Q: Is each human sperm and egg haploid?
A: Yes, (n=23) it is haploid as a result of meiosis. Fertilization restores the diploid condition by combining two haploid sets of chromosomes, and the human life cycle is repeated, generation after generation.
Q: So in how many daughter cells does meiosis result?
A: During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unreplicated chromosomes.
Q: How do sister chromatids stay together through meiosis I but separate from each other in meiosis II and mitosis?
A: Sister chromatids are attached along their lengths by protein complexes called cohesins. In mitosis, this attachment lasts until the end of metaphase, and in meiosis, the cohesions are cleaved at anaphase I and anaphase II, in two steps.
1. Offspring acquires genes from parents by inheriting chromosomes.
2. Fertilization and meiosis alternate in sexual life cycles.
3. Meiosis reduces the number of chromosome sets from diploid to haploid.
4. Genetic variation produced in sexual life cycles contributes to evolution.
5. Either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. Only diploid cells, however, can undergo meiosis because haploid cells have a single set of chromosomes that cannot be further reduced.
This diagram shows Metaphase II of meiosis. In this phase, the chromosomes are positioned on the metaphase plate as in mitosis,and as we can see, because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical. Also, the kinetochores of sister chromatins are attached to microtubules extending from opposite poles.
Meiosis is a basic and important process in sexually reproducing eukaryotic organisms, because it produces the haploid gametes that join to produce a new individual, and provides a mechanism for genetic variability, which is important for survival of the species.
Overall, meiosis involves one replication of the genetic material in the chromosomes, followed by two divisions of that genetic material. Therefore, the genetic material is reduced by half. The result of meiosis in a diploid cell is four haploid cells.
Chiasma: the x-shaped, microscopically visible region where homologous nonsister chromatids have exchanged genetic material through crossing over during meiosis, the two homologs remaining associated due to sister chromatic cohesion.
Homologous chromosomes: a pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother. Also called homologs, or a homologous pair.
Locus: a specific place along the length of a chromosome where a given gene is located.
Spore: in the life of a plant or alga undergoing alternation of generations, a haploid cell produced in the sporophyte by meiosis. A spore can divide by mitosis to develop into a multicellular haploid individual, the gametophyte, without fusing with another cell.
Synapsis: the pairing and physical connection of replicated homologous chromosomes during prophase I of meiosis.
Diploid cell: a cell containing two sets of chromosomes, one set inherited from each parent.
Haploid cell: a cell containing only one set of chromosomes.
Karyotype: a display of the chromosome pairs of a cell arranged by size and shape.
Sex chromosome: a chromosome responsible for determining the sex of an dindividual.
Crossing over: the reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.
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