DNA Tertiary Structure and RNA Types & Functions

Posted on Apr 3, 2025 in Biology

Advantages of DNA Tertiary Structure

  • Reduces the overall length of the DNA molecule, allowing it to fit within cellular compartments.
  • Facilitates DNA replication. Its clockwise winding (dextrorotatory) structure is efficiently unwound by enzymes that typically work in a counter-clockwise (levorotatory) manner.

Some organisms possess non-coding DNA fragments, known as supernumerary DNA.

DNA Structure Types and Packing

  • Single-stranded DNA (ssDNA): Composed of a single nucleotide chain.
  • Double-stranded DNA (dsDNA): Composed of two nucleotide chains (bicatenary), forming the typical double helix.

RNA (Ribonucleic Acid)

  • RNA is composed of nucleotides containing ribose (a pentose sugar), nitrogenous bases (Adenine, Guanine, Cytosine, Uracil – A, G, C, U), and a phosphate group.
  • It forms a chain of nucleotides linked by phosphodiester bonds in the 5′ to 3′ direction.
  • RNA generally has a lower molecular weight than DNA.
  • RNA is present in many viruses, as well as prokaryotic and eukaryotic cells.

RNA is classified into several types:

  • Double-stranded RNA (dsRNA): Found in some viruses, like reoviruses.
  • Single-stranded RNA (ssRNA): Includes various functional types:
    • Transfer RNA (tRNA)
    • Messenger RNA (mRNA)
    • Ribosomal RNA (rRNA)
    • Nucleolar RNA (noRNA) / Small nucleolar RNA (snoRNA)

Transfer RNA (tRNA)

  • tRNA typically contains 70-90 nucleotides.
  • It has a molecular weight of about 25,000 Daltons.
  • Found in the cytoplasm.
  • It is a single-stranded molecule that folds into a specific three-dimensional structure.
  • Structure:
    • There are many different types, specific to the amino acids they carry.
    • It folds into a characteristic L-shaped tertiary structure.
  • Function: tRNA transports specific amino acids to the ribosome during protein synthesis, matching them to the codons on the messenger RNA (mRNA) sequence.

Messenger RNA (mRNA)

  • mRNA is typically a linear, single-stranded molecule.
  • Its molecular weight varies greatly, ranging from approximately 20,000 Daltons up to several million.
  • Function: mRNA carries genetic information copied from DNA (a process called transcription) to the ribosomes, where it serves as a template for protein synthesis (a process called translation).
  • Its structure and processing differ between prokaryotes and eukaryotes.

Eukaryotic mRNA

  • Eukaryotic mRNA can have complex secondary structures (like hairpin loops) and is often associated with proteins.
  • Formation: Eukaryotic mRNA is processed from a precursor molecule called pre-mRNA or heterogeneous nuclear RNA (hnRNA). hnRNA contains coding regions (exons) interrupted by non-coding regions (introns).
  • During processing (splicing), introns are removed, and exons are joined together. Mature mRNA also typically receives a 5′ cap and a 3′ poly-A tail.

Prokaryotic mRNA

  • Prokaryotic genes typically do not contain introns.
  • Prokaryotic mRNA usually lacks the 5′ cap and 3′ poly-A tail found in eukaryotes.
  • It is often polycistronic, meaning a single mRNA molecule can carry the coding information for multiple proteins.

Ribosomal RNA (rRNA)

  • rRNA is the most abundant type of RNA, making up about 60% of the ribosome’s weight.
  • rRNA associates with ribosomal proteins to form the ribosomal subunits. These structures provide the framework and catalytic activity for protein synthesis, facilitating mRNA binding and tRNA interaction during translation.
  • rRNA molecules have complex secondary structures, including single-stranded regions and double-helical segments.
  • Molecular weight ranges from approximately 500,000 to over 1,700,000 Daltons (often measured by sedimentation coefficient in Svedberg units, S).

Nucleolar RNA (noRNA / snoRNA)

  • Found primarily in the nucleolus of eukaryotic cells.
  • Transcribed from specific DNA segments located in the nucleolar organizing regions (NORs).
  • Precursor rRNA (e.g., 45S in mammals) is transcribed in the nucleolus. This precursor associates with proteins (forming ribonucleoprotein complexes) and small nucleolar RNAs (snoRNAs) which guide its processing (cleavage and modification) into mature rRNA molecules (e.g., 18S, 5.8S, 28S). The 5S rRNA is transcribed separately and imported into the nucleolus.
  • Function: Nucleolar RNAs (specifically snoRNAs) are primarily involved in the processing and modification of rRNA and the assembly of ribosomes.

Note: Cytosol is the fluid portion of the cytoplasm.

Key Functions of RNA

  1. Transmission of Genetic Information: mRNA carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm.
  2. Template for Protein Synthesis: mRNA serves as the direct template sequence that dictates the order of amino acids during translation.
  3. Decoding Genetic Code (Translation): tRNA molecules read the mRNA codons and bring the corresponding amino acids to the ribosome. rRNA within the ribosome catalyzes peptide bond formation.
  4. Genetic Material Storage: In many viruses (retroviruses, RNA viruses), RNA serves as the primary repository of genetic information instead of DNA.
  5. Catalytic Activity: Some RNA molecules, known as ribozymes (like components of the ribosome itself), can act as biological catalysts, performing chemical reactions similar to protein enzymes.
  6. Regulatory Roles: Various small RNAs (like miRNA, siRNA) regulate gene expression.