Unit 3 Study Guide: Genetics, Molecular Biology, and Evolution

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DNA and Gene Expression

Structure of DNA and RNA

DNA (Deoxyribonucleic Acid) is a double-stranded helix composed of nucleotides, each containing a deoxyribose sugar, phosphate group, and nitrogenous base (adenine, thymine, cytosine, guanine).
RNA (Ribonucleic Acid) is usually single-stranded, contains ribose sugar, and uses uracil instead of thymine.
Base pairing: Adenine pairs with thymine (A-T) in DNA, and with uracil (A-U) in RNA; cytosine pairs with guanine (C-G).
DNA stores genetic information; RNA is involved in protein synthesis and gene regulation.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself during cell division.
Occurs during the S phase of the cell cycle, before mitosis or meiosis.
Key enzymes: Helicase unwinds the DNA, DNA polymerase synthesizes new strands, ligase joins fragments.
Replication is semi-conservative: each new DNA molecule contains one old and one new strand.
Equation:

Gene Expression: Transcription and Translation

Gene expression is the process by which information from a gene is used to synthesize a functional gene product (usually a protein).
Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus (eukaryotes) or cytoplasm (prokaryotes).
Translation: mRNA is translated into a polypeptide chain (protein) at the ribosome.
Gene expression occurs when a gene is "turned on" and its product is made; this can happen at different times and in different cell types.
Equation:

Protein Targeting and Localization

Proteins can be targeted to the cytoplasm, plasma membrane, or secreted outside the cell.
Signal sequences in the protein determine its final destination.
Proteins for secretion or membrane insertion are synthesized on ribosomes bound to the endoplasmic reticulum (ER).

Gene Regulation

Gene expression is regulated at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.
Different types of genes (structural, regulatory, housekeeping) are expressed as needed.
Prokaryotes often use operons (e.g., lac operon) for gene regulation; eukaryotes use enhancers, silencers, and epigenetic modifications.

Mutations

Mutations are changes in the DNA sequence that can affect gene function.
Types of point mutations: substitution, insertion, deletion.
Effects: silent (no change in protein), missense (change in one amino acid), nonsense (introduces stop codon).
Chromosomal mutations: deletions, duplications, inversions, translocations.
Mutations are important for genetic diversity and evolution, but can also cause disease.

Genetics

Mendelian Genetics

Allele: Different forms of a gene.
Dominant: An allele that masks the effect of a recessive allele.
Recessive: An allele whose effect is masked by a dominant allele.
Homozygous: Having two identical alleles for a gene.
Heterozygous: Having two different alleles for a gene.
Phenotype: Observable traits.
Genotype: Genetic makeup.

Punnett Squares and Genetic Crosses

Punnett squares are used to predict the probability of offspring inheriting certain traits.
Monohybrid cross: one trait; dihybrid (two-trait) cross: two traits.
Linked genes are inherited together; unlinked genes assort independently.
Sex-linked traits are associated with genes on sex chromosomes (e.g., X-linked).
Complex inheritance includes incomplete dominance (blending), codominance (both alleles expressed), and polygenic traits.

Pedigrees and Inheritance Patterns

Pedigrees are diagrams showing inheritance patterns in families.
Can be used to determine if a trait is autosomal or sex-linked, dominant or recessive.
A carrier is an individual who has one copy of a recessive allele but does not express the trait.

Epigenetic Inheritance

Epigenetics refers to heritable changes in gene expression that do not involve changes to the DNA sequence (e.g., DNA methylation, histone modification).
Epigenetic changes can be influenced by environment and can affect phenotype.

Biotechnology

Gene Cloning

Gene cloning involves copying a gene of interest for study or use.
Steps: chop (cut DNA with restriction enzymes), amplify (make copies), insert (into vector), grow (in host cells), identify (select cells with gene).

Transgenic Organisms

Transgenic organisms contain genes from other species.
Used in agriculture (e.g., pest-resistant crops), medicine (e.g., insulin production).

Gene Therapy

Gene therapy aims to treat diseases by introducing, removing, or altering genetic material.
In vivo: Genes delivered directly into the body.
Ex vivo: Cells modified outside the body and then returned.

CRISPR Technology

CRISPR is a genome editing tool that allows precise changes to DNA.
Uses a guide RNA and Cas9 enzyme to target and cut specific DNA sequences.

DNA Profiling and Sequencing

DNA profiling identifies individuals based on unique DNA patterns (used in forensics).
DNA sequencing determines the exact order of nucleotides in DNA.
Humans share a high percentage of DNA with other eukaryotes (e.g., ~98% with chimpanzees).

Microevolution

Evolutionary Thought and Population Genetics

Evolutionary theory has developed from early ideas (Lamarck, Darwin) to modern synthesis.
Population: Group of individuals of the same species in a given area.
Gene pool: All the alleles in a population.

Allele and Genotype Frequencies

Allele frequency: proportion of a specific allele in the gene pool.
Genotype frequency: proportion of a specific genotype in the population.
Equation: (where p and q are frequencies of two alleles)
Equation: (Hardy-Weinberg equilibrium for genotype frequencies)

Hardy-Weinberg Equilibrium

Describes a non-evolving population where allele and genotype frequencies remain constant.
Requirements: large population, random mating, no mutation, no migration, no selection.
Used as a null hypothesis to detect evolution in populations.

Mechanisms of Microevolution

Mutation: introduces new alleles.
Gene flow: movement of alleles between populations.
Genetic drift: random changes in allele frequencies (especially in small populations).
Natural selection: differential survival and reproduction.

Natural Selection

Three conditions: variation, heritability, differential reproductive success.
Outcomes: directional, stabilizing, disruptive selection.
Special types: sexual selection (mate choice), artificial selection (human-directed breeding).

Macroevolution

Microevolution vs. Macroevolution

Microevolution: changes in allele frequencies within populations over time.
Macroevolution: large-scale evolutionary changes, such as speciation and extinction.

Evolution of Multicellular Eukaryotes

Multicellular eukaryotes evolved over 1 billion years ago.
Major events: origin of animals (~600 million years ago), plants (~470 million years ago), fungi.

Taxonomy and Species Concepts

Taxonomy: science of classifying organisms.
Species definitions: biological (interbreeding), morphological (structure), phylogenetic (evolutionary history).
Reproductive isolation prevents gene flow between species (prezygotic and postzygotic barriers).

Speciation

Allopatric speciation: geographic separation leads to new species.
Sympatric speciation: new species arise without geographic isolation (e.g., polyploidy in plants).
Adaptive radiation: rapid diversification due to new habitats, mass extinctions, or innovations.

Evidence for Evolution and Phylogenetics

Evidence: fossils, comparative anatomy, molecular biology, biogeography.
Phylogenetic trees show evolutionary relationships among species.
Multiple hominin species have existed; Homo sapiens is not the only human species in Earth's history.

Mutation Type	Description	Effect
Point Mutation	Change in a single nucleotide	Silent, missense, or nonsense
Chromosomal Mutation	Large-scale changes (deletion, duplication, inversion, translocation)	May affect many genes

Inheritance Pattern	Example	Key Feature
Autosomal Dominant	Huntington's disease	Trait appears in every generation
Autosomal Recessive	Cystic fibrosis	Trait can skip generations; carriers possible
Sex-linked	Hemophilia	More common in one sex (often males)

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curricula.