DNA and the genome (AQA GCSE Biology): Revision Notes
DNA and the genome
What is DNA?
DNA stands for deoxyribonucleic acid. It is the genetic material found in living things. DNA contains all the instructions needed to build and run an organism.
The genome is all of an organism's genetic material put together. Think of it as the complete instruction manual for that living thing.
The human genome contains approximately 3 billion base pairs of DNA, yet it can fit inside the tiny nucleus of almost every cell in your body!
Where is DNA found?
DNA is found in the nucleus of cells. Most cells have a nucleus (these are called eukaryotic cells).
Inside the nucleus, DNA is tightly packed into structures called chromosomes. This tight packing helps fit lots of DNA into the small space of a cell nucleus.
If you could stretch out all the DNA in one human cell, it would be about 2 metres long! The tight packing into chromosomes is essential for fitting this enormous amount of information into such a tiny space.
The structure of DNA
DNA has a special shape called a double helix. This looks like a twisted ladder or spiral staircase.
DNA is made from smaller units called nucleotides. Each nucleotide contains three parts:
- A phosphate group
- A sugar
- A base
The DNA strand has alternating phosphate and sugar parts that form the "backbone". The bases stick out from this backbone.
Understanding DNA Structure: The Ladder Analogy
Imagine DNA as a twisted ladder:
- The sides of the ladder = phosphate and sugar backbone
- The rungs of the ladder = pairs of bases (A-T and G-C)
- The twist = the double helix shape that makes DNA more compact and stable
The four bases
DNA contains four different bases:
- A (Adenine)
- T (Thymine)
- G (Guanine)
- C (Cytosine)
The order of these bases is very important. A sequence of three bases creates the code for one amino acid. Amino acids join together to make proteins.
Base Sequences and Amino Acids
The sequence ATG might code for one amino acid, whilst TGC codes for a different amino acid.
This three-base system means there are 64 possible combinations (4³ = 64), which is more than enough to code for the 20 different amino acids used in proteins.
What are genes?
A gene is a small section of DNA found on a chromosome. Each gene contains the instructions to make a specific protein.
Different proteins do different jobs in the body. Some proteins:
- Help with digestion
- Fight infections
- Build muscles
- Control chemical reactions
Humans have approximately 20,000-25,000 genes, but this represents only about 2% of our total DNA. The rest of our DNA has various functions including regulation of gene expression.
The human genome project
Scientists have now studied the entire human genome. This means they know the complete DNA sequence of humans.
This research helps scientists:
- Find genes that cause diseases like heart disease or cancer
- Understand inherited conditions like cystic fibrosis
- Develop new treatments for genetic disorders
The Human Genome Project, completed in 2003, was one of the most significant scientific achievements in history. It has revolutionised medicine and our understanding of human biology, leading to personalised medicine approaches.
DNA and human migration
Scientists can use DNA to study how humans moved around the world in the past.
Mitochondrial DNA is especially useful for this. Mitochondria are tiny structures in cells that have their own DNA.
Inheritance Patterns:
- We inherit mitochondrial DNA from our mother only
- Men inherit their Y chromosome from their father only
- Scientists can trace these patterns to create maps showing human migration
These inheritance patterns act like molecular clocks, helping scientists track human migration patterns over thousands of years.
Remember!
Key Points to Remember:
- DNA is genetic material that contains instructions for building and running living things
- DNA has a double helix structure made from nucleotides containing bases
- The four bases are A, T, G, and C - their order creates the genetic code
- Genes are sections of DNA that code for specific proteins
- The human genome helps scientists understand diseases and develop treatments