AQA A-Level Biology Topic 4 Genetics

Eukaryotic vs Prokaryotic DNA Similarities

Identical nucleotide structure (deoxyribose, phosphate, base); adjacent nucleotides joined by phosphodiester bonds and complementary bases by hydrogen bonds; DNA in mitochondria and chloroplasts is short, circular, and not associated with proteins like prokaryotic DNA.

Eukaryotic vs Prokaryotic DNA Differences

Eukaryotic DNA is longer, linear, associated with histone proteins, and contains introns; prokaryotic DNA is shorter, circular, not associated with histones, and lacks introns.

Chromosome Definition

Long, linear DNA molecule coiled around its associated histone proteins, found in the nucleus of eukaryotic cells.

Gene Definition

A sequence of DNA (nucleotide) bases that codes for the amino acid sequence of a polypeptide or a functional RNA (such as rRNA or tRNA).

Locus Definition

The fixed position a gene occupies on a particular DNA molecule.

Nature of the Genetic Code

Triplet code: 3 DNA bases code for one amino acid; Universal: same triplets code for the same amino acids in all organisms; Non-overlapping: each base is part of only one triplet; Degenerate: an amino acid can be coded for by more than one triplet.

Non-Coding Base Sequences

DNA that does not code for amino acid sequences or polypeptides; found between genes (non-coding multiple repeats) and within eukaryotic genes (introns).

Exons vs Introns

Exons are base sequences of a gene coding for amino acid sequences in a polypeptide; introns are non-coding base sequences within eukaryotic genes that do not code for amino acids.

Genome vs Proteome

Genome is the complete set of genes in a cell (including mitochondria and chloroplasts); proteome is the full range of proteins that a cell can produce (coded for by its genome).

tRNA vs mRNA Structure

Both are single polynucleotide strands; tRNA is folded into a clover-leaf shape with hydrogen bonds, has a shorter fixed length, an anticodon, and an amino acid binding site, whereas mRNA is linear, variable in length, has codons, and lacks hydrogen bonds.

Transcription in Eukaryotic Cells

Hydrogen bonds between DNA bases break; one DNA strand acts as a template; free RNA nucleotides align to complementary bases (uracil replaces thymine); RNA polymerase joins adjacent RNA nucleotides forming phosphodiester bonds via condensation reactions to form pre-mRNA, which is spliced to remove introns.

Eukaryotic vs Prokaryotic mRNA Production

Eukaryotic cells produce pre-mRNA which undergoes splicing to remove introns to form mature mRNA; prokaryotic cells produce mRNA directly from DNA because their genes do not contain introns.

Translation (Polypeptide Synthesis)

mRNA attaches to a ribosome at a start codon; tRNA brings a specific amino acid and its anticodon binds a complementary mRNA codon; ribosome moves along mRNA and catalyses peptide bond formation between amino acids using energy from ATP hydrolysis; tRNA detaches until a stop codon is reached.

Role of ATP, tRNA, and Ribosomes in Translation

ATP hydrolysis provides energy to attach amino acids to tRNA and form peptide bonds; tRNA transports specific amino acids and pairs its anticodon with mRNA codons; ribosomes hold mRNA (space for 2 codons), allow tRNA binding, and catalyse peptide bond formation.

Gene Mutation and Mutagenic Agents

A gene mutation is a spontaneous change in the base sequence of DNA during DNA replication (e.g. base deletion or substitution); a mutagenic agent is a factor that increases mutation rate (e.g. UV light, alpha particles).

How a Mutation Leads to a Non-Functional Enzyme

Changes sequence of DNA base triplets and mRNA codons, changing the primary amino acid sequence; this alters the position of hydrogen, ionic, and disulphide bonds, changing the protein's tertiary structure and active site shape so the substrate is no longer complementary and cannot bind.

Effects of a Base Substitution Mutation

One DNA base is replaced by another, changing one triplet and one mRNA codon; may change one amino acid (altering tertiary structure if R-group bonds change) OR have no effect due to the degenerate genetic code or if located in an intron.

Effects of a Base Deletion Mutation

One DNA nucleotide is removed, causing a frameshift that alters all subsequent DNA triplets and mRNA codons after the mutation; this changes the primary amino acid sequence, alters hydrogen/ionic/disulphide bond positions, and changes the protein's tertiary structure.

Homologous Chromosomes

Chromosomes of the same length with the same genes at the same loci, though they may carry different alleles.

Diploid vs Haploid Cells

Diploid cells (2n) contain two complete sets of chromosomes; haploid cells (n) contain a single set of unpaired chromosomes.

Overview of Meiosis

After DNA replicates in interphase, Meiosis I separates homologous chromosomes (halving chromosome number) via crossing over and independent segregation, and Meiosis II separates sister chromatids, producing four genetically varied haploid daughter cells.

How Crossing Over Creates Genetic Variation

Homologous chromosomes pair up (forming a bivalent) and chiasmata form between non-sister chromatids; equal lengths of chromatids and alleles are exchanged, creating new combinations of maternal and paternal alleles.

How Independent Segregation Creates Genetic Variation

Homologous pairs align randomly along the cell equator during Meiosis I, so it is random which chromosome from each pair enters each daughter cell, creating new combinations of maternal and paternal chromosomes.

Random Fertilisation and Variation

Random fusion of haploid gametes from two parents creates new combinations of alleles and maternal/paternal chromosomes, increasing genetic variation without mutation.

Mitosis vs Meiosis Outcomes

Mitosis has 1 division producing 2 genetically identical daughter cells with the same chromosome number; meiosis has 2 divisions producing 4 genetically varied daughter cells with half the chromosome number.

Importance of Meiosis

Produces haploid gametes so the diploid chromosome number is restored at fertilisation (maintaining chromosome number across generations), while crossing over and independent segregation generate genetic variation.

Chromosome Non-Disjunction

Spontaneous failure of homologous chromosomes (in Meiosis I) or sister chromatids (in Meiosis II) to separate, producing gametes with an extra chromosome (n+1) or a missing chromosome (n-1).

Calculating Chromosome Combinations After Meiosis and Fertilisation

Possible chromosome combinations in gametes after meiosis = 2^n; possible combinations after random fertilisation of two gametes = (2^n)^2, where n is the number of pairs of homologous chromosomes.

Genetic Diversity, Alleles, and Population Definitions

Genetic diversity is the number of different alleles of genes in a population; alleles are variations of a gene at the same locus arising by mutation; a population is a group of organisms of the same species in the same place and time that can interbreed to produce fertile offspring.

Importance of Genetic Diversity and Evolution

Genetic diversity enables natural selection because advantageous alleles can increase survival and reproductive success in certain environments; evolution is the change in allele frequency in a population over many generations via natural selection.

Principles of Natural Selection

1. Random gene mutation produces a new allele; 2. In a specific environment, the allele gives a selective advantage; 3. Possessors are more likely to survive and reproduce; 4. Advantageous allele is inherited by offspring; 5. Allele frequency increases over many generations.

Three Types of Adaptations

Anatomical: structural/physical features; Physiological: internal processes/chemical reactions; Behavioural: ways an organism acts to increase survival.

Directional vs Stabilising Selection

Directional selection favours one extreme trait in a changing environment (e.g. antibiotic resistance in bacteria), shifting the mean; stabilising selection favours the average/modal trait in a stable environment (e.g. human birth weight), reducing variation around the mean.

Required Practical 6: Aseptic Techniques

Wash hands and disinfect surfaces, sterilise pipettes/spreaders and boil agar, flame bottle necks, work near a Bunsen burner (upward air current carries microbes away), and lift petri dish lids only slightly to prevent contamination.

Required Practical 6: Antimicrobial Investigation Method

Spread bacteria evenly onto agar with a sterile spreader, place equal-sized discs soaked in antimicrobials using sterile forceps, tape lid lightly (not fully sealed) to allow oxygen in and prevent growth of harmful anaerobic bacteria, incubate upside down at 25°C for 48 hours, and measure inhibition zone area (pi r^2).

Required Practical 6: Clear Zones and Controls

Clear zones form where antimicrobial diffuses into agar and kills/inhibits bacteria (larger zone = more effective); no clear zone means bacteria are resistant or antimicrobial is ineffective; a water-soaked disc acts as a control to show the paper itself does not inhibit growth.

Species Definition and Hybrid Infertility

A species is a group of organisms that can interbreed to produce fertile offspring; two different species cannot produce fertile offspring because they have different chromosome numbers, so homologous pairs cannot form and meiosis cannot occur to produce gametes.

Role of Courtship Behaviour

Allows recognition of members of the same species, attracts/recognises the opposite sex, synchronises mating and gamete release, indicates sexual maturity/fertility, and forms a pair bond to raise young.

Phylogenetic Classification System and Hierarchy

Arranges species into taxa based on evolutionary origins (common ancestors) using a hierarchy of smaller groups within larger groups with no overlap: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Binomial Naming System

Each species is universally identified by two Latin names—its Genus (capitalised) followed by its species (lowercase)—avoiding confusion from local common names.

Interpreting Phylogenetic Trees

Branch points represent common ancestors and branches represent evolutionary paths; species sharing a more recent common ancestor are more closely related.

Genome Sequencing and Immunology in Taxonomy

Comparing DNA base sequences or protein tertiary structures (via antibody binding) clarifies evolutionary relationships: fewer DNA differences or greater antibody binding indicate a more similar amino acid sequence and a more recent common ancestor.

Biodiversity, Community, and Species Richness

Biodiversity is the variety of living organisms (species, genetic, and ecosystem diversity); a community is all populations of different species in an area; species richness is the number of different species in a community.

Index of Diversity Formula and Advantage

d = N(N - 1) / sum of n(n - 1), where N = total organisms of all species and n = organisms of each species; more useful than species richness because it also accounts for population sizes of each species.

How Farming Reduces Biodiversity and Conservation Balance

Removing woodland/hedgerows, monoculture, herbicides, and pesticides reduce plant variety, habitats, niches, and variety of food sources; conservation increases biodiversity but can lower crop yields and farmer income, so grants/incentives are offered.

Increasing Biodiversity in Agriculture

Reintroduce field margins and hedgerows, reduce pesticide use, grow multiple crops together (intercropping), and use crop rotation with nitrogen-fixing crops instead of artificial fertilisers.

Measuring Genetic Diversity Within and Between Species

Compare frequency of observable characteristics, DNA base sequences, mRNA base sequences, or amino acid sequences of proteins; direct DNA/amino acid comparison is more accurate than observable traits because traits can be polygenic or influenced by the environment.

Quantitative Sampling of Variation

Divide area into a grid, use a random number generator for coordinates to avoid bias, use the same measurement method, measure a large sample size until the running mean becomes constant, and release organisms unharmed.

Processing and Analysing Quantitative Variation Data

Calculate mean and standard deviation (SD, which shows spread around the mean); if SDs overlap, differences may be due to chance; use a statistical test (e.g. Student's t-test, correlation coefficient, chi-squared) to determine if differences are statistically significant.