2.3 Mitosis, Meiosis & the Cell Cycle

2.3 Mitosis, Meiosis & the Cell Cycle

Quick Answer: Mitosis yields two identical diploid somatic cells for growth and repair. Meiosis yields four haploid, genetically diverse gametes for sexual reproduction. DNA replicates once in S phase before either process; meiosis includes two divisions and crossing over. Know PMAT stages, the G1–S–G2–M cycle, and when chromosome number is diploid (2n) vs haploid (n).

Cell division explains how a fertilized egg becomes a multi-trillion-cell person, how skin and gut continuously renew, how eggs and sperm form, and—when checkpoints fail—how cancer arises. For NLN NEX Science, this is classic Biology domain content: expect questions on cycle phases, mitosis stages, meiosis outcomes, and side-by-side comparisons.

Why Cells Divide

• Growth of tissues and the organism
• Repair and replacement of damaged or short-lived cells (epidermis, blood cells, intestinal lining)
• Reproduction: mitosis for asexual reproduction in some organisms; meiosis for gametes in humans

Not all adult human cells divide at the same rate. Neurons and most skeletal muscle fibers are largely post-mitotic; epithelial and hematopoietic cells divide frequently.

The Cell Cycle

The cell cycle is the ordered sequence of events from the formation of a cell to its own division into two daughters.

Interphase (most of the cycle)

Interphase is not “resting”—the cell is metabolically active and preparing to divide.

Some cells exit the cycle into G0 (quiescence)—a non-dividing state that may be temporary or long-term (e.g., many neurons).

M Phase

M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). Checkpoints at G1/S, G2/M, and the spindle (metaphase) checkpoint help ensure DNA is intact and chromosomes are properly attached before progression. Failure of these controls contributes to genomic instability and cancer.

Chromosome Vocabulary You Need

• Diploid (2n): two sets of chromosomes—one from each parent. Human somatic cells: 46 chromosomes (23 pairs).
• Haploid (n): one set of chromosomes. Human gametes: 23 chromosomes.
• Homologous chromosomes: a maternal and paternal pair that carry the same genes at the same loci (may have different alleles).
• Sister chromatids: identical copies of one chromosome produced in S phase, held at the centromere until separation.
• Centromere: constriction where sister chromatids attach and where spindle fibers bind via the kinetochore.

After S phase, a human cell still has 46 chromosomes by centromere count, but each has two chromatids (92 chromatids). Exam wording is picky: know whether the question means chromosomes or chromatids.

Mitosis: Stages (PMAT) + Cytokinesis

Mitosis divides the nucleus so each daughter gets an identical full diploid set.

Prophase

• Chromatin condenses into visible chromosomes (each with two sister chromatids)
• Nuclear envelope breaks down
• Mitotic spindle begins to form from microtubules (centrosomes move apart in animal cells)

Prometaphase (sometimes listed separately): spindle microtubules attach to kinetochores on chromosomes.

Metaphase

• Chromosomes line up at the metaphase plate (cell equator)
• Spindle checkpoint verifies bipolar attachment before anaphase

Anaphase

• Sister chromatids separate and are pulled toward opposite poles
• Once separated, each chromatid is counted as an individual chromosome
• Cell elongates

Telophase

• Chromosomes arrive at poles and begin to decondense
• Nuclear envelopes re-form around each set
• Spindle disassembles
• Mitosis of the nucleus is essentially complete

Cytokinesis

Cytokinesis divides the cytoplasm. In animal cells, a contractile ring of actin and myosin pinches the cell membrane inward, forming a cleavage furrow that splits one cell into two. Result: two diploid daughter cells genetically identical to the parent (barring mutation).

Meiosis: Reducing Chromosome Number for Gametes

Meiosis occurs in germline cells of the gonads (ovaries and testes) to produce eggs and sperm. DNA is replicated once (S phase), then the nucleus divides twice (meiosis I and meiosis II).

Meiosis I (Reductional Division)

Homologous chromosomes are separated; chromosome number is halved (2n → n).

• Prophase I: Homologs pair as tetrads (synapsis). Crossing over exchanges segments between non-sister chromatids of homologs → major source of genetic recombination/variation. Nuclear envelope breaks down; spindle forms.
• Metaphase I: Tetrads align at the metaphase plate; orientation of each pair is random (independent assortment).
• Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain attached.
• Telophase I / cytokinesis: Two haploid cells form; each chromosome may still consist of two chromatids.

Meiosis II (Equational Division)

Mechanically similar to mitosis, but starting with haploid cells:

• Prophase II → Metaphase II → Anaphase II (sister chromatids finally separate) → Telophase II + cytokinesis

End result of meiosis: four haploid cells, each genetically unique. In human males, four sperm can mature from one meiosis; in human females, asymmetric cytokinesis typically yields one ovum and polar bodies.

Genetic Variation

Meiosis generates diversity through:

1. Crossing over (prophase I)
2. Independent assortment of homologs (metaphase I)
3. Random fertilization of any egg by any sperm (post-meiosis)

Mitosis does not create new allele combinations between homologs in the same way; its job is faithful copying for somatic lineages.

Mitosis vs Meiosis Comparison

Clinical and Nursing Relevance

Cancer and Mitosis

Cancer is uncontrolled cell proliferation—cycle checkpoints fail, cells ignore growth-suppressing signals, and mitosis continues inappropriately. Chemotherapy and radiation often target rapidly dividing cells (DNA replication or mitotic spindle), which explains both anti-tumor effect and side effects on hair follicles, GI mucosa, and bone marrow.

Meiosis Errors and Aneuploidy

If chromosomes fail to separate properly (nondisjunction), gametes may have extra or missing chromosomes. Fertilization can produce aneuploidy (e.g., trisomy 21 / Down syndrome). Risk of some nondisjunction events rises with maternal age—a standard genetics/health connection.

Tissue Healing

Wound healing and recovery after surgery depend on orderly mitotic replacement of damaged cells plus connective tissue remodeling—cell division as the foundation of repair.

Exam Traps

• S phase timing: DNA replicates in interphase before mitosis or meiosis—not during prophase.
• Anaphase of mitosis vs anaphase I: Mitosis and meiosis II separate sister chromatids; meiosis I separates homologous chromosomes.
• Chromosome number after mitosis: Still 46 in human somatic daughters—not 23.
• Meiosis product count and ploidy: Four cells, haploid—not two diploid cells.
• Crossing over location: Prophase I of meiosis—not a defining feature of mitosis.
• Cytokinesis vs mitosis: Mitosis divides the nucleus; cytokinesis divides the cytoplasm—they are related but not identical terms.

Study Strategy for NEX

1. Draw the cell cycle and label G1, S, G2, M, and G0.
2. Sketch PMAT for mitosis and mark what is happening to the nuclear envelope and chromatids.
3. Draw meiosis I vs II focusing on what separates at each anaphase.
4. Drill the comparison table until you can recreate it from memory in under two minutes.

Master these patterns and you will convert dense-looking cell-division stems into straightforward process-of-elimination items—while building the cellular foundation for genetics, cancer biology, and reproductive A&P later in the Science section.