Central Dogma

• DNA is made up of nucleotides in a DNA double helix
• A DNA nucleotide consists of:

  • Phosphate group
  • 5-carbon sugar
  • Nitrogenous base
• DNA double helix structure:

  • "Rungs of ladder" made of nitrogenous bases (A, T, G, or C)
  • "Legs of ladder" made of phosphate and sugar backbone
• Nitrogenous bases in DNA:

  • Purines: Adenine (A) and Guanine (G)
  • Pyrimidines: Thymine (T) and Cytosine (C)
  • Base pairing: A = T (2 hydrogen bonds), C = G (3 hydrogen bonds)
• Chargaff's Rule:

  • Adenine pairs with Thymine
  • Guanine pairs with Cytosine
  • Amounts in a given DNA molecule will be about the same
• If there is 30% Adenine, there would be 20% Cytosine
• DNA Replication:

  • Takes place in the Synthesis Phase (S Phase) of the cell cycle
  • Involves replication forks and replication bubbles
  • Strand separation is facilitated by Helicase, Single-Strand Binding Proteins, and Topoisomerase
• Priming in DNA Replication:

  • RNA primers are needed to start the addition of new nucleotides
  • Primase is the enzyme that synthesizes the RNA primer
• Synthesis of the new DNA strands:

  • DNA Polymerase catalyzes the synthesis of a new DNA strand in the 5' to 3' direction
  • Leading Strand is synthesized continuously in the 5' to 3' direction
  • Lagging Strand is synthesized discontinuously against the overall direction of replication
  • Okazaki Fragments are short segments on the lagging strand
  • DNA ligase is the enzyme that catalyzes the formation of a covalent bond
• DNA replication involves the synthesis of new DNA strands
• DNA ligase is a linking enzyme that catalyzes the formation of a covalent bond from the 3’ to 5’ end of joining stands
• DNA ligase joins two Okazaki fragments together
• Proofreading: initial base-pairing errors are usually corrected by DNA polymerase
• Watson and Crick showed the semi-conservative model of DNA replication where the two strands of the parental molecule separate and each functions as a template for the synthesis of a new complementary strand
• Excision repair involves a damaged segment being excised by a repair enzyme, followed by DNA polymerase and DNA ligase replacing and bonding the new nucleotides together
• RNA differs from DNA in three ways:

  • RNA is single-stranded but can fold back upon itself to form secondary structures
  • In RNA, the sugar molecule is ribose rather than deoxyribose
  • In RNA, the fourth base is uracil rather than thymine
• Three types of RNA are involved in protein synthesis:

  • Messenger RNA [mRNA]: the template
  • Ribosomal RNA [rRNA]: structural component of the ribosome
  • Transfer RNA [tRNA]: the adapter
• RNA is transcribed from a DNA template after the bases of DNA are exposed by unwinding of the double helix

  • In a given region of DNA, only one of the two strands can act as a template for transcription
• Transcription has three phases: Initiation, Elongation, Termination
• Synthesis - Elongation:

  • RNA polymerase elongates the nascent RNA molecule in a 5’-to-3’ direction, antiparallel to the template DNA
  • Nucleotides are added by complementary base pairing with the template strand
  • The substrates, ribonucleoside triphosphates, are hydrolyzed as added, releasing energy for RNA synthesis
• Synthesis - Termination:

  • Special DNA sequences and protein helpers terminate transcription
  • The transcript is released from the DNA
  • The Primary Transcript is called the “pre-mRNA” and is processed to generate the mature mRNA
• Protein synthesis involves DNA serving as the master blueprint for protein synthesis

  • Genes are segments of DNA carrying instructions for a polypeptide chain
  • Triplets of nucleotide bases form the genetic library, with each triplet specifying coding for an amino acid
• Overview of Protein Synthesis:

  • DNA contains the information necessary to produce proteins
  • Transcription of one DNA strand results in mRNA, a complementary copy of the information needed to make a protein
  • The mRNA leaves the nucleus and goes to a ribosome
  • Amino acids are carried to the ribosome by tRNAs
  • Translation uses the information in mRNA to determine the number, kinds, and arrangement of amino acids in the polypeptide chain
• DNA contains the information necessary to produce proteins
• Transcription of one DNA strand results in mRNA, which is a complementary copy of the information in the DNA strand needed to make a protein
• The mRNA leaves the nucleus and goes to a ribosome
• Amino acids, the building blocks of proteins, are carried to the ribosome by tRNAs
• In the process of translation, the information contained in mRNA is used to determine the number, kinds, and arrangement of amino acids in the polypeptide chain
• Synthesis of mRNA, tRNA, and rRNA based on the nucleotide sequence in DNA
• Messenger RNA (mRNA) carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm
• Transfer RNAs (tRNAs) bound to amino acids base pair with the codons of mRNA at the ribosome to begin the process of protein synthesis
• Ribosomal RNA (rRNA) is a structural component of ribosomes
• An enzyme that oversees the synthesis of RNA is RNA polymerase
• RNA polymerase unwinds the DNA template and adds complementary ribonucleoside triphosphates on the DNA template
• RNA polymerase joins these RNA nucleotides together and encodes a termination signal to stop transcription
• Posttranscriptional processing modifies mRNA before it leaves the nucleus by removing introns and then splicing exons together with enzymes called spliceosomes
• Functional mRNA consists only of exons
• Alternative splicing produces different combinations of exons, allowing one gene to produce more than one type of protein
• Synthesis of proteins in response to the codons of mRNA
• Codon: a set of 3 nucleotides that codes for 1 amino acid during translation