Unit 4 - Genetic information, variation and relationships between organisms
Summary
Summary Genetic diversity
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Unit 4 - Genetic information, variation and relationships between organisms
Institution
AQA
Summary notes for AQA A-level Biology genetic diversity topic. Includes clear information on mutations, meiosis, genetic diversity, adaption (evolution) and the types of selection. Summarised from class notes and the official course textbook. From an A* student.
Unit 4 - Genetic information, variation and relationships between organisms
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Genetic diversity
Mutations
A mutation is any change to the quantity or the Mutation:
base sequence of DNA. 1. changes base sequence of gene
A gene mutation is any change in the DNA 2. Protein has a different primary structure
base sequence of chromosomes 3. Protein has a different tertiary structure
4. Changes ionic/disulphide/hydrogen bonds
Mutations in introns have no effect on proteins
as they are not translated
Mutations can occur:
• Randomly during replication
• Rate increases due to mutagenic agents: (substances that increase the
rate of mutations) e.g. toxic chemicals, ionising radiation, high energy
radiation
Substitution
• One base is swapped with another e.g. GTA to GCA.
• The polypeptide is produced with one different amino acid
• If the amino acid that has changed is important in the tertiary structure
of the protein produced or its in the active site of an enzyme then this mutation might have a
more harmful effect
• Due to the degenerate nature of DNA, the amino acid might not change at all
• Example of this mutation: sickle cell anaemia
Deletion
• A base is removed from the DNA sequence - number of bases changes
• It is a frame shift mutation as all subsequent codons and triplets are effected
• number of bases changes and all triplets are shifted
• The amino acid sequence is entirely different and so the polypeptide is unlikely to function
properly
• Example of this mutation: cystic fibrosis
Chromosome mutations
• Changes in the structure or number of whole chromosomes
• Inherited conditions as errors are present in the gametes
• Changes in the number of individual chromosomes = non-disjunction and is where
chromosomes don’t separate during meiosis. e.g. Down’s syndrome (extra 21)
• Multiple sets of chromosomes = polyploidy.
Meiosis
• Produces four genetically different haploid cells.
• They have half the number of chromosomes as the parent cell.
• In sexual reproduction, two gametes fuse. This means that gametes must have half the normal
amount of chromosomes.
The process
1. DNA unravels and replicates to form two copies of each chromosome
2. DNA condenses to form sister chromatids held together at the centromere
3. First division is where homologous pairs are separated
4. Second division is where the sister chromatids are separated
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