Cell Division, Genetics, and Inheritance: Study Guide for Biology 161 Test 4

Mitosis, the Cell Cycle, and Apoptosis

Key Terminology

• Sister chromatids: Identical copies of a 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 carries out normal functions.

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

• G2 phase: Cell prepares for division; checks for DNA errors.

• M phase (Mitosis): Division of the nucleus and cytoplasm.

Events in Mitosis

• 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 envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, forming two daughter cells.

Bacterial Cell Division (Binary Fission)

• Prokaryotes divide by binary fission, a simpler process involving DNA replication, segregation, and division of the cell.

Regulation of the Cell Cycle

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

• Checkpoints: Control points (G1, G2, M) where the cell assesses readiness to proceed; tumor suppressor genes (e.g., p53) help regulate these checkpoints.

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

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

Apoptosis – Programmed Cell Death

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

• Process involves DNA fragmentation, formation of apoptotic bodies, membrane blebbing, and activation of macrophages to clear debris.

Meiosis and Genetic Recombination

Key Terminology

• Haploid (n): One set of chromosomes.

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

• Aneuploid: Abnormal number of chromosomes.

Lifecycles: Animals vs Plants

• Animals: Gametes (sperm, egg) are haploid; fertilization restores diploid state.

• Plants: Alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations.

Events in Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate (similar to mitosis).

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

Differences Between Meiosis and Mitosis

• Mitosis produces two genetically 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: Exchange of genetic material between homologous chromosomes during Prophase I.

• Independent assortment: Random orientation of homologous pairs during Metaphase I.

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

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

• Studied inheritance in pea plants; deduced the laws of segregation and independent assortment.

Key Terminology

• Allele: Alternative form of a gene.

• Gene: Unit of heredity.

• 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 (dominant:recessive) in F2 generation.

• Dihybrid cross: 9:3:3:1 phenotypic ratio in F2 generation.

Testcross

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

Punnett Squares and Probability

• Visual tool to predict offspring genotypes and phenotypes.

• Multiplication rule: Probability of 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 (e.g., pink flowers from red and white parents).

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

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

• Polygenic traits: Multiple genes affect a trait (e.g., skin color).

• Epistasis: One gene affects expression of another gene.

• Pleiotropy: One gene influences multiple traits.

• Gene x environment: Environmental factors influence gene expression (e.g., temperature-sensitive fur color in rabbits).

Pedigree Analysis

• Used to determine inheritance patterns (autosomal or X-linked, dominant or recessive) in families.

Telomeres and Telomerase

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

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

DNA Proofreading and Repair Mechanisms

• DNA polymerases proofread and correct errors during replication.

• Repair mechanisms (e.g., mismatch repair, excision repair) fix DNA damage.

Higher Order Thinking Questions (with Academic Context)

1. Compare and contrast mitosis and meiosis: Both involve chromosome duplication and division, but mitosis produces two identical diploid cells for growth/repair, while meiosis produces four unique haploid cells for sexual reproduction.

2. Cell cycle regulation: Ensures cells divide only when appropriate; loss of control can lead to cancer or apoptosis.

3. Cytoplasmic factors and mitosis: Cytoplasm from dividing cells contains MPF (cyclin + cdk), which can induce mitosis in non-dividing cells by activating mitotic processes.

4. Crossing-over: Occurs during Prophase I of meiosis; increases genetic diversity but improper crossing-over can cause chromosomal mutations.

5. Oogenesis chromatid count: Human oogonium (2n) duplicates DNA (4n chromatids), then meiosis I and II reduce to one egg (n) with one chromatid per chromosome.

6. Genetic cross ratios:

   • 3:1 – Monohybrid cross (heterozygotes)

   • 9:3:3:1 – Dihybrid cross (independent assortment)

   • 1:1:1:1 – Testcross with dihybrid

   • 1:1 – Testcross with monohybrid

   • 1:2:1 – Incomplete dominance or codominance in monohybrid cross

7. Pedigree analysis: Sex-linked recessive traits often skip generations and affect more males; autosomal recessive traits affect both sexes equally. Dominant traits appear in every generation; X-linked dominant traits affect both sexes but often more females.

8. Dominant traits: Determined by allele function, not prevalence. Example: In Mendel’s peas, yellow seed color is dominant but not necessarily most common. Dominance results from the gene product’s function or mutation effects.

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

Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic composition	Identical to parent	Genetically unique
Chromosome number	Diploid (2n)	Haploid (n)
Role	Growth, repair	Gamete production

Key Equations

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

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