This is called synapsis. Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination , also know as "crossing over" see below occurs. This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below.
Anaphase I : Instead of chromatids splitting at the centromere, homologous chromosome pairs now shuffled by crossing over move along the spindle fibers to opposite poles.
Anaphase II : The chromatids split at the centromere and migrate along the spindle fibers to opposite poles. Telophase II : The cells pinch in the center and divide again. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere. A large structure called the meiotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell.
Between prophase I and metaphase I, the pairs of homologous chromosome form tetrads. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination.
Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes. In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate. Then, in anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.
During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Animals, plants and fungi are all eukaryotes. Eukaryotic cells have specialized components called organelles, such as mitochondria , chloroplasts and the endoplasmic reticulum. Each of these performs a specific function. 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. After pairing of homologous chromosomes, crossing over occurs resulting in the interchange of segments of chromatids of homologous pair.
At the end of meiosis-I, two daughter cells are formed having half the number of chromosomes present in diploid cell undergoing meiosis. Each daughter cell undergoes meiosis-II, producing two cells. In meiosis-II the separation of two chromatids occur so that equal number of chromatids in fact chromosome due to duplication of genetic material goes to each of the daughter cell.
Thus, at the end of meiosis-II, four daughter cells are formed. Each cell has half the number of chromosomes present in the diploid cell. Each cell is identical as far as the number of chromosomes is concerned.
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