Cell Biology, Evolution, and Primordial Earth: Key Concepts
Cell Division and Processes
Interphase (I): Distinct nucleus, normal cell functions.
Prophase (P): Nuclear membrane breaks down, chromosomes condense, spindles form.
Metaphase (M): Chromosomes line up across the equator of the cell.
Anaphase (A): Chromatids migrate to poles.
Telophase (T): Two new nuclei form, chromosomes decondense.
Cytokinesis (C): Cytoplasm divides, resulting in two new cells.
Translation
- mRNA binds to ribosome.
- mRNA advances until AUG (start codon) is at the P site.
- tRNA binds to codon at the anticodon, bringing the appropriate amino acid.
- Another tRNA binds at the A site.
- A peptide bond is formed between the two amino acids.
- Elongation: P site tRNA moves to the E site, A site tRNA moves to the P site (P>E, A>P).
- The process repeats.
- The tRNA at the E site is ejected.
- Repeat until a stop codon is reached.
- Release factor protein binds to the stop codon, terminating translation.
Transcription
- RNA polymerase binds at the promoter region of the gene.
- DNA unzips.
- RNA polymerase moves forward until it reaches the coding region.
- RNA polymerase produces RNA complements (5′ to 3′) on the template strand.
- The process continues until a termination sequence is reached.
- A hairpin loop forms.
- RNA is released.
- RNA is edited: introns are removed, and exons are spliced together to form functional mRNA, a process carried out by snRNPs (small nuclear ribonucleoproteins).
Primordial Earth and the Origin of Life
Primordial Earth Conditions: Highly energetic environment with high temperature, UV radiation (no ozone layer), and electrical activity.
- Atmosphere primarily composed of nitrogen gas, carbon dioxide, water vapor, and hydrogen. No free oxygen gas.
Haldane Hypothesis
Early atmosphere composed of methane, ammonia, hydrogen, and water vapor.
Energy from radioactivity, lightning, and cosmic rays caused these gases to react, forming organic molecules.
These organic molecules accumulated in oceans, forming a primordial soup.
Life further evolved by random chemical reactions and transformations in this complex soup.
Heterotrophs would have evolved first.
Steps to the Evolution of Life
- Requires a large supply of organic molecules produced by non-biological processes.
- Some process had to assemble those small molecules into complex polymers such as nucleic acids and proteins.
- Other processes had to organize the polymers into a self-replicating system.
Miller-Urey Experiment
Stanley Miller and Harold Urey designed an apparatus that held a mix of gases similar to those found in Earth’s early atmosphere over a pool of water, representing Earth’s early ocean. Electrodes delivered an electric current, simulating lightning, into the gas-filled chamber. After allowing the experiment to run for one week, they analyzed the contents of the liquid pool and found that several organic amino acids had formed spontaneously from inorganic raw materials.
Assumptions in Population Genetics
Diploid organisms, only sexual reproduction occurs, generations are non-overlapping, mating is random, population size is infinitely large, allele frequencies are equal in the sexes, there is no migration, mutation, or selection.
RNA World Hypothesis
RNA serves as both template and catalyst for replication. This has been partially reproduced in the lab, but no RNA molecule has been found that can completely self-replicate.
Primate Evolution
Fossil evidence suggests a split in Order Primates about 55 million years ago. At that point, monkeys, apes, and humans diverged from other primates (prosimians: lemurs, lorises, galagos).
Old World Monkeys: Mandrill, Macaque.
New World Monkeys: Capuchin Monkey, Howler Monkey.
Great Apes: Chimpanzee, Lowland Gorilla, Orangutan.
General Characteristics of Primates
- Opposable thumbs (and toes, sometimes).
- Fingernails rather than claws.
- Highly flexible/mobile shoulder and hip joints.
- Binocular vision.
- Color vision.
- Language/Communication.
- Usually one offspring at a time.
Humans vs. Other Primates
- Upright, bipedal locomotion.
- Complex vocal structures.
- Particularly large, complex brain.
- Hippocampus, critical for memory and learning, is more developed in humans than in simpler primates, especially prosimians.
Evidence for Evolution
- Skeletal Evidence: Homologous structures.
- Fossil Evidence: Transitional species.
- Molecular Evidence: DNA hybridization, DNA, and amino acid sequencing.
Skull Trends (Ape-like to Human)
- Decreasing facial angle.
- Decreasing brow ridge.
- Increasing cranial capacity.
- Movement of the foramen magnum.
- Smaller teeth and jaws.
Molecular Evidence
- DNA sequences.
- Amino acid sequences.
- Chromosomal comparison.
- DNA hybridization.
Evolutionary Theories and Concepts
Aristotle: Goal-oriented evolution.
Lamarck: Evolution by acquired characteristics.
Darwin: Evolution by natural selection.
Natural selection is the process by which heritable traits make it more likely that an organism will have greater reproductive success.
Evolution by Natural Selection Depends On
- More offspring are produced than can survive to reproduce.
- Individuals within a species show variation in traits.
- The differences within individuals are heritable.
- Some differences affect how well adapted an organism is to its environment.
- Differences in adaptation are reflected in the number of offspring successfully produced.
Sources of Variation in Individuals
- Sexual Reproduction: Recombination, crossing over during meiosis.
- Mutations in Gametes: Point mutations, translocations.
Evidence for Evolution
- Paleontology (fossil record).
- Biogeography (distribution of species).
- Comparative anatomy.
- Molecular biology.
Types of Evolution
- Convergent Evolution: The evolution of similar traits in unrelated organisms (e.g., body shape and fins in sharks and dolphins).
- Co-evolution: The changes in one organism driving the evolution of changes in another organism (e.g., predator-prey relationships).
- Divergent Evolution: The accumulation of differences between groups that can lead to new species.
Evolutionary Rates
- Gradualism (Darwin): Evolution proceeds through the slow and gradual accumulation of variation at a more or less constant (slow) rate.
- Punctuated Equilibrium (S.J. Gould): Evolution proceeds with relatively short bursts of rapid change followed by long periods of near stasis (stability).
Adaptation
An adaptation is a trait that increases an organism’s reproductive success (fitness).
Adaptation is the consequence of genetic variation in a population.
Mimicry
Harmless animals imitate each other in order to prevent being predated upon, often through warning coloration.
- Batesian Mimicry: A harmless species imitates a dangerous species.
- Mullerian Mimicry: Both species are dangerous, and mimicry reinforces adaptation.
- Example: Viceroy Butterfly and Monarch Butterfly.
Speciation
Differences between groups that can lead to new species.
- Convergent Evolution: The evolution of similar traits in unrelated organisms (e.g., body shape and fins in sharks and dolphins).
- Co-evolution: The changes in one organism driving the evolution of changes in another organism (e.g., predator-prey relationships).