SUMMARY / RELATED TOPICS

Mitosis

In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis is preceded by the S stage of interphase and is accompanied or followed by cytokinesis, which divides the cytoplasm and cell membrane into two new cells containing equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other; the process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, metaphase and telophase. During mitosis, the chromosomes, which have duplicated and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell; the result is two genetically identical daughter nuclei.

The rest of the cell may continue to divide by cytokinesis to produce two daughter cells. Producing three or more daughter cells instead of the normal two is a mitotic error called tripolar mitosis or multipolar mitosis. Other errors during mitosis can induce apoptosis or cause mutations. Certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Mitosis varies between organisms. For example, animal cells undergo an "open" mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Most animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include the gametes -- egg cells -- which are produced by meiosis.

Numerous descriptions of cell division were made during 18th and 19th centuries, with various degrees of accuracy. In 1835, the German botanist Hugo von Mohl, described cell division in the green alga Cladophora glomerata, stating that multiplication of cells occurs through cell division. In 1838, Matthias Jakob Schleiden affirmed that the formation of new cells in their interior was a general law for cell multiplication in plants, a view rejected in favour of Mohl model, due to contributions of Robert Remak and others. In animal cells, cell division with mitosis was discovered in frog and cat cornea cells in 1873 and described for the first time by the Polish histologist Wacław Mayzel in 1875. Bütschli and Fol might have claimed the discovery of the process presently known as "mitosis". In 1873, the German zoologist Otto Bütschli published data from observations on nematodes. A few years he discovered and described mitosis based on those observations; the term "mitosis", coined by Walther Flemming in 1882, is derived from the Greek word μίτος.

There are some alternative names for the process, e.g. "karyokinesis", a term introduced by Schleicher in 1878, or "equational division", proposed by August Weismann in 1887. However, the term "mitosis" is used in a broad sense by some authors to refer to karyokinesis and cytokinesis together. Presently, "equational division" is more used to refer to meiosis II, the part of meiosis most like mitosis; the primary result of mitosis and cytokinesis is the transfer of a parent cell's genome into two daughter cells. The genome is composed of a number of chromosomes—complexes of coiled DNA that contain genetic information vital for proper cell function; because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. Chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere; when mitosis begins, the chromosomes become visible.

In some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell disappears. Microtubules project from opposite ends of the cell, attach to the centromeres, align the chromosomes centrally within the cell; the microtubules contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes; as the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase. A new nuclear envelope forms around the separated daughter chromosomes, which decondense to form interphase nuclei. During mitotic progression after the anaphase onset, the cell may undergo cytokinesis. In animal cells, a cell membrane pinches inward between the two developing nuclei to produce two new cells. In plant cells, a cell plate forms between the two nuclei. Cytokinesis does not always occur.

The mitotic phase is a short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for the process of cell division. Interphase is divided into three phases: G1, S, G2. During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are

Segundo Romance Tour

Segundo Romance Tour was a concert tour performed by Luis Miguel during the last part of 1994 to promotionate his last album. He began the tour in Mexico before the official release of Segundo Romance, performing the new songs of the album on the 16 sold-out concerts at the National Auditorium in Mexico City. To promote the album, Miguel began his Segundo Romance Tour in August 1994 with 16 shows at the National Auditorium in Mexico City, which drew an audience of more than 155,000. Miguel performed throughout Mexico, the United States and Argentina until December 31, 1994, when the tour concluded in Acapulco; the first part of Miguel's set list featured contemporary ballads. In October 1995, Warner Music released the El Concierto live album and video, a compilation of Miguel's performances at the National Auditorium in Mexico City and his concert at the Estadio Vélez in Buenos Aires. Stephen Thomas Erlewine of AllMusic praised Miguel's performance. Note: Some dates and venues are missing due to the lack of reliable sources

Liaoning Provincial Museum

The Liaoning Provincial Museum is a prominent museum of history and fine arts located in Shenyang, the capital of China's Liaoning province. The institution was founded as the Northeast Museum by the Northeast People's Government and opened on July 7, 1949, it was renamed the "Liaoning Provincial Museum" in 1959. In July 2003, it moved to a new, specially-designed building on the east side of Government Square in Shenyang; the permanent collections include: the Dawn of Chinese Culture Gallery: archaeological exhibits related to the Hongshan and Xinle cultures the Northern Shang and Zhou Period Gallery: bronze tools and weapons the Bei Shan Tang Tablet Gallery: Chinese steles the Ancient Chinese Currency and Coins Gallery the Art Work Exhibition of the Ming and Qing Dynasties Gallery, including Prosperous Suzhou the Chinese Post-Unification Gallery: tomb relics The museum is open from 9 AM to 5 PM, except on Mondays. The museum is closed during the public holiday for Chinese New Year. Entry is free