Cell Division and Mendelian Genetics: Study Notes for Chapters 12-15

Cell Division: Mitosis and Meiosis

Order, Function, and Key Events of Mitosis Phases

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. It consists of several distinct phases:

• Prophase: Chromatin condenses into visible chromosomes; spindle fibers begin to form.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids are pulled apart toward opposite poles.

• Telophase: Nuclear envelopes reform around separated chromosomes; chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Key Function: Mitosis ensures equal distribution of genetic material to daughter cells, supporting growth, repair, and asexual reproduction.

Meiosis: Phases and Purpose

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique gametes. It consists of two sequential divisions:

• Meiosis I: Homologous chromosomes separate, reducing chromosome number.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Purpose: Meiosis generates genetic diversity and is essential for sexual reproduction.

Cell Cycle Phases: Gap 0, Gap 1, Synthesis, Gap 2

• Gap 0 (G0): Resting phase where cells may exit the cycle.

• Gap 1 (G1): Cell grows and prepares for DNA replication.

• Synthesis (S): DNA is replicated.

• Gap 2 (G2): Cell prepares for mitosis; checks for DNA errors.

Checkpoints: Critical control points (G1, G2, M) ensure proper cell cycle progression and prevent errors.

Sexual vs. Asexual Reproduction

• Sexual Reproduction: Involves meiosis and fertilization; increases genetic diversity.

• Asexual Reproduction: Involves mitosis; produces genetically identical offspring.

Advantages/Disadvantages: Sexual reproduction enhances adaptability; asexual reproduction is efficient but less diverse.

Homologs, Independent Assortment, and Crossing Over

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Independent Assortment: Random distribution of homologs during meiosis increases genetic variation.

• Crossing Over: Exchange of genetic material between homologs during prophase I of meiosis.

Example: Crossing over results in recombinant chromosomes, contributing to genetic diversity.

Chromosomal Disorders

• Nondisjunction: Failure of chromosomes to separate properly, leading to disorders such as Down syndrome (trisomy 21).

Additional info: Chromosomal disorders can result from errors in meiosis, affecting chromosome number or structure.

Mendelian Genetics and Inheritance

Key Terms in Genetics

• Characteristic: Observable feature (e.g., flower color).

• Trait: Specific variant of a characteristic (e.g., purple flowers).

• True-breeding: Organisms that produce offspring identical to themselves when self-pollinated.

• Allele: Alternative form of a gene.

• Dominant/Recessive: Dominant alleles mask recessive alleles in heterozygotes.

• Phenotype: Observable traits.

• Genotype: Genetic makeup (e.g., AA, Aa, aa).

Punnett Squares and Predicting Outcomes

• Punnett Square: Diagram used to predict the outcome of genetic crosses.

• Monohybrid Cross: Cross involving one trait.

• Dihybrid Cross: Cross involving two traits.

Example: Crossing two heterozygous pea plants (Aa x Aa) yields a 3:1 ratio of dominant to recessive phenotypes.

Mendel's Laws of Inheritance

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Application: Dihybrid crosses demonstrate independent assortment, resulting in a 9:3:3:1 phenotypic ratio.

Genetic Crosses and Pedigrees

• Test Cross: Crossing an individual with a dominant phenotype with a homozygous recessive to determine genotype.

• Pedigree Analysis: Chart showing inheritance patterns in families; used to track genetic disorders.

Complex Patterns of Inheritance

• Incomplete Dominance: Heterozygotes show intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are expressed (e.g., AB blood type).

• Polygenic Inheritance: Multiple genes influence a trait (e.g., skin color).

• Environmental Effects: Phenotype can be influenced by environment (e.g., nutrition affecting height).

Example: Human traits such as height and skin color are polygenic and influenced by environmental factors.

Human Traits and Inheritance Patterns

• Pedigree Analysis: Used to determine inheritance patterns of traits such as cystic fibrosis or sickle cell anemia.

• Patterns of Inheritance: Autosomal dominant, autosomal recessive, X-linked, and multifactorial traits.

Additional info: Understanding inheritance patterns is crucial for predicting genetic risks and counseling.

Summary Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Genetic Identity	Identical	Unique
Chromosome Number	Diploid (2n)	Haploid (n)
Role	Growth, repair, asexual reproduction	Sexual reproduction