Genetics: Inheritance and DNA
Types of Objects and Evolution
The world contains two main types of objects: living organisms and inert matter. A key difference is the ability of living organisms, such as human beings, to reproduce. These copies are not identical, creating diversity that allows adaptation to different environments and drives the evolution of species.
Darwin proposed that competition for resources among species influences the selection of characteristics. He initially believed in blended inheritance, where offspring inherit a mix of their parents’ traits. However, this contradicted the need for diversity for natural selection to operate.
Mendel’s Discovery: Genes
Mendel demonstrated that inheritance does not involve blending. Instead, units of heredity, called genes, retain their individuality across generations. This explains the reappearance of parental traits in later generations, disproving the blended inheritance theory. Mendel showed that traits are transmitted independently, with two versions of each gene (one from each parent), such as the gene for pea color (yellow or green).
A gene is a unit of hereditary information that controls a specific trait. Genes are located within the cell nucleus, on structures called chromosomes. Humans have 23 pairs of chromosomes, including one pair of sex chromosomes (XX for females and XY for males).
Genome Size and Complexity
The size and complexity of an organism’s genome (the complete set of genetic information) is not directly correlated with the organism’s complexity. Organisms with more genes or nucleotides are not necessarily more complex.
Genomics is the study of genomes and their relationship to diseases, such as cancer.
Fertilization and Genetic Endowment
Sex cells (gametes) contain only 23 chromosomes, half the number found in other body cells. During fertilization, the combination of sperm and egg restores the full set of 23 pairs.
DNA Structure and Replication
Genes are segments of DNA. DNA’s double helix structure was discovered through X-ray diffraction and Chargaff’s rules, which describe the pairing of DNA bases (A with T, and C with G).
DNA replication resembles a zipper: the two strands separate, and each strand serves as a template for creating a new, identical strand. This ensures accurate transmission of genetic information to offspring.
Genetic Code and Protein Synthesis
The genetic code uses sequences of DNA nucleotides to instruct the production of proteins. Each group of three nucleotides (a codon) codes for a specific amino acid. Proteins perform most functions in living organisms.
Genome Composition
The human genome contains about 23,000 genes, which make up about 5% of the total DNA. Within a gene, exons are the portions that code for proteins, while introns do not. Some DNA is considered “junk DNA” because it does not code for any known function.
Developmental Genetics
Developmental genetics studies how organisms develop. Animals develop in a modular fashion, with specific genes expressed in certain areas during development. This involves cell proliferation (replication of cells and their genomes) and differentiation (regulation of gene expression to create different tissues).