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AQA A Level Biology Summary Notes - DNA, Genes and Protein Synthesis
Summarised notes for AQA A Level Biology DNA, Genes and Protein Synthesis for students studying A Levels.
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DNA, genes & protein synthesis● A gene is a section of DNA that codes for a protein.
● They are found in chromosomes.
● Different versions of the same gene are called alleles - can be
dominant or recessive.
Redundancy of the genetic code:
● 3 bases/a triplet code = 1 amino acid.
● There are 64 combinations but only 20 amino acids can be
made so there is excess.
● Chain always starts with AUG, ends with stop codon.
Features of the genetic code:
● Code is degenerate because there is always more than 1 triplet code for each amino
acid. Code is universal in all organisms - code the same way.
● Non-overlapping - each base is part of only 1 triplet e.g. 123456 not 123,234.
Protein that packages DNA is called a histone. DNA is found in a chromosome.
Homologous pair:
● They are chromosomes that carry the same gene.
● How can you tell - same size/length, same gene location/loci
● Diploid number = total number of homologous pairs e.g. humans 46. Haploid - single.
Structure of a chromosome:
Eukaryotes - DNA is in the nucleus, DNA in chromosomes are coiled and packed tighter
because of histones so take up less space
Prokaryotes - DNA found as a free circular DNA/chromosome in the cytoplasm/plasmids that
contain DNA, no histones
Structure - DNA + histones = chromosome
Transcription: (making pre-mRNA)
1. The relevant section of DNA uncoils through the enzyme DNA helicase
2. One of the strands acts as a template strand
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