DNA

• DNA replication is a semi-conservative process:

  • One strand is from the original template molecule
  • One strand is newly synthesized
• Each nitrogenous base in DNA can only pair with its complementary partner:

  • Adenine (A) pairs with thymine (T)
  • Cytosine (C) pairs with guanine (G)
• The Meselson-Stahl experiment in 1958 confirmed the theory of semi-conservative DNA replication:

  • They used radioactive isotopes of nitrogen to test three proposed models of DNA replication: Conservative, Semi-Conservative, and Dispersive
  • Results after two divisions supported the semi-conservative model, as some DNA molecules consisted solely of the lighter 14N isotope
• Nucleic acids are composed of nucleotide monomers which are linked into a single strand via condensation reactions
• The phosphate group of one nucleotide attaches to the sugar of another nucleotide at the 3’– hydroxyl (-OH) group, forming a phosphodiester bond between the two nucleotides
• Successive condensation reactions result in the formation of long polynucleotide strands
• Two polynucleotide chains of DNA are held together via hydrogen bonding between complementary nitrogenous bases:

  • Adenine (A) pairs with Thymine (T) via two hydrogen bonds
  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds
• In DNA, the nitrogenous base is attached to the 1’– carbon atom, while the phosphate group is attached to the 5’– carbon atom
• The two strands of DNA are described as being antiparallel, meaning they run in opposite directions
• The double-stranded DNA forms a double helix structure with approximately 10-15 bases per twist
• The structural organization of the DNA molecule as a double helix was proposed by James Watson and Francis Crick in 1953
• DNA replication is a semi-conservative process where pre-existing strands act as templates for newly synthesized strands
• The process of DNA replication is coordinated by two key enzymes: helicase and DNA polymerase
• The polymerase chain reaction (PCR) is an artificial method of replicating DNA under laboratory conditions, amplifying large quantities of a specific DNA sequence
• Transcription is the process by which an RNA sequence is produced from a DNA template, occurring in the nucleus before the RNA moves to the cytoplasm for translation
• Translation is the process of protein synthesis where the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain
• The genetic code is universal, with the same codons coding for the same amino acids in almost every living organism
• Genes can be transferred between species due to the universality of the genetic code, allowing for the production of substances like human insulin in bacteria
• mRNA is a complementary copy of a DNA segment and can be used to deduce the gene sequence by applying the rules of complementary base pairing
• To translate an mRNA sequence into a polypeptide chain, it is crucial to establish the correct reading frame, typically starting with the AUG start codon
• An open reading frame starts with AUG and continues in triplets to a termination codon, allowing for the deduction of the corresponding amino acid sequence using the genetic code
• A blocked reading frame may be frequently interrupted by termination codons, affecting the translation process
• The mRNA sequence is read by the ribosome in triplets of bases called codons, where each codon codes for one amino acid in a polypeptide chain
• The order of the codons in an mRNA sequence determines the order of amino acids in a polypeptide chain, following the rules of the genetic code
• The genetic code identifies the corresponding amino acid for each codon combination, with 64 codon possibilities due to the three bases per codon
• Tables displaying the genetic code may occasionally show the sequence on the sense strand of DNA, which is identical to the mRNA codons except for thymine (T) being present instead of uracil (U)
• Ribosomes catalyze the formation of peptide bonds between adjacent amino acids during translation, moving along the mRNA molecule to synthesize a polypeptide chain until a stop codon is reached
• The final construction of a correct DNA molecule was heavily influenced by the X-ray crystallography data generated by Rosalind Franklin, confirming the arrangement of DNA strands into a helical structure
• Franklin is now recognized as a key contributor to the elucidation of DNA structure
• Early models of DNA structure had bases on the outside and sugar-phosphate residues in the center, with nitrogenous bases not initially configured correctly to demonstrate complementarity
• Watson and Crick's model building for DNA structure was based on trial and error, with early models showing faults like a triple helix structure and incorrect base configurations
• DNA and RNA are both polymers of nucleotides, differing in the number of strands present, composition of nitrogenous bases, and type of pentose sugar
• DNA replication is a semi-conservative process where pre-existing strands act as templates for newly synthesized strands, coordinated by helicase and DNA polymerase enzymes
• The polymerase chain reaction (PCR) is an artificial method of replicating DNA under laboratory conditions, amplifying large quantities of a specific DNA sequence through denaturation, annealing, and elongation steps