Study Guide: Chromosomal Basis of Inheritance, Molecular Basis of Inheritance, Gene Expression, and Evolution

Chapter 15: Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.

• Law of Segregation: During meiosis, homologous chromosomes (and thus gene alleles) separate so that each gamete receives only one allele of each gene.

• Law of Independent Assortment: Chromosomes (and their genes) assort independently into gametes, leading to genetic variation.

• Example: Mendel’s pea plant experiments demonstrated these laws, which were later explained by chromosome behavior during meiosis.

Sex Determination Systems

Sex is genetically determined by specific chromosomes or gene combinations, varying among species.

• Humans: XX (female), XY (male); the SRY gene on the Y chromosome triggers male development.

• Fruit Flies (Drosophila): XX (female), XY (male), but sex is determined by the ratio of X chromosomes to autosomes.

• Grasshoppers: XO system; females are XX, males are XO (only one X chromosome).

• Birds: ZW system; females are ZW, males are ZZ.

• Ants and Bees: Haplo-diploid system; females are diploid, males are haploid.

Sex-Linked Inheritance

Genes located on sex chromosomes (especially the X chromosome) show unique inheritance patterns.

• Sex-linked disorders (e.g., color blindness, hemophilia) are more common in males because they have only one X chromosome.

• Females need two copies of a recessive allele to express the disorder; males need only one.

Barr Bodies

A Barr body is an inactivated X chromosome in female mammalian cells, formed to balance gene dosage between males and females.

• Occurs via X-inactivation early in embryonic development.

Alterations of Chromosome Structure

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Movement of a segment to a nonhomologous chromosome.

Genomic Imprinting

Genomic imprinting is the phenomenon where certain genes are expressed in a parent-of-origin-specific manner due to epigenetic marks.

Other Locations of DNA in Cells

• Mitochondria (mitochondrial DNA)

• Chloroplasts (in plants and algae)

Chapter 16: Molecular Basis of Inheritance

DNA and RNA Structure

• DNA: Double helix, deoxyribose sugar, bases A, T, C, G.

• RNA: Usually single-stranded, ribose sugar, bases A, U, C, G.

Chargaff’s Rules

• In DNA, the amount of adenine (A) equals thymine (T), and cytosine (C) equals guanine (G):

$A = T$ $C = G$

• This base pairing explains the structure of the DNA double helix.

Semi-Conservative Model of DNA Replication

• Each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

DNA Replication Process

• Helicase: Unwinds the DNA double helix.

• Single-Strand Binding Proteins: Stabilize unwound DNA.

• Primase: Synthesizes RNA primers.

• DNA Polymerase III: Adds nucleotides to the growing DNA strand.

• DNA Polymerase I: Replaces RNA primers with DNA.

• Ligase: Joins Okazaki fragments on the lagging strand.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

Okazaki fragments are short DNA segments synthesized on the lagging strand.

Telomeres

• Telomeres are repetitive nucleotide sequences at chromosome ends that protect genes from erosion during replication.

• They are important for maintaining chromosome stability.

Roles of Nucleic Acid and Protein in Chromosomes

• Nucleic acids (DNA): Store genetic information.

• Proteins (histones): Package and organize DNA into chromatin.

Chapter 17: Gene Expression: From Gene to Protein

Central Dogma of Molecular Biology

• Information flows from DNA to RNA to protein.

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Genetic Code: Codons and Anticodons

• Codon: A sequence of three RNA nucleotides that codes for a specific amino acid.

• Anticodon: A sequence of three nucleotides on tRNA complementary to a codon on mRNA.

Transcription and Translation

• Transcription: Synthesis of RNA from a DNA template (occurs in the nucleus in eukaryotes).

• Translation: Synthesis of a polypeptide using mRNA as a template (occurs in the cytoplasm at ribosomes).

• End products: mRNA (from transcription), polypeptide (from translation).

Steps of Transcription

• Initiation: RNA polymerase binds to the promoter region.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

• Termination: RNA synthesis ends at a terminator sequence.

Roles of mRNA, tRNA, and rRNA

• mRNA: Carries genetic code from DNA to ribosome.

• tRNA: Brings amino acids to the ribosome; contains anticodon.

• rRNA: Structural and catalytic component of ribosomes.

mRNA Processing in Eukaryotes

• 5' Cap: Modified guanine nucleotide added to the 5' end.

• Poly-A Tail: Series of adenine nucleotides added to the 3' end.

• Splicing: Removal of introns and joining of exons.

Ribosomes

• Complexes of rRNA and proteins that facilitate translation by matching tRNA anticodons to mRNA codons and catalyzing peptide bond formation.

Initiation of Translation

• Small ribosomal subunit binds to mRNA and initiator tRNA; large subunit joins to form the complete ribosome.

Elongation of Polypeptide Chain

• Amino acids are added one by one to the growing chain as tRNAs bring them to the ribosome, matching codons and anticodons.

Types of Mutations Affecting mRNA

• Point mutations: Change a single nucleotide (can be silent, missense, or nonsense).

• Insertions/Deletions: Add or remove nucleotides, potentially causing frameshifts.

Chapter 22: Darwinian View of Life

Conventional View Before Darwin

• Species were considered fixed and unchanging; Earth was viewed as relatively young.

Evolution as a Scientific Theory

• Evolution is a well-supported explanation for the diversity of life, based on extensive evidence.

Evidence for Descent with Modification

• Fossil record

• Homologies (anatomical, molecular, developmental)

• Biogeography

• Direct observations of evolutionary change

Artificial Selection

• Humans breed organisms for desired traits (e.g., dog breeds, crop varieties).

Fossil Record and Its Limitations

• Shows changes in organisms over time, but is incomplete due to rare fossilization and erosion.

Homologies as Evidence

• Similarities due to shared ancestry (e.g., vertebrate limb bones).

Biogeography and Pangaea

• Geographic distribution of species explained by continental drift and isolation.

• Pangaea: Ancient supercontinent.

• Endemic species: Found only in a specific location.

Darwin’s Observations and Inferences

• Variation exists within populations.

• More offspring are produced than can survive.

• Individuals with advantageous traits survive and reproduce more.

• Favorable traits accumulate over generations.

Influence of Malthus and Artificial Selection

• Malthus: Populations grow faster than resources, leading to competition.

• Artificial selection showed that selection could change traits over generations.

Limits of Evolution

• Evolution does not produce perfect organisms due to constraints, trade-offs, and historical limitations.

Chapter 23: Evolution of Populations

Sources of Genetic Variation

• Mutation: Random changes in DNA sequence.

• Sexual reproduction: Recombines alleles through crossing over, independent assortment, and fertilization.

Gene Pool, Population, and Microevolution

• Gene pool: All alleles in a population.

• Population: Group of individuals of the same species in a given area.

• Microevolution: Change in allele frequencies in a population over time.

Hardy-Weinberg Equilibrium

• Describes a non-evolving population; allele and genotype frequencies remain constant if five conditions are met:

Condition	Description
No mutations	Gene pool is not changed by new alleles
Random mating	All individuals have equal chance to mate
No natural selection	All genotypes have equal fitness
Large population size	Genetic drift is negligible
No gene flow	No migration of alleles in or out

Hardy-Weinberg equation:

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

where $p$ and $q$ are allele frequencies.

Genetic Drift and Gene Flow

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Bottleneck effect: Population size is drastically reduced, leading to loss of genetic diversity.

• Founder effect: A few individuals start a new population with different allele frequencies.

• Gene flow: Movement of alleles between populations, reducing differences between them.

Natural Selection and Adaptive Evolution

• Natural selection is the only mechanism that consistently increases the frequency of advantageous alleles, leading to adaptation.

Types of Selection

Type	Description
Stabilizing selection	Favors intermediate phenotypes
Directional selection	Favors one extreme phenotype
Disruptive selection	Favors both extreme phenotypes

Sexual Selection

• Intrasexual selection: Competition among individuals of one sex (usually males) for mates.

• Intersexual selection: Mate choice by individuals of one sex (usually females).

• Both contribute to microevolution and can lead to speciation.