Dna And Evolution

DNA and Evolution DNA contains the genetic information that allows all modern living things to function, grow and reproduce. However, it is unclear how long in the 4-billion-year history of life DNA has performed this function, as it has been proposed that the earliest forms of life may have used RNA as their genetic material. RNA may have acted as the central part of early cell metabolism as it can both transmit genetic information and carry out catalysis as part of ribozymes. This ancient RNA world where nucleic acid would have been used for both catalysis and genetics may have influenced the evolution of the current genetic code based on four nucleotide bases. This would occur since the number of different bases in such an organism is a trade-off between a small number of bases increasing replication accuracy and a large number of bases increasing the catalytic efficiency of ribozymes. Unfortunately, there is no direct evidence of ancient genetic systems, as recovery of DNA from most fossils is impossible. This is because DNA will survive in the environment for less than one million years and slowly degrades into short fragments in solution. Claims for older DNA have been made, most notably a report of the isolation of a viable bacterium from a salt crystal 250 million years old, but these claims are controversial. The following table gives the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, human cells are diploid and have 22 different types of autosome, each present as two copies, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as bread wheat, which is hexaploid and has six copies of seven different chromosomes – 42 chromosomes in total. Chromosome numbers in some plants Plant Species # Arabidopsis thaliana 10 (diploid) Rye (diploid) 14 Maize (diploid) 20 Einkorn wheat (diploid) 14 Durum wheat 28 (tetraploid) Bread wheat 42 (hexaploid)[ Potato (tetraploid) 48 Cultivated tobacco 48 (diploid) Adder's Tongue Fern approx (diploid) 1,400

Chromosome numbers (2n) in some animals Species # Species # Common fruit fly 8 Guinea Pig 64 Chinese hamster (Cricetulus 22 Garden snail 54 griseus) Earthworm Octodrilus 36 Tibetan fox 36 complanatus Domestic cat 38 Domestic pig 38 Laboratory Laboratory mouse 40 42 rat Rabbit (Oryctolagus Syrian 44 44 cuniculus) hamster Hares 48 Human 46 Domestic Gorillas, Chimpanzees 48 54 sheep Elephants 56 Cow 60 Donkey 62 Horse 64 13 Dog 78 Kingfisher 2 100Goldfish Silkworm 56 104

Human chromosomes Human cells have 23 pairs of large linear nuclear chromosomes, giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database. Number of genes is an estimate as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.