Nucleic Acids Overview

Nucleic Acids

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers of nucleotides.
• Nucleotides consist of:
  • Pentose (5-carbon sugar)
  • Nitrogen-containing organic base
  • Phosphate group

DNA

• Components: deoxyribose, phosphate group, and one of the organic bases adenine, cytosine, guanine, or thymine.
• Structure: double helix composed of two polynucleotides joined by hydrogen bonds between complementary bases.

RNA

• Components: ribose, phosphate group, and one of the organic bases adenine, cytosine, guanine, or uracil.
• Structure: relatively short single polynucleotide chain.

ATP

• A nucleotide derivative consisting of ribose, adenine, and three phosphate groups.
• Energy is released when ATP is hydrolyzed to form ADP and a phosphate molecule.
• This process is catalyzed by ATP hydrolase.
• The inorganic phosphate can be used to phosphorylate other compounds, making them more reactive.
• Condensation of ADP and inorganic phosphate catalyzed by ATP synthase produces ATP during photosynthesis and respiration.

DNA Replication

• The semi-conservative replication of DNA ensures genetic continuity between generations of cells.
• Steps:
  • The double helix unwinds, and the hydrogen bonds between the complementary bases break using DNA helicase, separating the two strands of DNA.
  • Both strands serve as templates for new DNA strands, with complementary base pairing between template strands and free nucleotides.
  • Adjacent nucleotides are joined by phosphodiester bonds formed through condensation reactions, using DNA polymerase.

Genetic Code

• The order of bases on DNA is called the genetic code, consisting of triplets of bases.
• Each triplet codes for a particular amino acid and is known as a codon.
• Amino acids are joined together by peptide bonds, forming a polypeptide chain.
• A gene is a sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain.
• Non-coding sections of DNA are called introns, while coding regions are called exons.
• Features:
  • Non-overlapping: each triplet is read only once, and triplets don't share bases.
  • Degenerate: more than one triplet codes for the same amino acid, reducing the phenotypic effect of mutations.
  • Mutations can alter the amino acid sequence and the protein, potentially leading to various effects.

Protein Synthesis

• Two stages: transcription and translation

Transcription

• Occurs in the nucleus, involving DNA and mRNA.
• Steps:
  • The hydrogen bonds between complementary bases break, and the DNA uncoils, separating the two strands.
  • One DNA strand serves as a template for RNA polymerase to make the mRNA molecule (the antisense strand).
  • Free nucleotides line up by complementary base pairing, and adjacent nucleotides are joined by phosphodiester bonds by RNA polymerase, forming a single-stranded molecule of mRNA.
  • mRNA moves out of the nucleus through a pore and attaches to a ribosome in the cytoplasm, the site of translation.

Translation

• Involves mRNA, tRNA, and ribosomes.
• During translation, amino acids are assembled together to form a polypeptide chain/protein.
• Steps:
  • mRNA attaches to a ribosome, and transfer RNA (tRNA) collects amino acids from the cytoplasm and carries them to the ribosome.
  • tRNA is a single-stranded molecule with a binding site at one end, allowing it to carry only one type of amino acid, and a triplet of bases at the other end.
  • tRNA attaches to mRNA via complementary base pairing, with two molecules attaching to mRNA at a time.
  • The amino acids attached to two tRNA molecules join by a peptide bond, and then tRNA molecules detach themselves from the amino acids, leaving them behind.
  • This process repeats until a stop codon is reached on mRNA, ending protein synthesis.