Cell Division, Mendelian Genetics, and Chromosomal Inheritance: BIOL 105 Exam 3 Study Guide

The Cell Cycle and Mitosis

DNA Quantity During Interphase

During interphase, the cell prepares for division by growing and replicating its DNA. Interphase consists of three main phases: G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis). The quantity of DNA doubles during the S phase, resulting in each chromosome consisting of two sister chromatids by the end of interphase.

• G1 phase: Normal DNA content (one chromatid per chromosome).

• S phase: DNA replication occurs; DNA content increases.

• G2 phase: DNA content is doubled (two sister chromatids per chromosome).

Nuclear Envelope Degradation in Prophase

During prophase of mitosis, the nuclear envelope breaks down to allow spindle fibers to access and attach to chromosomes. This is essential for the proper segregation of chromosomes during cell division.

Phases of Mitosis: Identification and Key Events

• Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope degrades.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Chromosomes decondense, nuclear envelopes reform around daughter nuclei.

Plant vs. Animal Cell Cytokinesis

• Animal cells: Undergo cytokinesis via cleavage furrow formation, where the cell membrane pinches inward.

• Plant cells: Form a cell plate that develops into a new cell wall, dividing the cell in two.

Sister Chromatids During Mitosis

• Present: From the end of S phase through metaphase.

• Separated: During anaphase, sister chromatids are pulled apart to opposite poles.

Main Events of Interphase (DNA Focus)

• G1: Cell growth and normal function.

• S: DNA replication, forming sister chromatids.

• G2: Preparation for mitosis, error checking, and repair.

Cyclin-Dependent Kinase (CDK)

Cyclin-dependent kinases (CDKs) are enzymes that regulate the cell cycle. Their activity is controlled by binding to cyclins, which are regulatory proteins whose concentrations fluctuate throughout the cell cycle.

• Type: Protein kinase (adds phosphate groups to target proteins).

• Regulation: Activated only when bound to a cyclin.

Asexual Reproduction and Cell Division

Asexual reproduction involves the production of offspring from a single parent, typically through mitosis. Offspring are genetically identical to the parent.

Chromosome Number in Karyotypes and Gametes

• Karyotype: The complete set of chromosomes in a cell (diploid, 2n).

• Gametes: Sex cells (sperm or egg) contain half the chromosome number (haploid, n).

Abnormal Chromosome Numbers in Humans

Abnormal chromosome numbers, such as trisomy or monosomy, often result from errors during meiosis (nondisjunction).

Meiosis and Chromosomal Inheritance

Homologous Chromosomes

• Definition: Chromosomes that have the same genes at the same loci but may have different alleles.

• Alignment: Homologous chromosomes pair and align during metaphase I of meiosis.

Comparing Cells at Different Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Nondisjunction and Aneuploidy

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

• Aneuploidy: Presence of an abnormal number of chromosomes (e.g., trisomy 21 in Down syndrome).

Mendelian Genetics

Monohybrid and Dihybrid Crosses

• Monohybrid cross: Involves one gene with two alleles (e.g., Aa x Aa).

• Dihybrid cross: Involves two genes, each with two alleles (e.g., AaBb x AaBb).

• Difference: Dihybrid crosses show independent assortment of genes.

Law of Independent Assortment

The law of independent assortment states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.

Punnett Squares and Genotype/Phenotype Prediction

• Punnett square: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Dominant/Recessive: Dominant alleles mask the effect of recessive alleles.

• Example: AaBB x aaBb cross; 1/2 of offspring will have the recessive phenotype for gene a and the dominant phenotype for gene b.

Genetic Terms: Incomplete Dominance, Pleiotropy, Epistasis

• Incomplete dominance: Heterozygotes show an intermediate phenotype (e.g., red x white flowers produce pink offspring).

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene affects the expression of another gene.

Pedigree Analysis

A pedigree chart is a diagram that shows the inheritance of a trait through generations of a family. It is used to predict genotypes and phenotypes of individuals based on family history.

• Symbols: Squares represent males, circles represent females; shaded symbols indicate affected individuals.

• Analysis: Patterns of inheritance (dominant, recessive, X-linked) can be inferred from the pedigree.

Sex-Linked Characteristics and Morgan's Experiments

• Thomas Hunt Morgan: Demonstrated that genes are located on chromosomes using fruit fly crosses, especially for sex-linked traits.

• Sex-linked traits: Traits controlled by genes on sex chromosomes (e.g., X-linked recessive traits).

• Punnett squares: Used to predict inheritance of X-linked traits (e.g., XNXn x XnY).

Interpreting Sex-Linked Genotype Shorthand

• Notation: XNXN (female, homozygous dominant), XnY (male, recessive allele).

Predicting Outcomes of Nondisjunction

• Meiosis I error: Homologous chromosomes fail to separate; results in gametes with n+1 or n-1 chromosomes.

• Meiosis II error: Sister chromatids fail to separate; similar consequences.

Term	Definition	Example
Monohybrid Cross	Cross involving one gene	Aa x Aa
Dihybrid Cross	Cross involving two genes	AaBb x AaBb
Incomplete Dominance	Heterozygote shows intermediate phenotype	Red x White = Pink
Pleiotropy	One gene affects multiple traits	Sickle cell gene affects blood, organs
Epistasis	One gene affects expression of another	Coat color in mice

Additional info: For more detailed examples and practice problems, refer to textbook chapters on cell division, Mendelian genetics, and chromosomal inheritance. The pedigree chart provided is a classic example for practicing genotype and phenotype predictions in family studies.