Problem 61 - Chemistry The Molecular Science

Question

One strand of DNA contains the base sequence \(\mathrm{T}-\mathrm{C}-\mathrm{G}\). Draw a structure of this section of DNA that shows the hydrogen bonding between the base pairs of this strand and its complementary strand.

Step-by-Step Solution

Answer

The complementary sequence is A-G-C, with hydrogen bonds: T-A (2 bonds), C-G (3 bonds).

Step 1: Identify the Complementary Strand Sequence

For the given DNA sequence, identify the complementary bases. The base pairing rules are: Thymine (T) pairs with Adenine (A) and Cytosine (C) pairs with Guanine (G). Therefore, the complementary strand for the sequence T-C-G is A-G-C.

Step 2: Understand Hydrogen Bonding Between Base Pairs

Each pair of complementary bases forms hydrogen bonds. Specifically, Adenine (A) and Thymine (T) form 2 hydrogen bonds, whereas Cytosine (C) and Guanine (G) form 3 hydrogen bonds.

Step 3: Draw the DNA Strand Structure

Draw two parallel lines to represent the sugar-phosphate backbones of the DNA double helix. On one strand, write the provided sequence T-C-G, and on the opposite strand, write the complementary sequence A-G-C, aligning each base with its pair.

Step 4: Illustrate the Hydrogen Bonds

Between each pair of complementary bases, draw lines to represent the hydrogen bonds. For T-A pairs, draw 2 lines, and for C-G pairs, draw 3 lines. This illustrates the specific hydrogen bonding interactions between the nucleotides.

Key Concepts

Hydrogen Bonding in DNA

Hydrogen bonds play a critical role in holding the two strands of DNA together. These are weak interactions that are essential for the structure and stability of the DNA double helix. Hydrogen bonds form between complementary nitrogenous bases in opposite DNA strands.

Each pair of bases—Adenine (A) with Thymine (T) and Cytosine (C) with Guanine (G)—connect across the strands by hydrogen bonds:

• A-T pairs utilize two hydrogen bonds. This connection helps maintain a less rigid but stable linkage that holds the strands together.
• C-G pairs have three hydrogen bonds. This makes them stronger and more stable than A-T pairs, contributing to the overall stability of the DNA.

When drawing the DNA structure, lines are used to depict these vital bonds between the base pairs. These bonds, though individually weak, collectively play a vital role in maintaining the DNA's double helix, allowing it to be both stable and flexible for biological mechanisms like replication and transcription.

Complementary DNA Strands

Complementary DNA strands are a hallmark feature of the DNA double helix. The idea here is that the sequence of one DNA strand precisely determines the sequence of its partner strand.

This complementarity is defined by specific base pairing rules:

• Thymine (T) always pairs with Adenine (A).
• Cytosine (C) always pairs with Guanine (G).

Thus, if you have a DNA sequence like T-C-G on one strand, the complementary strand will be A-G-C. These connections ensure that during DNA replication, the sequence can be accurately copied.

The complementary nature of the strands also means that if you know the sequence of one strand, you can easily deduce the sequence of the other. This is vital for genetic information to be passed on accurately within biological processes.

Nucleotide Interactions

Nucleotide interactions are fundamental to the structure and function of DNA. A nucleotide is composed of three components: a sugar molecule, a phosphate group, and a nitrogenous base. Each nucleotide connects to its partner through these components.

In DNA:

• The backbone is formed by sugar-phosphate connections. This creates a strong framework for the DNA structure.
• The bases, which are attached to the sugars, reach inward and form hydrogen bonds with bases on the opposite DNA strand.

These interactions are crucial because they offer the perfect structural balance needed for the DNA's functions in the cell. The sugar-phosphate backbone provides stability, while the hydrogen bonding between bases allows the double helix to open for replication and transcription, processes essential for life.

Understanding these interactions helps to realize how the unique features of DNA strands contribute to its central role in genetics and cellular functions.