Recombination allows chromosomes to exchange their genetic information, producing new combinations of genes, which is efficient for the natural selection process. It allows for new proteins to evolve, some of which will be evolutionarily advantageous, while some will not. The most common type of crossover is called a homologous crossover, where two very similar chromosomal segments exchange information. In general, non-homologous recombination is not evolutionarily advantageous and usually damages the cell or causes genetic abnormalities. The enzymes that do the recombination process are called recombinases, of which there are several types. They break a double strand by using an endonuclease and join the broken strand to another strand of DNA. Only strands that have similar polarity will exchange during the recombination process. There are two types of cleavage reactions, one that cleaves both strands of DNA, while another that cleaves just one strand of the DNA. Either will allow for genetic diversity that can be advantageous to the offspring. The DNA genome contains all of the genetic information that allows the cell and the organism to survive and reproduce. It is believed that the earliest forms of life just used RNA and didn’t involve DNA. Eventually, organisms were based on DNA and the RNA was responsible for making the proteins in the ribosomes. No one knows how or where the original base pairs came from or how they were created to make complex forms of life. All of this happened billions of years ago and DNA fragments don’t last very long so they can’t be isolated from fossils.
Genes A gene is a segment or region of DNA that is made of up nucleotides that together have the potential to create a protein used by the cell. Genes make up the genotype of the cell and are ultimately responsible for the physical features or “phenotype” of the organism. Genes represent the molecular units of heredity. Most biological features are the result of multiple genes acting together but there are some genes that act alone to create a certain protein or feature in an organism. Some genetic traits are easily seen, like hair and eye color, the number of limbs, and the color of the skin. Other genes code for things that aren’t recognizable, like those for intracellular enzymes and those for a person’s blood type. Genes can become mutated leading to different alleles or variations of the gene. In the population, these different alleles account for a genetic variation, natural selection, and, in many cases, the survival of the fittest—which are those organisms that have the very best alleles that allow for some improvement in the way the organism functions. Genes are usually closely connected to their regulatory regions but this just doesn’t have to be the case, as was relatively recently discovered. Coding regions related to a specific gene can be separated into several exons that are spliced together in the coding process. Some viruses will store their entire genome in a segment or several segments of RNA instead of DNA and some 15
