Molecular Biology Lecture 1

Molecular Biology Lecture 1

• Topic: Nucleic Acids, the Genetic Code, and Synthesis of Macromolecules
• Central Dogma: DNA → RNA → Protein
• Transcription: DNA to RNA
• Translation: RNA to protein
• RNA Types: mRNA, rRNA, tRNA

Structure of Nucleic Acids

• Nucleic Acids: Formed by polymerization of nucleotides

• Nucleotides: Composed of a phosphate group, a sugar (ribose or deoxyribose), and a nitrogenous base

• Sugars: Ribose is in RNA, deoxyribose in DNA

• Bases: Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine, Uracil)

• Nucleosides: Base + Sugar

• Nucleotides: Nucleoside + Phosphate

• Phosphodiester Bonds: Link nucleotides in a nucleic acid

• 5' End: Phosphate group attached to the 5' carbon

• 3' End: Hydroxyl group attached to the 3' carbon

• Directionality: Nucleic acids are synthesized in the 5' to 3' direction

• Nucleotides of DNA: Adenine, guanine, cytosine, thymine.

• nucleotides of RNA: adenine, guanine, cytosine, uracil

Native DNA

• Double Helix: Complementary antiparallel chains
• Base Pairing: G-C, A-T (in DNA), A-U (in RNA)
• Antiparallel Chains: Run in opposite directions (5' to 3' and 3' to 5')
• DNA Forms: B-form (right-handed helix), A-form (right-handed helix, dehydrating), Z-form (left-handed helix)
• Grooves: Major and minor grooves on the outside of the DNA helix

DNA Strand Separation

• Denaturation: DNA unwinding to single strands by high temperature or chemicals (formamide, urea)
• Renaturation: Double-stranded DNA reformed from single strands
• Melting Temperature (Tm): Temperature at which half of the DNA is single-stranded
• G-C Content: Higher G-C content leads to higher Tm (stronger base pairs)

Circular DNA

• Some DNA molecules are circular, i.e. closed loops

RNA Structure and Function

• RNA structure: Varies – secondary structures (stem-loops, hairpins), tertiary

Protein Synthesis

• Proteins: chains of amino acids
• Synthesis of protein: Starts from amino(NH2) and ends at carboxyl (COOH) terminus in proteins and starts at 5' and ends at 3' in nucleic acids
• Modification: Proteins and nucleic acids are often modified after synthesis

Nucleic Acid Synthesis

• Copying: DNA and RNA are made by copying template strands
• Direction: The strands grow in the 5' to 3' direction
• Initiation: RNA polymerase can start strand growth
• DNA polymerase cannot start RNA growth

Organization of Genes

• Prokaryotes: Genes arranged in operons
• Eukaryotes: Genes can be interrupted (exons and introns). mRNA is processed (Introns removed).

RNA in Protein Synthesis

• Messenger RNA (mRNA): Carries genetic code for protein synthesis.
• Transfer RNA (tRNA): Transports amino acids to ribosomes (cloverleaf shape)
• Ribosomes: Protein synthesis takes place on ribosomes (RNA + proteins)
• Genetic Code: 3-letter code (codons) for amino acids. Degenerate, meaning multiple codons can code for the same amino acids
• Aminoacyl-tRNA Synthetases: Enzymes that attach the correct amino acid to the tRNA molecule
• Reading frame: Sequence of codons that specifies the protein sequence.