CB3.2 — DNA and the genome (Edexcel 1SC0)
DNA structure
DNA is a double helix made of two polynucleotide strands wound around each other.
Each nucleotide consists of:
- A deoxyribose sugar
- A phosphate group
- One of four nitrogenous bases: A, T, C, G
Base pairing rules: A pairs with T (two hydrogen bonds); C pairs with G (three hydrogen bonds). This complementary base pairing holds the two strands together.
Gene
A gene is a section of DNA that codes for a specific protein. The sequence of bases determines the sequence of amino acids in the protein.
The genome
The genome is the complete set of genetic information in an organism (all the DNA in all chromosomes).
The Human Genome Project (completed 2003) mapped all ~3 billion base pairs in human DNA. Benefits:
- Identifying genes linked to inherited diseases.
- Developing targeted treatments and gene therapy.
- Understanding human evolution.
DNA replication
Before cell division, DNA replicates:
- Double helix unwinds and hydrogen bonds break (unzipping).
- Each strand acts as a template.
- Free nucleotides attach by complementary base pairing.
- Two identical DNA molecules are formed.
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