The S phase is next, during which the DNA of the chromosomes is replicated. This replication produces two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate.
Finally, during the G 2 phase also called the second gap phase , the cell undergoes the final preparations for meiosis. During prophase I, chromosomes condense and become visible inside the nucleus. As the nuclear envelope begins to break down, homologous chromosomes move closer together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes.
The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.
Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
Crossover between homologous chromosomes : Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes. Synapsis holds pairs of homologous chromosomes together : Early in prophase I, homologous chromosomes come together to form a synapse.
The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. The key event in prometaphase I is the formation of the spindle fiber apparatus where spindle fiber microtubules attach to the kinetochore proteins at the centromeres. Microtubules grow from centrosomes placed at opposite poles of the cell.
The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes at the kinetochores. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole.
In addition, the nuclear membrane has broken down entirely. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator.
This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set.
There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In this case, there are two possible arrangements at the equatorial plane in metaphase I.
The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes.
In anaphase I, the microtubules pull the attached chromosomes apart. The sister chromatids remain tightly bound together at the centromere.
The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart. In telophase I, the separated chromosomes arrive at opposite poles. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division.
In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division.
The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes.
Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together.
Unlike prokaryotes, eukaryotic DNA is packed within a central compartment called the nucleus. Within the eukaryotic nucleus, long double-helical strands of DNA are wrapped tightly around proteins called histones. This forms a rod-like structure called the chromosome. Cells in the human body have 23 pairs of chromosomes, or 46 in total. This includes two sex chromosomes: two X chromosomes for females and one X and one Y chromosome for males. Because each chromosome has a pair, these cells are called "diploid" cells.
On the other hand, human sperm and egg cells have only 23 chromosomes, or half the chromosomes of a diploid cell. Thus, they are called "haploid" cells. When the sperm and egg combine during fertilization, the total chromosome number is restored. That's because sexually reproducing organisms receive a set of chromosomes from each parent: a maternal and paternal set.
Each chromosome has a corresponding pair, orhomolog. Eukaryotes are capable of two types of cell division: mitosis and meiosis. Mitosis allows for cells to produce identical copies of themselves, which means the genetic material is duplicated from parent to daughter cells.
Mitosis produces two daughter cells from one parent cell. Single-celled eukaryotes, such as amoeba and yeast, use mitosis to reproduce asexuallyand increase their population. Multicellular eukaryotes, like humans, use mitosis to grow or heal injured tissues.
Meiosis, on the other hand, is a specialized form of cell division that occurs in organisms that reproduce sexually. As mentioned above, it produces reproductive cells, such as sperm cells, egg cells, and spores in plants and fungi. In humans, special cells called germ cells undergo meiosis and ultimately give rise to sperm or eggs. Germ cells contain a complete set of 46 chromosomes 23 maternal chromosomes and 23 paternal chromosomes.
By the end of meiosis, the resulting reproductive cells, or gametes , each have 23 genetically unique chromosomes. The overall process of meiosis produces four daughter cells from one single parent cell. Each daughter cell is haploid, because it has half the number of chromosomes as the original parent cell. Andrew Hoyt, a biologist and professor at Johns Hopkins University.
Unlike in mitosis, the daughter cells produced during meiosis are genetically diverse. Homologous chromosomes exchange bits of DNA to create genetically unique, hybrid chromosomes destined for each daughter cell. Before meiosis begins, some important changes take place within the parent cells. During anaphase II the sister chromatids are separated to opposite poles. Similarities Mitosis Diploid parent cell Consists of interphase, prophase, metaphase, anaphase and telophase In metaphase individual chromosomes pairs of chromatids line up along the equator.
Ends with cytokinesis. Meiosis Diploid parent cell Consists of interphase, prophase, metaphase, anaphase and telophase but twice! In metaphase II individual chromosomes pairs of chromatids line up along the equator.
Related Content:. What is mitosis? What is meiosis? What is a cell? What is a stem cell? How helpful was this page?
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