Genetics, Gene Expression, Evolution, and Speciation: Study Guide

Chapter 15: Chromosomal Basis of Inheritance

Mendel’s Laws and Chromosome Behavior

Mendel’s laws of inheritance describe how traits are passed from parents to offspring. These laws are explained by the behavior of chromosomes during meiosis and fertilization.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

• Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation, provided they are on different chromosomes.

• Chromosome Theory of Inheritance: Genes are located on chromosomes, and their behavior during meiosis explains Mendel’s laws.

• Morgan’s Experiments: Thomas Hunt Morgan used Drosophila melanogaster (fruit flies) to show that genes are located on chromosomes. His work with red and white-eyed flies demonstrated sex-linked inheritance.

Example: Morgan observed that only male fruit flies had white eyes, indicating the gene was on the X chromosome.

Sex-Linked Genes and X Inactivation

Sex-linked genes are located on sex chromosomes, often the X chromosome in mammals. X inactivation occurs in female mammals to balance gene dosage.

• Sex-Linked Inheritance: Traits controlled by genes on the X chromosome show different patterns in males and females.

• X Inactivation: In female mammals, one X chromosome is randomly inactivated in each cell, forming a Barr body.

Example: Calico cats display patches of color due to X inactivation.

Linkage and Crossing Over

Genes located close together on the same chromosome tend to be inherited together (linkage), but crossing over during meiosis can separate them.

• Parental Phenotypes: Offspring with combinations of traits matching the parents.

• Nonparental (Recombinant) Phenotypes: Offspring with new combinations of traits due to crossing over.

Example: Linked genes can be separated by recombination, producing recombinant phenotypes.

Alterations of Chromosome Number and Structure

Changes in chromosome number or structure can cause genetic disorders.

• Polyploidy: More than two sets of chromosomes (e.g., triploid = 3n).

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy = extra chromosome, monosomy = missing chromosome).

Example: Down syndrome is caused by trisomy 21.

Non-Mendelian Inheritance

Some inheritance patterns do not follow Mendel’s laws.

• Organelle Genes: Genes in mitochondria and chloroplasts are inherited maternally.

• Imprinted Genes: Expression depends on whether the gene is inherited from the mother or father.

Example: Mitochondrial diseases are inherited from the mother.

Chapter 16: The Molecular Basis of Inheritance

DNA Structure and Evidence as Genetic Material

DNA is the hereditary material in all living organisms. Its structure was determined through multiple lines of evidence.

• Double Helix: DNA consists of two antiparallel strands forming a double helix.

• Base Pairing: Adenine pairs with thymine, and guanine pairs with cytosine.

• Evidence: Experiments by Griffith, Avery, Hershey, and Chase demonstrated DNA is the genetic material.

DNA Replication and Enzymes

DNA replication is the process by which DNA makes a copy of itself during cell division.

• Semiconservative Replication: Each new DNA molecule consists of one old and one new strand.

• Key Enzymes:

  • Helicase: Unwinds DNA

  • Primase: Synthesizes RNA primers

  • DNA Polymerase: Synthesizes new DNA

  • Ligase: Joins DNA fragments

• Repair Mechanisms: Proofreading and mismatch repair correct errors.

• End Replication Problem: Telomerase extends telomeres in eukaryotes.

• Prokaryotic vs Eukaryotic Replication: Prokaryotes have a single origin of replication; eukaryotes have multiple origins.

Equation:

$\text{DNA Polymerase:} \quad (dNTP)_n + dNTP \rightarrow (dNTP)_{n+1} + PP_i$

Chromatin Packing in Eukaryotes

DNA is packaged into chromatin in eukaryotic cells, with several levels of organization.

• Nucleosome: DNA wrapped around histone proteins.

• 30-nm Fiber: Nucleosomes coil to form a thicker fiber.

• Looped Domains: 30-nm fibers form loops attached to a scaffold.

• Metaphase Chromosome: Highest level of packing during cell division.

Chapter 17: Gene Expression: From Gene to Protein

Evidence and Process of Gene Expression

Gene expression is the process by which information from a gene is used to synthesize a functional product, usually a protein.

• One Gene-One Polypeptide Hypothesis: Each gene encodes a single polypeptide.

• Evidence: Beadle and Tatum’s experiments with Neurospora mutants.

Transcription

Transcription is the synthesis of RNA from a DNA template.

• Key Molecules: RNA polymerase, transcription factors, promoter regions.

• Steps: Initiation, elongation, termination.

Eukaryotic RNA Processing

In eukaryotes, pre-mRNA undergoes processing before becoming mature mRNA.

• 5' Capping

• 3' Polyadenylation

• Splicing: Removal of introns and joining of exons.

Translation

Translation is the synthesis of a polypeptide using mRNA as a template.

• Key Components: Ribosomes, tRNA, amino acids.

• Process: Initiation, elongation, termination.

• Polyribosomes: Multiple ribosomes can translate a single mRNA simultaneously in both bacteria and eukaryotes.

Mutations and the Concept of a Gene

• Types of Mutations: Point mutations, insertions, deletions, frameshifts.

• Effects: Can alter protein structure and function.

• Gene: A region of DNA that can be expressed to produce a functional product.

Chapter 18: Regulation of Gene Expression

trp and lac Operons

Operons are clusters of genes regulated together in prokaryotes.

• trp Operon: Repressible operon; turned off when tryptophan is present.

• lac Operon: Inducible operon; turned on in the presence of lactose.

• Structural Components: Promoter, operator, structural genes.

Regulation of Eukaryotic Gene Expression

Gene expression in eukaryotes is regulated at multiple stages.

• Chromatin modification

• Transcriptional control

• RNA processing

• mRNA stability

• Translational control

• Post-translational modifications

Differential Gene Expression

Different cell types arise because different genes are expressed in each cell type.

Chapter 19: Viruses

Virus Structure

Viruses are infectious particles consisting of genetic material enclosed in a protein coat.

• Capsid: Protein shell

• Genetic Material: DNA or RNA

• Some have envelopes derived from host membranes

Viral Replication and Evolution

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome and replicates with it.

Prions

Prions are infectious proteins that cause neurodegenerative diseases.

Chapter 22: Descent with Modification

Darwin’s Theory vs. Prevailing Ideas

Darwin’s concept of descent with modification differed from earlier ideas by proposing natural selection as the mechanism of evolution.

• Prevailing Ideas: Species were fixed and unchanging.

• Darwin: Species change over time through natural selection.

Natural Selection and Adaptation

Natural selection leads to adaptation, where organisms become better suited to their environment.

Evidence for Evolution

• Fossil record

• Homologous structures

• Biogeography

• Molecular evidence

Chapter 23: The Evolution of Populations

Genetic Variation

Genetic variation arises through mutation, recombination, and sexual reproduction. It is essential for evolution.

Hardy-Weinberg Equilibrium

Describes a population that is not evolving. At equilibrium, allele and genotype frequencies remain constant.

Equation:

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

$p + q = 1$

Mechanisms of Evolution

• Natural Selection: Differential survival and reproduction.

• Genetic Drift: Random changes in allele frequencies.

• Gene Flow: Movement of alleles between populations.

Adaptation

Natural selection increases the frequency of advantageous traits.

Chapter 24: The Origin of Species

Biological Species Concept and Reproductive Barriers

A species is a group of populations whose members can interbreed and produce fertile offspring. Reproductive barriers prevent gene flow between species.

• Prezygotic Barriers: Prevent mating or fertilization (e.g., habitat, temporal, behavioral isolation).

• Postzygotic Barriers: Prevent hybrid offspring from developing into fertile adults.

Allopatric vs. Sympatric Speciation

• Allopatric Speciation: Occurs when populations are geographically separated.

• Sympatric Speciation: Occurs without geographic separation, often via polyploidy or habitat differentiation.

Hybrid Zones and Outcomes

Hybrid zones are regions where different species meet and mate, producing hybrids. Outcomes include reinforcement, fusion, or stability of the hybrid zone.

Rates and Genetic Basis of Speciation

• Speciation can be rapid (punctuated equilibrium) or gradual.

• Can involve changes in few or many genes.