Chapter 12: Mitosis & Cell Cycle

Overview of the Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It consists of interphase (G1, S, G2 phases) and the mitotic (M) phase, which includes mitosis and cytokinesis.

• Interphase: The period of cell growth and DNA replication. It includes:

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

  • S phase: DNA is replicated.

  • G2 phase: Cell prepares for mitosis.

• Mitosis (M phase): Division of the nucleus, followed by cytokinesis (division of the cytoplasm).

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets. The phases are:

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

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform around each set of chromosomes, chromosomes decondense.

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

Labeling and Describing Phases of Mitosis and the Cell Cycle

Students should be able to label diagrams of the cell cycle and mitosis, and provide brief descriptions of each phase. For example:

• Metaphase: Chromosomes line up at the cell's equator, attached to spindle fibers.

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

Chromatids and Chromosome Number

During mitosis, the number of chromatids and chromosomes changes as follows:

• Before S phase: Each chromosome consists of one chromatid.

• After S phase (before mitosis): Each chromosome consists of two sister chromatids.

• After anaphase: Sister chromatids separate, each becoming an individual chromosome.

In a species with a haploid number (n), the diploid number is 2n. Chromosome and chromatid numbers double during S phase and halve after cell division.

Origin of Cancer in Cell Cycle Dysregulation

Cancer can arise when the cell cycle is dysregulated, leading to uncontrolled cell division. Mutations in genes that regulate the cell cycle, such as proto-oncogenes and tumor suppressor genes, can result in cancer.

• Proto-oncogenes: Genes that promote cell division; mutations can make them oncogenes, causing excessive division.

• Tumor suppressor genes: Genes that inhibit cell division; mutations can disable these brakes.

Example: Mutation in the p53 tumor suppressor gene is common in many cancers.

Chapter 13: Meiosis & Alternation of Generations

Overview of Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid cells (gametes). It is essential for sexual reproduction and genetic diversity.

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Comparison of Mitosis and Meiosis

Key differences between mitosis and meiosis:

• Number of divisions: Mitosis (1), Meiosis (2)

• Number of daughter cells: Mitosis (2, identical), Meiosis (4, genetically unique)

• Genetic variation: Mitosis (none), Meiosis (crossing over and independent assortment create variation)

Crossing Over and Genetic Diversity

Crossing over occurs during prophase I of meiosis, where homologous chromosomes exchange genetic material. This increases genetic diversity in offspring.

• Chiasmata: Sites where crossing over occurs.

• Result: New combinations of alleles in gametes.

Role of Sex Chromosomes in Sex Differentiation

Sex chromosomes (X and Y in humans) determine an individual's sex. The presence or absence of certain chromosomes and genes (such as SRY on the Y chromosome) leads to male or female development.

• XX: Female

• XY: Male

Some species have different sex determination systems (e.g., ZW in birds, XO in insects).

Sex-Linked Inheritance

Genes located on sex chromosomes exhibit unique inheritance patterns. Sex-linked traits are often associated with the X chromosome, as it carries more genes than the Y.

• Example: Color blindness and hemophilia are X-linked recessive traits in humans.

Genetics: Pedigrees, Karyotypes, and Punnett Squares

Karyotypes

A karyotype is an organized profile of an individual's chromosomes. It is used to detect chromosomal abnormalities and determine sex.

• Example: A human karyotype shows 46 chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).

Pedigree Analysis

A pedigree is a diagram that shows the inheritance of a trait through generations of a family. It helps determine the mode of inheritance (dominant, recessive, sex-linked).

• Symbols: Squares (males), circles (females), shaded (affected), unshaded (unaffected).

Punnett Squares

A Punnett square is a tool used to predict the genotypes and phenotypes of offspring from a genetic cross.

• Genotype: The genetic makeup (e.g., AA, Aa, aa).

• Phenotype: The observable trait (e.g., tall, short).

Example: Crossing two heterozygous parents (Aa x Aa):

Genotypic ratio: 1 AA : 2 Aa : 1 aa Phenotypic ratio (if A is dominant): 3 dominant : 1 recessive

Calculating Expected Populations and Chi-Square Analysis

To determine if observed genetic ratios match expected ratios, use the chi-square () test:

• Formula:

• O = observed value, E = expected value

• Compare calculated to critical value to accept or reject hypothesis

Simulated Evidence and Application of Genetics

Simulated Evidence for Genetic Inheritance

Students may be asked to analyze simulated data to determine inheritance patterns, using terms such as dominant, recessive, autosomal, and sex-linked.

Application: Sex-Linked Inheritance and Human Health

Understanding sex-linked inheritance is important for predicting the likelihood of inheriting certain diseases and for genetic counseling.

• Example: A carrier mother (XNXn) and a normal father (XNY) have a 25% chance of having a son with an X-linked recessive disorder.

Summary Table: Mitosis vs. Meiosis

Additional info:

• Students should be familiar with interpreting scientific figures and diagrams, as referenced in the questions (e.g., Figures 12.6, 12.7, 13.10, 14.5, 14.8).

• Understanding the concept of alternation of generations is important for plant life cycles, though not detailed here.