Nucleic Acids and Nucleotides

Nucleic Acids

• Nucleic acids are large biomolecules that play essential roles in all cells and viruses, storing and expressing genomic information that determines inherited characteristics.
• There are two main classes of nucleic acids: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).

Structure of Nucleotides

• Nucleotides are the building blocks of DNA and RNA, consisting of:
  • Nitrogenous base (purines: adenine and guanine; pyrimidines: cytosine, thymine, and uracil)
  • Pentose sugar (deoxyribose in DNA and ribose in RNA)
  • Phosphate group

DNA Structure

• DNA is composed of a phosphate-deoxyribose sugar backbone and the nitrogenous bases adenine (A), guanine (G), cytosine (C), and thymine (T).
• DNA forms a double helix with two strands twisted around each other, held together by hydrogen bonding between complimentary base pairs (A-T and G-C).

RNA Structure

• RNA is a single-stranded nucleic acid polymer of the four nucleotides A, C, G, and U, with a backbone of alternating phosphate and ribose sugar residues.
• RNA is made by copying the base sequence of a section of double-stranded DNA, called a gene, into a piece of single-stranded nucleic acid through transcription.

Types of RNA Molecules

• There are three main classes of RNA molecules:
  • Messenger RNA (mRNA): copies DNA's instructions and carries them to ribosomes for protein assembly.
  • Transfer RNA (tRNA): carries amino acids in the cytoplasm to ribosomes for protein assembly.
  • Ribosomal RNA (rRNA): makes up the large and small subunits of ribosomes.
• Other classes of RNA molecules include:
  • Small nuclear RNA (snRNA)
  • Micro RNA (miRNA)
  • Small interfering RNA (siRNA)
  • Heterogeneous nuclear RNA (hnRNA)

Gene Expression

• Gene expression is the process by which genetic information stored in a gene is used to synthesize a functional protein.
• Proteins synthesis occurs in two steps:
  • Transcription: DNA → RNA
  • Translation: RNA → protein

Transcription and Translation

• Transcription is the process of copying a gene's DNA sequence to make an RNA molecule, performed by enzymes called RNA polymerases.
• Translation is the process of translating the transcript (genetic information) from mRNA into a specific sequence of amino acids in the growing chain of a polypeptide.
• The three general steps of translation are:
  • Initiation
  • Elongation
  • Termination