Question

This questions are from Evolution class

1. In a short answer: What does the evidence point to as the ancestors of eukaryotes? How did eukaryotes form? Your answer

2. Mitochondria have their own Genes; in humans, only 13 genes. What is significant about this related to evolutionary origin of mitochondria AND why might be an ecological/evolutionary reason for there be so few mitochondrial Genes?

3. What is the principle of the selfish gene and how is our common way of framing genetics a skyhook?

Answer 1

1. The eukaryotes developed at least 2.7 billion years ago, following some 1 to 1.5 billion years of prokaryotic evolution. Studies of their DNA sequences indicate that the archaebacteria and eubacteria are as different from each other as either is from present-day eukaryotes. Eukaryotes are believed to have formed from the divergence of three lines of descent from a common ancestor, giving rise to archaebacteria, eubacteria, and eukaryotes. Interestingly, many archaebacterial genes are more similar to those of eukaryotes than to those of eubacteria, indicating that the archaebacteria and eukaryotes share a common line of evolutionary descent and are more closely related to each other than either is to the eubacteria.

A critical step in the evolution of eukaryotic cells was the acquisition of membrane-enclosed subcellular organelles, allowing the development of the complexity characteristic of these cells.The hypotheses that eukaryotic cells evolved from a symbiotic association of prokaryotes—endosymbiosis—which are thought to have evolved from bacteria living in large cells. Mitochondria and chloroplasts contain their own DNA, which encodes some of their components. The mitochondrial and chloroplast DNAs are replicated each time the organelle divides, and the genes they encode are transcribed within the organelle and translated on organelle ribosomes. Mitochondria and chloroplasts thus contain their own genetic systems, which are distinct from the nuclear genome of the cell. An endosymbiotic origin for these organelles is now generally accepted, with mitochondria thought to have evolved from aerobic bacteria and chloroplasts from photosynthetic bacteria, such as the cyanobacteria. The acquisition of aerobic bacteria would have provided an anaerobic cell with the ability to carry out oxidative metabolism. Through time, most of the genes originally present in these bacteria apparently became incorporated into the nuclear genome of the cell, so only a few components of mitochondria and chloroplasts are still encoded by the organelle genomes. Multicellular organisms evolved from unicellular eukaryotes at least 1.7 billion years ago. Some unicellular eukaryotes form multicellular aggregates that appear to represent an evolutionary transition from single cells to multicellular organisms. For instance, the cells of many algae (e.g., the green alga Volvox) associate with each other to form multicellular colonies , which are thought to have been the evolutionary precursors of present-day plants.

1. Mitochondria, which are found in almost all eukaryotic cells, are the sites of oxidative metabolism and are responsible for generating most of the ATP derived from the breakdown of organic molecules. The genome contains 2 rRNA genes, 22 tRNA genes, and 13 protein-coding sequences. Compared to nuclear, chloroplast and bacterial genomes, the mitochondrial genome has certain surprising features.

1. Dense gene packing

2. Relaxed codon usage and

3. Variant genetic code.

The relatively small size of the human mitochondrial genome made it a particularly attractive target for early DNA-sequencing projects, and in 1981, the complete sequence of its 16,569 nucleotides was published. By comparing this sequence with known mitochondrial tRNA sequences and with the partial amino acid sequences available for proteins encoded by the mitochondrial DNA, all of the human mitochondrial genes were mapped on the circular DNA.

The genetic code is nearly the same in all organisms provides strong evidence that all cells have evolved from a common ancestor. There are few differences in the genetic code in many mitochondria. The reason is the mitochondrial genetic code is different in different organisms. In the mitochondrion with the largest number of genes , that of the protozoan Reclinomonas, the genetic code is unchanged from the standard genetic code of the cell nucleus. Yet UGA, which is a stop codon elsewhere, is read as tryptophan in mitochondria of mammals, fungi, and invertebrates. Similarly, the codon AGG normally codes for arginine, but it codes for stop in the mitochondria of mammals and codes for serine in the mitochondria of Drosophila. Such variation suggests that a random drift can occur in the genetic code in mitochondria. Presumably, the unusually small number of proteins encoded by the mitochondrial genome makes an occasional change in the meaning of a rare codon tolerable, whereas such a change in a large genome would alter the function of many proteins and thereby destroy the cell.

Comparisons of DNA sequences in different organisms reveal that the rate of nucleotide substitution during evolution has been 10 times greater in mitochondrial genomes than in nuclear genomes, which presumably is due to a reduced fidelity of mitochondrial DNA replication, inefficient DNA repair. Because only about 16,500 DNA nucleotides need to be replicated and expressed as RNAs and proteins in animal cell mitochondria, the error rate per nucleotide copied by DNA replication, maintained by DNA repair, transcribed by RNA polymerase, or translated into protein by mitochondrial ribosomes can be relatively high without damaging one of the relatively few gene products. This could explain why the mechanisms that perform these processes are relatively simple compared with those used for the same purpose elsewhere in cells. The presence of only 22 tRNAs and the unusually small size of the rRNAs are expected to reduce the fidelity of protein synthesis in mitochondria.

3. The gene-centered view of evolution is a synthesis of the theory of evolution by natural selection, the particulate inheritance theory, and the non-transmission of acquired characters. It states that those alleles whose phenotypic effects successfully promote their own propagation will be favorably selected relative to their competitor alleles within the population. This process produces adaptations for the benefit of alleles that promote the reproductive success of the organism, or of other organisms containing the same allele or even its own propagation relative to the other genes within the same organism