Central Dogma of Molecular Biology

• Central Dogma


  The basic underlying principle in the field of genetics that explains that DNA codes for RNA, which codes for proteins
• Components of Central Dogma

  • DNA
  • RNA
  • Proteins
• DNA (deoxyribonucleic acid)


  •Genetic blueprint passed on from parents to offspring which contains instructions necessary for survival of every organism.

  •found in the nucleoid region of the cytoplasm in prokaryotes such as bacteria
• DNA

  • Located in nucleus and some are in the mitochondria of eukaryotes
  • Has a double helix structure (Francis Crick and James Watson, 1953)
  • Nucleotide- building block of DNA composed of a phosphate group, a pentose sugar (5C), and nitrogenous bases

  
• Deoxyribose sugar

  Deoxy- means that ribose has lost an oxygen atom, the 5 carbon structure is numbered based on its carbon atom
• Nitrogenous bases

  Can be classified as purines (double ring) or pyrimidines (single ring), undergo complementary base pairing where A is paired with T and G is paired with C, U replaces T in RNA
• RNA (ribonucleic acid)


  Single-stranded molecule also composed of nucleotides with a few modifications, sugar backbone is ribose, A-U instead of A-T, functions include creating proteins, acting as enzymes, and helping in regulating various cell processes

  
• Types of RNA

  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
• Proteins


  Final products in the central dogma of molecular biology, building blocks of life with diverse functions including structural support, transporting molecules, acting as enzymes, and acting as passageways
• Proteins


  • Composed of polymers of amino acids known as polypeptides
  • 3-dimensional structure defines size, shape, and function
  • Properties determined by order of amino acids in polypeptide
  • Genetic code translates order of nitrogenous bases in DNA/RNA into specific amino acids
• Codon


  1. letter combination in the mRNA
• Stop codons

  UAG, UGA, UAA
• states that information flows in one direction from DNA to RNA to proteins

  Central Dogma states that information flows in one direction from DNA to RNA to proteinsnformation flows in one direction from DNA to RNA to proteins
• Replication

  1. Occurs in the nucleus

  2. Ensures each cell has complete set of DNA during cell division

  3. Described as semiconservative as half the DNA is old template and half is new complementary strand
• Replication Process

  1. DNA molecules separate into two complementary strands

  2. Both strands serve as template for new strand

  3. Enzymes unzip DNA by breaking hydrogen bonds between base pairs

  4. Each DNA molecule produces one original and one new strand
• A strand with bases ACGTTA would produce a complementary strand with the bases TGCAAT
• Transcription


  1. The process by which RNA is synthesized from DNA

  2. Transcription happens when a DNA portion is copied to form its complementary mRNA sequence

  3. For prokaryotes, transcription occurs in the cytoplasm, in eukaryotes it occurs in the nucleus

  4. There are 3 major steps: initiation, elongation, termination
• Initiation


  Transcription requires RNA polymerase, which binds to DNA at specific sequences called promoters to start transcription
• Elongation


  Only one DNA strand acts as template for mRNA synthesis, different nucleotides are added to growing RNA chain in 5'-3' direction
• Termination


  Transcription stops when RNA polymerase reaches the terminator site, which contains a specific sequence that signals the end of transcription
• Modification


  Before translating to mRNA to proteins, it undergoes modification by RNA splicing, 5' end capping, and adding a poly-A tail
• RNA Splicing


  Large pieces known as introns or intervening sequences are cut out while still in the nucleus, the remaining exons or expressed sequences are spliced back together to form the final mRNA
• 5' End Capping


  Protects the mRNA from exonuclease activity, regulates nuclear transport, and promotes translation and intron excision
• Poly-A Tail


  Addition of multiple adenosine monophosphates at the end of the mRNA molecule, protects mRNA from enzymatic degradation and aids in termination
• Translation


  The sequence of nucleotide bases in mRNA serves as a code for the order of amino acids to be joined together, involves the ribosomes and can be summarized into 3 steps: initiation, elongation, termination
• Initiation


  mRNA is released into the cytoplasm, the small ribosomal subunit binds to the 5' strand of mRNA until it encounters the start codon AUG, the initiator tRNA with anticodon complementary to AUG binds to the P site
• Elongation


  As each codon moves through the ribosomes, the proper amino acid is brought in by tRNA and attached to the growing polypeptide chain

  
• Termination


  Continuous attachment of tRNA allows the polypeptide chain to elongate until it encounters a STOP codon (UAA, UAG, or UGA), which signals protein release factors to release the polypeptide from the ribosome
• Gene expression happens when DNA is transcribed into mRNA, which is then translated into a defined sequence of amino acids in a protein