Cell Cycle, Mitosis, Meiosis, and Chromosomal Variation

The Cell Cycle

Cell Size Limitations

The size of a cell is limited by the relationship between its surface area and volume. This relationship affects the cell's ability to exchange materials with its environment and impacts metabolic efficiency.

• Surface Area: Determines the rate at which materials (e.g., H2O, O2, CO2) can enter or leave the cell.

• Volume: Relates to the amount of metabolic activity and waste production.

• Smaller cells maximize surface area relative to volume, facilitating efficient exchange.

Prokaryotic Cell Cycle (e.g., Bacteria)

Prokaryotes reproduce primarily by binary fission, a simple process involving the duplication of a single circular chromosome.

• Growth: Cell increases in size.

• DNA Replication: Single chromosome duplicates.

• Division: Cell splits into two identical daughter cells.

Eukaryotic Cell Cycle

Eukaryotic cells have a more complex cell cycle, involving multiple phases and structures such as the nucleus and multiple chromosomes.

• Interphase: Cell grows, replicates DNA, and prepares for division.

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

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

Eukaryotic Genome

The eukaryotic genome consists of multiple linear chromosomes, each containing DNA tightly coiled around proteins called histones.

• Chromatin is loosely coiled during interphase and tightly coiled during mitosis.

• Chromosomes are protected and easily moved during cell division.

Chromosome Changes Through the Cell Cycle

Chromatid and Chromosome Number

During DNA replication, each chromosome duplicates to form two sister chromatids. Mitosis separates these chromatids, resulting in two chromosomes per original chromosome.

• DNA replication does not change chromosome number, only chromatid (copy) number.

Phases of the Cell Cycle

• Interphase: Nucleus is distinct, DNA is in chromatin form, and cell grows.

• G1 (Gap 1): Cell growth and preparation for DNA replication.

• S (Synthesis): DNA replication occurs.

• G2 (Gap 2): Additional growth and preparation for mitosis.

Mitosis

Mitosis is the process by which eukaryotic cells divide their nucleus and chromosomes to produce two genetically identical daughter cells.

• Prophase: Chromatin coils into chromosomes, nuclear envelope breaks down, spindle fibers form.

• Metaphase: Chromosomes align at the cell's equator (metaphase plate).

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

• Telophase: Chromosomes uncoil, nuclear envelope reforms, spindle apparatus breaks down.

• Cytokinesis: Cytoplasm divides, forming two daughter cells.

Ploidy and Chromosome Sets

Definitions

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

The number of chromosomes in a set is represented by the variable "n". For humans, n = 23, so diploid cells have 46 chromosomes.

Chromosome Variation

• Chromosomes differ in size, centromere position, staining patterns, and gene content.

• Autosomes: Non-sex chromosomes, carry genes for most traits.

• Sex Chromosomes: Determine sex (XX = female, XY = male).

Meiosis

Overview

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing haploid gametes or spores. It consists of two nuclear divisions: Meiosis I and Meiosis II.

• Meiosis I: Separates homologous chromosomes, reducing ploidy from diploid to haploid.

• Meiosis II: Separates sister chromatids, similar to mitosis.

Phases of Meiosis

• Prophase I: Chromatin coils into chromosomes, nuclear envelope breaks down, homologous chromosomes pair up (synapsis), crossing over occurs.

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

• Anaphase I: Homologous chromosomes separate to opposite poles.

• Telophase I: Chromosomes uncoil, nuclear envelope reforms, cytokinesis produces two haploid cells.

• Prophase II: Chromatin coils into chromosomes, spindle apparatus forms.

• Metaphase II: Chromosomes align at the equator.

• Anaphase II: Sister chromatids separate to opposite poles.

• Telophase II: Chromosomes uncoil, nuclear envelope reforms, cytokinesis produces four haploid cells.

Cytokinesis in Females and Males

• Males: Even cytokinesis produces 4 small sperm cells.

• Females: Uneven cytokinesis produces 1 large egg cell and 3 smaller non-gametes.

Sources of Genetic Variation

Source	Effect
Mutation	New combinations of base sequence information
Crossover	New combinations of genes on a chromosome
Homolog Shuffling	New combinations of chromosomes in a gamete
Random Fertilization	New combinations of gametes upon fertilization to form zygotes

Meiotic Errors and Chromosomal Mutations

Nondisjunction

Nondisjunction is the failure of chromosomes to separate properly during meiosis, resulting in aneuploidy (abnormal chromosome number).

• Can occur with homologs in meiosis I or sister chromatids in meiosis II.

Chromosomal Mutations

Type	Description	Effect
Deletion	Chromosome segments missing	Loss of genetic material
Duplication	Extra segments present	Additional genetic material
Inversion	Chromosome segment reversed	Gene order changed
Translocation	Segment ends up on wrong chromosome	Genes moved to new location

Key Terms and Concepts

• Chromatid: One of two identical halves of a replicated chromosome.

• Centromere: Region where sister chromatids are joined.

• Spindle Apparatus: Structure that separates chromosomes during cell division.

• Synapsis: Pairing of homologous chromosomes during meiosis I.

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

Formulas and Equations

• Number of chromosomes in a diploid cell: $2n$

• Number of chromosomes in a haploid cell: $n$

Examples

• Human Somatic Cell: 46 chromosomes (diploid, 2n = 46)

• Human Gamete: 23 chromosomes (haploid, n = 23)

Additional info: Chromosome mutations and meiotic errors can lead to genetic disorders such as Down syndrome (trisomy 21), Turner syndrome (monosomy X), and others.