28.2 Base Pairing in DNA

DNA's double helix structure is a marvel of nature. Its complementary base pairing and unique features ensure genetic information is stored and passed on accurately. This intricate design allows for the stability and fidelity crucial in biological processes.

The central dogma of molecular genetics explains how genetic info flows in cells. It outlines three key processes: DNA replication, transcription to RNA, and translation to proteins. This fundamental concept ties together the roles of DNA, RNA, and proteins in life.

DNA Structure and Function

Base-pairing rule in DNA

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• DNA composed of two antiparallel polynucleotide strands form a double helix
• Each nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base (nucleobase) (adenine, thymine, guanine, or cytosine)
• Complementary base pairing occurs between nitrogenous bases on opposite strands
  • Adenine (A) pairs with thymine (T) via two hydrogen bonds (A=T)
  • Guanine (G) pairs with cytosine (C) via three hydrogen bonds (G≡C)
• Specific base pairing ensures stability and fidelity of DNA double helix allows for accurate replication and transcription

Features of DNA double helix

• Right-handed spiral with diameter of ~2 nm
• Two polynucleotide strands run antiparallel with 5' end and 3' end
• Sugar-phosphate backbones on outside of helix, nitrogenous bases face inward
• Major groove and minor groove important for protein interactions (transcription factors)
• Complete turn every 10.5 base pairs, with rise of 3.4 Å per base pair
• Stacking of aromatic rings of nitrogenous bases contributes to stability via hydrophobic interactions and $\pi-\pi$ stacking (base stacking)

DNA Base Classification and Structure

• Nucleobases in DNA are classified into two categories:
  • Purines: larger, double-ring structures (adenine and guanine)
  • Pyrimidines: smaller, single-ring structures (thymine and cytosine)
• Watson-Crick model describes the specific base pairing between purines and pyrimidines in DNA
• Base stacking interactions between adjacent base pairs contribute to DNA stability

Central Dogma of Molecular Genetics

Central dogma of molecular genetics

• Describes flow of genetic information within biological system
• DNA transcribed into RNA, which is then translated into proteins
• Three fundamental processes:
  1. Replication
     • Cell makes exact copy of genetic material before cell division
     • Semiconservative, each daughter DNA molecule contains one original strand and one newly synthesized strand
     • Ensures genetic information passed on to daughter cells (mitosis, meiosis)
  2. Transcription
     • Synthesis of RNA from DNA template catalyzed by RNA polymerase
     • Complementary RNA strand using one DNA strand as template
     • Resulting RNA molecule called messenger RNA (mRNA) carries genetic information from DNA to ribosomes
     • Also produces other RNA types (tRNA, rRNA, miRNA)
  3. Translation
     • Synthesis of proteins using genetic information carried by mRNA
     • Ribosomes read mRNA sequence in triplets called codons, each coding for specific amino acid
     • Transfer RNA (tRNA) molecules carry specific amino acids, recognize codons, deliver amino acids to growing polypeptide chain
     • Sequence of amino acids in polypeptide chain determines structure and function of resulting protein (enzymes, structural proteins, signaling molecules)