Molecular Biology Lecture 1

Molecular Biology Lecture 1

• Lecture delivered by Prof. Sami Mohamed
• Course: Molecular Biology, Level 2
• University: Badr University in Cairo, School of Biotechnology
• Lecture 1: Nucleic Acids, the Genetic Code, and the Synthesis of Macromolecules

Central Dogma

• DNA undergoes transcription to RNA
• RNA undergoes translation to protein
• mRNA, rRNA, and tRNA are types of RNA

Structure of Nucleic Acids

• Polymerization of nucleotides forms nucleic acids

• Nucleotides consist of a phosphate, a ribose or deoxyribose sugar, and a nitrogenous base

• Ribose is present in RNA

• Deoxyribose is present in DNA

• Purines include adenine (A) and guanine (G)

• Pyrimidines include cytosine (C), uracil (U), and thymine (T)

• Nucleosides are formed by linking a base to a sugar

• Nucleotides are formed by linking a nucleoside to a phosphate group

• One, two, or three phosphate groups are attached to the 5' carbon of the sugar

• Phosphodiester bonds link nucleotides together

• The 5' end of a nucleic acid strand has a free phosphate group

• The 3' end has a free hydroxyl group

• The nucleic acid strand has an end-to-end chemical orientation

Native DNA

• Double helix of complementary antiparallel chains
• Bases are regularly spaced (0.34 nm apart)
• Helix makes a complete turn every 3.4 nm (approximately 10 base pairs per turn)
• Major and minor grooves on the outside of the helix
• Variants of DNA: A-form and Z-form
• A-form: dehydrated, tilted stacked bases
• Z-form: short DNA, alternating purine-pyrimidine nucleotides, bases zigzag

DNA Strand Separation

• DNA can undergo reversible strand separation (denaturation or melting)
• Factors include high temperature, alkaline solutions, and concentrated solutions of formamide or urea
• Denaturation separates strands, renaturation reforms double helix
• Melting temperature (Tm) depends on G-C content

Circular DNA Molecules

• Many DNA molecules are circular
• Two polynucleotide strands can form either a right-handed or a left-handed helix

RNA Molecules

• RNA molecules exhibit varied conformations and functions
• Secondary structure: double-helical stem regions, hairpin, and stem-loop
• Tertiary structure: Folding, pseudoknot

Synthesis of Biopolymers

• Proteins and nucleic acids are made up of a limited number of different monomeric building blocks
• Monomers are added one at a time
• Synthesis starts at a specific “start” point and proceeds to a fixed terminus
• Monomer addition: in proteins from the amino (NH2-) to the carboxyl (COOH-) terminus
• Monomer addition: in nucleic acids from the 5' end to the 3' end

Modifications

• Primary product is often modified: cleavage, ligation, splicing, cross-linking
• Proteins and Nucleic Acid Synthesis is often modified

Nucleic Acid Synthesis

• Both DNA and RNA chains are produced by copying of template DNA strands
• Nucleic Acid strands grow in the 5' to 3' direction

RNA Polymerases

• RNA polymerases can initiate strand growth, but DNA polymerases cannot
• Binding of RNA polymerase
• Separation of DNA
• Base pairing
• Binding of second nucleoside triphosphate
• Binding and addition of third nucleoside triphosphate
• Continuation of process along template

Organization of Genes

• Prokaryotes: operons—transcription to form new RNA strands (e.g., trp)
• Eukaryotes: many trp operons
• Eukaryotic primary RNA transcripts are processed
• Exons consist of coding segments
• Introns consist of non-coding segments
• Splicing joins exons together

Translation

• Whole process by which mRNA base sequence is used to join amino acids
• mRNA carries information from DNA
• Three-letter genetic code (triplet code)
• Genetic code is degenerate (redundant, different codons may code for the same amino acid) and universal
• Sequence of codons (reading frame) runs from start to termination

tRNA

• tRNA has a folded structure (cloverleaf)
• Three-dimensional structure: L form
• Anticodon loop (complementary to mRNA codon)
• Acceptor stem (binds to amino acid)
• Nonstandard base pairing between codons (and anticodons) is common
• 30-40 different tRNAs in bacterial cells
• 61 different tRNAs in animal and plant cells
• Several amino acids have multiple tRNAs

Aminoacyl-tRNA Synthetases

• Enzymes link amino acids to specific tRNAs
• Synthetase complexed with aminoacyl-AMP
• One synthetase per amino acid
• Net result: Amino acid selected by codon

Ribosomes

• Protein-synthesizing machines
• Constructed from rRNA and proteins
• Prokaryotic and eukaryotic ribosomes differ in structure
• Both have similar functional components in stem loops

Explore the fundamentals of nucleic acids, their structure, and the central dogma of molecular biology in this quiz based on the first lecture delivered by Prof. Sami Mohamed. This quiz is designed for Level 2 Molecular Biology students, covering essential concepts like DNA, RNA, and macromolecule synthesis.