AP Biology : Understanding the Sugar-Phosphate Backbone

Study concepts, example questions & explanations for AP Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #69 : Dna, Rna, And Proteins

The most prevalent negative charge on DNA can be found on which of the following molecular components?

Possible Answers:

Phosphate backbone

Hydrogen bonds between base pairs

Hydroxyl residues

Phosphodiester linkage

Ribose sugar ring

Correct answer:

Phosphate backbone

Explanation:

The phosphate backbone of DNA is negatively charged due to the bonds created between the phosphorous atoms and the oxygen atoms. Each phosphate group contains one negatively charged oxygen atom, therefore the entire strand of DNA is negatively charged due to repeated phosphate groups. 

Example Question #1 : Understanding The Sugar Phosphate Backbone

Please complete the analogy.

Nitrogen : Nucleic Acids :: Phosphorous : ______________.

Possible Answers:

Phospholipids

ATP

DNA

ADP

All answer choices

Correct answer:

All answer choices

Explanation:

Nitrogen is essential to create all the nucleic acids, and phosphorous is essential to create phospholipids (an obvious choice), ATP and ADP (they are the same class of molecule, and the P stands for phosphate), and DNA (for the phosphate-sugar backbone).

Example Question #2 : Understanding The Sugar Phosphate Backbone

Which of the following is not true of a DNA molecule?

Possible Answers:

Adenine and thymine are held together by phosphodiester bonds

Complementary strands are held together by hydrogen bonds

Uracil is not a component of the molecule

A purine or pyrimidine is bound to each sugar-phosphate group

Correct answer:

Adenine and thymine are held together by phosphodiester bonds

Explanation:

DNA is a polymer composed of nucleotide monomers. Each nucleotide is formed from a deoxyribose sugar, a phosphate, and a nitrogenous base. There are two types of nitrogenous bases: purines and pyrimidines. The purines are adenine and guanine, while the pyrimidines are thymine and cytosine (and uracil). Adenine will always bind thymine and cytosine will always bind guanine. Uracil is only found in RNA, and is absent from DNA.

During DNA replication and synthesis, nucleotides align so that the nitrogenous bases are correctly paired. The bases bind to one other via hydrogen bonding to secure the nucleotide to the template strand. The protein DNA ligase then fuses the sugar-phosphate groups of adjacent nucleotides to create the DNA backbone. These bonds are known as phosphodiester bonds.

The only false statement concerns the identity of bonding between nitrogenous bases. Bases are held together by hydrogen bonds, and the DNA backbone is held together by phosphodiester bonds.

Example Question #2 : Understanding The Sugar Phosphate Backbone

A __________ bond between the sugar of one nucleotide and the phosphate of an adjacent nucleotide stabilizes the backbone of the DNA. 

Possible Answers:

ionic

weak

covalent

metallic

hydrogen

Correct answer:

covalent

Explanation:

The bond formed between the sugar of one nucleotide and the phosphate of an adjacent nucleotide is a covalent bond. A covalent bond is the sharing of electrons between atoms. A covalent bond is stronger than a hydrogen bond (hydrogen bonds hold pairs of nucleotides together on opposite strands in DNA). Thus, the covalent bond is crucial to the backbone of the DNA. 

Learning Tools by Varsity Tutors