Molecular Biology: DNA Replication

• Molecular biology studies the molecular basis of biological activity.
• It is related to genetics and biochemistry.
• Molecular biology focuses on the interactions between a cell's systems including DNA, RNA, and protein biosynthesis, and how these interactions are regulated.

DNA Replication

• DNA replication copies a DNA molecule to produce two identical DNA molecules.
• The process is essential for cell division during growth and tissue repair.
• DNA replication requires many enzymes and proteins.
• Helicase unwinds the DNA double helix at the start.
• Single-strand binding proteins prevent separated DNA strands from re-annealing.
• DNA polymerase adds nucleotides to the 3' end of the template strand, using each strand as a template.
• The leading strand is synthesized continuously in the 5' to 3' direction.
• The lagging strand is synthesized discontinuously, creating Okazaki fragments.
• DNA ligase joins Okazaki fragments.
• DNA replication is semi-conservative, resulting in each new DNA molecule having one original and one new strand.
• It's highly accurate, with an error rate of about one in a billion base pairs.
• DNA polymerase proofreading removes incorrectly incorporated nucleotides, ensuring high accuracy.

Protein Synthesis

• Protein synthesis, or translation, is how cells create proteins.
• It decodes mRNA to assemble amino acids into a polypeptide chain.
• Protein synthesis occurs on ribosomes, which consist of rRNA and proteins.
• The three main stages are initiation, elongation, and termination.
• Initiation starts when the ribosome binds to mRNA and finds the start codon (usually AUG).
• A special initiator tRNA carrying methionine binds to the start codon.
• Elongation involves sequentially adding amino acids to the growing polypeptide chain.
• tRNA recognizes each mRNA codon and brings the corresponding amino acid.
• The ribosome moves along the mRNA, and tRNA delivers amino acids to the ribosome.
• Amino acids are added to the polypeptide chain, and tRNA is released.
• Termination happens when the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA.
• Release factors bind to the ribosome because there are no tRNAs for stop codons.
• The polypeptide chain is released, and the ribosome dissociates.
• Proteins may undergo post-translational modifications like folding, glycosylation, or phosphorylation, affecting their function.

Gene Expression

• Gene expression synthesizes a functional gene product, usually a protein, from the information in a gene.
• It is tightly regulated, allowing cells to control protein production in terms of type, timing, and quantity.
• The two main stages are transcription and translation.
• Transcription copies the DNA sequence of a gene into RNA.
• RNA polymerase catalyzes this process, synthesizing a complementary RNA molecule from a DNA template.
• In eukaryotes, pre-mRNA is processed into mature mRNA through capping, splicing, and polyadenylation.
• Translation uses the information in mRNA to synthesize a protein.
• It takes place on ribosomes, using tRNA to bring amino acids in the correct order.
• Regulation occurs at multiple levels: transcription, RNA processing, translation, and post-translational modification.
• Transcriptional regulation is a major control mechanism, where transcription factors bind to specific DNA sequences.
• Binding of transcription factors can either activate or repress transcription.
• Other control mechanisms include RNA interference (RNAi) and epigenetic modifications.

Molecular Genetics

• Molecular genetics studies gene structure and function at the molecular level.
• Techniques of molecular biology are used to study gene inheritance, variation, and expression.
• Molecular genetics has led to a greater understanding of heredity and has facilitated advances in medicine and biotechnology.
• Key concepts: gene structure and organization, DNA replication, mutation, and gene expression.
• Mutations are changes in the DNA sequence that can result in altered gene products.
• Mutations can be spontaneous or induced by exposure to mutagens.
• Mechanisms of genetic recombination are studied, which facilitate gene exchange between chromosomes.
• Genetic recombination is important for generating genetic diversity and for repairing damaged DNA.
• Molecular genetics has been key in developing gene therapy for treating or preventing disease.

Biotechnology Applications

• Biotechnology uses living systems and organisms to develop or make products.
• Modern biotechnology is based largely on molecular biology and genetic engineering techniques.
• Biotechnology has diverse applications across medicine, agriculture, industry, and environmental science.
• In medicine, biotechnology develops new drugs, diagnostic tests, and therapies.
• Examples include insulin production for diabetes, vaccine development, and gene therapy for genetic disorders.
• In agriculture, biotechnology improves crop yields, enhances nutritional content, and develops pest-resistant plants.
• Genetically modified (GM) crops are widely used.
• There are concerns about GM crops' potential impacts on the environment and human health.
• In industry, biotechnology produces enzymes, biofuels, and bioplastics.
• Bioremediation uses microorganisms to clean up pollutants.
• Biotechnology is used in forensic science with DNA fingerprinting to identify individuals and solve crimes.
• Ethical issues are an important consideration, particularly in genetic engineering and stem cell research.