Question

Describe the double-helical structure of DNA. What type of bonding occurs within the chain of each strand of the double helix? What type of bonding exists between strands to link them together?

Short answer

The double-helical structure of DNA consists of two strands twisted around each other, forming a double helix shape. Each strand contains nucleotides with a phosphate group, a sugar molecule, and one of the four nitrogenous bases (A, T, C, G). Covalent bonds (phosphodiester bonds) occur within each strand, connecting the phosphate group and sugar molecule of adjacent nucleotides to create a sugar-phosphate backbone. The bonding between strands is due to hydrogen bonds, formed between complementary nitrogenous base pairs (A-T and C-G), enabling the double helix structure to maintain its stability.

Step-by-step solution

Introducing DNA Structure

Deoxyribonucleic acid (DNA) is a molecule that carries the genetic instructions for the growth, development, functioning, and reproduction of all known living organisms. DNA structure consists of two strands, which form a double helix shape. Each strand is made up of 4 types of molecular building blocks called nucleotides, which contain a phosphate group, a sugar molecule, and one of the four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

Explain Bonding within Each Strand

The bonding within each strand of the double helix involves covalent bonds, also known as phosphodiester bonds. These bonds form between the phosphate group of one nucleotide and the sugar molecule (deoxyribose) of the next nucleotide. This creates a sugar-phosphate backbone along the strand, with the nitrogenous bases sticking out from it.

Explain Nitrogenous Base Pairing

In the DNA double helix, the nitrogenous bases of each strand face one another, and they are specifically paired through hydrogen bonds. Adenine (A) forms two hydrogen bonds with thymine (T), while cytosine (C) forms three hydrogen bonds with guanine (G). This base pairing plays a crucial role in maintaining the stability of the DNA structure.

Describe Bonding between Strands

The bonding between the two strands of the double helix is due to the hydrogen bonds formed between complementary nitrogenous base pairs, as mentioned previously. The complementary base pairing allows the strands to twist around each other and form the double helix structure. These hydrogen bonds between strands can be easily broken and reformed, enabling processes such as DNA replication and transcription.

Summary

In summary, the double-helical structure of DNA consists of two strands twisted around each other, connected by hydrogen bonds between complementary nitrogenous base pairs (A-T and C-G). The strands themselves are held together by covalent bonds (phosphodiester bonds) that form the sugar-phosphate backbone.

Key concepts

Double Helix DNA

Imagine unzipping a twisted ladder and you'll have a picture of DNA's double helix structure. Each side of this 'ladder' is formed by a long chain of nucleotides, which are the building blocks of DNA. These nucleotides link together in a sequence that provides the instructions for life processes. A nucleotide is composed of three parts: a phosphate group, a sugar molecule called deoxyribose, and a nitrogenous base. The arrangement of these nucleotides in a particular order is what carries genetic information.

The 'twisted ladder' shape, or double helix, arises as two DNA strands wind around each other, with certain nitrogenous bases from each strand attracting each other to form the 'rungs' of the ladder. The helix retains its shape due to these interactions between the bases, which are critically important for the next concept, nucleotide bonding.

Nucleotide Bonding

Within each DNA strand, nucleotides are linked by covalent bonds between the phosphate group of one nucleotide and the sugar molecule of the next. This bond is known as a phosphodiester bond, and it forms a strong backbone for the DNA strand, giving it structure and resilience. These covalent bonds are much stronger than the hydrogen bonds that connect the two strands, which is why the shape of the DNA strand remains intact, even when the hydrogen bonds are broken during processes like DNA replication.

This sugar-phosphate backbone is critical as it frees the nitrogenous bases to interact with each other, which is crucial for nitrogenous bases pairing, a process we will look at shortly.

Hydrogen Bonds in DNA

Hydrogen bonds may be weaker than covalent bonds, but they have a crucial role in DNA structure and function. They form between the complementary nitrogenous bases of two strands, with adenine (A) pairing with thymine (T) and cytosine (C) with guanine (G). This specific pairing through hydrogen bonds maintains DNA's spiral shape and facilitates crucial processes like DNA replication because they can be easily broken and reformed. These bonds are not randomly formed but follow strict rules which are a key feature of nitrogenous bases pairing, ensuring the correct replication and transcription of genetic information.

DNA Replication

DNA's ability to copy itself, known as DNA replication, is essential for cell division and growth. During this process, the two strands of DNA are separated, breaking the hydrogen bonds between the bases. Each strand then serves as a template for creating a new complementary strand. Enzymes such as DNA polymerase are key players in this process, joining new nucleotides to the existing strand's bases forming new phosphodiester bonds. The unwinding of the strands, as well as the formation of new strands, relies on the specific pairing rules of nitrogenous bases, ensuring an accurate copy of genetic material.

Nitrogenous Bases Pairing

In the heart of DNA's structure are the rules of nitrogenous bases pairing. These rules stipulate that adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). A and T form two hydrogen bonds between them, while C and G form three, creating a uniform distance between the DNA strands. This precise interaction not only stabilizes the DNA structure but also ensures that during DNA replication, the genetic code is accurately copied. Any deviation from these rules can lead to mutations which could have serious implications for the organism.