DNA from a newly discovered virus was purified, and UV light absorption was foll

Question

DNA from a newly discovered virus was purified, and UV light absorption was followed as the molecule was slowly heated. The absorbance increase at the melting temperature was only 10%. Part A What does this result tell you about the structure of the viral DNA? Check all that apply. Check all that apply.

1) The DNA is mostly single-stranded.

2) There may be a small amount of intramolecular base-pairing.

3) There is no intramolecular base-pairing.

4) There may be a great amount of intramolecular base-pairing.

5) The DNA is mostly double-stranded.

It more than one answer to this problem. The number 1 option is not the only answer because it is incorrect.

Explanation / Answer

Analysis:

The melting point (Tm) is the temperature where 50% of the DNA is denatured, and the absorbance of UV light is 50% between the maximum and minimum. Data says to us, that absorbance increase was just 10% at its melting point.

The right interpretation of this is that the DNA at his melting point suffered a decrease of absorbance also known as the hypochromic effect.

And this effect describes the decrease in the absorbance of ultraviolet light in a double stranded DNA compared to its single stranded counterpart. G-C base pairing have 3 hydrogen bonds between them while A-T base pairs have only 2. DNA with a higher G-C content, whether because of its source will have a higher melting temperature than DNA with a higher A-T content.

Answers:

1. The DNA is mostly single stranded, this option is not a possibility, since if it were single stranded the UV light absorbance would be higher.

2. There may be a small amount of intramolecular base pairing, this option could be a posibility but when with talk about base pairing it refers to an increase in melting point and not in absorbance.

3. There is no intramolecular base-pairing. This is not a possibility the DNA must have intereactions.

4. There may be a great amount of intramolecular base-pairing. This is not a possibility since the higher the base pairing the higher the melting point.

5. The DNA is mostly double-stranded. Yes this makes sense since we have a decrease in the absorbance of ultraviolet light in a double stranded DNA from theory.

Hope this helps you!

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