Skip to main content
Back

Key Concepts in Chromosomal Inheritance, DNA Replication, Gene Expression, and Evolution

NotesPracticeVideo lessons

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Chapter 15: Chromosomal Inheritance

Chromosome Theory of Inheritance

The chromosome theory of inheritance connects Mendel’s laws of genetics to the physical behavior of chromosomes during meiosis. This theory explains how genes are transmitted from parents to offspring.

  • Chromosome Theory: Genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.

  • Mendel’s Laws: The law of segregation and the law of independent assortment are explained by the separation and random assortment of chromosomes.

  • Example: The inheritance of eye color in Drosophila melanogaster (fruit flies) demonstrates the link between genes and chromosomes.

Sex-Linked Inheritance

Sex-linked traits are associated with genes found on sex chromosomes, most commonly the X chromosome. Males are more likely to express recessive X-linked traits because they have only one X chromosome.

  • Sex-Linked Traits: Traits controlled by genes on the X or Y chromosome.

  • Expression in Males: Males (XY) express recessive X-linked traits if their single X chromosome carries the allele.

  • Example: Red-green color blindness is more common in males.

SRY Gene and Sex Determination

The SRY gene (Sex-determining Region Y) is a gene on the Y chromosome that triggers male development in humans.

  • SRY Gene: Initiates the development of testes and male characteristics.

  • Absence of SRY: Leads to female development.

Barr Body

A Barr body is an inactivated X chromosome found in the nuclei of female mammals. This ensures dosage compensation between males and females.

  • Dosage Compensation: Equalizes expression of X-linked genes in males and females.

  • Example: Female cells have one active X chromosome; the other becomes a Barr body.

Chromosomal Mutations

Chromosomal mutations are large-scale changes in chromosome structure that can affect many genes.

  • Types of Mutations:

    • 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.

Chapter 16: DNA Replication

Semi-Conservative Replication

DNA replication is semi-conservative, meaning each new DNA molecule consists of one old (parental) strand and one newly synthesized strand.

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

  • Evidence: Meselson-Stahl experiment demonstrated semi-conservative replication.

Enzymes in DNA Replication

Several enzymes coordinate the process of DNA replication:

  • Helicase: Unwinds the DNA double helix.

  • DNA Polymerase: Synthesizes new DNA strands by adding nucleotides.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Leading and Lagging Strands

DNA replication is continuous on the leading strand and discontinuous on the lagging strand, which forms short segments called Okazaki fragments.

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

  • Lagging Strand: Synthesized in short fragments (Okazaki fragments) that are later joined.

DNA vs RNA

DNA and RNA are nucleic acids with distinct differences:

  • Sugar: DNA contains deoxyribose; RNA contains ribose.

  • Bases: DNA uses thymine (T); RNA uses uracil (U).

  • Structure: DNA is double-stranded; RNA is usually single-stranded.

Telomeres

Telomeres are repetitive nucleotide sequences at the ends of chromosomes that protect them from deterioration.

  • Function: Prevent loss of genetic information during replication.

  • Enzyme: Telomerase extends telomeres in germ cells and some stem cells.

Chapter 17: Gene Expression

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

  • Process: DNA → RNA → Protein

  • Significance: Explains how genetic information results in functional proteins.

Transcription and Translation

Gene expression involves two main steps:

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

  • Translation: Synthesis of protein from mRNA at the ribosome.

Codons and Anticodons

Codons are three-nucleotide sequences on mRNA that specify amino acids; anticodons are complementary sequences on tRNA.

  • Codon: Triplet of bases on mRNA (e.g., AUG codes for methionine).

  • Anticodon: Triplet on tRNA that pairs with the codon.

mRNA Processing

In eukaryotes, pre-mRNA undergoes several modifications before becoming mature mRNA:

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

  • Poly-A Tail: String of adenine nucleotides added to the 3’ end.

  • Splicing: Removal of introns and joining of exons.

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene expression:

  • Missense Mutation: Changes one amino acid in the protein.

  • Nonsense Mutation: Introduces a premature stop codon.

  • Frameshift Mutation: Insertion or deletion shifts the reading frame.

Chapter 22: Darwinian Evolution

Descent with Modification

Darwin’s theory of evolution proposes that species change over time through descent with modification from common ancestors.

  • Descent with Modification: Accumulation of inherited changes over generations.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution:

  • Fossils: Show changes in organisms over time.

  • Homology: Similar structures due to common ancestry.

  • Biogeography: Geographic distribution of species.

  • Anatomy: Comparative anatomy reveals evolutionary relationships.

Natural Selection

Natural selection is the process by which individuals with advantageous traits survive and reproduce more successfully.

  • Variation: Individuals in a population vary in traits.

  • Survival and Reproduction: Traits that enhance survival become more common.

Limits of Evolution

Evolution does not produce perfect organisms; it works with existing variation and is constrained by historical and environmental factors.

Chapter 23: Evolution of Populations

Hardy-Weinberg Equilibrium

The Hardy-Weinberg equation describes the genetic makeup of a non-evolving population. It provides a baseline to measure genetic change.

  • Equation:

  • p: Frequency of the dominant allele

  • q: Frequency of the recessive allele

  • p^2: Frequency of homozygous dominant genotype

  • 2pq: Frequency of heterozygous genotype

  • q^2: Frequency of homozygous recessive genotype

Conditions for Hardy-Weinberg Equilibrium

Five conditions must be met for a population to remain in Hardy-Weinberg equilibrium:

  • No mutations

  • Random mating

  • No natural selection

  • Extremely large population size

  • No gene flow (no migration)

Genetic Drift

Genetic drift is random change in allele frequencies, especially in small populations.

  • Bottleneck Effect: Sudden reduction in population size changes allele frequencies.

  • Founder Effect: A few individuals colonize a new area, leading to different allele frequencies.

Gene Flow and Natural Selection

Gene flow is the movement of alleles between populations, while natural selection increases the frequency of advantageous alleles.

Types of Selection

Natural selection can take different forms:

  • Stabilizing Selection: Favors intermediate phenotypes.

  • Directional Selection: Favors one extreme phenotype.

  • Disruptive Selection: Favors both extreme phenotypes over intermediates.

Pearson Logo

Study Prep