Cell Cycle, Cell Division, and Genetics: Exam 4 Study Guide

Cell Cycle and Its Phases

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (M). Some cells enter a resting phase called G0.

• G1 Phase (First Gap): Cell grows, produces proteins and organelles. Most variable in length.

• S Phase (Synthesis): DNA is replicated; chromosomes duplicate.

• G2 Phase (Second Gap): Cell prepares for division; checks for DNA errors, synthesizes proteins needed for mitosis.

• M Phase (Mitosis): Cell divides its nucleus and cytoplasm.

• G0 Phase: Non-dividing state; cells may remain here permanently (e.g., neurons).

Example: Liver cells may spend years in G0, while skin cells cycle rapidly.

Mitosis

Stages of Mitosis

Mitosis is the process by which somatic cells divide, producing two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle forms.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

• Cytokinesis: Division of cytoplasm.

Purpose: Growth, repair, and asexual reproduction.

Meiosis

Stages and Events of Meiosis

Meiosis produces gametes (sperm and eggs) with half the chromosome number of the parent cell. It consists of two divisions: Meiosis I and Meiosis II.

• Meiosis I: Homologous chromosomes separate. Includes synapsis, crossing over (2-3 times per chromosome), chiasmata formation.

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

Purpose: Genetic diversity and reduction of chromosome number.

Regulation of the Cell Cycle

MPF, Cyclins, CDKs, Kinases

Cell cycle progression is regulated by proteins:

• MPF (Maturation Promoting Factor): Triggers entry into mitosis.

• Cyclins: Proteins whose concentration fluctuates during the cell cycle.

• CDKs (Cyclin-Dependent Kinases): Enzymes activated by cyclins; phosphorylate target proteins.

• Kinases: Enzymes that add phosphate groups to proteins.

Example: MPF is high during mitosis, cyclin levels rise and fall.

Comparison of Mitosis and Meiosis

Key Differences

Feature	Mitosis	Meiosis
Daughter Cells	2, identical	4, genetically unique
Chromosome Number	2N (diploid)	N (haploid)
Purpose	Growth, repair	Gamete production
Stages	1 division	2 divisions
Genetic Variation	None	Crossing over, independent assortment

Chromosome Structure

Key Components

• Centromere: Region where sister chromatids are joined.

• Chromatids: Each of the two identical halves of a duplicated chromosome.

Cancer Cells vs. Normal Cells

Differences in Division

• Normal Cells: Divide only when needed, require growth factors.

• Cancer Cells: Divide uncontrollably, can grow without growth factors, ignore cell cycle checkpoints.

Benign vs. Malignant Tumors

Characteristics

Tumor Type	Characteristics
Benign	Non-invasive, do not spread
Malignant	Invasive, can migrate (metastasize) to other sites

Somatic vs. Gametic Cells

Comparison

Cell Type	Purpose	Genetic Material	Autosomes	Sex Chromosomes
Somatic	Body functions	2N (diploid)	Full set	2
Gametic	Reproduction	N (haploid)	Half set	1

Key Terms in Genetics and Cell Biology

Definitions

• Genome: Complete set of genetic material.

• Karyotype: Chromosome arrangement in a cell.

• Gametophyte: Haploid phase in plant life cycle.

• Zygote: Fertilized egg cell.

• Clone: Genetically identical organism.

• Spore: Reproductive cell in plants/fungi.

• Sporophyte: Diploid phase in plant life cycle.

• Homologous Chromosomes: Chromosome pairs with same genes.

• Spindle: Structure that separates chromosomes.

• Haploid: One set of chromosomes (N).

• Diploid: Two sets of chromosomes (2N).

Life Cycles

Alternation of Generations

Many plants and some algae alternate between haploid (gametophyte) and diploid (sporophyte) generations.

• Gametophyte: Produces gametes by mitosis.

• Sporophyte: Produces spores by meiosis.

Apoptosis

Process and Molecules Involved

Apoptosis is programmed cell death, involving DNA and protein fragmentation, packaging into vesicles, and removal by phagocytosis.

• Proteases: Enzymes that break down proteins (e.g., caspases).

• Ced-3, Ced-4, Ced-9: Regulate apoptosis; active Ced-9 inhibits apoptosis, loss of Ced-4 prevents apoptosis.

• P53: Tumor suppressor; activates genes in apoptosis pathway. Elephants have more copies, leading to lower cancer rates.

Cell Signaling

Stages and Components

• Signal Reception: Cell detects signaling molecule (ligand).

• Signal Transduction: Pathways relay signal inside cell, often using secondary messengers.

• Cellular Response: Cell changes behavior (e.g., gene expression).

• Types of Receptors: Membrane-bound, intracellular.

• Kinase: Enzyme that attaches phosphate to proteins.

Tumor Suppressor Genes and Cancer

Key Genes and Proteins

• BRCA1, BRCA2: Repair DNA damage.

• Ras Protein: Relays growth factor signals.

• Cancer: Results from multiple mutations, including proto-oncogene amplification and tumor suppressor inactivation.

• Proto-oncogenes: Regulate cell division; mutations can turn them into oncogenes, causing cancer.

Genetics: Key Terms and Concepts

Definitions

• Allele: Variant form of a gene.

• Trait: Observable characteristic.

• Phenotype: Physical expression of genes.

• Genotype: Genetic makeup.

• Recessive: Expressed only when homozygous.

• Heterozygous: Two different alleles.

• Homozygous: Two identical alleles.

• Dominant: Expressed when present.

• Testcross: Cross with homozygous recessive to determine genotype.

• Punnett Square: Diagram to predict genetic crosses.

• Progeny: Offspring.

• Dihybrid: Individual heterozygous for two traits.

Degrees of Dominance

Types and Examples

• Complete Dominance: One allele masks the other (e.g., Mendel's pea plants).

• Co-dominance: Both alleles expressed (e.g., blood type AB).

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell disease).

• Epistasis: One gene affects expression of another (e.g., coat color in mice).

Mendel's Crosses

P, F1, F2 Generations

• P Generation: Parental.

• F1 Generation: First filial, all dominant phenotype.

• F2 Generation: Second filial, 3:1 phenotype ratio, 1:2:1 genotype ratio.

Gene Distribution and Polygenic Inheritance

Chromosome Location and Effects

• Genes on Same Chromosome: Tend to be inherited together.

• Polygenic Inheritance: Multiple genes affect one trait (e.g., skin color).

Environmental Effects and Multifactorial Traits

Definitions

• Multifactorial: Traits influenced by genes and environment.

• Locus: Location of a gene on a chromosome.

Law of Independent Assortment

Principle

Genes for different traits are inherited independently if they are on different chromosomes.

Punnett Squares for Multiple Traits

Application

Punnett squares can be used to predict inheritance of two or more traits (dihybrid crosses).

Sex-Linked Traits and Chromosomal Systems

Inheritance and Chromosome Systems

• X-linked Traits: Inherited differently in males (hemizygous) and females (homozygous).

• Aneuploidy: Abnormal chromosome number (e.g., trisomic, monosomic).

• Polyploidy: More than two sets of chromosomes.

• Methylation: Addition of methyl groups to DNA, affects gene expression.

• SRY: Triggers male development.

• Barr Body: Inactivated X chromosome in female somatic cells.

Organism	Sex Determination System
Mammals	XX/XY
Birds	ZZ/ZW
Insects	XX/XO
Plants	Various systems

Linked Genes and Chromosome Alterations

Crossing Over and Structural Changes

• Linked Genes: Genes close together on a chromosome; new combinations arise from crossing over.

• Nondisjunction: Failure of chromosomes to separate; leads to aneuploidy.

• Alterations: Deletion, inversion, translocation.

Karyotypes

Best Time for Analysis

Karyotypes are best obtained when chromosomes are condensed and not yet separated (metaphase).

Additional info: Academic context and definitions were expanded for clarity and completeness.