Genophores & Chromosomes
Cell central holds the principal genetic material, but not all of a cell’s genome. For a prokaryote, cell central is the nucleoid, with the primary genetic package in a genophore. For a eukaryote, cell central is the nucleus, containing chromosomes.
A prokaryote’s nucleoid holds a cell’s genome in a single genophore: a large double-stranded DNA molecule, generally circular in shape. A nucleoid is not enclosed in a membrane. Having only a single copy of each gene makes a prokaryote haploid.
A typical prokaryote has 2,000 to 4,000 genes; a housefly, mouse, or human: ~20,000. An ocean bacterium, Pelagibacter ubique, has the most efficient genome known: 1,354 genes; no clutter, no noncoding sequences, no duplicate entries, no viral genes, nor any introns.
220 human genes come courtesy of horizontal gene transfer from a prokaryotic pal: bacteria. This was a direct transfer, not a product of ancestral lineage.
A fundamental concept in biology is that heritable material, DNA, is passed from parent to offspring, a process called vertical gene transfer. An alternative mechanism of gene acquisition is through horizontal gene transfer (HGT), which involves movement of genetic material between different species. HGT is well known in single-celled organisms such as bacteria. HGT has contributed to the evolution of many, perhaps all, animals and that the process is ongoing. ~ English biochemist Alastair Crisp et al