Biochemistry · MU Pleven

9. Nucleic acid: RNA. Types, Synthesis, Structure and Function.

Covers RNA as a single-stranded intermediary between DNA and proteins, then its ribonucleotide structure and the 2'-hydroxyl that makes it less stable than DNA. It goes on to describe mRNA, rRNA, tRNA, snRNA, miRNA and siRNA, with a table comparing siRNA and miRNA. Transcription is explained in initiation, elongation and termination, with the RNA polymerases of eukaryotes, and the note closes with the functions of RNA.

Inside: 3 sections · 8 labelled pictures · 1 table · 25 flashcards · 6 min

Topic 9 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 3 sections, about 841 words (6 min of reading), with 25 flashcards.

In this note

  1. Nucleic acid: RNA. Types, Synthesis, Structure and Function.
  2. Nucleic Acid: RNA
  3. V. Functions of RNA

Nucleic Acid: RNA

I. Introduction

  • RNA (Ribonucleic Acid) is a single-stranded nucleic acid that plays a key role in the expression of genetic information.
  • It acts as an intermediary between DNA and proteins, carrying instructions from the DNA for protein synthesis.
  • Unlike DNA, which mainly serves as genetic storage, RNA functions in information transfer, protein synthesis, and catalysis.
  • RNA is primarily found in the cytoplasm, although it is synthesized in the nucleus.

II. Structure of RNA

  • RNA is composed of repeating units called ribonucleotides.
  • Each ribonucleotide consists of:
  • A ribose sugar (with –OH at the 2’ carbon),
  • A phosphate group,
  • A nitrogenous base (adenine, guanine, cytosine, or uracil).
  • Uracil (U) replaces thymine (T) found in DNA.
  • The sugar-phosphate backbone forms the chain, linked by 3’–5’ phosphodiester bonds.
  • RNA is usually single-stranded, though it can fold into secondary structures (loops, hairpins) due to intra-chain base pairing.
  • Some RNA molecules have double-stranded regions or even enzymatic activity (ribozymes).
  • The presence of the 2’-hydroxyl group makes RNA less stable than DNA and more prone to hydrolysis.

III. Types of RNA and Their Functions

1. Messenger RNA (mRNA)

  • mRNA carries genetic information from DNA to the ribosomes, where it directs protein synthesis.
  • It serves as a template for the arrangement of amino acids during translation.
  • The sequence of bases in mRNA is complementary to the DNA template strand.
  • In eukaryotes, the primary transcript (pre-mRNA) undergoes processing to form mature mRNA, which includes:
  • Addition of a 5’ cap (methylated guanine),
  • Addition of a poly-A tail (adenine residues) at the 3’ end,
  • Splicing to remove introns (non-coding regions).
  • Mature mRNA exits the nucleus and binds to ribosomes for translation.
  • mRNA has a short half-life, allowing rapid adjustment of protein synthesis.

2. Ribosomal RNA (rRNA)

  • rRNA is the most abundant RNA type in the cell (~80% of total RNA).
  • It combines with proteins to form ribosomes, the site of protein synthesis.
  • rRNA provides both structural support and catalytic activity (as a ribozyme) in forming peptide bonds.
  • Different rRNA species are found in ribosomal subunits:
  • Prokaryotic ribosomes (70S): 16S, 23S, and 5S rRNAs.
  • Eukaryotic ribosomes (80S): 18S, 28S, 5.8S, and 5S rRNAs.
  • rRNA ensures proper alignment of mRNA and tRNAs during translation.

3. Transfer RNA (tRNA)

  • tRNA is the smallest RNA molecule (about 70–90 nucleotides).
  • It functions as an adapter molecule that carries specific amino acids to the ribosome during translation.
  • tRNA has a characteristic cloverleaf structure with several important regions:

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