Question

a student listed some characteristics of a dna double helix structure.

1. the purine bases always pair with the pyrimidine bases
2. the nitrogenous bases are attached to the sugar of the dna molecule.
3. the two strands of the dna double helix are held together by phosphate bonds.
4. the sugar molecules on the two strands of the dna helix are linked to each other by hydrogen bonds.

which listed characteristics of dna are true? (select all that are true)
1
2
3
4

Explanation:

• Characteristic 1: In DNA, purine bases (adenine and guanine) always pair with pyrimidine bases (thymine and cytosine). Adenine pairs with thymine (A - T) via two hydrogen bonds, and guanine pairs with cytosine (G - C) via three hydrogen bonds.
• Characteristic 2: A DNA molecule is made up of nucleotides. Each nucleotide has a nitrogenous base, a deoxyribose sugar, and a phosphate group. The nitrogenous base is attached to the 1' - carbon of the deoxyribose sugar.
• Characteristic 3: The two strands of the DNA double helix are held together by hydrogen bonds between the nitrogenous bases, not phosphate bonds. Phosphate bonds (phosphodiester bonds) link the nucleotides within a single strand of DNA (connecting the 3' - carbon of one deoxyribose sugar to the 5' - phosphate group of the next nucleotide).
• Characteristic 4: The sugar molecules on the two strands of the DNA helix are linked to each other via phosphodiester bonds (along the sugar - phosphate backbone of each strand), not hydrogen bonds. Hydrogen bonds are between the nitrogenous bases.

Answer:

1. 1
2. 2