Comprehensive Higher Human Biology Study Guide

Cell Division and Differentiation

Your body contains two main types of cells that behave very differently. Somatic cells make up most of your body (skin, muscle, organs) and divide through mitosis to create identical copies - any mutations here only affect you personally.

Germline cells are special because they eventually become gametes (sperm and eggs) through meiosis. This process creates cells with half the normal chromosomes, and mutations here can be passed to your children.

Cell differentiation is how identical cells become specialised - a muscle cell, brain cell, or liver cell all started from the same embryonic cell. They develop different functions by switching on specific genes to make the proteins they need for their particular job.

Key Point: Embryonic stem cells are pluripotent, meaning they can become any cell type because all their genes are still active - this is why they're so valuable for medical research.

Cancer Cells and Tumours

Cancer happens when cells ignore the normal "stop dividing" signals and multiply out of control. These rogue cells form a tumour - essentially a mass of abnormal cells that shouldn't be there.

Not all tumours are equally dangerous though. Benign tumours stay put and don't spread, making them much easier to treat. Malignant tumours are the scary ones because their cells can metastasise - break away and travel through your body to form secondary tumours elsewhere.

The key difference is that malignant cancer cells lose their ability to stick together properly, allowing them to invade healthy tissue and spread through your bloodstream or lymphatic system.

Remember: The ability to metastasise is what makes cancer so dangerous - it's not just one localised problem anymore.

DNA Structure and Replication

DNA is like a twisted ladder (double helix) where the rungs are made of paired bases - A pairs with T, and C pairs with G - held together by hydrogen bonds. The two sides run in opposite directions (antiparallel), which is crucial for how it copies itself.

DNA replication needs several key players: a template strand to copy from, a primer to get started, DNA polymerase to add new nucleotides, and ligase to join fragments together. The process happens in a specific order.

First, the DNA unwinds and separates. Then primers attach to give polymerase something to grip onto. New nucleotides get added in the 3' to 5' direction, but because the strands are antiparallel, one side (the leading strand) can be copied continuously while the other (the lagging strand) has to be made in fragments.

Study Tip: Remember that DNA polymerase can only add nucleotides to the 3' end - this limitation creates the leading/lagging strand difference.

Polymerase Chain Reaction (PCR)

PCR is like a molecular photocopier that can make millions of copies of a specific DNA sequence in just a few hours. You need four things: the DNA template you want to copy, free nucleotides as building blocks, primers to mark where copying starts, and heat-tolerant DNA polymerase that won't break down at high temperatures.

The process uses three temperature stages in a cycle. At 92-98°C, the DNA separates into single strands. At 50-65°C, the primers attach to their target sequences. At 70-80°C, the heat-tolerant polymerase adds nucleotides to build new DNA strands.

Each cycle doubles the amount of target DNA, so after 30 cycles you've got over a billion copies. This technique is essential for everything from forensics to medical diagnosis to paternity testing.

Real-world Application: PCR is used in COVID-19 testing to amplify viral RNA so there's enough to detect.

Gene Expression - From DNA to RNA

Gene expression is the process of turning genetic information into working proteins. It starts with transcription, where RNA polymerase reads DNA and creates mRNA - think of it as making a working copy of the instruction manual.

RNA is similar to DNA but with key differences: it uses ribose sugar instead of deoxyribose, has uracil (U) instead of thymine (T), and is single-stranded. There are three main types you need to know.

mRNA carries the genetic message from nucleus to ribosome. rRNA combines with proteins to build ribosomes. tRNA is folded into a specific shape and carries amino acids to the ribosome - each tRNA has an anticodon that matches with a specific codon on the mRNA.

During transcription, RNA polymerase attaches to the promoter region, unwinds the DNA, and builds a complementary RNA strand using the exposed bases.

Memory Trick: Think of mRNA as a messenger carrying instructions, tRNA as a delivery truck bringing materials, and rRNA as part of the factory machinery.

RNA Processing and Translation

Before mRNA can be used, it needs editing through RNA splicing. The initial transcript contains introns (non-coding regions that interrupt the message) and exons (the actual coding regions that get expressed). The introns get removed and exons joined together to create a mature transcript ready for protein-making.

Translation happens at ribosomes and converts the mRNA code into proteins. Each set of three bases (a codon) specifies one amino acid. The process starts when a ribosome binds to the 5' end of mRNA at the START codon.

tRNA molecules bring specific amino acids to match their complementary codons. Peptide bonds form between amino acids, creating a growing chain. Used tRNA molecules leave to collect more amino acids, and the process continues until a STOP codon is reached.

The final steps involve folding the amino acid chain (polypeptide) into its functional 3D shape to create the finished protein.

Key Concept: Remember that three bases = one codon = one amino acid in the final protein.

Mutations and Their Effects

Mutations are changes in DNA that can alter proteins and affect how organisms function. They're caused by mutagenic agents and come in different types with varying consequences.

Single gene mutations affect individual nucleotides. Missense mutations swap one amino acid for another (might change protein function), while nonsense mutations create premature stop codons (usually breaking the protein completely). Splice site mutations mess up the intron removal process.

Frameshift mutations are particularly disruptive - deletions remove nucleotides and insertions add them, shifting the entire reading frame and scrambling the protein code from that point onwards.

Chromosome mutations affect larger DNA sections: duplications add extra chromosome pieces, deletions remove sections, inversions reverse chromosome segments, and translocations move pieces between different chromosomes.

Important: Frameshift mutations usually have more severe effects than point mutations because they alter every codon downstream from the mutation site.

Genomics and Its Applications

Modern genomics relies heavily on bioinformatics - using powerful computers to analyse the massive amounts of genetic data we can now generate. Genome sequencers can read entire genetic codes in just hours, a process that used to take years.

This technological revolution has practical applications that directly affect healthcare. Pharmacogenetics uses your genetic information to choose the most effective drugs and doses for your specific genetic makeup, reducing side effects and improving treatment outcomes.

Understanding genomics is becoming essential for personalised medicine, where treatments are tailored to individual genetic profiles rather than using one-size-fits-all approaches.

Future Focus: Pharmacogenetics could revolutionise medicine by ensuring you get the right drug at the right dose based on your genetic profile.

Metabolic Pathways and Enzyme Control

Cell metabolism encompasses all the biochemical reactions keeping you alive, with enzymes acting as biological catalysts to speed things up. These protein machines have active sites that bind specifically to their substrates using the induced fit model - the active site changes shape slightly to accommodate the substrate perfectly.

Enzymes can work inside cells (intracellular) or outside them (extracellular). They lower activation energy to help reactions occur faster while remaining unchanged themselves, meaning they can be reused repeatedly.

Metabolic pathways are controlled in three main ways: making most steps reversible so the pathway can respond to changing needs, producing enzymes only when the pathway is actually required, and modifying enzyme activity through feedback inhibition.

Feedback inhibition is particularly clever - when the end product reaches a critical concentration, it inhibits an earlier enzyme in the pathway, preventing overproduction and maintaining balance.

Think of it: Feedback inhibition is like a thermostat - when the temperature reaches the set point, it switches off the heating to prevent overheating.

Enzyme Inhibition

Enzyme activity can be controlled through different types of inhibitors that work in distinct ways. Competitive inhibitors have similar structures to the normal substrate, so they compete for the same active site - imagine two keys trying to fit the same lock.

Non-competitive inhibitors work differently by binding to a separate site (allosteric site) away from the active site. This binding changes the enzyme's shape, altering the active site so the normal substrate can no longer bind properly.

The concentration effects differ between inhibitor types. With competitive inhibition, increasing substrate concentration can overcome the inhibition because substrates and inhibitors compete directly. With non-competitive inhibition, only the inhibitor concentration matters since it's not competing with the substrate.

Feedback inhibition specifically occurs when the pathway's end product reaches critical levels and shuts down earlier enzymes, preventing wasteful overproduction and maintaining cellular balance.

Study Strategy: Remember that competitive = competition for the same site, non-competitive = different site that changes enzyme shape.