Meiosis Explored
In previous posts, we explored the cell cycle and mitosis. You can find those posts here:
Mitosis allows for the growth and repair of organisms by producing two daughter cells that are genetically identical. Meiosis, on the other hand, is a type of cell division that creates daughter cells with half as many chromosomes and ensures genetic diversity in sexually reproducing organisms.
Meiosis occurs in germ cells—the cells that give rise to sperm and eggs in animals. While mitosis occurs in somatic cells, meiosis ensures that the resulting reproductive cells, or gametes, have half the chromosome number of the original cell.
What Are Chromosomes?
A chromosome is a DNA molecule bound with proteins called histones. Eukaryotic cells have multiple, linear chromosomes housed in the nucleus. The number of chromosomes varies among species—for example, humans have 46, tigers have 38, fruit flies have 8, and chickens have 78.
Before division, a cell duplicates its chromosomes during interphase. This produces two identical sister chromatids attached at a region called the centromere. It's important to remember that even replicated chromosomes count as a single chromosome, and the centromere is key in chromosome counting.
Cells also contain homologous chromosomes, which are chromosomes of the same size and shape that carry the same genes but may have different versions, called alleles. This genetic variation is what allows for different traits among organisms.
What Is Ploidy?
The ploidy of an organism or cell refers to the number of chromosome sets it contains. Organisms with two versions of each chromosome are diploid (2n), while those with one version are haploid (n). Humans, for instance, have a diploid number of 46 chromosomes (2n = 46), with each parent contributing 23 chromosomes.
Some organisms can have more than two sets of chromosomes, such as triploid (3n) or tetraploid (4n) cells, but for now, we will focus on haploid and diploid cells.
Mitosis maintains the ploidy of a cell, whereas meiosis is a reduction division, cutting the chromosome number in half, transforming diploid cells into haploid gametes. These gametes then fuse during fertilization, restoring the diploid number in the resulting zygote.
Sexual Reproduction and Genetic Diversity
Sexual reproduction involves the fusion of two haploid gametes (sperm and egg) to form a genetically unique offspring. Each gamete contributes half of the zygote’s chromosomes—one set from the mother and one set from the father.
Meiosis promotes genetic diversity through processes like crossing over and the random alignment of chromosomes during division. This diversity is essential for evolution and adaptation in sexually reproducing species.
Meiosis
Phases of Meiosis I
Interphase: Similar to mitosis, the cell grows and replicates its chromosomes.
Early Prophase I: Chromosomes condense, the spindle apparatus forms, and homologous chromosomes pair up, forming tetrads. Each tetrad consists of two homologs with two chromatids each.
Late Prophase I: Crossing over occurs between non-sister chromatids at sites called chiasmata, increasing genetic variation.
Metaphase I: Tetrads align randomly at the metaphase plate. This random alignment leads to further genetic diversity.
Anaphase I: Homologous chromosomes separate and move to opposite ends of the cell.
Telophase I and Cytokinesis: Chromosomes finish moving, and the cell divides, producing two haploid cells.
Phases of Meiosis II
Prophase II: The spindle apparatus reforms in each cell, but crossing over does not occur again.
Metaphase II: Chromosomes align at the metaphase plate.
Anaphase II: Sister chromatids separate and move to opposite ends of the cell.
Telophase II and Cytokinesis: Chromosomes reach the poles, and the cells divide again, producing four genetically unique haploid daughter cells.
Consequences of Meiosis
Meiosis produces four genetically distinct haploid cells through crossing over and the random alignment of chromosomes during Metaphase I. This diversity is crucial for the survival and adaptability of species that reproduce sexually.
Compare and Contrast Mitosis to Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Type of Division | Single cell division | Two cell divisions (Meiosis I and Meiosis II) |
| Purpose | Growth, repair, and asexual reproduction | Sexual reproduction (formation of gametes) |
| Number of Daughter Cells | 2 | 4 |
| Genetic Composition | Genetically identical to the parent cell | Genetically different from the parent cell |
| Ploidy of Daughter Cells | Diploid (2n) | Haploid (n) |
| Occurs In | Somatic (body) cells | Germ line cells (sperm and egg) |
| Phases | Prophase, Metaphase, Anaphase, Telophase | Meiosis I: Prophase I, Metaphase I, etc.; Meiosis II: Prophase II, Metaphase II, etc. |
| Crossing Over | Does not occur | Occurs in Prophase I |
| Homologous Chromosomes | Do not pair | Pair and form tetrads in Prophase I |
| Chromosome Number | Maintained (2n → 2n) | Halved (2n → n) |
| Creates Genetic Variation | No | Yes, through crossing over and independent assortment of chromosomes |
