Chapter 10: The Cell Cycle

The physical location of these units was eventually traced to chromosomes, where their pairing and separation during meiosis correlate perfectly with Mendelian principles, as theorized by Sutton. Further research using organisms like Drosophila demonstrated that genes residing on the same chromosome form linkage groups, but this linkage is often incomplete due to crossing over (genetic recombination), a process that allows genetic mapping based on recombination frequency. The definitive chemical identity of the gene was resolved through key experiments, including Griffith’s bacterial transformation and the Hershey–Chase bacteriophage study, which established DNA as the carrier of genetic information, overturning the earlier tetranucleotide theory. The ensuing Watson–Crick model depicted DNA as an antiparallel, complementary double helix, held together by specific hydrogen-bonded base pairs, consistent with Chargaff’s rules. The chapter explores how DNA’s structure supports its functions in genetic storage, replication, and expression, noting that the molecule’s topology is regulated by enzymes called topoisomerases that manage supercoiling. Analysis of genome complexity relies on techniques like DNA denaturation and renaturation, revealing that eukaryotic genomes are highly complex, containing highly repeated elements (such as microsatellite short tandem repeats, or STRs, implicated in diseases like Huntington’s), moderately repeated dispersed sequences (SINEs and LINEs), and nonrepeated single-copy genes. Genomic change is constant, driven by events like whole-genome duplication (polyploidization) and gene duplication via unequal crossing over, leading to the evolution of multigene families and pseudogenes. Comparative genomics between humans and chimpanzees reveals high sequence similarity but significant differences in structural variation and Copy Number Variations (CNVs), such as the duplication of the AMY1 gene. Genetic variation within the human population is defined by polymorphisms, most commonly single-nucleotide polymorphisms (SNPs), which are often inherited in blocks known as haplotypes, essential tools for genome-wide association studies (GWAS) focused on complex diseases. Finally, modern advancements include synthetic biology, such as the creation of minimal genomes, and the use of natural gene transfer mechanisms, like those employed by Agrobacterium tumefaciens, for genetic engineering in plants.