Question

1. Fossils are the remains and traces of past life that have been preserved in the Earth's crust. What type of fossils is most important in supporting the principle of evolution? Explain your answer and give examples.

2. Describe the 3 main steps of DNA replication in the correct order. Be sure to include the key structures involved in the process.

*MUST USE scholarly sources*

Answer 1

1. Transitional fossils are remnants of an organism that came in between a known version of a species and the current species. Allegedly, transitional fossils be evidence for evolution because it would show intermediate forms of a species and they changed and accumulated adaptations at a slow pace.

Examples are Archaeopteryx and Tiktaalik . Archaeopteryx possesses a combination of traits that clearly place it as a transitional form between non-avian dinosaurs and birds. Its similarities to non-avian dinosaurs include a long feathered tail and small teeth. Unlike non-avian dinosaurs however, Archaeopteryx also has flight feathers and wings, just like a modern bird. The discovery of the furcula, or fused clavicle bone, in Archaeopteryx was a firm confirmation of the relationship between birds and dinosaurs, as they are the only two groups to have this anatomical feature.

Tiktaalik is an ancient fish called a sarcopteryigian, or lobe-finned fish. Although it bears many similarities to fish like gills, scales, and fins, other key characteristics link Tiktaalik to land animals. While it did have fins, the bones inside the fins are homologous to the bones of the human hand and wrist, indicating it may have been able to bear weight. The animal also had a mobile neck and a strong ribcage, two critical traits that allowed four-legged (tetrapod) creatures to move onto land.

2.   DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. The three steps in the process of DNA replication are initiation, elongation and termination.

Initiation

Replication begins at a location on the double helix known as “oriC” to which certain initiator proteins bind and trigger unwinding. Enzymes known as helicases unwind the double helix by breaking the hydrogen bonds between complementary base pairs, while other proteins keep the single strands from rejoining. The “topoisomerase” proteins surround the unzipping strands and relax the twisting that might damage the unwinding DNA. The cell prepares for the next step, elongation, by creating short sequences of RNA called primers that provide a starting point of elongation.

Elongation

With the primer as the starting point for the leading strand, a new DNA strand grows one base at a time. The existing strand is a template for the new strand. For example, if the next base on the existing strand is an A, the new strand receives a T. The enzyme DNA polymerase controls elongation, which can occur only in the leading direction. The lagging strand unwinds in small sections that DNA polymerase replicates in the leading direction. The resulting small “Okazaki fragments” can contain 1,000 to 2,000 bases in bacteria, but eukaryotes -- organisms having cells with nuclei -- have fragments of only 100 to 200 bases. The fragments terminate in an RNA primer that is subsequently removed so that enzymes can stitch the fragments into an elongating strand.

Termination

After elongation is complete, two new double helices have replaced the original helix. During termination, the last primer sequence must be removed from the end of the lagging strand. This last portion of the lagging strand is the telomere section, containing a repeating non-coding sequence of bases. Enzymes snip off a telomere at the end of each replication, leading to shorter strands after each cycle. Finally, enzymes called nucleases “proofread” the new double helix structures and remove mispaired bases. DNA polymerase then fills in the gaps created by the excised bases.