Summary
Highlights
This section introduces the fundamental components of heredity. It differentiates between eukaryotic and prokaryotic DNA, explaining that eukaryotic DNA is long, linear, associated with histones, and found in the nucleus, forming chromosomes. Prokaryotic DNA is short, circular, and not associated with histones, often accompanied by plasmids. Mitochondrial and chloroplast DNA are also discussed, noting their similarity to prokaryotic DNA in being short, circular, and histone-free. The definition of a gene as a DNA base sequence coding for polypeptide amino acid sequences or functional RNA is highlighted, along with concepts like locus, universal, non-overlapping, and degenerate genetic code.
This part details the two main stages of protein synthesis: transcription and translation. Transcription occurs in the nucleus, where DNA's hydrogen bonds break to expose a template strand. Free nucleotides align via complementary base pairing (with uracil replacing thymine), and RNA polymerase forms phosphodiester bonds to create pre-mRNA. Introns are then removed through splicing to form mature mRNA, which exits the nucleus. Translation takes place in the cytoplasm on ribosomes, where mRNA attaches, and tRNA molecules with specific amino acids bind to complimentary mRNA codons. Peptide bonds are formed between amino acids (using ATP energy), leading to a polypeptide chain.
Mutations, defined as changes in the DNA base sequence, are explored. They occur spontaneously during DNA replication and can be sped up by mutagenic agents like radiation. Two types are discussed: base substitution (one base replaced by another) and base deletion (removal of a base). Base substitution might lead to a different amino acid or no change due to degeneracy, potentially altering protein structure depending on the position. Base deletion causes a frameshift mutation, drastically altering the amino acid sequence downstream and significantly changing the protein's tertiary structure.
Meiosis is introduced as a cell division producing haploid gametes (sex cells) from diploid cells, halving the chromosome number. Key stages involve DNA replication, homologous chromosomes lining up, first meiotic division (separating homologous chromosomes), and second meiotic division (separating sister chromatids), resulting in four genetically distinct haploid gametes. Meiosis contributes to variation through independent segregation of homologous chromosomes (2^N possibilities) and crossing over (exchange of genetic material between homologous chromosomes). Random fertilization further increases variation by allowing any sperm to fertilize any egg.
Genetic diversity is defined as the number of different alleles of genes in a population. Higher genetic diversity allows populations to better adapt to changing conditions through natural selection. Natural selection occurs when beneficial random mutations (new alleles) increase an individual's chance of survival and reproduction, thus increasing the allele's frequency in the population. Adaptations can be anatomical (physical features), physiological (internal processes), or behavioral (actions).
This section explains two types of natural selection. Directional selection, exemplified by antibiotic resistance in bacteria, shifts the mean phenotype towards one extreme. For instance, in bacteria, antibiotic exposure selects for individuals with higher resistance, shifting the population's resistance level. Stabilizing selection, illustrated by human birth weight, favors intermediate phenotypes and reduces variation at both extremes. Babies with very low or very high birth weights have lower survival rates, leading to a population with more intermediate birth weights over time.
Species are defined as organisms that can mate and produce fertile offspring. Courtship behaviors are discussed as mechanisms to recognize own species, identify fertile mates, and synchronize mating. The classification system (taxonomy) organizes species hierarchically from domain to species. The binomial system (genus and species) provides a universal naming convention. Biodiversity refers to the range and variety of living organisms in an area, measured by species richness and the index of diversity. The formula for the index of diversity, considering both the number of species and individuals per species, is explained with examples.
Genetic diversity and variation can be investigated by comparing measurable characteristics (e.g., height, mass) or molecular data. Comparing DNA base sequences, mRNA base sequences, or amino acid sequences of proteins between species reveals their relatedness. A phylogenetic tree visually represents the evolutionary relationships between species, with branching points indicating common ancestors. Higher DNA similarity suggests a more recent divergence into separate species.