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

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 located on sex chromosomes, most commonly the X chromosome in humans.

• Expression in Males: Males (XY) express recessive X-linked traits more often because they have only one X chromosome.

• Example: Red-green color blindness and hemophilia are X-linked recessive disorders.

SRY Gene and Sex Determination

The SRY gene (Sex-determining Region Y) is crucial for male development in mammals.

• Function: The SRY gene triggers the development of testes, leading to male characteristics.

• Location: Found on the Y chromosome.

Barr Body

A Barr body is an inactivated X chromosome found in the nuclei of female mammals.

• Purpose: Dosage compensation ensures females (XX) do not produce twice as many X-linked gene products as males (XY).

• Mechanism: One X chromosome condenses and becomes transcriptionally inactive.

Chromosomal Mutations

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

• Types:

  • Deletion: Loss of a chromosome segment.

  • Duplication: Repetition of a chromosome segment.

  • Inversion: Reversal of a chromosome segment.

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

• Example: Chronic myelogenous leukemia (CML) is caused by a translocation between chromosomes 9 and 22.

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.

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

Enzymes in DNA Replication

Several enzymes coordinate the accurate and efficient replication of DNA.

• Helicase: Unwinds the DNA double helix.

• DNA Polymerase: Synthesizes new DNA strands by adding nucleotides to a template strand.

• Ligase: Joins Okazaki fragments on the lagging strand.

Leading and Lagging Strands

DNA polymerase can only synthesize DNA in the 5’ to 3’ direction, resulting in continuous and discontinuous synthesis.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously as short Okazaki fragments, later joined by ligase.

DNA vs RNA

DNA and RNA are nucleic acids with distinct structures and functions.

• DNA: Double-stranded, contains deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, contains ribose sugar, bases A, U, C, G.

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 from DNA to RNA to protein.

• Process: DNA is transcribed into RNA, which is then translated into protein.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Transcription and Translation

Gene expression involves two main steps: transcription and translation.

• Transcription: Occurs in the nucleus; DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: Occurs at the ribosome; mRNA is decoded to build a polypeptide chain (protein).

Codons and Anticodons

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

• Example: The codon AUG codes for methionine and serves as the start signal for translation.

mRNA Processing

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

• 5’ Cap: Modified guanine nucleotide added to the 5’ end for stability and ribosome binding.

• Poly-A Tail: String of adenine nucleotides added to the 3’ end for stability and export from the nucleus.

• Splicing: Removal of non-coding introns and joining of coding exons.

Mutation Types

Mutations are changes in the DNA sequence that can affect gene expression and protein function.

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

• Nonsense Mutation: Introduces a premature stop codon, truncating the protein.

• Frameshift Mutation: Insertion or deletion of nucleotides shifts the reading frame, altering downstream amino acids.

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.

• Mechanism: Natural selection acts on heritable variation, leading to adaptation.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution.

• Fossils: Show changes in organisms over time.

• Homology: Similar structures in different species due to common ancestry.

• Biogeography: Geographic distribution of species reflects evolutionary history.

• Anatomy: Comparative anatomy reveals homologous and analogous structures.

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 their traits.

• Survival: Traits that enhance survival and reproduction 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.

• Example: The panda’s thumb is an adaptation, but not an ideal structure.

Chapter 23: Evolution of Populations

Hardy-Weinberg Equilibrium

The Hardy-Weinberg equation describes the genetic makeup of a non-evolving population.

• Equation: $p^2 + 2pq + q^2 = 1$

• 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 evolution).

• 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 due to a disaster, reducing genetic diversity.

• Founder Effect: A few individuals colonize a new area, leading to a different allele frequency than the original population.

Gene Flow and Natural Selection

Gene flow and natural selection are important mechanisms of evolution.

• Gene Flow: Movement of alleles between populations through migration.

• Natural Selection: Differential survival and reproduction based on heritable traits.

Types of Selection

Natural selection can take different forms, shaping the distribution of traits in a population.

• Stabilizing Selection: Favors intermediate phenotypes.

• Directional Selection: Favors one extreme phenotype.

• Disruptive Selection: Favors both extreme phenotypes over intermediates.