Structure of DNA and RNA

DNA and RNA are essential information-carrying molecules in all living organisms. 

DNA stores genetic information, while RNA transfers this genetic information from DNA to ribosomes, where proteins are synthesised.

Both DNA and RNA are polymers made up of repeating units called nucleotides. A nucleotide consists of:

• A pentose sugar (either deoxyribose in DNA or ribose in RNA)

• A phosphate group

• A nitrogenous base (which differs between DNA and RNA)

Assessed diagram:

The general structure of a nucleotide.

Nucleotides are linked together through condensation reactions that form phosphodiester bonds between the pentose sugar of one nucleotide and the phosphate group of the next. 

This process creates a sugar-phosphate backbone, with the nitrogenous bases extending outward. 

The sequence of these bases can vary, allowing nucleotides to join together in any order along the chain.

Structure of DNA

DNA is composed of nucleotides, each containing three parts: 

• A phosphate group

• A deoxyribose sugar

• A nitrogenous base, either adenine (A), thymine (T), guanine (G), or cytosine (C).

DNA nucleotide.

Key features of DNA structure:

• Double-stranded: DNA consists of two polynucleotide chains that run in opposite directions (antiparallel) and form a double helix. This makes DNA a stable molecule, protecting it from damage.

• Complementary base pairing: The two strands are held together by hydrogen bonds between complementary bases. Adenine (A) pairs with thymine (T) via two hydrogen bonds, while cytosine (C) pairs with guanine (G) via three hydrogen bonds. This ensures that genetic information can be accurately copied and checked for errors.

• Sequence variability: The four different bases can appear in any order along the DNA strand, allowing the sequence to encode vast amounts of genetic information.

• Length: DNA is a very long molecule, enabling it to store large quantities of genetic information.

DNA base pairing.

The DNA double helix.

Structure of RNA

RNA is similar to DNA but has some key differences in its structure:

• Nucleotide structure: RNA nucleotides consist of:

  • a phosphate group

  • a ribose sugar

  • a nitrogenous base, either adenine (A), uracil (U) (instead of thymine), guanine (G), and cytosine (C).

• Single-stranded: Unlike DNA, RNA is single-stranded, which makes it more versatile but less stable than DNA.

• Complementary base pairing: In RNA, adenine (A) pairs with uracil (U) instead of thymine (T).

• Shorter: RNA is much shorter than DNA and is often involved in short-term tasks, such as transferring genetic information.

RNA nucleotide.

Ribosomes and protein synthesis

Ribosomes, which are made from ribosomal RNA and proteins, play a key role in translating the genetic information in RNA into a polypeptide (protein chain). 

This process is explored further in Topic 4, where the role of RNA in protein synthesis will be covered in more detail.

Definitions