DNA Structure and Organisation

Ever wondered why human cells are so much more complex than bacterial cells? It all comes down to how DNA is organised. In eukaryotic cells (like yours), DNA is linear, much longer, and wrapped around special proteins called histones to form chromosomes. Prokaryotic cells (bacteria) keep things simpler with shorter, circular DNA that floats freely without any protein packaging.

Here's something interesting - your mitochondria and chloroplasts (if you were a plant) actually contain DNA that looks exactly like bacterial DNA. This supports the theory that these organelles were once independent bacteria that got absorbed into larger cells millions of years ago.

Genes are specific sequences of DNA bases that code for either amino acid sequences in proteins or functional RNA molecules. Each gene has a fixed address on a chromosome called a locus - think of it like a postcode for genetic information.

Key Point: The way DNA is packaged tells us a lot about how complex an organism's cells are - neat packaging usually means more complexity!

The Genetic Code Explained

The genetic code is essentially nature's language for converting DNA instructions into proteins. Since there are only four DNA bases (G, C, T, A) but 20 different amino acids to code for, DNA uses a triplet system - three bases together code for one amino acid.

The maths is quite elegant: 4³ = 64 possible combinations, which is more than enough for 20 amino acids. This creates a degenerate code where multiple triplets can code for the same amino acid. This redundancy is actually brilliant - if a mutation changes one base, you might still get the same amino acid, so no harm done!

The code is also non-overlapping, meaning each base belongs to only one triplet. Think of reading a sentence where each three-letter word has a clear meaning - there's no confusion about where one word ends and another begins.

Introns and exons make eukaryotic genes more complex. Exons are the coding sequences that actually make proteins, whilst introns are non-coding sections that get removed during processing.

Key Point: The genetic code's redundancy acts like a safety net - many mutations won't actually change the final protein!

RNA and Protein Synthesis

Your genome (complete set of genes) stays constant throughout your life, but your proteome (all the proteins you can make) changes constantly depending on what your cells need. This flexibility is what allows the same DNA to create different cell types.

mRNA acts as a messenger, carrying genetic instructions from DNA in the nucleus to protein-making machinery in the cytoplasm. It's single-stranded and complementary to the DNA template. tRNA molecules are the delivery service - each one carries a specific amino acid and has an anticodon that matches up with codons on the mRNA.

Transcription happens in the nucleus where DNA helicase unzips the double helix, and RNA polymerase builds a complementary mRNA strand. The initial transcript gets processed by removing introns before leaving the nucleus.

Translation occurs at ribosomes in the cytoplasm. The ribosome reads mRNA codons one by one, matching them with tRNA molecules carrying the correct amino acids, then links the amino acids together with peptide bonds.

Key Point: Think of transcription as photocopying instructions, and translation as following those instructions to build something!

Mutations and Their Effects

Gene mutations are random changes in DNA sequence that occur during replication, though exposure to mutagenic agents increases the likelihood. These mutations can involve base substitutions or deletions, each having different consequences.

Base substitutions might have no effect thanks to the degenerate genetic code - the new triplet might still code for the same amino acid. However, base deletions cause frameshift mutations where all subsequent codons are altered, potentially changing multiple amino acids in the final protein.

Chromosome mutations involve changes in whole chromosome numbers, occurring when chromosomes fail to separate properly during meiosis (non-disjunction). This can result in polyploidy (extra chromosome sets) or aneuploidy (extra individual chromosomes).

Meiosis creates genetic variation through two nuclear divisions, producing four genetically different haploid gametes from one diploid parent cell. This contrasts with mitosis, which produces two identical diploid cells.

Key Point: Most mutations are harmful or neutral, but the rare beneficial ones provide the raw material for evolution!

Natural Selection and Evolution

Natural selection drives evolution by changing allele frequencies in populations over generations. When random mutations create new alleles that improve survival chances, those individuals are more likely to reproduce and pass on the advantageous genes.

The process follows a clear pattern: random mutations create new alleles, beneficial ones increase survival and reproduction rates, advantageous alleles get passed to offspring, and over many generations, these alleles become more common in the population.

Directional selection favours one extreme trait when environments change - think of bacteria evolving antibiotic resistance. Stabilising selection favours the average trait in stable environments, reducing variation by selecting against extremes.

Species are defined as groups that can produce fertile offspring together. Courtship behaviours ensure species recognition and successful mating, which is essential for species survival and passing on beneficial alleles.

Key Point: Evolution isn't random - it's the non-random survival of random mutations that happen to be beneficial!

Classification and Biodiversity

Phylogenetic classification organises species according to evolutionary relationships rather than just physical similarities. This system tells us how closely related different species are and when they shared common ancestors.

The binomial system gives each species a two-part scientific name: genus followed by species (written in italics). The classification hierarchy runs from Domain (largest) down to Species (smallest): Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.

Simpson's diversity index measures biodiversity in ecosystems using the formula D = N$N-1$/Σn$n-1$, where N is the total number of organisms and n is the number in each species. Higher values indicate greater diversity.

Understanding biodiversity is crucial for conservation efforts and ecosystem management. Different ecosystems support different levels of diversity, and human activities can significantly impact these natural patterns.

Key Point: Classification systems help us understand the tree of life - every species has a unique evolutionary story that connects it to all other life forms!