Cell Division, Genetics, and Regulation: Study Guide for Biology 161

Mitosis, the Cell Cycle, and Apoptosis

Terminology

• Sister chromatids: Two identical copies of a single chromosome, connected by a centromere, formed during DNA replication.

• Homologous chromosomes: Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.

Phases of the Cell Cycle

• G1 phase: Cell grows and prepares for DNA replication.

• S phase: DNA is replicated, forming sister chromatids.

• G2 phase: Cell prepares for mitosis, checks for DNA errors.

• M phase: Mitosis and cytokinesis occur.

Events in Mitosis

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cytokinesis

• Division of the cytoplasm, resulting in two daughter cells.

Bacterial Cell Division (Binary Fission)

• Bacteria divide by binary fission, a simpler process than mitosis.

• DNA is replicated, cell elongates, and divides into two identical cells.

Regulation of the Cell Cycle

• Maturation Promoting Factor (MPF): A complex of cyclin and cyclin-dependent kinase (cdk) that triggers mitosis.

• Checkpoints: Control points where the cell verifies whether to proceed; tumor suppressor genes (e.g., p53) help regulate these.

• Social control/signal transduction: Proto-oncogenes promote cell division; different cyclins and cdks regulate specific phases.

• Cancer: Characterized by uncontrolled cell division, often due to mutations in regulatory genes; follows the multi-hit hypothesis (multiple mutations required).

• Environmental effects: Factors like radiation or chemicals can increase mutation rates and cancer risk.

Apoptosis – Programmed Cell Death

• Occurs during development, in response to DNA damage, or to remove unneeded cells.

• Process includes DNA fragmentation, formation of apoptotic bodies, membrane blebbing, and activation of macrophages for cleanup.

Meiosis and Genetic Recombination

Terminology

• Haploid: Cells with one set of chromosomes (n).

• Diploid: Cells with two sets of chromosomes (2n).

• Aneuploid: Cells with abnormal chromosome numbers.

Lifecycles: Animals vs Plants

• Animals: Diploid dominant; gametes produced by meiosis.

• Plants: Alternation of generations; both haploid and diploid multicellular stages.

Events in Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

• Order: Prophase I, Metaphase I, Anaphase I, Telophase I, then Prophase II, Metaphase II, Anaphase II, Telophase II.

Difference Between Meiosis and Mitosis

• Mitosis produces two identical diploid cells; meiosis produces four genetically unique haploid cells.

• Meiosis includes crossing-over and independent assortment, increasing genetic diversity.

Sources of Genetic Variation

• Crossing-over: Occurs during Prophase I; homologous chromosomes exchange segments.

• Independent assortment: Occurs during Metaphase I; random orientation of homologs.

• Random fertilization: Any sperm can fertilize any egg, further increasing variation.

Differences in Oogenesis vs Spermatogenesis

• Oogenesis: Produces one egg and polar bodies; occurs in ovaries.

• Spermatogenesis: Produces four sperm; occurs in testes.

Genetics (Mendelian and Non-Mendelian)

Mendel’s Experiments and Laws

• Mendel studied pea plants, deducing the laws of segregation and independent assortment.

• 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.

Terminology

• Allele: Variant form of a gene.

• Gene: DNA segment coding for a trait.

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

• Dominant: Expressed allele in heterozygote.

• Recessive: Masked allele in heterozygote.

• Genotype: Genetic makeup.

• Phenotype: Observable traits.

Genotypic and Phenotypic Ratios

• Monohybrid cross: 3:1 phenotypic ratio, 1:2:1 genotypic ratio.

• Dihybrid cross: 9:3:3:1 phenotypic ratio.

Testcross

• Used to determine genotype of an individual with dominant phenotype by crossing with a homozygous recessive.

Punnett Square

• Visual tool to predict offspring genotypes and phenotypes.

Rules of Multiplication and Addition

• Multiplication rule: Probability of two independent events occurring together is the product of their probabilities.

• Addition rule: Probability of either of two mutually exclusive events is the sum of their probabilities.

Non-Mendelian Inheritance Patterns

• Incomplete dominance: Heterozygote shows intermediate phenotype.

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

• Multiple allelism: More than two alleles for a gene (e.g., ABO blood group).

• Polygenic traits: Controlled by multiple genes (e.g., skin color).

• Epistatic traits: One gene affects expression of another.

• Pleiotropic traits: One gene affects multiple traits.

• Gene x environment: Expression affected by environmental factors.

Pedigree Analysis

• Used to identify inheritance patterns: autosomal or X-linked, dominant or recessive.

• Sex-linked traits often show different patterns in males and females.

Telomeres and Telomerase

• Telomeres: Protective ends of chromosomes; shorten with each division.

• Telomerase: Enzyme that extends telomeres, active in stem cells and cancer cells.

DNA Proofreading and Repair Mechanisms

• DNA polymerase proofreads during replication.

• Repair mechanisms fix mismatches and damage (e.g., excision repair).

Higher Order Thinking Questions

• Compare and contrast mitosis and meiosis: Both involve chromosome separation, but mitosis produces identical cells, meiosis produces genetically diverse gametes.

• Cell cycle regulation: Ensures proper division; loss of control can lead to cancer or apoptosis.

• MPF and cell division: Cytoplasm from dividing cells contains MPF, which can induce mitosis in non-dividing cells; cyclin and cdk must be activated, checkpoints must be passed.

• Crossing-over: Occurs in Prophase I; increases genetic diversity, but can lead to chromosomal mutations if errors occur.

• Chromatid number in oogenesis: Changes as DNA replicates and meiosis progresses; starts with 46 chromatids, doubles after replication, reduces after meiosis.

• Genetic cross ratios:

  • 3:1 – Monohybrid cross (dominant/recessive)

  • 9:3:3:1 – Dihybrid cross (two traits)

  • 1:1:1:1 – Dihybrid testcross

  • 1:1 – Testcross for one trait

  • 1:2:1 – Incomplete dominance or codominance

• Pedigree analysis: Sex-linked recessive traits often skip generations and affect males more; autosomal traits affect both sexes equally.

• Dominant traits: Determined by gene function, not prevalence; e.g., in Mendel’s crosses, dominant allele produces functional protein, recessive does not.

• Extensions to Mendel’s laws: Simple dominance (one allele masks another), incomplete dominance (blended phenotype), codominance (both alleles expressed).

Example Table: Genetic Cross Ratios

Type of Cross	Phenotypic Ratio	Genotypic Ratio
Monohybrid (dominant/recessive)	3:1	1:2:1
Dihybrid	9:3:3:1	Additional info: 1:2:1:2:2:4:1:2:1 (inferred)
Testcross (monohybrid)	1:1	1:1
Dihybrid testcross	1:1:1:1	1:1:1:1
Incomplete dominance	1:2:1	1:2:1

Key Equations

• Probability of independent events: $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of mutually exclusive events: $P(A \text{ or } B) = P(A) + P(B)$

Additional info:

• Some details about dihybrid genotypic ratios and cell cycle checkpoints were inferred for completeness.