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

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Mitosis, the Cell Cycle, and Apoptosis

Terminology

Sister chromatids: Identical copies of a chromosome connected by a centromere, formed during DNA replication.
Homologous chromosomes: Chromosome pairs (one from each parent) with the same genes but possibly different alleles.

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, resulting in two daughter cells.
In animals: cleavage furrow forms; in plants: cell plate forms.

Bacterial Cell Division (Binary Fission)

Prokaryotes divide by binary fission, a simpler process than mitosis.
DNA replicates, 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 (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: Uncontrolled cell division due to mutations in proto-oncogenes (become oncogenes) or loss of tumor suppressor function; follows the multi-hit hypothesis (multiple mutations required); environmental factors can contribute.

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 to clear debris.

Meiosis and Genetic Recombination

Terminology

Haploid (n): One set of chromosomes (e.g., gametes).
Diploid (2n): Two sets of chromosomes (e.g., somatic cells).
Aneuploid: Abnormal number of chromosomes (e.g., trisomy 21).

Lifecycles: Animals vs Plants

Animals: Gametes are haploid; fertilization restores diploid state.
Plants: Alternation of generations between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

Events in Meiosis

Meiosis I: Homologous chromosomes separate (reductional division).
Meiosis II: Sister chromatids separate (similar to mitosis).
Order: Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II.
Difference from mitosis: Meiosis produces four non-identical haploid cells; mitosis produces two identical diploid cells.

Sources of Genetic Variation

Crossing-over: Exchange of genetic material between homologous chromosomes during Prophase I; increases genetic diversity.
Independent assortment: Random orientation of homologous pairs during Metaphase I; leads to varied combinations of chromosomes in gametes.
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; begins before birth in humans.
Spermatogenesis: Produces four sperm; occurs in testes; continuous after puberty.

Genetics (Mendelian and Non-Mendelian)

Mendel’s Experiments and Laws

Studied inheritance in pea plants; deduced the Law of Segregation and Law of Independent Assortment.

Key Terminology

Allele: Different forms of a gene.
Gene: Unit of heredity encoding a trait.
Homozygous: Two identical alleles for a gene.
Heterozygous: Two different alleles for a gene.
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 both occurring:
Addition rule: Probability of either event occurring:

Non-Mendelian Inheritance Patterns

Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red × white).
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 × 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.
Autosomal recessive: Often skips generations; affects both sexes equally.
Sex-linked recessive: More common in males; affected males often have carrier mothers.
Autosomal dominant: Appears in every generation; both sexes affected.
Sex-linked dominant: Affected fathers pass trait to all daughters, not sons.

Telomeres and Telomerase

Telomeres: Repetitive DNA sequences at chromosome ends; protect from degradation.
Telomerase: Enzyme that extends telomeres; active in germ cells, stem cells, and cancer cells.

DNA Proofreading and Repair Mechanisms

DNA polymerases proofread and correct errors during replication.
Repair mechanisms include mismatch repair, nucleotide excision repair, and base excision repair.

Higher Order Thinking Questions (with Academic Context)

Mitosis vs Meiosis: Both involve chromosome duplication and division; mitosis produces identical cells for growth/repair, meiosis produces genetically unique gametes for reproduction.
Cell Cycle Regulation: Ensures proper division; loss of control can lead to cancer or apoptosis.
MPF and Cell Cycle: Cytoplasm from dividing cells contains MPF, which can induce mitosis in non-dividing cells by activating cyclin/cdk complexes and passing checkpoints.
Crossing-over: Occurs in Prophase I of meiosis; increases genetic diversity but can also cause chromosomal mutations if misaligned.
Oogenesis Chromatid Number: Oogonium (2n) duplicates DNA (4n), then meiosis I (2n), meiosis II (n) to form egg.
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
Pedigree Analysis: Sex-linked recessive traits show more affected males; autosomal traits affect both sexes equally; dominant traits appear in every generation.
Dominant Traits: Dominance is determined by allele expression, not prevalence; e.g., yellow pea color is dominant in Mendel’s crosses, but not always most common in nature.
Extensions to Mendel’s Laws: Simple dominance (one allele masks another), incomplete dominance (blending), codominance (both alleles expressed).

Inheritance Pattern	Key Features	Example
Autosomal Dominant	Appears in every generation; both sexes affected	Huntington's disease
Autosomal Recessive	May skip generations; both sexes affected	Cystic fibrosis
Sex-linked Recessive	More males affected; carrier mothers	Hemophilia
Sex-linked Dominant	Affected fathers pass to all daughters	Rett syndrome

Additional info: Academic context and examples have been added to clarify and expand on the original study guide points.