Cell Division: Mitosis, Meiosis, and Related Concepts (Biology 110 Unit Three Study Guide)

Cell Division and the Cell Cycle

Overview of Cell Division

Cell division is a fundamental process in biology, allowing organisms to grow, repair tissues, and reproduce. The two main types of cell division in eukaryotes are mitosis and meiosis, each serving distinct purposes and resulting in different outcomes.

• Mitosis: Produces two genetically identical daughter cells, used for growth, repair, and asexual reproduction.

• Meiosis: Produces four genetically unique daughter cells with half the chromosome number, essential for sexual reproduction.

• Cytokinesis: The physical process of cell division, separating the cytoplasm into two daughter cells after nuclear division.

Key Point: Mitosis and meiosis differ in their purpose, process, and outcome. Cytokinesis is distinct from both, as it refers to the division of the cytoplasm.

The Cell Cycle

The cell cycle describes the sequence of events that cells undergo as they grow and divide. It consists of interphase and the mitotic phase.

• Interphase: The period of cell growth and DNA replication, subdivided into G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• Mitotic Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Cells perform their assigned functions during interphase, particularly in G1 and G2.

Chromosomes, Chromatids, and Genome

Chromosomes are structures composed of DNA and proteins, carrying genetic information. Chromatids are the two identical halves of a duplicated chromosome, joined at the centromere.

• Genome: The complete set of genetic material in an organism.

• Human diploid (2N) cells: 46 chromosomes.

• Human haploid (N) cells: 23 chromosomes.

• Chromosome and chromatid counts: Vary during the cell cycle; after DNA replication, each chromosome consists of two sister chromatids.

Example: During metaphase of mitosis, a human cell has 46 chromosomes, each with two chromatids (92 chromatids total).

Mitosis vs. Meiosis

Stages of Mitosis and Meiosis (PMAT)

Both mitosis and meiosis involve a series of stages: Prophase, Metaphase, Anaphase, and Telophase (PMAT). However, meiosis consists of two rounds: Meiosis I and Meiosis II.

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Chromatids (mitosis) or homologous chromosomes (meiosis I) separate.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Comparison Table:

Feature	Mitosis	Meiosis
Number of divisions	1	2
Daughter cells	2	4
Chromosome number	Diploid (2N)	Haploid (N)
Genetic variation	None	High
Function	Growth, repair	Sexual reproduction
Occurs in	Somatic cells	Germ cells

Plant vs. Animal Cell Mitosis

• Animal cells: Undergo cytokinesis via cleavage furrow.

• Plant cells: Form a cell plate during cytokinesis due to rigid cell wall.

Example: Onion root tip cells are commonly used to study plant mitosis.

Prokaryotic vs. Eukaryotic Cell Division

Prokaryotic Cell Division

Prokaryotes (bacteria) divide by binary fission, a simpler process than mitosis.

• Chromosome: Typically one circular chromosome.

• Binary fission: DNA replicates, cell elongates, and divides into two.

• Reason for no mitosis: Prokaryotes lack a nucleus and complex chromosome structure.

Example: Escherichia coli divides rapidly by binary fission.

Genetic Variation in Meiosis

Homologous Chromosomes and Genetic Diversity

Homologs are pairs of chromosomes with the same genes but possibly different alleles. Meiosis increases genetic diversity through:

• Crossing over: Exchange of genetic material between homologs during prophase I.

• Independent assortment: Random distribution of homologs to daughter cells.

Example: Siblings inherit different combinations of parental genes due to meiosis.

Sex Cell Production: Oogenesis and Spermatogenesis

Oogenesis vs. Spermatogenesis

Oogenesis and spermatogenesis are processes that produce female and male gametes, respectively.

• Oogenesis: Occurs in ovaries, produces one ovum and three polar bodies per cycle.

• Spermatogenesis: Occurs in testes, produces four sperm per cycle.

• Difference: Oogenesis results in unequal cytoplasmic division; spermatogenesis produces equal-sized sperm.

Example: Human females produce eggs monthly; males produce millions of sperm daily.

Nondisjunction and Aneuploidy

Chromosome Number Abnormalities

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

• Diseases associated with aneuploidy: Down syndrome (trisomy 21), Turner syndrome (monosomy X), Klinefelter syndrome (XXY).

• Mechanism: Nondisjunction can occur in meiosis I or II, affecting gamete chromosome number.

Example: A child with Down syndrome has three copies of chromosome 21.

Summary Table: Chromosome and Chromatid Counts in Human Cells

Cell Cycle Stage	Chromosomes	Chromatids
G1 (before DNA replication)	46	46
S phase (after DNA replication)	46	92
Metaphase (mitosis)	46	92
After mitosis	46	46
Meiosis I (metaphase I)	46	92
Meiosis II (metaphase II)	23	46
After meiosis II	23	23

Additional info: Chromatid number doubles after DNA replication; chromosome number is halved after meiosis.

Key Terms and Definitions

• Mitosis: Division of a cell's nucleus resulting in two identical daughter cells.

• Meiosis: Division of a cell's nucleus resulting in four genetically unique gametes.

• Cytokinesis: Division of the cytoplasm.

• Genome: Complete set of genetic material.

• Homologous chromosomes: Chromosome pairs with the same genes.

• Nondisjunction: Failure of chromosomes to separate.

• Aneuploidy: Abnormal chromosome number.