Chromosomal Inheritance, DNA Replication, Gene Expression, and Evolution: Study Notes

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.

• Key Point 1: Genes are located on chromosomes, which segregate and assort independently during meiosis, mirroring Mendel’s laws.

• Key Point 2: The behavior of chromosomes during gamete formation explains patterns of inheritance.

• Example: The independent assortment of chromosomes during metaphase I of meiosis leads to genetic variation in gametes.

Sex-Linked Inheritance

Sex-linked inheritance refers to genes located on sex chromosomes, especially the X chromosome. Males (XY) are more likely to express recessive X-linked traits because they have only one X chromosome.

• Key Point 1: Males express recessive X-linked traits more often than females because they lack a second X chromosome to mask the effect.

• Key Point 2: Examples include color blindness and hemophilia.

SRY Gene and Sex Determination

The SRY gene (Sex-determining Region Y) is found on the Y chromosome and triggers male development.

• Key Point 1: Presence of SRY initiates testes development and male characteristics.

• Key Point 2: Absence of SRY leads to female development.

Barr Body

A Barr body is an inactivated X chromosome found in female cells, ensuring dosage compensation between males and females.

• Key Point 1: Only one X chromosome remains active in each cell; the other condenses into a Barr body.

• Key Point 2: This process is called X-inactivation.

Chromosomal Mutations

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

• Key Point 1: Deletion: Loss of a chromosome segment.

• Key Point 2: Duplication: Repetition of a chromosome segment.

• Key Point 3: Inversion: Reversal of a segment within a chromosome.

• Key Point 4: Translocation: Movement of a segment from one chromosome to another.

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.

• Key Point 1: Ensures genetic continuity between generations of cells.

• Key Point 2: Each strand serves as a template for the new strand.

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.

• Key Point 1: Leading strand is synthesized in the same direction as the replication fork movement.

• Key Point 2: Lagging strand is synthesized in short fragments, later joined by ligase.

DNA vs RNA

DNA and RNA are nucleic acids with distinct differences:

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Strands	Double-stranded	Single-stranded
Function	Genetic storage	Protein synthesis, regulation

Telomeres

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

• Key Point 1: Telomeres shorten with each cell division.

• Key Point 2: The enzyme telomerase can extend telomeres in certain 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.

• Key Point 1: Transcription: DNA → RNA (in the nucleus)

• Key Point 2: Translation: RNA → Protein (at the ribosome)

Codons and Anticodons

A codon is a sequence of three nucleotides on mRNA that codes for a specific amino acid. An anticodon is a complementary sequence on tRNA.

• Key Point 1: Codons determine the amino acid sequence of proteins.

• Key Point 2: Anticodons ensure correct pairing during translation.

mRNA Processing

Before mRNA leaves the nucleus, it undergoes several modifications:

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

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

• Splicing: Removal of non-coding introns; exons are joined together.

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, 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, where new species arise from ancestral forms.

• Key Point 1: All organisms share a common ancestor.

• Key Point 2: Modifications accumulate 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.

• Key Point 1: Variation exists within populations.

• Key Point 2: 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.

Chapter 23: Evolution of Populations

Hardy-Weinberg Principle

The Hardy-Weinberg equation describes genetic equilibrium in a population:

• Equation:

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

• p: Frequency of dominant allele

• q: Frequency of 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.