Problem 5 - Campbell Biology

Question

Enzymes that break down DNA catalyze the hydrolysis of the covalent bonds that join nucleotides together. What would happen to DNA molecules treated with these enzymes? (A) The two strands of the double helix would separate. (B) The phosphodiester linkages of the polynucleotide backbone would be broken. (C) The pyrimidines would be separated from the deoxyribose sugars. (D) All bases would be separated from the deoxyribose sugars.

Step-by-Step Solution

Answer

The correct answer is (B).

Step 1: Understand the role of enzymes

Enzymes that break down DNA are known as DNases. Their role is to hydrolyze DNA by breaking the covalent bonds in the polynucleotide chain.

Step 2: Identify the specific bonds

The covalent bonds that hold the nucleotides together in the DNA polynucleotide backbone are the phosphodiester linkages. These linkages connect the 3' carbon atom of one sugar molecule to the 5' carbon atom of another sugar through a phosphate group.

Step 3: Determine what happens when these bonds are broken

Breaking the phosphodiester linkages in the DNA backbone would result in the nucleotide chain being cleaved into smaller fragments.

Step 4: Analyze the given options

(A) The two strands of the double helix separating does not directly involve breaking the covalent bonds between nucleotides. (B) Breaking the phosphodiester linkages matches our understanding from step 3. (C) Separating pyrimidines from deoxyribose sugars involves breaking glycosidic bonds, not phosphodiester bonds. (D) Separating all bases from deoxyribose sugars also involves breaking glycosidic bonds.

Step 5: Select the correct answer

Based on the analysis, the correct answer is (B). The phosphodiester linkages of the polynucleotide backbone would be broken.

Key Concepts

DNA structure

DNA, or deoxyribonucleic acid, is the molecule that carries genetic instructions in living organisms. It is composed of two long strands forming a double helix. Each strand is made of simpler molecules called nucleotides. Nucleotides have three parts:

• A phosphate group
• A deoxyribose sugar
• A nitrogenous base (adenine, thymine, cytosine, or guanine)

The nitrogenous bases pair up across the two strands: adenine with thymine and cytosine with guanine. Hydrogen bonds connect these base pairs, holding the two strands together.
The sequence of these bases encodes genetic information. The backbone of the DNA strand is formed by the sugar and phosphate groups of nucleotides.

enzymatic function

Enzymes are biological catalysts that speed up biochemical reactions. DNases are a specific type of enzyme that targets DNA molecules. Their primary function is to hydrolyze the phosphodiester bonds in the DNA.
Hydrolysis is a reaction where water is used to break bonds. For DNases, this means breaking the covalent bonds that link nucleotides together. When these bonds are broken, the DNA chain is cut into smaller fragments. DNases are essential in processes that require DNA degradation, such as DNA repair, replication, and apoptosis.

phosphodiester linkages

Phosphodiester linkages are critical for the stability and integrity of the DNA structure. These are the covalent bonds that connect the 3' carbon atom of one sugar molecule to the 5' carbon atom of another sugar molecule through a phosphate group.
This creates a backbone for the DNA strand, giving it a consistent structure and stability. The hydrolysis of these bonds by enzymes like DNases disrupts the DNA backbone, resulting in the cleavage of the nucleotide chain into smaller parts. Breaking these linkages is crucial for many cellular processes, including DNA replication and repair.