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Unlock This CourseEvery living thing on Earth carries a set of instructions that makes it unique. These instructions are stored in something called DNA, which is organised into genes and genomes. Think of it like a massive recipe book - your genome is the entire cookbook, while individual genes are specific recipes that tell your body how to make different proteins and control various characteristics.
Understanding genome and gene structure is crucial because it explains how traits are passed from parents to offspring, why you might have your mum's eyes or your dad's height and how genetic disorders can occur.
Key Definitions:
Humans have approximately 3.2 billion base pairs of DNA and around 20,000-25,000 genes. That's enough information to fill about 200 telephone directories! Yet amazingly, only about 2% of our genome actually codes for proteins - the rest has other important functions or is still being studied.
DNA has a famous double helix structure, like a twisted ladder. The 'rungs' of this ladder are made of four different bases: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These bases always pair up in the same way - A with T and G with C. This pairing rule is crucial for DNA replication and inheritance.
DNA doesn't just float around loose in your cells. It's carefully packaged and organised in a hierarchy:
The basic twisted ladder structure containing the genetic code in sequences of A, T, G and C bases.
Specific sections of DNA that code for particular proteins or traits. Like chapters in a book.
Entire DNA molecules wrapped around proteins. Humans have 23 pairs (46 total) in most cells.
If you stretched out all the DNA in one human cell, it would be about 2 metres long! Yet it's packed into a nucleus that's only about 0.00001 metres across. That's like fitting 40 kilometres of thread into a tennis ball.
Genes are like instruction manuals for making proteins. Each gene contains the code for a specific protein and proteins do most of the work in your body - from building muscle to fighting infections. The process of turning a gene's code into a protein happens in two main steps: transcription and translation.
Not all genes are 'switched on' all the time. Different genes are active in different cells and at different times. For example, the genes for making insulin are mainly active in pancreas cells, while genes for making keratin (the protein in hair and nails) are active in skin cells.
Most genes come in different versions called alleles. Some alleles are dominant (they show up even if you only have one copy) while others are recessive (you need two copies for the trait to show). Brown eyes are usually dominant over blue eyes, which is why brown eyes are more common.
The way characteristics are passed from parents to children follows predictable patterns. Understanding these patterns helps us predict what traits offspring might have and explains why family members often look similar.
Gregor Mendel, working with pea plants in the 1860s, discovered the basic rules of inheritance that still apply today. He found that traits are controlled by 'factors' (now called genes) that come in pairs, with one from each parent.
Each parent has two copies of each gene, but only passes one copy to each offspring.
Genes for different traits are inherited independently of each other.
Some alleles are dominant and will be expressed even if only one copy is present.
Huntington's disease is caused by a dominant allele on chromosome 4. This means that if you inherit just one copy of the faulty gene from either parent, you will develop the disease. This demonstrates how understanding gene structure and inheritance patterns is crucial for medical genetics and family planning.
Although all humans share 99.9% of their DNA, that tiny 0.1% difference creates all the variation we see between people. This variation comes from several sources and is essential for evolution and adaptation.
Genetic variation arises through several mechanisms during reproduction and occasionally through mutations.
When gametes (sex cells) are formed, chromosomes are shuffled and mixed, creating new combinations of genes. This is why siblings can look quite different despite having the same parents.
Sometimes errors occur when DNA is copied, creating new alleles. Most mutations are harmless, but some can cause genetic disorders or, rarely, provide advantages that help organisms survive better.
Understanding genome and gene structure has revolutionised medicine, agriculture and forensic science. Today, we can read entire genomes, edit genes and use genetic information to diagnose diseases and develop treatments.
Scientists have successfully treated some genetic disorders by introducing healthy copies of genes into patients' cells. For example, gene therapy has been used to treat severe combined immunodeficiency (SCID), allowing children born without functioning immune systems to live normal lives.
We can now test for many genetic conditions before symptoms appear. This raises important ethical questions about privacy, discrimination and what we should do with genetic information. Genetic counsellors help families understand test results and make informed decisions.