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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Genome and Gene Structure
DNA (deoxyribonucleic acid) is the molecule that contains all the genetic instructions needed for an organism to develop, function and reproduce. It's like a massive instruction manual for building and maintaining a living thing!
Key Definitions:
DNA has a double helix structure, like a twisted ladder. The 'rungs' of this ladder are made of pairs of chemicals called bases: adenine (A), thymine (T), guanine (G) and cytosine (C). A always pairs with T and G always pairs with C. The 'sides' of the ladder are made of alternating sugar and phosphate molecules.
The sequence of bases in DNA forms a code. This code is like a language that tells cells how to make proteins. Each three-base sequence (called a codon) corresponds to a specific amino acid, which are the building blocks of proteins.
Think of your genome as a library, chromosomes as books and genes as chapters in those books. Let's explore how these are organised:
The basic building block. In humans, if stretched out, the DNA from a single cell would be about 2 metres long!
Sections of DNA that code for proteins. Humans have about 20,000-25,000 genes. Each gene can range from a few hundred to over 2 million base pairs in length.
Structures that contain DNA tightly packed with proteins. Humans have 23 pairs of chromosomes (46 in total).
If you could type out the human genome at a rate of one letter per second, it would take you over 30 years to type it all! The human genome contains about 3 billion base pairs.
Genes are the functional units of heredity. Let's look at their structure and how they work:
A typical gene has several key parts:
Genes work by being 'read' and translated into proteins through a process called protein synthesis. These proteins then carry out various functions in the body, from building structures to controlling chemical reactions.
Protein synthesis is how your cells turn the information in your DNA into actual proteins. It happens in two main stages:
What happens: DNA is copied into messenger RNA (mRNA)
Where it happens: In the nucleus
How it works:
What happens: mRNA is used to build a protein
Where it happens: At ribosomes in the cytoplasm
How it works:
Sickle cell anaemia shows how a tiny change in DNA can have big effects. Just one base change in the gene for haemoglobin (from A to T) changes a single amino acid in the protein. This causes red blood cells to become sickle-shaped when oxygen levels are low, leading to blocked blood vessels and pain. This demonstrates how crucial the precise sequence of DNA is for proper protein function.
The genetic code is how the sequence of bases in DNA or RNA determines the sequence of amino acids in a protein. Here are its key features:
This international research project, completed in 2003, mapped all the genes in human DNA. It took 13 years and cost nearly $3 billion! The project has revolutionised medicine by helping us understand genetic diseases and develop new treatments. It's also helped us understand human evolution and migration patterns.
Interestingly, genome size doesn't always correlate with organism complexity:
This shows that it's not just the amount of DNA that matters, but how it's organised and used!
Let's recap the journey from DNA to proteins:
Understanding this process helps us understand how our bodies develop and function and how changes in DNA can lead to changes in proteins that can affect our health.
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