Cell Division and Stem Cells

Every cell in your body falls into one of two categories: somatic cells (all your regular body cells) or germline cells (reproductive cells and their precursors). Somatic cells divide through mitosis, creating two identical diploid cells for growth and repair. Germline cells use both mitosis and meiosis, with meiosis producing four genetically different haploid gametes.

Differentiation is how cells become specialised - they switch on specific genes to develop unique functions. Think of it like choosing a career path; once a cell differentiates, it's committed to that role. Pluripotent stem cells (like embryonic stem cells) can become any cell type, whilst multipotent stem cells (tissue stem cells) are more limited but crucial for repair and renewal.

Stem cells offer exciting therapeutic possibilities for treating damaged organs and provide valuable research models for studying diseases. However, when cells lose control and ignore regulatory signals, they divide excessively to form tumours - and if these spread, they become cancerous.

Key Insight: Understanding cell division helps explain both how we grow and heal, and what goes wrong in cancer.

DNA Structure and Replication

DNA consists of two anti-parallel strands held together by hydrogen bonds between complementary base pairs $A-T and G-C$. When cells need to replicate, DNA polymerase unwinds the double helix and builds new strands using free nucleotides. The enzyme needs primers (short DNA sequences) to start working and can only add nucleotides to the 3' end.

Replication happens differently on each strand - one continuous, the other in fragments that DNA ligase joins together. This process is essential for cell division and passing genetic information to offspring.

PCR (Polymerase Chain Reaction) uses repeated heating and cooling cycles to amplify specific DNA regions. It's revolutionised forensics, paternity testing, and genetic diagnosis by allowing scientists to make millions of copies from tiny DNA samples.

Real-World Application: PCR technology is what makes COVID-19 testing possible - amplifying viral DNA to detectable levels.

Gene Expression

Only a fraction of your genes are active at any time - gene expression determines which proteins get made when and where. Transcription occurs in the nucleus, where RNA polymerase creates an mRNA copy of the gene. This primary transcript gets processed through RNA splicing, removing non-coding regions (introns) and joining coding regions (exons).

Translation happens at ribosomes, where tRNA molecules carry specific amino acids to match their anticodons with mRNA codons. Each triplet of bases codes for one amino acid, building polypeptide chains that fold into functional proteins.

Alternative RNA splicing allows one gene to produce multiple protein variants by including different combinations of exons. This dramatically increases protein diversity without expanding genome size. Your phenotype - all your observable characteristics - results from the proteins produced through gene expression.

Mind-Blowing Fact: Alternative splicing means humans can produce over 100,000 different proteins from just ~20,000 genes.

Mutations

Mutations are DNA changes that can alter protein production. Single gene mutations involve substitutions, insertions, or deletions of nucleotides. Insertions and deletions cause frameshift mutations, changing all amino acids downstream. Substitutions affect single codons and can be missense (different amino acid), nonsense (premature stop), or splice-site (affecting intron-exon boundaries).

Chromosomal structure mutations involve larger DNA segments and include duplications, deletions, inversions, and translocations. These substantial changes are often lethal because they disrupt multiple genes simultaneously.

Understanding mutations is crucial for genetic counselling and developing treatments for genetic disorders. Many mutations are harmless, but some can cause serious diseases or, occasionally, provide evolutionary advantages.

Important Note: Most mutations are neutral or harmful, but they're also the raw material for evolution and genetic diversity.

Human Genomics

Your genome contains all hereditary information encoded in DNA - both protein-coding genes and regulatory sequences. Genomic sequencing determines the exact order of nucleotide bases, whilst bioinformatics uses computer programs to analyse and compare sequence data.

Personalised medicine uses individual genome sequences to predict disease risk and select optimal treatments. Pharmacogenetics applies this knowledge to drug selection and dosing, ensuring medications work effectively whilst minimising side effects.

This field is transforming healthcare by moving from one-size-fits-all treatments to personalised approaches based on genetic profiles. It's particularly valuable for cancer treatment, where tumour genetics guide therapy choices.

Future Impact: Genomic medicine will make treatments more effective and reduce adverse drug reactions.

Metabolic Pathways

Metabolic pathways are controlled sequences of enzyme reactions that keep cells functioning. They include anabolic reactions (building larger molecules using energy) and catabolic reactions (breaking down molecules and releasing energy). Pathways have reversible steps, irreversible steps, and alternative routes.

Enzyme activity follows the induced fit model - active sites adjust to better accommodate substrates. Substrates have high affinity for active sites, whilst products have low affinity, ensuring reactions proceed forward. Competitive inhibitors block active sites, whilst non-competitive inhibitors change enzyme shape.

Feedback inhibition provides pathway control - when end products accumulate, they inhibit earlier enzymes, preventing overproduction. This elegant system maintains cellular balance without waste.

Clever Design: Feedback inhibition is like a thermostat - it automatically maintains optimal levels without external control.

Cellular Respiration

Cellular respiration extracts energy from glucose through three stages. Glycolysis breaks glucose into pyruvate in the cytoplasm, requiring initial ATP investment but producing a net ATP gain. Under aerobic conditions, pyruvate enters mitochondria and forms acetyl coenzyme A.

The citric acid cycle occurs in the mitochondrial matrix, where acetyl groups combine with oxaloacetate to form citrate. Through enzyme-controlled reactions, citrate regenerates oxaloacetate whilst producing ATP and CO₂. Dehydrogenase enzymes transfer hydrogen ions and electrons to NAD, forming NADH.

The electron transport chain on the inner mitochondrial membrane produces most ATP. Energy from electron transfer pumps hydrogen ions across the membrane, and their return through ATP synthase generates ATP. Oxygen serves as the final electron acceptor, combining with hydrogen to form water.

Energy Fact: The electron transport chain produces about 28-30 ATP molecules per glucose - that's roughly 85% of cellular respiration's total yield.

Energy Systems in Muscle Cells

During vigorous exercise, oxygen supply can't meet demand, so pyruvate converts to lactate instead of entering aerobic respiration. This fermentation regenerates NAD needed for continued glycolysis and ATP production, but lactate accumulation causes muscle fatigue.

Slow-twitch muscle fibres excel at endurance activities, using aerobic respiration with abundant mitochondria, blood supply, and myoglobin (oxygen-storing protein). They contract slowly but sustain activity for long periods using fats as fuel.

Fast-twitch fibres generate powerful, rapid contractions for short bursts. They rely primarily on glycolysis, have fewer mitochondria, and use stored glycogen. They're perfect for sprinting but fatigue quickly.

Training Tip: Understanding fibre types explains why some athletes excel at marathons whilst others dominate sprints - it's partly genetic!