(a) How are the two strands of DNA helix held together?

(b) In chemical terms what are DNA and RNA?

To understand how the two strands of a DNA helix are held together, we need to look at the structure of DNA itself. DNA is composed of two long strands that twist around each other to form a double helix. These strands are held together by specific interactions between the bases that make up the DNA sequence.

Base Pairing in DNA

The two strands of DNA are held together primarily through hydrogen bonds between complementary nitrogenous bases. Each strand consists of a backbone made of sugar and phosphate groups, but it’s the bases that play a crucial role in the stability of the double helix.

• Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) through three hydrogen bonds.

This specific pairing is known as Chargaff's rules, which state that the amount of adenine equals thymine, and the amount of cytosine equals guanine in a given DNA molecule. The hydrogen bonds, while relatively weak individually, collectively provide significant stability to the DNA structure, allowing it to maintain its double helical form.

Role of the Sugar-Phosphate Backbone

In addition to base pairing, the sugar-phosphate backbone of each strand contributes to the overall stability of the DNA molecule. The phosphodiester bonds between the sugar and phosphate groups create a strong covalent bond that holds the nucleotides together, ensuring the integrity of each strand.

Understanding DNA and RNA in Chemical Terms

When we talk about DNA and RNA in chemical terms, we are referring to their composition and structure as nucleic acids. Both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are polymers made up of nucleotide monomers, but they have distinct differences.

Structure of DNA

DNA is a double-stranded molecule, as mentioned earlier, and its nucleotides consist of:

• A deoxyribose sugar
• A phosphate group
• A nitrogenous base (A, T, C, or G)

The absence of an oxygen atom on the second carbon of the sugar (deoxyribose) is what distinguishes DNA from RNA.

Structure of RNA

RNA, on the other hand, is typically single-stranded and contains:

• A ribose sugar
• A phosphate group
• A nitrogenous base (A, U, C, or G)

In RNA, uracil (U) replaces thymine (T), which is found in DNA. The presence of an additional oxygen atom in ribose makes RNA more reactive and less stable than DNA, which is one reason why RNA is usually found in a transient form within cells.

Functional Implications

The structural differences between DNA and RNA lead to different functions in biological systems. DNA serves as the long-term storage of genetic information, while RNA plays various roles, including acting as a messenger (mRNA), a structural component (rRNA), and a catalyst (tRNA). This functional diversity is crucial for the processes of transcription and translation, which are essential for protein synthesis.

In summary, the two strands of DNA are held together by hydrogen bonds between complementary bases, while both DNA and RNA are nucleic acids made up of nucleotides, differing in their sugar components and base pairs. These structural features are fundamental to their roles in genetics and cellular function.