DNA: Storage, Replication, and Transmission

• In 1928, Frederick Griffith discovered that a chemical factor from heat-killed bacteria could alter the inherited traits of another strain, a process termed transformation.
• Griffith's experiment led him to conclude that the transforming factor was a gene, as the ability to cause disease was passed on to subsequent generations.
• In 1944, Oswald Avery identified DNA as the substance responsible for bacterial transformation, proving that it stores and transmits genetic information.
• Bacteriophages are viruses that infect bacteria by injecting their genetic material into the host cell.
• Alfred Hershey and Martha Chase used radioactive tracers in 1952 to confirm that DNA, not protein, is the genetic material of bacteriophages.
• Hershey and Chase's findings reinforced Avery's results, establishing DNA as the universal genetic material in all living cells.

Role of DNA

• Genes' DNA is capable of storing, copying, and transmitting genetic information within a cell.

Components of DNA

• DNA consists of nucleotides linked by covalent bonds, arranged in any sequence.
• A nucleotide comprises a nitrogenous base, a deoxyribose sugar, and a phosphate group.
• Four nitrogenous bases are found in DNA: adenine, guanine, cytosine, and thymine.

Solving the Structure of DNA

• Erwin Chargaff found that adenine and thymine, as well as guanine and cytosine, exist in almost equal percentages in DNA.
• Rosalind Franklin's X-ray diffraction images revealed DNA's double-helix structure.
• James Watson and Francis Crick developed a model that elucidated DNA's structure.

Double-Helix Model

• The double helix consists of two strands running in opposite directions, with nitrogenous bases in the middle.
• Each strand contains a nucleotide sequence, acting as a four-letter alphabet for genetic information.
• Hydrogen bonds between base pairs hold the two strands together but can be easily broken to allow separation.
• Base pairing occurs specifically between adenine and thymine, and guanine and cytosine.
• Nucleic acids contain hydrogen, oxygen, nitrogen, carbon, and phosphorus.
• Nucleic acids are polymers of nucleotides.
• There are two kinds of nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
• A nucleotide has three parts: a 5-carbon sugar, a phosphate (–PO4) group, and a nitrogenous base.
• Nucleic acids store and transmit hereditary (genetic) information.

Copying the Code

• Since each strand can be used to make the other strand, the strands are said to be complementary.
• Each strand of DNA contains the instructions to reconstruct the other through base pairing.
• DNA replication involves unzipping the double helix at replication forks and adding new bases according to base pairing rules (A with T, G with C).
• Each new molecule consists of an original strand and a newly synthesized strand.
• Helicase unwinds the DNA molecule by breaking hydrogen bonds.
• Single-stranded binding proteins prevent the separated DNA strands from re-annealing.
• Primase lays down RNA primer, creating either leading or lagging strands (Okazaki Fragments).
• DNA polymerase joins nucleotides to produce a new DNA strand in the 5’ to 3’ direction only.
• Ligase joins fragmented DNA segments together.

Replication in Living Cells

• Prokaryotes have a single, circular DNA molecule in the cytoplasm, while eukaryotes contain much more DNA organized into chromosomes within the nucleus.
• Replication in prokaryotes initiates from a single point and proceeds bidirectionally until the entire chromosome is copied.
• In eukaryotes, replication starts at multiple points on the DNA molecule, proceeding bidirectionally until each chromosome is fully replicated.

Role of RNA

• RNA, or ribonucleic acid, is a nucleic acid composed of a long chain of nucleotides.
• The sequence of bases in RNA determines the protein production.
• Cell proteins ultimately determine phenotypic traits.
• RNA contains ribose sugar instead of deoxyribose.
• RNA is single-stranded, unlike the double-stranded DNA.
• RNA contains uracil (U) instead of thymine (T).
• RNA serves as a disposable copy of a DNA segment and is primarily involved in protein synthesis.
• Messenger RNA (mRNA) carries instructions for polypeptide synthesis from the nucleus to ribosomes.
• Ribosomal RNA (rRNA) forms part of the ribosome structure, facilitating protein assembly.
• Transfer RNA (tRNA) transports amino acids to the ribosome, matching them to the mRNA code.

RNA Synthesis

• Transcription is the mechanism of RNA synthesis, where DNA segments serve as templates for complementary RNA molecules.
• In prokaryotes, RNA and protein synthesis occur in the cytoplasm.
• In eukaryotes, RNA production occurs in the nucleus before moving to the cytoplasm for protein synthesis.
• RNA polymerase binds to DNA during transcription, separates the DNA strands, and uses one strand as a template to assemble a complementary RNA strand.
• RNA polymerase binds to promoters, DNA regions with specific base sequences that signal where to start RNA synthesis.
• Transcription stops when specific signals indicate the completion of the new RNA molecule.
• RNA is often edited before use, with introns removed and exons spliced together to form the final mRNA.

The Genetic Code

• A specific base sequence in DNA dictates the formation of a polypeptide, which is a chain of amino acids.
• The sequence and type of amino acids determine the properties of the protein.
• The mRNA base sequence forms the genetic code.
• The four bases (A, C, G, and U) act as "letters."
• The code is read in three-letter words called codons, each corresponding to a single amino acid.
• Some codons indicate the start and stop signals for protein synthesis.

Translation

• Ribosomes use mRNA codons to assemble amino acids into polypeptide chains, a process called translation.
• mRNA is transcribed in the nucleus and then enters the cytoplasm.
• Translation begins at the start codon on the ribosome. An anticodon, a complementary base sequence on tRNA, is attracted to each codon.
• Each tRNA carries a specific amino acid; the codon-anticodon match ensures the correct amino acid is added.
• Amino acids bond together as the ribosome moves along the mRNA, exposing new codons.
• Upon reaching a stop codon, the newly formed polypeptide and mRNA are released from the ribosome.