DNA and Chromosomes

DNA is the basis of life - it means deoxyribose nucleic acid. DNA codes for proteins; the building blocks our bodies. 

In these AQA AS Biology: Biol2 notes, we will focus on DNA structure, nucleotide structure, the function of DNA, genes, the triplet code, chromosomes and homologous chromosomes, alleles, meiosis, and recombination by crossing over.

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DNA: The Structure

DNA - Heriditary material responsible for passing on genetic information from cell to cell and down generations

Nucleotide - The structure of DNA, containing deoxyribose sugar, a phosphate group, and an organic base

• DNA is made up of nucleotides 
• Nucleotides contain three components: deoxyribose sugar (pentagon), phosphate molecule (circle), and an organic base (rectangle)
• The three molecules are bonded together by condensation reactions
• Nucleotides join together to make dinucleotides or polynucleotides
• So how is DNA formed through this? Quite simply. Two nucleotides, or strands, are combined by hydrogen bonds between bases. 
• Theory: 1953 - Watson and Crick took credit for Rosalind Franklin's work on DNA by figuring out structure on X-ray diffraction patterns

Pairing of bases

Complementary base pairs - two DNA bases which pair up due to their ring structures

• There are four bases, made up of nitrogen
• These are Adenine, Thymine, Guanine, and Cytosine
• The two bases Adenine and Guanine have double ring structures, so they are considered purine bases
• The two bases Thymine and Cytosine have single ring structures so they are considered pyrimidine bases
• One purine pairs up with one pyrimidine 
• A + T with two hydrogen…