enzyme.


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What is Chargaff's Rule of DNA?

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Erwin Chargaff was an Australian biochemist who analyzed nucleic acids of many organisms. From his research, he concluded that the number of purine bases (A, G) equals the pyrimidine bases (C, T) in any double stranded DNA of an organism.

It means,

  •  A+G / T+C = 1; 
  • A+T / C+G may vary from organism to organism.
  • % A = % T, and % G = % C

Based on Chargaff's rule, many questions are asked in various competitive exams including AIPMT, CSIR NET, GATE, etc.

Q1. One of the strands of a double stranded DNA has the following number of bases:
A= 3800; T= 2600. What would be the base composition of the double stranded DNA?

Solution:

From the question, it is a double stranded DNA (Chargaff's rule is valid for only double stranded DNA), and only one strand with its base composition is given. So, the total number of bases would be 20k (10k bases on each strand).

Now, A and T bases with their numbers are given on one strand, so A will have T and T will have A on their complementary DNA. It means, on one strand, the total number of A would be 3800+2600 because % A = % T. Similarly, the total number of T would be 2600 + 3800 = 6400

The total number of A and T on both strands would be 6400 + 6400 = 12800

The remaining bases would be G and C = 20,000 - 12800 = 7200

Since % G =  % C; G = 7200/2 = 3600; C = 3600




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