Genetic Codes
▫ A gene is a conceptual entity, not a physical chemical complex. Genes are just ideas in the minds of geneticists.
▫ Genes were first conceived as molecular packages of trait heredity. They were later redefined as recipes for making proteins. Once other biological products were discovered to be important, and their templates stored in DNA, the definition of gene was extended to include them as non-protein coding genes. Now genes are conceived as molecular packages of trait heredity.
▫ Historically, a gene was assumed to be a certain strand of DNA that cogently carried a coding sequence for synthesizing a protein. Such facilely imagined localized genes are relatively rare, and their codes are not necessarily faithfully transcribed.
1st, physical trait data may be in multiple places. 2nd, translating genetic instructions into the intended product is a tortuous path, where much material goes unused or is waylaid in the expression process.
▫ DNA sequences act as instructions for specific manufactures. Gene expression is the process of using the information in a gene to synthesize a functional bioproduct, which is typically a protein.
▫ Prodigious research efforts have been expended toward gene mapping: pinpointing the loci of coding sequences for traits. The quest is mostly quixotic. Inherited traits are commonly the product of a network of genes, and gene expression is far more than just transcription at a gene’s locus. A trait may be the outcome of many genes, either activated or suppressed, in a long sequence of reactions leading to a result, with innumerable other factors in play.
▫ An allele is a gene variation at the same locus. Different alleles may result in different traits, but not necessarily. One allele may be dominant, while another recessive. A dominant allele may completely mask a recessive allele.
▫ A genome is the total complement of genes in a cell or organism. Individual cells have their own genomes. How that comes to be is not known.
▫ A prokaryotic cell has a single genophore stuffed with genetic material. Prokaryotes readily pick up new genetic material in non-genophore plasmid packages, which are exchanged with other microbes via horizontal gene transfer (HGT). Eukaryotes also employ HGT, though not nearly as frequently. A eukaryotic cell keeps its genome packed in chromosomes.
The genetic makeup of a multicellular organism is multifarious. Each cell has its own individualized genomic package (pergenome). Genetic variation among cells is termed chromosomal mosaicism.
▫ Evolutionary descent frequently involves carrying more genetic baggage. While over 90% of the genome of prokaryotes is employed, less than 2% of human genes are used for protein production.
DNA not directly recognized as used in protein production is termed noncoding. Such genetic material is often repetitive, sometimes with slight variations. This unobvious genetic material is employed in subtle ways that are just beginning to be appreciated.
▫ Algae, slime molds, fungi, and plants practice alternation of generations: alternate reproductive and genetic modes during their life cycle: asexual spore production while haploid, and sexual reproduction while diploid. A few animals may alternate reproductive modes – sexual and asexual – but all animals are always diploid.
▫ There is a genomic protection system that maintains the integrity of a cell’s genome, especially against damage that may be caused by self-serving jumping genes that otherwise proliferate.
