The crossover events are the first source of genetic variation in the nuclei produced by meiosis. The result is an exchange of genetic material between homologous chromosomes. Crossover occurs between non-sister chromatids of homologous chromosomes. Crossing over occurs at chaiasmata (singular = chiasma), the point of contact between non-sister chromosomes of a homologous pair (Figure 3).Īt the end of prophase I, the pairs are held together only at the chiasmata (Figure 3) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other (Figure 2). In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair.) The tight pairing of the homologous chromosomes is called synapsis. (Recall that, in mitosis, homologous chromosomes do not pair together. 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.Īs the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Early in prophase I, homologous chromosomes come together to form a synapse. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.īecause the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. Meiosis employs many of the same mechanisms as mitosis. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. While many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitosis, many single-celled organisms and most multicellular organisms reproduce regularly using another method: meiosis. In mitosis, both the parent and the daughter nuclei are at the same ploidy level-diploid for most plants and animals. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)Īs you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. After trillions of cell divisions, each of us develops into a complex, multicellular organism. Each of us, like these other large multicellular organisms, begins life as a fertilized egg. In kind does not generally mean exactly the same. Hippopotamuses give birth to hippopotamus calves, Joshua trees produce seeds from which Joshua tree seedlings emerge, and adult flamingos lay eggs that hatch into flamingo chicks. In kind means that the offspring of any organism closely resemble their parent or parents. The ability to reproduce in kind is a basic characteristic of all living things.
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