Question

How are the two strands of nucleic acid in DNA held together?

Short answer

The two strands of DNA are held together by hydrogen bonds between complementary bases (A-T and C-G).

Step-by-step solution

Identify the Structure of DNA

DNA is composed of two strands that coil around each other to form a double helix.

Understand the Components of DNA

Each strand of DNA is made up of nucleotides, which include a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base.

Determine Base Pair Interactions

In the double helix, the nitrogenous bases of one strand form hydrogen bonds with the bases of the opposite strand following specific pairing rules: adenine (A) pairs with thymine (T) with two hydrogen bonds, and cytosine (C) pairs with guanine (G) with three hydrogen bonds.

Describe Hydrogen Bonding

Hydrogen bonds between the complementary bases are what hold the two strands of DNA together. These are weak bonds that are strong enough to keep the double helix stable but can be easily broken for processes like DNA replication and transcription.

Key concepts

Double Helix

DNA, or deoxyribonucleic acid, has a very specific shape called a double helix. Imagine two long ribbons that twist around each other, forming a spiral staircase. This shape allows DNA to be incredibly compact yet very stable.
This double helix structure was discovered by scientists James Watson and Francis Crick in 1953. It's important because it forms the basis of how DNA stores genetic information. The twisting nature of the double helix helps protect the genetic code inside while allowing it to be accessed when needed.
In summary:

• Two strands coil around each other
• Looks like a spiral staircase
• Discovered in 1953

Nucleotides

The building blocks of DNA are called nucleotides. These are the small units that come together to form the two strands of the double helix. Each nucleotide is made up of three components:

• A phosphate group
• A sugar molecule called deoxyribose
• A nitrogenous base

The sugar and phosphate form the backbone or outer strands of the helix, while the nitrogenous bases stick out like rungs on a ladder.
There are four different types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Each nucleotide contains one of these bases.
Understanding nucleotides is crucial because they are the alphabet of DNA, spelling out genetic instructions in code.

Hydrogen Bonds

Hydrogen bonds play a vital role in holding the two strands of DNA together. These bonds form between nitrogenous bases on opposite strands:
When adenine (A) on one strand pairs with thymine (T) on the other strand, they are held together by two hydrogen bonds. Similarly, cytosine (C) pairs with guanine (G) through three hydrogen bonds.
Hydrogen bonds are relatively weak compared to other types of chemical bonds. This is beneficial because it allows the DNA strands to separate easily during DNA replication and transcription:

• Two hydrogen bonds for A-T pairs
• Three hydrogen bonds for C-G pairs
• Weak enough to break easily when needed

Even though they are weak, the sheer number of hydrogen bonds collectively makes the DNA molecule very stable.

Base Pairing

One of the most important features of DNA is its base pairing rules. These rules dictate how the nitrogenous bases on one strand of DNA interact with those on the other strand:
Adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This is known as complementary base pairing. Because of this specific pairing:

• A pairs with T
• C pairs with G

Complementary base pairing ensures that DNA can be accurately copied during the process of replication. Each strand of the double helix can serve as a template for a new strand, making sure that genetic information is passed on correctly.
In summary, these base pairs are like puzzle pieces that fit perfectly together, ensuring the integrity of genetic information.