Cell Division and Differentiation

Your body contains two main types of cells that divide in completely different ways. Somatic cells are everywhere in your body (except reproductive organs) and divide through mitosis to maintain exactly 46 chromosomes. Germline cells are your reproductive cells that use both mitosis and meiosis to create gametes with only 23 chromosomes each.

Cellular differentiation is how a single cell transforms into specialised cells like brain neurons or muscle fibres. This happens when cells switch on specific genes to produce the proteins they need for their particular job.

Stem cells are the superstars of regenerative medicine because they can become multiple cell types. Embryonic stem cells are pluripotent (can become any cell type), while tissue stem cells like those in your bone marrow are multipotent (limited to specific tissue types). Scientists are using stem cells to repair damaged corneas and regenerate burnt skin.

Quick Fact: Your bone marrow stem cells constantly produce red blood cells, platelets, and immune cells to keep your blood healthy.

Unfortunately, when cell division goes wrong, cancer develops. Primary tumours form when cells ignore regulatory signals and divide excessively, while secondary tumours spread when cancer cells break away and travel through your body.

DNA Structure and Replication

DNA might look complex, but it's basically a twisted ladder made of nucleotides - each containing a sugar, phosphate, and one of four bases. The two strands run in opposite directions $anti-parallel$ and are held together by hydrogen bonds between complementary bases.

DNA replication follows a precise three-step process. First, the double helix unwinds and hydrogen bonds break. Then primers attach to the 3' ends of both template strands. Finally, DNA polymerase adds new nucleotides, but there's a catch - the leading strand replicates continuously while the lagging strand forms in fragments that ligase must join together.

PCR (Polymerase Chain Reaction) is like a photocopier for DNA, amplifying tiny samples millions of times. It uses three temperature stages: heating to 92-98°C separates DNA strands, cooling to 60-65°C allows primers to bind, and heating to 70-80°C lets heat-resistant polymerase replicate the DNA.

Real-World Application: PCR helps solve crimes through DNA fingerprinting, settles paternity disputes, and diagnoses genetic disorders from tiny tissue samples.

Gene Expression and Protein Synthesis

Your DNA contains the instructions, but gene expression is how those instructions become actual proteins. Only a fraction of your genes are active at any time, and the process involves three types of RNA working together.

mRNA (messenger RNA) carries genetic information from your nucleus to ribosomes. tRNA (transfer RNA) brings specific amino acids to the ribosome, with each tRNA having an anticodon that matches specific codons on mRNA. rRNA (ribosomal RNA) forms the ribosome structure where protein synthesis happens.

Transcription occurs when RNA polymerase unwinds DNA and creates a primary mRNA transcript. The clever bit is RNA splicing - your cells remove non-coding sections (introns) and keep coding sections (exons) to create mature mRNA. Alternative RNA splicing lets one gene produce different proteins by keeping different combinations of exons.

Translation starts at a start codon and ends at a stop codon. As ribosomes read mRNA codons, tRNA molecules bring matching amino acids that join together with peptide bonds to form proteins.

Key Point: Your proteins fold into specific 3D shapes held together by hydrogen bonds - this shape determines exactly what job each protein can do.

Mutations and Genetic Changes

Mutations are changes in your DNA that can dramatically affect protein production. Single gene mutations involve substitution (replacing one base), insertion (adding a base), or deletion (removing a base) in the DNA sequence.

Substitution mutations can have three effects: missense (changing one amino acid), nonsense (creating a premature stop codon), or splice-site (affecting how exons and introns are processed). While substitutions change just one amino acid, insertion and deletion mutations cause frameshift mutations that alter every codon after the mutation point.

Chromosome structure mutations involve much larger changes that are often lethal. Duplication adds chromosome sections $causing Pallister-Killian syndrome$, deletion removes sections $causing Cri-du-Chat syndrome$, inversion reverses chromosome sections (linked to Haemophilia A), and translocation moves sections between non-homologous chromosomes.

Important: Frameshift mutations from insertions or deletions typically have more severe effects than substitution mutations because they affect multiple amino acids.

These mutations explain why genetic disorders occur and why understanding your genetic makeup is becoming increasingly important for personalised medicine.

Human Genomics and Personalised Medicine

Your genome contains your entire genetic blueprint - about 3 billion base pairs of DNA stored in 23 chromosome pairs. Surprisingly, only 4% actually codes for proteins, while the remaining 96% includes regulatory sequences that control gene expression.

Genomic sequencing determines the exact order of bases in DNA, enabling scientists to map entire genomes. Bioinformatics uses powerful computers to analyse and compare these massive datasets, identifying genes by finding similarities to known sequences.

Personalised medicine is revolutionising healthcare through pharmacogenetics - using your genetic information to choose the right drugs and doses for your unique metabolism. Doctors can now scan for disease markers like BRCA1 and BRCA2 genes that increase breast cancer risk, allowing families to make informed decisions about prevention and monitoring.

The Human Genome Project (1990-2004) mapped our entire genetic code and paved the way for modern genomic medicine. This international effort sequenced multiple organisms, from bacteria to humans, creating the foundation for today's genetic research.

Future Impact: Soon, doctors might scan your entire genome to predict disease risks and customise treatments specifically for your genetic makeup.

Risk prediction for conditions like diabetes, heart disease, and cancer is becoming more accurate as scientists identify more genetic markers associated with these conditions.

Metabolic Pathways and Enzyme Control

Metabolism encompasses thousands of biochemical reactions that keep your cells functioning. Metabolic pathways are carefully coordinated sequences of enzyme-controlled reactions that can be anabolic (building complex molecules using energy) or catabolic (breaking down molecules to release energy).

Enzymes control these pathways through induced fit - their active sites change shape to perfectly match substrates, lowering the activation energy needed for reactions. During reactions, substrates have high affinity for active sites while products have low affinity, allowing enzymes to be reused efficiently.

Enzyme activity depends on temperature, pH, substrate concentration, and inhibitors. Competitive inhibitors block active sites directly, non-competitive inhibitors change active site shape by binding elsewhere, and feedback inhibition occurs when end products inhibit earlier enzymes to prevent overproduction.

Pathways can be reversible or irreversible, with alternative routes providing flexibility. The presence of substrates or removal of products drives reactions in specific directions.

Control Mechanism: Feedback inhibition works like a thermostat - when you've made enough product, the pathway automatically slows down to prevent waste.

Understanding these pathways explains how your body maintains chemical balance and why enzyme deficiencies cause metabolic disorders.

Cellular Respiration and Energy Production

Cellular respiration extracts energy from glucose through three interconnected stages that occur in different cellular locations. This catabolic process systematically breaks down glucose to produce ATP - your cells' universal energy currency.

Glycolysis happens in the cytoplasm, where glucose is phosphorylated using ATP during the energy-investment phase, then broken down to pyruvate while producing 4 ATP molecules and reducing NAD to NADH. The net gain is 2 ATP molecules per glucose.

The citric acid cycle occurs in the mitochondrial matrix, where pyruvate is converted to acetyl-CoA and fed into enzyme-controlled cyclic reactions. This stage produces more NADH and releases CO₂ as waste.

The electron transport chain in the inner mitochondrial membrane is where most ATP is made. NADH donates electrons that provide energy to pump hydrogen ions across the membrane. These ions then flow back through ATP synthase, driving the phosphorylation of ADP to ATP. Oxygen acts as the final electron acceptor, forming water.

Energy Yield: One glucose molecule can produce up to 38 ATP molecules through complete aerobic respiration - that's why oxygen is essential for life.

This process explains why you breathe oxygen and produce CO₂, and why mitochondria are called the powerhouses of cells.