The fact that all species utilize the same genetic code to synthesize proteins argues for a common ancestry to all life on earth. • Comparing Chromosome banding patterns, DNA sequences, or proteins. • DNA sequences change over time as mutations occur. The fewer differences, the more closely related the two species – the more recently they diverged from a common ancestor. • Assumes that it is highly unlikely that two unrelated species would happen to evolve precisely the same sequence of DNA nucleotides or amino acids simply by chance.
Comparing Genomes (all the DNA of an organism) • 1. humans, chimpanzees, and bonobos (pygmy chimps) share 99.5 percent of the genes that encode proteins. However, the 0.5 percent difference has major effects on phenotype.
Comparing Chromosomes • 1. Scientists compare the band patterns and gene orders on stained chromosomes as a measure of species relatedness. • 2. Chromosomes, genes or proteins that are identical or very similar in different species are said to be highly conserved. • 3. Examples • All mammals have identically banded X chromosomes. • Segments of chromosome # 1 in humans, apes, and monkeys are similar to parts of chromosomes in cats and mice. • 4. Gene order (synteny) is better to look at than banding patterns.
Comparing Protein Sequences • 1. The amino acid sequence in the proteins of different species can be compared to analyze relatedness. • 2. Chimpanzee proteins and human proteins are alike in 99% of their amino acids. Several are virtually identical. • 3. Cytochrome C – involved in cellular respiration (extracting energy from nutrients)
Cytochrome C • 20 of a total of 104 amino acids occupy identical positions in the cytochrome C of ALL Eukaryotes. • The more closely related two species are, the more alike their cytochrome C amino acid sequence is. • Differences in Cytochrome C amino acid sequence between human & : Horse 12 amino acids Kangaroo 8 amino acids Chimpanzee identical
Homeobox Proteins • Protein that ensures that anatomical parts- whether a leg, petal, or segment of a larva-develop in the appropriate places. • Encoded by homeobox DNA sequences. • If there are mutations they result in legs on the heads of flies, leukemia (from misplaced white blood cells), DiGeorge syndrome (missing thymus and parathyroid glands) • Fly with missing eyes gets mouse gene and grows eyes
Comparing DNA Sequences • DNA hybridization- compares how complementary DNA sequences are. • Researcher unwinds DNA double helixes from two species and mixes them. • The rate at which the helices re-form is a direct measure of how similar they are in sequence. When there are more mismatches the DNA takes longer to reform into helices. • Ancient DNA • Need to use PCR (polymerase chain reaction) to amplify or replicate the DNA. It is very important that the ancient DNA is not contaminated.