Cell Division, Meiosis, and Mendelian Genetics: Study Notes

Chapter 12: The Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It ensures the accurate duplication and distribution of the cell's genetic material to daughter cells.

• Key Roles of Cell Division: Growth, repair, and reproduction in organisms.

• Genetically Identical Daughter Cells: Produced by mitosis, each with the same genetic information as the parent cell.

Genome, Chromosomes, and Chromatin

• Genome: The complete set of an organism's DNA, including all of its genes.

• Chromosomes: Structures that organize and carry genetic information; made of DNA and proteins.

• Chromatin: The complex of DNA and proteins that makes up chromosomes; condenses during cell division.

Somatic Cells vs. Gametes

• Somatic Cells: All body cells except reproductive cells; diploid (2n).

• Gametes: Reproductive cells (sperm and egg); haploid (n).

Sister Chromatids and Centromere

• Sister Chromatids: Two identical copies of a chromosome, joined at the centromere, formed during DNA replication.

• Centromere: The region where sister chromatids are most closely attached.

Mitosis vs. Meiosis

• Mitosis: Division of the nucleus resulting in two genetically identical daughter cells.

• Meiosis: Division that reduces chromosome number by half, producing four genetically distinct gametes.

Phases of the Cell Cycle

• Interphase: Period of cell growth and DNA replication; consists of G1, S, and G2 phases.

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for division.

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

Stages of Mitosis

• Prophase: Chromatin condenses into chromosomes; mitotic spindle begins to form.

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

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelopes reform; chromosomes decondense.

Mitotic Spindle, Centrosomes, and Kinetochores

• Mitotic Spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Centrosomes: Microtubule-organizing centers; duplicate during interphase.

• Kinetochore: Protein structure on chromatids where spindle fibers attach.

Cytokinesis

• Animal Cells: Cleavage furrow forms, pinching the cell in two.

• Plant Cells: Cell plate forms, dividing the cell.

Binary Fission in Bacteria

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

Cancer Cells and Cell Cycle Control

• Cell Cycle Control System: Regulates the timing of cell cycle events.

• Checkpoints: Control points where stop and go-ahead signals regulate the cycle (e.g., G1, G2, M checkpoints).

• G0 Phase: Non-dividing state cells may enter if they do not pass the G1 checkpoint.

• Benign vs. Malignant Tumors: Benign tumors do not spread; malignant tumors invade tissues and can metastasize.

• Metastasis: Spread of cancer cells to other parts of the body.

Chapter 13: Meiosis

Genes, Gametes, and Chromosomes

• Genes: Units of heredity made up of DNA.

• Gametes: Haploid reproductive cells (sperm and egg).

• Somatic Cells: Diploid body cells.

• Locus: Specific location of a gene on a chromosome.

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent.

• Sexual Reproduction: Offspring have unique combinations of genes from two parents.

Karyotype, Homologous Chromosomes, and Chromosome Types

• Karyotype: Ordered display of an individual's chromosomes.

• Homologous Chromosomes: Chromosome pairs, one from each parent, with genes for the same traits.

• Autosomes: Non-sex chromosomes.

• Sex Chromosomes: Determine an individual's sex (e.g., X and Y in humans).

Diploid vs. Haploid

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

• Haploid (n): One set of chromosomes.

Life Cycles and Meiosis

• Meiosis occurs in germ cells to produce gametes.

• After meiosis I and II, four haploid, genetically distinct daughter cells are produced.

Phases of Meiosis

• Prophase I: Homologous chromosomes pair up and exchange segments (crossing over at chiasmata).

• Metaphase I: Homologous pairs align randomly at the metaphase plate.

• Anaphase I: Homologous chromosomes separate to opposite poles.

• Meiosis II resembles mitosis, separating sister chromatids.

Comparison of Mitosis and Meiosis

• Similarities: Both involve chromosome duplication and division.

• Differences: Meiosis has two divisions, produces four non-identical haploid cells; mitosis produces two identical diploid cells.

Genetic Variation Mechanisms

• Independent Assortment: Random orientation of homologous pairs during metaphase I.

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I.

• Random Fertilization: Any sperm can fertilize any egg, increasing genetic diversity.

Chapter 14: Mendel and the Gene Idea

Gregor Mendel and His Experiments

• Gregor Mendel: Father of genetics; studied inheritance in pea plants.

• Characters and Traits: Characters are heritable features; traits are variants of characters.

• True-Breeding: Organisms that produce offspring of the same variety when self-pollinated.

• P, F1, F2 Generations: Parental, first filial, and second filial generations in Mendel's crosses.

Mendel's Findings

• Crossing true-breeding individuals resulted in all F1 offspring showing the dominant trait; F2 generation showed a 3:1 ratio of dominant to recessive traits.

• Dominant vs. Recessive Traits: Dominant traits mask recessive traits in heterozygotes.

Mendel's Four Concepts

1. Alternative versions of genes (alleles) account for variations in inherited characters.

2. Each organism inherits two alleles, one from each parent.

3. If alleles differ, the dominant allele determines the organism's appearance.

4. Law of Segregation: The two alleles for a gene segregate during gamete formation.

Genotype and Phenotype

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

Testcross and Dihybrid Cross

• Testcross: Breeding an individual with a dominant phenotype with a homozygous recessive to determine genotype.

• Dihybrid Cross: Cross between individuals differing in two characters.

Deviations from Mendelian Genetics

• Incomplete Dominance: Heterozygotes show intermediate phenotype (e.g., pink flowers from red and white parents).

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

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

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

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

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

• Environmental Impact: Environment can influence phenotype (e.g., hydrangea flower color depends on soil pH).

Inherited Disorders

• Recessive Alleles: Disorders expressed only in homozygous recessive individuals (e.g., cystic fibrosis, sickle cell disease).

• Carriers: Heterozygotes who carry a recessive allele but do not show symptoms.

• Dominantly Inherited Disorders: Expressed in individuals with one or two copies of the dominant allele (e.g., Huntington's disease).

• Multifactorial Disorders: Influenced by genetic and environmental factors (e.g., heart disease, diabetes).

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

Example: Sickle Cell Disease

• Caused by a recessive allele; heterozygotes are carriers and may have some resistance to malaria.

Additional info: The above notes expand on the provided key concepts with definitions, examples, and a comparison table for clarity and completeness.