2.1 DNA structure, replication, and gene expression

DNA is the blueprint of life, encoding genetic instructions for all living organisms. Its double helix structure, composed of nucleotides and held together by hydrogen bonds, forms the basis for genetic inheritance and cellular function.

Gene expression transforms DNA's information into functional proteins through transcription and translation. This process, guided by the central dogma of molecular biology, allows cells to respond to their environment and carry out essential biological processes.

DNA Structure and Function

Structure and components of DNA

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• Double helix structure consists of two antiparallel strands coiled in right-handed fashion
• Nucleotides comprise deoxyribose sugar, phosphate group, and nitrogenous bases (purines: Adenine, Guanine; pyrimidines: Thymine, Cytosine)
• Base pairing forms A-T and G-C complementary pairs through hydrogen bonding
• Backbone composed of sugar-phosphate bonds runs 5' to 3' directionality
• Major and minor grooves form along helix
• DNA helix measures approximately 2 nm in diameter

DNA replication process

1. Semi-conservative replication model ensures genetic continuity
2. Initiation begins at origin of replication (ori) with helicase unwinding DNA strands
3. Elongation involves leading strand synthesis and lagging strand synthesis (Okazaki fragments)
4. DNA polymerase III catalyzes nucleotide addition while primase generates RNA primers
5. Termination occurs at telomeres in eukaryotes
6. Process enables growth, repair, and inheritance of traits

Gene Expression

Central dogma of molecular biology

• Genetic information flows from DNA to RNA to proteins
• Transcription converts DNA to RNA
• Translation converts RNA to protein
• Reverse transcription occurs in retroviruses (HIV)
• RNA replication happens in some viruses (influenza)
• Exceptions include prions (misfolded proteins) and RNA editing (ADAR enzymes)

Transcription vs translation

• Transcription occurs in nucleus (eukaryotes) or nucleoid (prokaryotes) while translation happens in cytoplasm
• RNA polymerase synthesizes RNA from DNA template during transcription
• Ribosomes, tRNA, and amino acids facilitate protein synthesis from mRNA during translation
• Transcription involves promoter regions and transcription factors
• RNA processing in eukaryotes includes 5' capping, 3' polyadenylation, and splicing
• Translation proceeds through initiation, elongation, and termination phases
• Post-translational modifications alter protein structure and function (glycosylation, phosphorylation)