General Biology Study Guide: Transcription, Translation, Evolution, and Ecology

Transcription

Overview of Transcription

Transcription is the process by which genetic information encoded in DNA is copied into messenger RNA (mRNA) for protein synthesis. This process is fundamental to gene expression in all living organisms.

• Template vs. Coding Strand: The template strand of DNA is used by RNA polymerase to synthesize a complementary RNA strand. The coding strand has the same sequence as the RNA (except T is replaced by U).

• Direction of Synthesis: RNA is synthesized in the 5' to 3' direction.

• Primers: Unlike DNA polymerases, RNA polymerases do not require a primer to initiate synthesis.

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region of DNA, unwinding the DNA helix.

• Elongation: RNA nucleotides are added to the growing RNA chain.

• Termination: Transcription ends when RNA polymerase reaches a terminator sequence.

Prokaryotes vs. Eukaryotes

• Promoters: Prokaryotic promoters contain -10 and -35 elements; eukaryotic promoters are more complex.

• Initiation Factors: Prokaryotes use sigma factors; eukaryotes use transcription factors.

• Location: Transcription occurs in the cytoplasm in prokaryotes and in the nucleus in eukaryotes.

• RNA Processing: Eukaryotic mRNA undergoes capping, polyadenylation, and splicing; prokaryotic mRNA does not.

Translation

Overview of Translation

Translation is the process by which the sequence of an mRNA molecule is used to direct the synthesis of a protein. This occurs at the ribosome and involves tRNA molecules that bring amino acids to the growing polypeptide chain.

• Prokaryotes vs. Eukaryotes: In prokaryotes, transcription and translation can occur simultaneously; in eukaryotes, they are separated by the nuclear envelope.

tRNA and the Genetic Code

• tRNA Structure: tRNAs have an anticodon that pairs with the mRNA codon and an attached amino acid.

• Wobble Hypothesis: Some tRNAs can pair with more than one codon due to flexible base pairing at the third position.

Ribosome Function

• Sites: Ribosomes have three sites: A (aminoacyl), P (peptidyl), and E (exit).

• Initiation, Elongation, Termination: These are the three main stages of translation. Initiation involves assembly of the ribosome on the mRNA, elongation adds amino acids, and termination releases the completed polypeptide.

Protein Modification

• After translation, proteins may be modified by folding, cleavage, or addition of chemical groups.

Natural Selection

What is Evolution?

Evolution is the change in the heritable characteristics of biological populations over successive generations. Charles Darwin described evolution as "descent with modification" through natural selection.

• Evidence for Evolution:

  • Geologic time and fossils

  • Extinction events

  • Transitional features

  • Vestigial traits

  • Homology (structural, developmental, genetic)

  • Biogeography (similar species in the same geographic area)

Darwin's Four Postulates

• Variation exists among individuals in a population.

• Some variation is heritable.

• More offspring are produced than can survive.

• Individuals with advantageous traits survive and reproduce more successfully.

Misconceptions about Evolution

• Individuals do not adapt; populations evolve.

• Natural selection acts on phenotypes, not genotypes.

• Evolution is not goal-directed.

• Traits are not always adaptive.

• Natural selection is not the only process of evolution.

Evolutionary Processes

Types of Selection

• Stabilizing Selection: Favors intermediate phenotypes.

• Directional Selection: Favors one extreme phenotype.

• Disruptive Selection: Favors both extreme phenotypes.

• Balancing Selection: Maintains genetic diversity (e.g., heterozygote advantage, frequency-dependent selection).

• Sexual Selection: Selection for traits that increase mating success.

Genetic Drift

• Random changes in allele frequencies, especially in small populations.

• Examples: Bottleneck effect, founder effect.

Gene Flow

• Movement of alleles between populations, which can increase genetic diversity.

Mutation

• Restores genetic diversity by creating new alleles.

• Types: Point mutations, chromosomal mutations, horizontal gene transfer.

Hardy-Weinberg Principle

• Describes a non-evolving population.

• Assumptions: No mutation, no selection, no gene flow, infinite population size, random mating.

Equation:

$p^2 + 2pq + q^2 = 1$

where p and q are the frequencies of two alleles in a population.

Speciation

What is Speciation?

Speciation is the process by which one species splits into two or more distinct species. It involves the evolution of reproductive isolation.

Species Concepts

• Biological Species Concept: Species are groups of interbreeding populations that are reproductively isolated from other groups.

• Reproductive Isolation:

  • Prezygotic barriers: Habitat, temporal, behavioral, mechanical, gametic isolation.

  • Postzygotic barriers: Hybrid inviability, hybrid sterility.

• Morphological Species Concept: Based on physical characteristics.

• Phylogenetic Species Concept: Based on evolutionary history and monophyly.

Modes of Speciation

• Allopatric Speciation: Occurs when populations are geographically separated (e.g., vicariance, dispersal).

• Sympatric Speciation: Occurs without geographic separation, often via polyploidy or disruptive selection.

Ecology

Population Ecology

Population ecology studies the distribution and abundance of organisms and the factors that regulate population size.

• Population Density: Number of individuals per unit area.

• Population Growth: Determined by birth rates, death rates, immigration, and emigration.

• Carrying Capacity (K): Maximum population size an environment can support.

• Density-dependent Factors: Factors whose effects change with population density (e.g., competition, disease).

• Density-independent Factors: Affect populations regardless of density (e.g., weather, natural disasters).

Community Ecology

Community ecology examines interactions between species and their effects on population dynamics and ecosystem structure.

• Species Interactions:

  • Mutualism (+/+): Both species benefit.

  • Commensalism (+/0): One benefits, the other is unaffected.

  • Competition (-/-): Both species are harmed.

  • Consumption (+/-): Includes predation, herbivory, and parasitism.

• Fundamental vs. Realized Niche: The fundamental niche is the full range of conditions a species can use; the realized niche is where it actually exists due to competition.

• Keystone Species: Species that have a disproportionately large effect on community structure.

Ecosystem Ecology

• Nutrient and Carbon Cycling: Movement of nutrients and carbon through ecosystems.

• Human Impact: Human activities influence atmospheric carbon dioxide and global climate.