What is Meiosis?: The Basics
Meiosis is a process of nuclear division in eukaryotic (plant, animal, etc...) cells. Cells that reproduce using meiosis undergo sexual reproduction as opposed to asexual reproduction.
Hint: If you are unfamiliar with any of these terms, check the "Terms to Know" list at the bottom of the page.
The purpose of meiosis is to divide a cell nucleus into four separate cells, known as daughter cells. As opposed to mitosis, which produces clones (exact copies) of a cell, meiosis creates daughter cells that differ in their genetic makeup, creating genetic diversity. Meiosis accomplishes this genetic diversity by randomly but equally dividing the original cell's information storing molecules, deoxyribonucleic acid (DNA), between the four new daughter cells.
Where it Takes Place:
Meiosis occurs in the nucleus of eukaryotic cells. Meiosis only takes place in in cells specialized for reproduction (sperm, ova, etc...) called gametes. It does not occur in somatic cells, which are any cells that do not serve a function in sexual reproduction.
Structure of a Chromosome:
Chromosomes are actually quite basic in their structure. They are made of two chromatids, which is a set of a "short arm and long arm" seen in the picture above. Paired chromatids in a chromosome are called sister chromatids. Note that each chromatid runs vertically, they are not "U" shaped. These chromatids consist of molecules of DNA that are tightly coiled by wrapping around and protein called a histone. The two chromatids are identical and attach to a centered object called a centromere (this is the black square in the center of the picture above). Chromatids also contain protein structures called kinetochores that are vital to meiosis, as we'll see in just a moment.
Meiosis occurs in two main steps, meiosis I and meiosis II. Each step has it's own phases.
- Interphase - Interphase is a period of rapid growth for the cell, occurring in three parts; G1 (first gap) phase, in which the cell increases its mass and volume, S (synthesis) phase, in which DNA is replicated, and G2 (second gap) phase, in which the cell continues to grow and begins synthesizing proteins.
- Prophase I - Organized structures of coiled DNA called chromosomes form pairs with each other, which are called synapses. Two chromosomes that pair with each other are called homologous, because they contain DNA that is used to express the same gene (IE - they both contain a gene for eye color). In humans, one chromosome in a pair comes from the mother and one comes from the father. The nuclear envelope then disappears and kinetochores form on the chromosomes. An apparatus called the meiotic spindle forms at opposite ends (poles) of the cell and spindle fibers attach to the kinetochores on each chromosome. Note: recombination through crossing over occurs during prophase. This is when portions of two homologous chromosomes, also called homologs, swap positions with each other, resulting in higher genetic diversity.
- Metaphase I - With the spindle fibers attached, each homolog moves to the center of the cell, aligning at the metaphase plate. It's important to note that the metaphase plate is not a physical structure, but just an imaginary dividing line in the cell, much like Earth's equator. In order to increase the chances of greater genetic diversity, chromosomes from the mother and father orient themselves randomly on either side of the metaphase plate. This is called independent assortment. This ensures that daughter cells could end up with either the mother's or father's chromosome.
- Anaphase I - The meiotic spindle then pulls the homologs apart, with half the chromosomes heading to one pole, and the other half heading to the other pole.
- Telophase I - Telophase is essentially the completion of anaphase, with half the chromosomes stationed at one pole, and half stationed at the other pole.
- Cytokinesis - The nuclear membrane reforms and begins to pinch the cell right in the middle, eventually splitting the original (parent) cell into two daughter cells, each with half the number of chromosomes as the parent cell. Note that because the daughter cells only have one copy of each chromosome instead of the two homologs the parent cell had, they are now haploid cells rather than diploid.
Meiosis II: The second step of meiosis is very similar to the first, but DNA replication, through interphase, does not occur.
- Prophase II - Similarly to prophase I, the chromosomes in each daughter cell condense, the meiotic spindle forms, and the nuclear envelop breaks down. However, since each daughter cell only has one copy of each chromosome, no homologous pairs are produced and no recombination occurs.
- Metaphase II - Again, the chromosomes align at the metaphase plate in each daughter cell.
- Anaphase II - This time, instead of homologous chromosomes being separated, the meiotic spindle splits sister chromatids from each other, pulling one chromatid (half the chromosome) to one pole, and half to the other pole.
- Telophase II - Nuclei then form at each pole and cytokinesis occurs. This splits each daughter cell into two new cells, creating four haploid cells each with half the chromatids of the original daughter cell and one fourth the chromatids of the original parent cell.
Four haploid daughter cells: The four daughter cells created by meiosis are known as gametes, and are called ova (eggs) or sperm depending on if meiosis occurred in a female or male respectively. Ova and sperm are used in sexual reproduction to produce offspring. The recombination that took place during the formation of these gametes during meiosis is what allows for genetic diversity in sexually reproducing organisms.
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Terms to Know:
Asexual reproduction: The creation of a new individual from a single organism without gametes; forms clones of the original organism.
Centromere: the point on a chromosome where two chromatids are attached.
Chromatid: a threadlike coil of DNA that carries genetic information. Two chromatids form a chromosome.
Chromosome: a structure of DNA made of two chromatids, a protein kinetochore, and a centromere. Chromosomes carry genetic information.
Daughter cell: a cell formed by the division of a previous cell.
Diploid: A cell or nucleus that contains two chromosomes containing the same gene, one from each parent.
DNA: An information storing nucleic acid.
Gamete: a haploid sex cell of a sexually reproducing organism.
Gene: a distinct sequence of DNA that encodes information (ie - eye color, height, etc...)
Haploid: A cell or nucleus that contains one chromosome for each gene randomly from either parent.
Homologous chromosomes: also called homologs; two chromosomes that have the same relative position, structure, and gene sequence.
Kinetochore: a protein structure that forms on the centromere of a chromosome; attaches to the meiotic spindle.
Mitosis: asexual cell division that results in two daughter cells that are clones of the original parent cell; occurs in somatic cells.
Parent cell: a cell that divides to become multiple daughter cells.
Sexual reproduction: The creation of a new individual from a single organism with gametes; forms genetic diversity.
Somatic cell: any cell that is not specialized for reproduction; undergoes mitosis.