Cells and genomes: how genes shape organisms
Key ideas • Key ideas
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Cells as the fundamental units of life
A cell is the smallest unit that shows all the processes of living organisms. Single-celled organisms perform all life functions within one cell; multicellular organisms perform specialised functions by organising many cells into tissues and organs. Cells appear in two broad groups: prokaryotic cells (smaller, no nucleus) and eukaryotic cells (larger, with a nucleus) . Organelles inside eukaryotic cells carry out distinct roles. The nucleus stores chromosomes and DNA; mitochondria release energy by respiration; ribosomes synthesise proteins. Structural differences between cell types cause functional specialisation, so cell structure determines cell function and supports organisation into tissues and systems .
Genome, chromosomes and genes
A genome is one complete copy of all the DNA in a diploid body cell and is organised into pairs of chromosomes. Humans have 23 pairs of chromosomes; each chromosome contains many genes, which are short sections of DNA that code for proteins . DNA consists of four bases (A, T, C and G) arranged as base pairs in a double helix; sequences of three bases (triplet code) specify amino acids during protein synthesis. Chromosomes compactly coil long DNA molecules so large genomes fit into cell nuclei, and some DNA regions do not code for proteins (non-coding DNA) .
How genes determine characteristics
Genes provide the instructions to build polypeptides and proteins. The processes of transcription (making an RNA copy of a gene) and translation (assembling amino acids into a polypeptide) convert a gene’s base sequence into a functional protein; that protein influences a measurable characteristic such as enzyme activity or pigment production . Different alleles of the same gene give rise to variation in the protein produced. Dominant and recessive relationships between alleles affect the expressed phenotype, and most organisms inherit one allele from each parent, producing characteristic patterns of inheritance and predictable genotype–phenotype relationships .
Variation, mutation and inheritance
Sexual reproduction and meiosis generate genetic variation by producing gametes with different combinations of alleles; offspring inherit a unique genome made from parental contributions, so individuals normally carry two alleles per gene . Mutations are permanent changes to DNA base sequences. Mutations can substitute, add or remove bases and can alter proteins by changing amino acids or shifting reading frames; mutations can be neutral, harmful or occasionally advantageous and provide raw material for evolutionary change .
Environmental influence on phenotype
Observable characteristics (phenotype) result from interactions between genotype and environment. Identical genomes can produce different phenotypes under different environmental conditions, so nutrition, temperature and acquired physical changes alter growth, physiology and appearance without changing DNA sequence . Natural selection acts on phenotypic variation that affects survival and reproduction. Environmental pressures favour certain variants, so adaptation arises when genetic variation that increases fitness becomes more common in a population over generations .
Limitations and precise definitions
A gene is a section of DNA that provides the code to make a protein; an allele is one version of a gene. The genome denotes the full DNA complement in body cells, not the immediate environment or learned changes; phenotype denotes measurable characteristics produced by protein function and environmental effects . Non-coding DNA comprises most of human DNA and can vary between species; not all DNA sequence differences change proteins. Careful distinction between genotype and phenotype prevents confusion when interpreting inherited traits and environmental effects .
Key notes
Important points to keep in mind