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Practice Listening: Multiple Choice, Single Answer ID: #40082 Short DNA
Instructions ›
You will see a video explaining the molecular structure of DNA. Listen to the recording and answer the multiple-choice question by selecting the correct response. Only one response is correct.

Transcript:

The structure of DNA is a double helix polymer, a spiral consisting of two DNA strands twisted around each other. Each strand is composed of a long chain of four different types of subunits, called nucleotides. Structurally, the nucleotides are similar. Each consists of a deoxyribose sugar molecule attached to a phosphate group and one of four nitrogenous bases: adenine, guanine, cytosine, and thymine. Then holding the strands together are the bases: adenine always joined to thymine by two hydrogen bonds, and guanine always joined to cytosine by three.

How are the strands of DNA held together?

* Listen to the recording and select the correct answer

Result:

AI explanationShows the answer. Tap to open.
Option 3: "By hydrogen bonds between specific pairs of nitrogenous bases."

Explanation: This option is correct because the text explicitly states that adenine is joined to thymine by two hydrogen bonds and guanine is joined to cytosine by three hydrogen bonds. This clearly indicates that the strands of DNA are held together by hydrogen bonds between specific pairs of nitrogenous bases.

Option 1: "By covalent bonds between the sugar and phosphate groups."

Explanation: This option is incorrect because the text specifies that the strands of DNA are held together by hydrogen bonds between the nitrogenous bases, not by covalent bonds. Covalent bonds do exist in the structure of DNA, but they connect the sugar and phosphate groups within each nucleotide, not between the strands.

Option 2: "By ionic bonds between the nucleotides."

Explanation: This option is incorrect as the text does not mention ionic bonds as a means of holding the DNA strands together. The correct mechanism is through hydrogen bonds between the nitrogenous bases, which is a different type of bond altogether.

Option 4: "By peptide bonds between the adenine and cytosine."

Explanation: This option is incorrect because peptide bonds are not involved in the structure of DNA. Peptide bonds are used to link amino acids in proteins, not to connect nucleotides in DNA. The text does not support this option, as it focuses on hydrogen bonds between nitrogenous bases.

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