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 several distinct phases, each with specific characteristics and functions.

• G1 Phase (First Gap): Cell grows, produces RNA and proteins, prepares for DNA synthesis. Most variable in length.

• S Phase (Synthesis): DNA replication occurs, chromosomes duplicate.

• G2 Phase (Second Gap): Cell continues to grow, prepares for mitosis, checks for DNA errors.

• M Phase (Mitosis): Cell divides its nucleus and cytoplasm, resulting in two daughter cells.

• G0 Phase: Non-dividing state; cells exit the cycle and do not prepare for division (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 a cell divides its nucleus and distributes identical genetic material to two daughter cells.

• Prophase: Chromosomes condense, spindle forms, nuclear envelope breaks down.

• 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, forming two cells.

Structures: Spindle fibers, centromeres, chromatids.

Purpose: Growth, repair, asexual reproduction.

Meiosis

Stages and Events of Meiosis

Meiosis is a two-part cell division process that produces gametes (sperm and eggs) with half the chromosome number of the original cell.

• 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, reduction of chromosome number.

Structures: Homologous chromosomes, chromatids, spindle.

Regulation of the Cell Cycle

MPF, Cyclins, CDKs, Kinases

Cell cycle progression is regulated by proteins and enzymes.

• MPF (Maturation Promoting Factor): Triggers mitosis; composed of cyclin and CDK.

• 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: Cyclin levels rise during S and G2, fall after M phase.

Comparison: Mitosis vs. 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
Cell Type	Somatic	Gametic

Chromosome Structure

Key Components

• Centromere: Region joining sister chromatids.

• Chromatids: Each half of a duplicated chromosome.

Cancer Cells vs. Normal Cells

Division and Tumor Types

• Cancer Cells: Divide uncontrollably, can grow without external signals.

• Normal Cells: Require signals, stop dividing when crowded.

• Benign Tumors: Do not invade other tissues.

• Malignant Tumors: Can migrate (metastasize) to other sites.

Somatic vs. Gametic Cells

Characteristics

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

Key Terms in Genetics and Cell Biology

• Genome: Complete set of genetic material.

• Karyotype: Chromosome arrangement; best obtained when chromosomes are condensed.

• Gametophyte: Haploid phase in plant life cycle.

• Zygote: Fertilized egg, diploid.

• Clone: Genetically identical organism.

• Spore: Reproductive cell in plants/fungi.

• Sporophyte: Diploid phase in plant life cycle.

• Homologous Chromosomes: Chromosome pairs, one from each parent.

• 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 (Programmed Cell Death)

Process and Molecules Involved

• Fragmentation: DNA and proteins are cut up.

• Vesicle Formation: Cell contents are packaged into vesicles.

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

• Ced-3, Ced-4, Ced-9: Regulate apoptosis; active Ced-9 inhibits apoptosis, non-functioning Ced-4 prevents apoptosis.

• P53: 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 involves secondary messengers.

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

• Types of Receptors: Membrane-bound, intracellular.

• Kinase: Enzyme that attaches phosphate to proteins.

Cancer Genetics

Genes and Mutations

• Tumor Suppressor Genes: Repair DNA damage; examples include BRCA1 and BRCA2.

• Ras Protein: Relays signal from growth factor receptor.

• Cancer: Results from multiple mutations; amplification of proto-oncogene and inactivation of tumor suppressor gene.

• Proto-oncogenes: Regulate cell division; mutation can convert to oncogene, causing cancer.

Mendelian Genetics

Key Terms and Concepts

• Allele: Variant form of a gene.

• Trait: Observable characteristic.

• Phenotype: Physical expression of genes.

• Genotype: Genetic makeup.

• Recessive/Dominant: Recessive alleles are masked by dominant alleles.

• Heterozygous/Homozygous: Two different/same alleles.

• Testcross: Cross to determine genotype.

• Punnett Square: Diagram to predict genetic outcomes.

• Progeny: Offspring.

• Dihybrid: Two traits.

Degrees of Dominance

• Complete Dominance: One allele masks the other.

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

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene affects expression of another.

Mendel's Crosses

• P Generation: Parental.

• F1 Generation: First filial; all dominant phenotype.

• F2 Generation: Second filial; 3:1 phenotype ratio.

Gene Distribution and Inheritance

Chromosomal Distribution

• Genes on different chromosomes assort independently.

• Genes close together on same chromosome are linked.

• Polygenic Inheritance: Multiple genes affect one trait.

• Environmental Effects: Environment can influence gene expression.

Key Terms in Inheritance

• Multifactorial: Traits influenced by genes and environment.

• Locus: Location of a gene on a chromosome.

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

Punnett Squares and Probability

Using Punnett Squares

Punnett squares can be used to determine the probability of inheritance for multiple traits.

Sex-Linked Traits and Chromosomal Systems

X-Linked and Sex-Linked Traits

• 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: Chemical modification affecting gene expression.

Chromosomal Systems for Sex Determination

• Mammals: XX (female), XY (male).

• Birds: ZW (female), ZZ (male).

• Insects: Various systems.

• Plants: Diverse mechanisms.

SRY: Triggers male development.

Barr Body: Inactive X chromosome in female somatic cells.

Linked Genes and Chromosome Alterations

Linked Genes and Crossing Over

• New combinations arise from crossing over.

• Nondisjunction can cause aneuploidy.

Alterations in Chromosome Structure

• Deletion: Loss of chromosome segment.

• Inversion: Reversal of segment.

• Translocation: Segment moves to another chromosome.

Karyotypes

Obtaining Karyotypes

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

Additional info: Academic context was added to clarify and expand on brief points, including definitions, examples, and explanations of processes.