Question

Why is \(T_{m}\) related to base composition?

Short answer

Answer: The melting temperature of DNA is related to its base composition because the stability of the DNA, determined by the strength of the hydrogen bonds between the base pairs, depends on the proportion of A-T and C-G base pairs present. DNA with a higher GC content, due to stronger hydrogen bonding, will have a higher melting temperature compared to DNA with a higher AT content.

Step-by-step solution

Understanding the DNA structure and base pairs

DNA is a double-stranded molecule composed of nucleotides, with each nucleotide containing a phosphate group, a sugar molecule, and a nitrogenous base. There are four types of nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair up in specific combinations in the double helix structure, with adenine pairing up with thymine (A-T) and cytosine pairing up with guanine (C-G). These base pairs are held together by hydrogen bonds.

Base pairs and bonding strength

The hydrogen bonds between the nitrogenous bases are crucial for the stability of the double helix structure. The strength of these bonds depends on the number of hydrogen bonds holding the base pairs together. Between adenine and thymine (A-T), there are two hydrogen bonds, while between cytosine and guanine (C-G), there are three hydrogen bonds. This means that C-G base pairs have stronger bonding than A-T base pairs.

Base composition and melting temperature

The melting temperature (\(T_m\)) of DNA is the temperature at which 50% of the DNA is in a double-stranded state and the other 50% is in a single-stranded state. The stability of DNA, and consequently its melting temperature, is mainly determined by the strength of the hydrogen bonds holding the base pairs together. Since C-G base pairs have stronger bonding (three hydrogen bonds) than A-T base pairs (two hydrogen bonds), DNA with a higher GC content (i.e., a higher proportion of C-G base pairs) will require more energy and a higher temperature to break the hydrogen bonds and denature the double helix structure.

Conclusion

In conclusion, the melting temperature (\(T_m\)) is related to the base composition of DNA because the stability of the DNA, determined by the strength of the hydrogen bonds between the base pairs, depends on the proportion of A-T and C-G base pairs present. DNA with a higher GC content, due to stronger hydrogen bonding, will have a higher melting temperature compared to DNA with a higher AT content.