describe the structure of an RNA molecule, using the example of messenger RNA (mRNA)

Structure of Nucleic Acids

Nucleic acids are polymers composed of nucleotides. Each nucleotide consists of three parts:

• A five‑carbon sugar (deoxyribose in DNA, ribose in RNA)
• A nitrogenous base (purine or pyrimidine)
• A phosphate group

The backbone of both DNA and RNA is formed by alternating sugar and phosphate groups linked by phosphodiester bonds. The sequence of nitrogenous bases encodes genetic information.

DNA Replication – Overview

DNA replication is a semi‑conservative process that occurs during the S‑phase of the cell cycle. The main steps are:

1. Initiation at origins of replication
2. Unwinding of the double helix by helicase
3. Stabilisation of single strands by single‑strand binding proteins
4. Synthesis of new strands by DNA polymerases (leading and lagging strands)
5. Removal of RNA primers and replacement with DNA
6. Ligation of Okazaki fragments

Structure of RNA Molecules

RNA (ribonucleic acid) differs from DNA in three key ways:

• The sugar is ribose, which has a hydroxyl group at the 2' carbon.
• Uracil (U) replaces thymine (T) as the pyrimidine base that pairs with adenine (A).
• RNA is typically single‑stranded, although it can form intramolecular base‑pairing.

The general formula for a ribonucleotide is \$\text{Ribonucleotide: } \mathrm{C5H{10}O5\!-\!PO3^{2-}}\$ where the base is attached to the 1' carbon of ribose.

Messenger RNA (mRNA)

Messenger RNA is the transcript that carries genetic information from DNA to the ribosome for protein synthesis. Its structure can be described in three functional regions:

1. 5' Cap – a modified guanine nucleotide (7‑methylguanosine) added to the 5' end, protecting the mRNA from degradation and assisting ribosome binding.
2. Coding Region – a series of codons, each consisting of three nucleotides, that specify the amino‑acid sequence of the protein. The coding strand is read in the 5'→3' direction.
3. 3' Poly‑A Tail – a stretch of adenine residues (typically 50–250) added post‑transcriptionally, enhancing stability and export from the nucleus.

During transcription the following simplified reaction occurs:

\$\$

\text{DNA template (3'→5')} \xrightarrow{\text{RNA polymerase}} \text{mRNA (5'→3')} + \text{PP_i}

\$\$

Key Features of mRNA

• Single‑stranded with occasional hairpin loops formed by complementary regions.
• Contains the start codon AUG (methionine) and three stop codons (UAA, UAG, UGA).
• Processed in eukaryotes: 5' capping, splicing of introns, and poly‑adenylation.

Comparison of DNA and RNA

Summary

Messenger RNA is a single‑stranded nucleic acid that conveys the genetic code from DNA to the ribosome. Its structure includes a 5' cap, a coding region of codons, and a 3' poly‑A tail, all built from ribonucleotides containing ribose and uracil. Understanding the differences between DNA and RNA, and the specific features of mRNA, is essential for grasping how genetic information is expressed in cells.