Identify and describe What is DNA?, Chromosomes: Packaging DNA and The 23rd Pair: Sex Chromosomes in the context of dNA and the genome.
BEEZYREVISOR
GCSE Biology Revision
Learn it. Recall it. Revise it.
GCSE Biology revision
DNA and the genome
Reproduction
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Explain the relationships between What is DNA?, Chromosomes: Packaging DNA and The 23rd Pair: Sex Chromosomes and how they contribute to dNA and the genome.
Apply and analyse knowledge of What is DNA?, Chromosomes: Packaging DNA and The 23rd Pair: Sex Chromosomes to interpret unfamiliar information about dNA and the genome.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What is DNA?
- DNA stands for deoxyribonucleic acid and is the chemical that all genetic material is made of
- DNA is a polymer, meaning it is made up of many similar repeating units joined together
- DNA consists of two strands twisted together to form a double helix — a double- stranded spiral structure
- If the DNA from a single cell were unravelled into one long strand, it would be over 2 metres long
Chromosomes: Packaging DNA
- To fit inside the nucleus, DNA is coiled tightly into 46 separate sections called chromosomes
- Each chromosome is a highly compact coil of DNA, keeping the genetic material organised and preventing tangling
- Humans have 46 chromosomes arranged into 23 pairs, with one chromosome in each pair inherited from each parent
- Chromosomes only appear in their familiar X-shape just before cell division (eg during mitosis or meiosis); at rest they appear as loose threads
The 23rd Pair: Sex Chromosomes
- The 23rd pair of chromosomes are the sex chromosomes, which determine biological sex
- Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY)
Genes and Proteins
- A gene is a small section of DNA that codes for a particular protein by specifying a sequence of amino acids
- There are only 20 different amino acids, but they can be combined in thousands of different sequences to produce thousands of different proteins
- The proteins a cell produces determine what type of cell it becomes — for example, red blood cells produce large amounts of haemoglobin to transport oxygen
What is a Genome?
- A genome is the entire set of genetic material found in an organism
- Every individual's genome is unique, though it is more similar to close relatives such as parents, and identical in identical twins
- Scientists have now sequenced the complete human genome, meaning the entire DNA code of humans is known
Genomes and Disease
- Some genes directly cause inherited diseases, which are passed from parents to offspring through their genes
- Other genes increase the risk of certain diseases — for example, the BRCA genes increase the risk of developing breast cancer
- Understanding a person's genome allows doctors to choose more effective, personalised treatments and helps scientists develop better therapies
Tracing Human History with Genomes
- Although all humans share most of their genome, small differences between populations can reveal when groups separated from one another
- Genomic data has been used to trace key events in human migration, such as when humans first left Africa or travelled to North America