Why do some substitution mutations not change the amino acid sequence of a protein?

Because the genetic code is degenerate, so the new codon may still code for the same amino acid

Under what condition do addition or deletion mutations cause a frameshift?

When the number of bases added or deleted is not a multiple of three

What is a frameshift mutation?

A mutation that changes the nature of all base triplets downstream from the mutation point

Why do frameshift mutations typically have more severe effects on a polypeptide than substitution mutations?

Because they alter every codon after the mutation point, changing the entire downstream amino acid sequence

AQA A-Level Biology 7402

Biological Molecules

Topic overview Monomers, polymers, condensation and hydrolysis Monosaccharides, disaccharides and glycosidic bonds Alpha and beta glucose, amylose and amylopectin How starch, glycogen and cellulose suit their functions Cellulose microfibrils and tests for sugars and starch Triglycerides, fatty acids and phospholipid structure Properties of lipids and the emulsion test The general structure of amino acids Peptide bonds, polypeptides and bonds in proteins Levels of protein structure, the biuret test and chromatography Activation energy, induced fit and enzyme specificity Enzyme and substrate concentration and inhibition pH, temperature, initial rates and measurement uncertainty DNA, RNA and the components of a nucleotide Base pairing in DNA and RNA Phosphodiester bonds and the DNA double helix Doubts about DNA, base ratios and RNA structure Semi-conservative replication, helicase and DNA polymerase Steps of replication and the Meselson-Stahl experiment ATP structure, hydrolysis and resynthesis Water as a metabolite, solvent and coolant Hydrogen, iron, sodium and phosphate ions

Cells

Topic overview Cell-surface membrane, nucleus and mitochondria Chloroplasts, Golgi apparatus and lysosomes Ribosomes, endoplasmic reticulum, cell walls and vacuoles Cell specialisation, levels of organisation and organelle roles Prokaryotic cell structure and virus particles Optical and transmission electron microscopes Scanning electron microscopes, magnification and graticules Cell fractionation, homogenisation and ultracentrifugation Order of sedimentation, artefacts and fractionation conditions The cell cycle, interphase and the stages of mitosis Spindle fibres, cytokinesis and uncontrolled cell division Binary fission, viral replication and root tip squashes Mitotic index, mitosis stages and size calculations The fluid-mosaic model and cholesterol Simple diffusion, facilitated diffusion and membrane proteins Osmosis, water potential and active transport Co-transport in the ileum and adaptations for rapid transport Transport rates, dilution series and membrane permeability Antigens, cell recognition and phagocytosis Antigen-presenting cells, T cells and clonal selection Antibody structure, agglutination and antigen-antibody complexes Memory cells, vaccination and active and passive immunity HIV structure and replication AIDS, monoclonal antibodies, ELISA and clinical trials

Organisms Exchange Substances with Their Environment

Topic overview Surface area to volume ratio, metabolic rate and calculations Gas exchange in single-celled organisms, insects and fish Leaf gas exchange, insect water loss and xerophytes Trachea, bronchi, alveoli and lung dissection Inspiration, expiration and pulmonary ventilation rate Lung disease, correlation, insect tracheae and Spearman's rank Digestion of carbohydrates and lipids Endopeptidases, exopeptidases and dipeptidases Co-transport of amino acids, micelles and chylomicrons Haemoglobin, red blood cells and the dissociation curve Cooperative binding, the Bohr effect and different haemoglobins Double circulation and the major blood vessels Heart chambers, the cardiac cycle and heart dissection Arteries, arterioles, veins and capillaries Tissue fluid, cardiovascular risk factors and semilunar valves Xylem structure and the cohesion-tension theory Phloem structure, companion cells and translocation Mass flow from source to sink, tracers and ringing Evaluating mass flow, potometers and transpiration rate

Genetic Information, Variation and Relationships Between Organisms

Topic overview Prokaryotic and eukaryotic DNA, chromosomes and histones DNA in mitochondria and chloroplasts, genes and loci Features of the genetic code, exons and introns Genome, proteome, mRNA and tRNA Transcription, RNA polymerase and splicing Translation at the ribosome and the role of tRNA Gene mutations, mutagens and chromosome non-disjunction Meiosis, independent segregation and crossing over Meiosis compared with mitosis and random fertilisation Genetic diversity and the spread of advantageous alleles Directional and stabilising selection Defining adaptations and how antibiotic resistance spreads Selection on birth weight, aseptic technique and antimicrobials Classifying anatomical, physiological and behavioural adaptations Defining a species, courtship and phylogenetic classification The taxonomic hierarchy, binomial names and molecular evidence Species richness, the index of diversity and farming Comparing genetic diversity, standard deviation and the t-test

Energy Transfers in and Between Organisms

Topic overview Photoionisation and ATP from the light-dependent reaction Chemiosmosis, photolysis and products for the Calvin cycle Carbon fixation, triose phosphate and limiting factors Glasshouse conditions, pigment chromatography and DCPIP Glycolysis and its products Anaerobic respiration and the oxidation of pyruvate Acetylcoenzyme A and the Krebs cycle Oxidative phosphorylation, other substrates and respirometers Biomass and gross and net primary production Net production of consumers and productivity Farming practices, transfer efficiency and the production equations Saprobionts and the phosphorus cycle Mycorrhizae, ammonification and nitrification Nitrogen fixation, denitrification and fertilisers Leaching, eutrophication and nutrient cycles reviewed

Organisms Respond to Changes in Their Internal and External Environments

Topic overview IAA, gravitropism and phototropism Taxes, kineses, reflex arcs and choice chambers The Pacinian corpuscle and generator potentials Rods, cones, colour vision and visual acuity The SAN, AVN, bundle of His and Purkyne tissue Chemoreceptors, baroreceptors and cardiac output Motor neurone structure and the resting potential Depolarisation, action potentials and saltatory conduction The refractory period and speed of conduction Synapse structure and the neuromuscular junction Transmission at a cholinergic synapse Acetylcholinesterase, summation and inhibitory synapses Neuromuscular junctions compared, agonists and antagonists Drug effects and potassium channels in hyperpolarisation Antagonistic muscle pairs and muscle fibre structure Sarcomere bands, actin, myosin and tropomyosin The sliding filament cycle, ATP and phosphocreatine Slow twitch and fast twitch muscle fibres Control systems and why temperature, pH and glucose matter Negative and positive feedback Glycogenesis, glycogenolysis and gluconeogenesis How insulin, glucagon and adrenaline act The second messenger model, diabetes and the sugar debate Glucose calibration curves and hormones affecting blood glucose Osmoregulation, osmoreceptors and ADH Nephron structure and ultrafiltration The proximal convoluted tubule and the loop of Henle The distal tubule, collecting duct and urine concentration

Genetics, Populations, Evolution and Ecosystems

Topic overview Genotype, phenotype and types of allele Monohybrid and dihybrid crosses, sex linkage and autosomal linkage Epistasis, genetic diagrams and the chi-squared test Gene pools, allele frequency and Hardy–Weinberg Sources of variation and selection pressures Types of selection and allopatric and sympatric speciation Genetic drift, founder effects and the route to speciation Communities, ecosystems, niches and carrying capacity Competition, predation, quadrats and mark-release-recapture Mark-release-recapture assumptions and succession Succession, conservation and evaluating the evidence

The Control of Gene Expression

Topic overview Types of gene mutation and mutagenic agents Silent, frameshift, missense and nonsense mutations Totipotent, pluripotent and induced pluripotent stem cells Transcription factors, oestrogen, epigenetics and twin studies DNA methylation, histone acetylation and cancer RNA interference and epigenetic drug targets Benign and malignant tumours, suppressor genes and oncogenes Abnormal methylation, oestrogen and evidence on causes of cancer Genome projects, the proteome and sequencing methods Transgenic organisms and making DNA fragments The polymerase chain reaction Promoters, restriction endonucleases, sticky ends and ligase Transformation, marker genes and uses of recombinant DNA The humanitarian debate, cDNA and in vivo cloning DNA probes, hybridisation and screening for alleles Genetic counselling, personalised medicine and screening ethics VNTRs, PCR and gel electrophoresis Using and interpreting genetic fingerprints

The Control of Gene Expression

Silent, frameshift, missense and nonsense mutations

3 things to know
Papers 2 and 3

Overview

Recall questions on why degeneracy makes some substitutions silent, frameshifts from additions and deletions, missense and nonsense mutations, and why a change at an active site matters most.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Evaluation

Why does a base substitution not always change the protein?

Key term

What does it mean that the genetic code is degenerate?

Key term

What is a silent mutation?

Questions from this subtopic

Each one with the model answer underneath.

1
What does it mean that the genetic code is degenerate?
Model answer
Multiple different triplet codons can code for the same amino acid
2
Why do some substitution mutations not change the amino acid sequence of a protein?
Model answer
Because the genetic code is degenerate, so the new codon may still code for the same amino acid
3
What is a silent mutation?
Model answer
A mutation where a base change does not alter the amino acid encoded, due to degeneracy of the genetic code
4
Under what condition do addition or deletion mutations cause a frameshift?
Model answer
When the number of bases added or deleted is not a multiple of three
5
What is a frameshift mutation?
Model answer
A mutation that changes the nature of all base triplets downstream from the mutation point
6
Why do frameshift mutations typically have more severe effects on a polypeptide than substitution mutations?
Model answer
Because they alter every codon after the mutation point, changing the entire downstream amino acid sequence

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Biological Molecules

Cells

Organisms Exchange Substances with Their Environment

Genetic Information, Variation and Relationships Between Organisms

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phosphodiesterglycosidicpeptide

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AOne glycerol molecule.

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Genotype means the genetic constitution of an organism.

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