Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 13.3 Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid
Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically distinct haploid cells. This process ensures genetic diversity in sexually reproducing organisms.
Overview of Meiosis
- Meiosis is preceded by DNA replication in interphase
- Followed by two consecutive divisions: meiosis I and meiosis II
- Result: four haploid daughter cells with one set of chromosomes each
- Unlike mitosis (which produces two identical cells), meiosis produces genetically unique cells
Key Terminology
- Sister chromatids: identical copies of a chromosome connected at the centromere
- Homologous chromosomes: one from each parent, carry the same genes at corresponding loci
- Alleles: different versions of the same gene
- Chiasmata: sites where crossing over occurs between homologous chromosomes
Meiosis I: Separates Homologous Chromosomes
- Prophase I
- Homologous chromosomes pair up in a process called synapsis
- Crossing over occurs, exchanging DNA between nonsister chromatids
- Chiasmata form; spindle attaches to kinetochores
- Metaphase I
- Homologous pairs align at the metaphase plate
- Independent assortment occurs—random orientation of maternal/paternal homologs
- Anaphase I
- Homologous chromosomes (not sister chromatids) separate and move to opposite poles
- Telophase I and Cytokinesis
- Two haploid cells form, each with duplicated chromosomes
- No further chromosome duplication occurs before meiosis II
Meiosis II: Separates Sister Chromatids
- Similar to mitosis but begins with haploid cells
- Prophase II: spindle forms
- Metaphase II: chromosomes align at the metaphase plate
- Anaphase II: sister chromatids finally separate
- Telophase II and Cytokinesis: four genetically unique haploid cells are produced
Crossing Over and Genetic Diversity
- Occurs during prophase I
- Involves the exchange of DNA between nonsister chromatids
- Requires proteins like cohesins and the synaptonemal complex
- Results in chiasmata, holding homologs together until separation
Crossing over combined with independent assortment greatly increases genetic variation in gametes.
Comparison: Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Number of daughter cells | 2 (diploid, identical) | 4 (haploid, unique) |
| Crossing over | No | Yes (prophase I) |
| Homolog pairing | No | Yes (synapsis) |
| Role | Growth, repair | Gamete formation |
In a Nutshell
Meiosis reduces the chromosome number from diploid to haploid through two cell divisions. It introduces genetic variation through independent assortment and crossing over, producing four genetically diverse gametes essential for sexual reproduction.