Biodiversity within a community

Biodiversity means the variety of living organisms in an area. Scientists measure it at any scale — from a single pond to the whole planet.

Genetic information, variation and relationships between organisms

Biodiversity within a community

Species richness tells you how many different species live in a community. A woodland with 40 different plant species has a higher species richness than one with only 10.

Genetic information, variation and relationships between organisms

Biodiversity within a community

An index of diversity is a single number that measures biodiversity. It accounts for both how many species exist and how many individuals belong to each species.

Genetic information, variation and relationships between organisms

Biodiversity within a community

The index of diversity (d) uses the formula d = N(N–1) / Σn(n–1), where N is the total number of all organisms counted and n is the count for each individual species. The symbol Σ means 'sum of', so you calculate n(n–1) separately for every species and then add all those values together. Follow these steps for any dataset: 1. Count all organisms across every species to get N. Calculate N(N–1). 2.

Genetic information, variation and relationships between organisms

Biodiversity within a community

Modern farming methods increase food production, but they also destroy habitats and remove species. This drives down the variety of living organisms in an area.

Genetic information, variation and relationships between organisms

Biodiversity within a community

Farming feeds people but damages biodiversity. Conservation protects biodiversity but can limit food production. Society must find ways to do both at the same time.

Genetic information, variation and relationships between organisms

Biodiversity within a community

Measuring biodiversity — the variety of living organisms in an area — requires more than simply counting how many different species are present; it also matters how individuals are distributed across those species. An index of diversity captures both of these factors in a single calculated value, giving a more meaningful picture of how healthy and stable a community is than species richness alone. Understanding how human activities such as intensive farming reduce biodiversity then sets up the real-world tension between food production and conservation — the protection of natural communities — that you will need to evaluate.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

The genome is the complete set of genes found in a cell. In humans, this means roughly 20,000–25,000 genes, all encoded in the DNA across 46 chromosomes. Every body cell carries the same genome — a liver cell and a neuron contain identical DNA. The proteome, however, is the full range of proteins a particular cell is able to produce. Unlike the genome, the proteome is not fixed. Consider a liver

Genetic information, variation and relationships between organisms

DNA and protein synthesis

mRNA is a single-stranded molecule that carries a copy of a gene's instructions. tRNA is a small molecule that reads those instructions and delivers the correct amino acid to build a protein.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

Transcription is the process where a cell copies a gene from DNA into a single-stranded molecule called mRNA. The enzyme RNA polymerase carries out this copying.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

In prokaryotic cells, transcription produces a finished mRNA molecule straight away. No editing steps happen before the mRNA is used to make a protein.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

In eukaryotic cells, copying a gene first produces a rough draft called pre-mRNA. The cell then cuts out non-coding sections and joins the useful parts together to make the final mRNA.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

Translation is the process where a ribosome reads the mRNA sequence and builds a protein. Transfer RNA molecules bring the correct amino acids, joining them into a chain called a polypeptide.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

You do not need to memorise which codon codes for which amino acid. AQA will always give you that information in the exam if you need it.

Genetic information, variation and relationships between organisms

DNA and protein synthesis

Every cell contains a genome — the complete set of genes — but it only ever produces a specific proteome, meaning the particular range of proteins that cell actually makes, and understanding how that happens is what this subtopic is about. The base sequence of DNA is first copied into mRNA (a single-stranded messenger molecule) during transcription, and in eukaryotes — organisms with a nucleus — this involves an editing step called splicing before the mRNA is ready to use. That mRNA is then read by ribosomes during translation, where tRNA molecules (which carry amino acids) match their anticodons to the codons on the mRNA, assembling a polypeptide chain. Mastering this flow of information from DNA to protein underpins almost everything else in genetics, from how mutations cause disease to how cells become specialised.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Prokaryotes are simple cells with no nucleus, such as bacteria. Their DNA forms a small, closed loop and floats freely in the cell, unattached to any proteins.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Eukaryotic cells (cells that contain a membrane-bound nucleus, such as human liver cells or plant leaf cells) store their genetic information as chromosomes inside that nucleus. Each chromosome consists of a single, extremely long DNA molecule wound tightly around spool-like proteins called histones. This winding is essential: a single human DNA molecule, if stretched out, would be roughly 5 cm lo

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Mitochondria and chloroplasts carry their own DNA. That DNA is short, circular, and not wrapped around any proteins.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

A gene is a specific sequence of DNA bases. It either carries instructions for building a protein, or it produces an RNA molecule that the cell uses directly.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Every gene sits at a precise, unchanging location on a chromosome. Biologists call this fixed address the gene's locus.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Every group of three bases in a DNA strand acts as a code word for one specific amino acid. Amino acids are the building blocks that join together to make proteins.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Every living organism uses the same DNA code to build proteins. Each set of three bases is read separately, and multiple triplets can code for the same amino acid.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Most DNA in a eukaryotic cell's nucleus does not carry instructions for making proteins. Large stretches of DNA sit between genes and do not code for any polypeptide.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Inside a gene, coding sections called exons carry the instructions for building a protein. Non-coding sections called introns sit between the exons and do not code for amino acids.

Genetic information, variation and relationships between organisms

DNA, genes and chromosomes

Genetic information is stored as DNA, but its organisation differs between cell types — in eukaryotic cells (cells with a nucleus), DNA is long, linear and wound around proteins called histones to form chromosomes, whereas in prokaryotic cells and in organelles like mitochondria, DNA is short and circular. A gene is a specific sequence of DNA bases that either codes for a polypeptide (a chain of amino acids that folds into a protein) or produces a functional RNA molecule, with every three bases — a triplet — specifying a particular amino acid. Understanding this structure is the foundation for everything that follows in this section, from how proteins are built to how mutations arise and drive genetic diversity.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Genetic diversity measures how many different versions of genes exist across all individuals in a population. More different versions means higher genetic diversity.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Natural selection needs variation to work on. Genetic diversity — the range of different alleles in a population — supplies that variation, giving selection something to act on.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Natural selection follows a clear causal chain: 1. A random mutation — a change in the DNA base sequence — produces a new allele (a new version of a gene). 2. By chance, that allele gives some individuals an advantage in their current environment. For example, a mutation in a bacterium might alter the active site of an enzyme that an antibiotic normally targets, making the antibiotic ineffective.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Directional selection happens when one extreme version of a trait gives organisms a survival advantage. Over generations, that extreme trait becomes more common in the population.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Stabilising selection is a type of natural selection that favours average individuals. It removes extreme traits from a population, keeping most individuals close to the middle of the range.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Natural selection acts as a filter over many generations. Individuals with traits that suit their environment survive and reproduce more, so those traits become more common in the species.

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Adaptations are features that help an organism survive and reproduce in its environment. They fall into three types: structural (body shape or parts), chemical/functional (internal processes), or behavioural (what the organism does).

Genetic information, variation and relationships between organisms

Genetic diversity and adaptation

Genetic diversity — the total number of different alleles, meaning different versions of genes, present in a population — is the raw material that makes natural selection possible. When an allele gives an organism a survival or reproductive advantage in its environment, that allele is passed on more frequently until it becomes more common across generations. This process drives two distinct patterns: directional selection, where one extreme trait is favoured (such as antibiotic resistance in bacteria), and stabilising selection, where intermediate traits are favoured (such as average birth weight in humans). Understanding these mechanisms explains how populations gradually accumulate adaptations — anatomical, physiological or behavioural features that suit them to their environment.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

A gene mutation is a permanent change to the sequence of DNA bases in a chromosome. This change alters the genetic instructions carried by that gene.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

During DNA replication, copying errors can occur by accident. These errors — called mutations — change the sequence of bases in the DNA, and they happen without any external cause.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

The genetic code is degenerate — multiple codons can code for the same amino acid. So swapping one DNA base for another sometimes makes no difference to the protein produced.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

Some chemicals and types of radiation damage DNA. They are called mutagenic agents, and they make gene mutations happen more often than normal.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

Sometimes chromosomes fail to separate properly during meiosis. This error, called non-disjunction, produces sex cells with the wrong number of chromosomes.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

Meiosis is a type of cell division that makes sex cells. Each sex cell it produces carries a unique combination of genetic information.

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

Meiosis starts with one diploid parent cell — a cell containing two full sets of chromosomes (46 in humans). It produces four haploid daughter cells, each with only one set (23 in humans). These daughter cells become gametes: sperm or eggs. The process involves two sequential nuclear divisions: 1. Before division begins, DNA replicates so each chromosome consists of two identical sister chromati

Genetic information, variation and relationships between organisms

Genetic diversity can arise as a result of mutation or during meiosis

Genetic diversity — the range of different alleles (versions of genes) present within a population — can arise through two main routes: gene mutations, which are changes to the DNA base sequence that may alter the protein an organism produces, and meiosis, the type of cell division that produces sex cells. During meiosis, processes called independent segregation (the random sorting of chromosome pairs) and crossing over (the exchange of DNA segments between paired chromosomes) shuffle alleles into new combinations, ensuring offspring are genetically unique. Understanding these mechanisms explains where heritable variation comes from — the raw material that natural selection acts on.

Genetic information, variation and relationships between organisms

Investigating diversity

Genetic diversity refers to the range of different alleles — alternative versions of a gene — present within or between species. Scientists use four methods to measure it: 1. **Observable or measurable characteristics** — traits such as shell banding patterns in snails, or height in humans, can be recorded across a population. A wider spread of trait frequencies indicates greater diversity. 2. **

Genetic information, variation and relationships between organisms

Investigating diversity

When biologists investigate variation within a species — for example, measuring the shell length of garden snails — they follow a structured approach to make their conclusions reliable. 1. Collect data from random samples. Random sampling means every individual has an equal chance of selection. This removes bias and makes the sample representative of the whole population. Techniques include rando

Genetic information, variation and relationships between organisms

Investigating diversity

You will never need to calculate a standard deviation in an AQA Biology exam. You do need to read, interpret, and compare standard deviation values that are given to you.

Genetic information, variation and relationships between organisms

Investigating diversity

Genetic diversity — the range of alleles, or different versions of genes, present within or between species — can be measured by comparing DNA base sequences, the mRNA transcribed from them, or the amino acid sequences of the proteins they encode. The more these sequences differ, the greater the genetic diversity. When investigating variation within a species, scientists collect data from random samples and use the mean and standard deviation — a measure of how spread out the values are around the average — to draw reliable conclusions. These methods give you the tools to turn biological observations into evidence you can actually interpret and evaluate.

Genetic information, variation and relationships between organisms

Species and taxonomy

Two organisms belong to the same species if they can mate and produce offspring that are themselves able to reproduce. Producing offspring is not enough — that offspring must also be fertile.

Genetic information, variation and relationships between organisms

Species and taxonomy

Before animals mate, they perform courtship behaviours. These rituals allow individuals to identify others of the same species, so that mating produces fertile offspring.

Genetic information, variation and relationships between organisms

Species and taxonomy

Phylogenetic classification groups species by their evolutionary history — how closely related they are. Species that share a more recent common ancestor sit in the same group.

Genetic information, variation and relationships between organisms

Species and taxonomy

Classification arranges living things into a series of nested groups. Every small group sits entirely inside a larger one, and no group ever belongs to two different larger groups at the same time.

Genetic information, variation and relationships between organisms

Species and taxonomy

Biologists sort living things into named groups. Each of those groups — at any level — is called a taxon. The plural of taxon is taxa.

Genetic information, variation and relationships between organisms

Species and taxonomy

Biologists sort all living things into eight ranked groups. From broadest to most specific, these are: domain, kingdom, phylum, class, order, family, genus and species.

Genetic information, variation and relationships between organisms

Species and taxonomy

Every species gets a unique two-part Latin name called a binomial. The first part names the genus; the second names the species — for example, Homo sapiens.

Genetic information, variation and relationships between organisms

Species and taxonomy

AQA does not ask you to memorise different classification systems. You only need to know the single hierarchy taught in this subtopic.

Genetic information, variation and relationships between organisms

Species and taxonomy

Organisms are grouped into the same species if they can interbreed to produce fertile offspring, with courtship behaviour playing a key role in ensuring individuals recognise suitable mates. To make sense of the enormous diversity of life, biologists use phylogenetic classification — a system that organises species into nested groups called taxa (singular: taxon) based on their shared evolutionary history. This hierarchy runs from the broadest level (domain) down to the most specific (species), and every species is given a unique two-part Latin name — its binomial — made up of its genus and species, such as Homo sapiens.

Genetic information, variation and relationships between organisms