Answer- DNA Structure-A sample of human DNA is subjected to increasing temperature


A sample of human DNA is subjected to increasing temperature until the major fraction exhibits optical density changes due to disruption of its helix (melting or denaturation). A smaller fraction is atypical in that it requires a much higher temperature for melting. This smaller, atypical fraction of DNA must contain a higher content of

A. Adenine plus Cytosine

B. Cytosine plus Guanine

C. Adenine plus Thymine

D. Cytosine plus Thymine

E. Adenine plus Guanine

Answer-The answer is- B- Cytosine plus Guanine.

Deoxyribonucleic acid (DNA) is the chemical basis of heredity and is organized into genes, the fundamental units of genetic information.

Single stranded stucture of DNA

Figure-1 Single stranded structure of DNA

Salient features of DNA structure

  • DNA contains four Deoxynucleotides- deoxyadenylate, deoxyguanylate, deoxycytidylate, and thymidylate.
  • These monomeric units of DNA are held in polymeric form by 3′, 5′-phosphodiester bridges constituting a single strand (figure-1)
  • The informational content of DNA (the genetic code) resides in the sequence in which these monomers—purine and pyrimidine deoxyribonucleotides—are ordered.
  • The polymer possesses a polarity; one end has a 5′-hydroxyl or phosphate terminal while the other has a 3′-phosphate or hydroxyl terminal (figure-1)
  • The observation of Chargaff that in DNA molecules the concentration of deoxyadenosine (A) nucleotides equals that of thymidine (T) nucleotides (A = T), while the concentration of deoxyguanosine (G) nucleotides equals that of deoxycytidine (C) nucleotides (G = C), led Watson, Crick, and Wilkins to propose in the early 1950s a model of a double-stranded DNA molecule (figure-2)
  • The two strands of this double-stranded helix are held in register by both hydrogen bonds between the purine and pyrimidine bases of the respective linear molecules and by van der Waals and hydrophobic interactions between the stacked adjacent base pairs (figure-2)
  • The pairings between the purine and pyrimidine nucleotides on the opposite strands are very specific and are dependent upon hydrogen bonding of A with T and G with C.
  • This common form of DNA is said to be right-handed because as one looks down the double helix, the base residues form a spiral in a clockwise direction.
  • In the double-stranded molecule, restrictions imposed by the rotation about the phosphodiester bond, the favored anticonfiguration of the glycosidic bond and the predominant tautomers of the four bases (A, G, T, and C) allow A to pair only with T and G only with C.
  • This base-pairing restriction explains that in a double-stranded DNA molecule the content of A equals that of T and the content of G equals that of C.
  • The two strands of the double-helical molecule, each of which possesses a polarity, are antiparallel; i.e., one strand runs in the 5′ to 3′ direction and the other in the 3′ to 5′ direction.
  • In the double-stranded DNA molecules, the genetic information resides in the sequence of nucleotides on one strand, the template strand.
  • This is the strand of DNA that is copied during ribonucleic acid (RNA) synthesis. It is sometimes referred to as the noncoding strand.
  • The opposite strand is considered the coding strand because it matches the sequence of the RNA transcript (but containing uracil in place of thymine that encodes the protein.
  • The two strands, in which opposing bases are held together by interstrand hydrogen bonds, wind around a central axis in the form of a double helix. Double-stranded DNA exists in at least six forms (A–E and Z).
  • The B form is usually found under physiologic conditions (low salt, high degree of hydration).
  • A single turn of B-DNA about the axis of the molecule contains ten base pairs. The distance spanned by one turn of B-DNA is 3.4 nm (34 Å).
  • The width (helical diameter) of the double helix in B-DNA is 2 nm (20 Å).
  • Three hydrogen bonds hold the deoxyguanosine nucleotide to the deoxycytidine nucleotide, whereas the other pair, the A–T pair, is held together by two hydrogen bonds (figure-2).
  • Thus, the G–C bonds are much more resistant to denaturation, or “melting,” than A–T-rich regions


Figure-2- Base pairing between deoxyadenosine and thymidine involves the formation of two hydrogen bonds. Three such bonds form between deoxycytidine and deoxyguanosine.

DNA denaturation

The double-stranded structure of DNA can be separated into two component strands (melted) in solution by increasing the temperature or decreasing the salt concentration. Not only do the two stacks of bases pull apart but the bases themselves unstack while still connected in the polymer by the phosphodiester backbone. Concomitant with this denaturation of the DNA molecule is an increase in the optical absorbance of the purine and pyrimidine bases—a phenomenon referred to as hyperchromicity of denaturation. The strands of a given molecule of DNA separate over a temperature range. The midpoint is called the melting temperature, or Tm. The Tm is influenced by the base composition of the DNA and by the salt concentration of the solution. DNA rich in G–C pairs, which have three hydrogen bonds, melts at a higher temperature than that rich in A–T pairs, which have two hydrogen bonds.

In the given situation

There can not be – Adenine plus Cytosine, Cytosine plus thymine or adenine plus guanine base pairs.Base pairing depends upon the tautomers of the 4 bases and always purines pair with pyrimidines. Cytosine plus thymine both are pyrimidines and adenine and guanine both are purines, hence physiologically these combinations are not possible. The answer can not be Adenine plus Thymine, because they are linked together by two hydrogen bonds they would require less temperature for strand separation.

Hence the right option is cytosine plus guanine.

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