MPSC Notes - Nucleic Acids (DNA and RNA Review) Parts - II

11. What is the numeric relation between pyrimidine and purine bases in the DNA molecule? Is that relation valid in RNA molecules?

 The DNA molecule is made of two bound polynucleotide chains that form a helical structure (the double helix). The binding of the two chains is between their nitrogen-containing bases and it always obeys the following rules: adenine (A), a purine base, binds with thymine (T), a pyrimidine base, and guanine (G), a purine base, binds to cytosine (C), a pyrimidine base. Therefore in one molecule of DNA there will be the same number of adenine (A) and thymine (T) and same number of cytosine (C) and guanine (G). The quantities of purine and of pyrimidine bases then will also be the same in a 50% proportion for each type. The relation A = T and C = G, or A/T = C/G = 1, is called Chargaff’s relation and the pairing rules described above are known as Chargaff’s rules.

 In RNA there are not two nucleotide chains. RNA is a simple chain molecule and there is no necessary proportionality of nitrogen-containing bases to form it.

12. Which type of chemical bond maintains the pairing of each chain in the DNA molecule?

 To form the DNA molecule, purine bases bind to pyrimidine bases by intermolecular bonds called hydrogen bonds. Hydrogen bonds occur when there is hydrogen near one of these electronegative elements: fluorine, oxygen or nitrogen.

 In such conditions hydrogen looks like having lost electrons for those elements and a very strong polarization is created. The highly positive hydrogen attracts pairs of electrons of other molecules making a hydrogen bond.

13. What is the completing sequence of nitrogen-containing bases for a AGCCGTTAAC fragment of a DNA chain?
TCGGCAATTG.

14. What is the name of the DNA duplication process? What is the main enzyme that participates in it? The process of copying, or duplication, of the DNA molecule is called replication. The enzyme that participates in the formation of a new DNA chain is the DNA polymerase. There are also other important enzymes in the replication process, the helicase, the gyrase and the ligase.

15. Why is it not correct to assert that DNA self-replicates?

 DNA is not completely autonomous in its duplication process because the replication does not occur without enzymatic activity. So it is not entirely correct to assert that DNA self-replicates.

16. How do the two complementary nucleotide chains of the DNA facilitate the replication process of the molecule?
The fact that the DNA molecule is made of two polynucleotide chains whose nitrogen-containing bases form hydrogen bonds facilitates the duplication of the molecule. During the DNA replication, the binding of the two chains is broken and each of them serves as a template for the formation of a new nucleotide sequence along it, with the help of the enzyme DNA polymerase and obeying the pairing rule A-T, C-G. At the end of the process two double helix of DNA are produced, each made of an original template chain and of a new synthesized polynucleotide chain.

17. What are the chemical bonds of the DNA molecule that are broken for the replication process to occur?
During the DNA replication process, hydrogen bonds between nitrogen-containing bases of the polynucleotide chains are broken.

18. As a result of DNA replication two DNA molecules come into existence. Why is it not correct to assert that two “new” DNA molecules are created? What is the name given to the process concerning that fact?

During replication each chain of the DNA molecule acts by pairing new nucleotides and after the process two newly formed chains made with the union of these nucleotides appear. Then two DNA molecules are created, each with one chain from the original molecule and one new chain formed by new nucleotides. Thus it is not entirely correct to assert that the replication produces two new molecules of DNA. It is better to affirm that two new half-molecules are created.

For this phenomenon DNA replication is called semiconservative replication.

19. Does DNA replication occur in cell division?

Yes. DNA replication occurs in mitosis as well in meiosis.

20. One characteristic of the DNA molecule is its replication capability. What are the consequences of failures during DNA replication?

Ideally a DNA molecule should replicate in a perfect way. Sometimes however failures in the duplication occur, with alteration (deletion, addition or substitution) of one or more nucleotides in the molecule.

Those mistakes, or mutations, therefore make changes in the protein synthesis process too. For example, the production of an important protein for cells or tissues may be suppressed, new utile or inutile proteins can be created, etc. The mistake in the DNA duplication and the resulting production of altered genetic material are some of the main creative forces for the biological evolution and the diversity of species.

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