Biology AS AQA
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1-1-biological-molecules-carbohydrates11 主题
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1-1-1-biological-molecules-key-terms
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1-1-2-biological-molecules-reactions
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1-1-3-monosaccharides
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1-1-4-glucose
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1-1-5-the-glycosidic-bond
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1-1-6-chromatography-monosaccharides
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1-1-7-disaccharides
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1-1-8-starch-and-glycogen
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1-1-9-cellulose
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1-1-10-biochemical-tests-sugars-and-starch
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1-1-11-finding-the-concentration-of-glucose
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1-1-1-biological-molecules-key-terms
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1-2-biological-molecules-lipids3 主题
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1-3-biological-molecules-proteins5 主题
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1-4-proteins-enzymes12 主题
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1-4-1-many-proteins-are-enzymes
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1-4-2-enzyme-specificity
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1-4-3-how-enzymes-work
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1-4-4-required-practical-measuring-enzyme-activity
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1-4-5-drawing-a-graph-for-enzyme-rate-experiments
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1-4-6-using-a-tangent-to-find-initial-rate-of-reaction
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1-4-7-limiting-factors-affecting-enzymes-temperature
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1-4-8-limiting-factors-affecting-enzymes-ph
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1-4-10-limiting-factors-affecting-enzymes-enzyme-concentration
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1-4-11-limiting-factors-affecting-enzymes-substrate-concentration
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1-4-12-limiting-factors-affecting-enzymes-inhibitors
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1-4-14-control-of-variables-and-uncertainty
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1-4-1-many-proteins-are-enzymes
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1-5-nucleic-acids-structure-and-dna-replication8 主题
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1-5-2-nucleotide-structure-and-the-phosphodiester-bond
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1-5-3-dna-structure-and-function
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1-5-4-rna-structure-and-function
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1-5-5-ribosomes
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1-5-6-the-origins-of-research-on-the-genetic-code
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1-5-8-the-process-of-semi-conservative-replication
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1-5-9-calculating-the-frequency-of-nucleotide-bases
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1-5-10-the-watson-crick-model
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1-5-2-nucleotide-structure-and-the-phosphodiester-bond
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1-6-atp-water-and-inorganic-ions4 主题
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2-1-cell-structure7 主题
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2-2-the-microscope-in-cell-studies4 主题
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2-3-cell-division-in-eukaryotic-and-prokaryotic-cells8 主题
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2-4-cell-membranes-and-transport9 主题
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2-4-1-the-structure-of-cell-membranes
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2-4-3-the-cell-surface-membrane
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2-4-4-diffusion
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2-4-5-osmosis
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2-4-7-osmosis-in-animal-cells
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2-4-9-required-practical-investigating-water-potential
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2-4-10-active-transport-and-co-transport
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2-4-11-adaptations-for-rapid-transport
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2-4-13-required-practical-factors-affecting-membrane-permeability
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2-4-1-the-structure-of-cell-membranes
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2-5-cell-recognition-and-the-immune-system7 主题
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2-6-vaccines-disease-and-monoclonal-antibodies6 主题
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3-1-adaptations-for-gas-exchange6 主题
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3-2-human-gas-exchange14 主题
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3-2-5-the-alveolar-epithelium
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3-2-1-the-human-gas-exchange-system
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3-2-2-dissecting-the-gas-exchange-system
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3-2-3-microscopy-and-gas-exchange-surfaces
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3-2-4-investigating-gas-exchange
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3-5-5-investigating-heart-rate
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3-5-6-blood-vessels
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3-5-7-capillaries-and-tissue-fluid
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3-5-8-cardiovascular-disease-data
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3-2-10-risk-factor-data
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3-2-11-correlations-and-causal-relationships
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3-2-6-ventilation-and-gas-exchange
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3-2-8-the-effects-of-lung-disease
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3-2-9-pollution-and-smoking-data
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3-2-5-the-alveolar-epithelium
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3-3-digestion-and-absorption5 主题
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3-4-mass-transport-in-animals6 主题
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3-5-the-circulatory-system-in-animals4 主题
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3-6-mass-transport-in-plants6 主题
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4-1-dna-genes-and-chromosomes10 主题
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4-2-dna-and-protein-synthesis3 主题
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4-3-genetic-diversity-mutations-and-meiosis7 主题
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4-4-genetic-diversity-and-adaptation6 主题
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4-5-species-and-taxonomy4 主题
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4-6-biodiversity9 主题
4-2-6-nucleic-acid-and-amino-acid-sequence-comparison
Exam code:7401
Linking DNA to polypeptides
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When comparing the genetic code to amino acid sequences, mRNA codons are often used
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The four bases found in RNA molecules (adenine, uracil, cytosine and guanine) can form 64 different codons, yet there are only 20 amino acids that are coded for
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Therefore, the genetic code is degenerate
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Multiple mRNA codons can encode the same amino acid; e.g., UGU and UGC both code for the amino acid cysteine
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This means that a change in the genetic code doesn’t necessarily result in a change in the amino acid sequence
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Some codons send important signals to the translation machinery
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The START codon marks the start of the protein and therefore initiates the process of translation from the right location (this is always the amino acid methionine in eukaryotic cells, coded for by the codon AUG)
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STOP codons cause translation to terminate at the end of the protein and do not code for any amino acids, e.g. UAA
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The genetic code is non-overlapping
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Each base is only read once in the codon that it is part of
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The number of amino acids in a protein can be calculated using the number of coding nucleotides in the mRNA molecule and vice versa:
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Each amino acid is coded for by a triplet (codon) of 3 nucleotides
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The last codon is a STOP codon, which does not code for an amino acid
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When given the number of amino acids, multiply by 3 and add three (for the STOP codon)
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Worked Example
A segment of mRNA contains 303 coding nucleotides.
How many amino acids will be in the polypeptide produced?
Step 1: When given the number of coding mRNA nucleotides, divide by 3
303 ÷ 3 = 101 codons
Step 2: Minus 1, to take into account the STOP codon
101 – 1 = 100 amino acids
(Assuming one of the codons is a stop codon and doesn’t code for an amino acid)
Working out an amino acid sequence
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An amino acid sequence can be derived when given a genetic code table (or codon table)
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The bases in the codon are read left to right across the table, and the corresponding amino acids can be found
Worked Example
A strand of DNA has the following sequence:
TAC CCG AAA ACT
Use a codon table to identify the sequence of amino acids coded for by this mRNA.
Step 1: Convert the DNA sequence into mRNA codons using complementary base pairing
TAC CCG AAA ACT → AUG GGC UUU UGA
Step 2: Use the codon table to translate the mRNA codons into an amino acid sequence
AUG = Met
GGC = Gly
UUU = Phe
UGA = Stop (does not code for an amino acid)
Therefore, the amnio acid sequence = Met – Gly – Phe
Examiner Tips and Tricks
You will not be required to memorise specific codons and the amino acids for which they code. When using the codon table, remember you need to use the mRNA codons, and not the DNA triplet codes. The mRNA sequence will have been formed from the DNA template strand during transcription and so will have a complementary sequence.
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