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

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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: Each parent has two alleles for each gene, which segregate during gamete formation so each gamete receives one allele.

  • Law of Independent Assortment: Genes on different chromosomes are inherited independently of each other due to the random orientation of chromosomes during meiosis.

  • Chromosomal Basis: These laws are explained by the movement and separation of chromosomes during meiosis.

Sex Determination and the SRY Gene

Sex is genetically determined in humans by the presence or absence of the Y chromosome, specifically the SRY gene.

  • SRY Gene: Located on the Y chromosome, it triggers male development.

  • Human System: XX = female, XY = male.

  • Other Systems:

    • Fruit flies: Sex determined by ratio of X chromosomes to autosomes.

    • Grasshoppers: XO system (XX = female, XO = male).

    • Birds: ZW system (ZZ = male, ZW = female).

    • Ants and Bees: Haplodiploidy (fertilized eggs = female, unfertilized = male).

Sex-Linked Inheritance

Sex-linked genes are located on sex chromosomes, often the X chromosome.

  • Patterns: Males (XY) express recessive X-linked disorders more frequently because they have only one X chromosome.

  • Examples: Color blindness, hemophilia.

Barr Body Formation

A Barr body is an inactivated X chromosome found in female cells.

  • Formation: One X chromosome in each cell is randomly inactivated during early development.

  • Significance: Ensures dosage compensation between males and females.

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 from one chromosome to another.

Genomic Imprinting

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

  • Mechanism: Chemical modifications (such as methylation) silence one allele.

Other Locations of DNA in Cells

  • Mitochondria

  • Chloroplasts (in plants)

Chapter 16: Molecular Basis of Inheritance

Structures of DNA and RNA

  • DNA: Double-stranded helix, deoxyribose sugar, bases: adenine, thymine, cytosine, guanine.

  • RNA: Single-stranded, ribose sugar, bases: adenine, uracil, cytosine, guanine.

Chargaff’s Rules

Chargaff’s rules state that in DNA, the amount of adenine equals thymine, and the amount of cytosine equals guanine.

  • Implication: Base pairing (A-T, C-G) is fundamental to DNA structure.

Semi-Conservative Model of DNA Replication

Each new DNA molecule consists of one old strand and one new strand.

  • Model: Parental strands serve as templates for new strands.

DNA Replication Process

  • Enzymes:

    • Helicase: Unwinds DNA.

    • Primase: Synthesizes RNA primer.

    • DNA Polymerase: Adds nucleotides to growing DNA strand.

    • Ligase: Joins Okazaki fragments.

  • Leading Strand: Synthesized continuously.

  • Lagging Strand: Synthesized in short segments called Okazaki fragments.

  • Okazaki Fragment: Short DNA segment on the lagging strand.

Telomeres

Telomeres are repetitive DNA sequences at chromosome ends.

  • Importance: Protect chromosome ends from deterioration and fusion.

Roles of Nucleic Acid and Protein in Chromosomes

  • Nucleic Acid: Stores genetic information.

  • Protein: Organizes and regulates DNA (e.g., histones).

Chapter 17: Gene Expression: From Gene to Protein

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

  • Transcription: DNA is transcribed to RNA.

  • Translation: RNA is translated to protein.

Genetic Code: Codons and Anticodons

  • Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

  • Anticodon: Three-nucleotide sequence in tRNA complementary to codon.

Transcription and Translation Locations

  • Transcription: Occurs in the nucleus.

  • Translation: Occurs in the cytoplasm (on ribosomes).

  • End Products: Transcription produces mRNA; translation produces polypeptides.

Transcription Process

  • Initiation: RNA polymerase binds to promoter.

  • Elongation: RNA polymerase synthesizes RNA.

  • Termination: RNA polymerase releases RNA at terminator sequence.

Roles of mRNA, tRNA, and rRNA

  • mRNA: Carries genetic code from DNA to ribosome.

  • tRNA: Brings amino acids to ribosome; matches codon with anticodon.

  • rRNA: Structural and catalytic component of ribosome.

Eukaryotic mRNA Processing

  • 5' Cap: Added to mRNA for stability and initiation.

  • Poly-A Tail: Added to 3' end for stability.

  • Splicing: Removal of introns, joining of exons.

Ribosomes

  • Definition: Complex of rRNA and proteins; site of protein synthesis.

  • Function: Facilitates translation by aligning mRNA and tRNA.

Translation Initiation

  • Process: Small ribosomal subunit binds mRNA; initiator tRNA binds start codon; large subunit joins.

Amino Acid Addition to Polypeptide Chain

  • Step-by-Step:

    1. tRNA brings amino acid to ribosome.

    2. Peptide bond forms between amino acids.

    3. Ribosome moves to next codon.

Types of Mutations Affecting mRNA

  • Substitution: One base replaced by another; may cause silent, missense, or nonsense mutation.

  • Insertion/Deletion: Addition or loss of bases; may cause frameshift mutation.

Chapter 22: Darwinian View of Life

Conventional View of Earth and Life (Pre-Darwin)

  • Static Species: Species were thought to be unchanging.

  • Young Earth: Earth was believed to be only a few thousand years old.

Evolution as a Scientific Theory

  • Significance: Explains the diversity and adaptation of organisms.

  • Evidence-Based: Supported by multiple lines of evidence.

Evidence for Descent with Modification

  • Fossil Record: Shows changes in species over time.

  • Homologies: Similarities due to shared ancestry.

  • Biogeography: Geographic distribution of species.

  • Direct Observation: Evolution observed in living populations.

Artificial Selection

  • Definition: Humans select traits in organisms.

  • Examples: Dog breeds, crop plants.

Fossil Record and Its Limitations

  • Evidence: Transitional forms, extinct species.

  • Incomplete: Not all organisms fossilize; gaps exist.

Homologies as Evidence

  • Anatomical: Similar structures (e.g., vertebrate limbs).

  • Molecular: DNA and protein similarities.

Biogeography and Pangaea

  • Pangaea: Ancient supercontinent.

  • Endemic Species: Species found only in one location.

  • Distribution: Evolution explains why related species are found in similar regions.

Darwin’s Observations and Inferences

  • Observation: Variation exists; more offspring produced than survive.

  • Inference: Individuals with advantageous traits survive and reproduce.

Influence of Malthus and Artificial Selection

  • Malthus: Population growth exceeds resources.

  • Artificial Selection: Shows how selection can change traits.

Imperfect Adaptation

  • Reason: Evolution works with existing variation; environmental changes; trade-offs.

Chapter 23: Evolution of Populations

Sources of Genetic Variation

  • Mutation: Changes in DNA sequence.

  • Sexual Reproduction: Recombination of alleles.

Definitions

  • Gene Pool: All alleles in a population.

  • Population: Group of individuals of same species in an area.

  • Microevolution: Change in allele frequencies over time.

Hardy-Weinberg Equilibrium

  • Conditions:

    1. No mutations

    2. Random mating

    3. No natural selection

    4. Large population size

    5. No gene flow

  • Equation:

Genetic Drift and Gene Flow

  • Genetic Drift: Random changes in allele frequencies.

  • Bottleneck Effect: Population size reduced; genetic diversity lost.

  • Founder Effect: New population started by few individuals; allele frequencies differ.

  • Gene Flow: Movement of alleles between populations; increases genetic similarity.

Natural Selection and Adaptive Evolution

  • Natural Selection: Only mechanism consistently leading to adaptation.

Types of Selection

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Directional Selection: Favors one extreme phenotype.

  • Disruptive Selection: Favors both extreme phenotypes.

Sexual Selection

  • Intrasexual Selection: Competition among same sex (e.g., males).

  • Intersexual Selection: Mate choice by opposite sex (e.g., females).

  • Contribution: Drives microevolution and speciation.

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