4.2+Cell+reproduction

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the cell repro duction

Most human cells Most human cells are frequently reproduced and replaced during the life of an individual. However, the process varies with the kind of cell. **Somatic**, or body cells, such as those that make up skin, hair, and muscle, are duplicated by **mitosis**. The **sex cells**, sperm and ova, are produced by **meiosis** in special tissues of male testes and female ovaries. Since the vast majority of our cells are somatic, mitosis is the most common form of cell replication.  The cell division process that produces new cells for growth, repair, and the general replacement of older cells is called mitosis. In this process, a somatic cell divides into two complete new cells that are identical to the original one. Human somatic cells go through the 6 phases of mitosis in 1/2 to 1 1/2 hours, depending on the kind of tissue being duplicated.
 * Mitosis **

=Eukaryotic Cell Reproduction = The life of eukaryotic cells is characterized by a cell cycle with two major phases: the **interphase** and **cell division**. During the interphase, the cell takes in nutrients, grows, and duplicates its chromosomes. During the cell division phase, the nucleus divides in a process called mitosis and then the divided nuclei are established in separate cells in a process called cytokinesis.

=​ Prokaryotic Cell Reproduction = Prokaryotic cells reproduce by a process that is called binary fission. The DNA in such cells is contained in a single circular chromosome called a plasmid within the cytoplasm. The reproductive process starts with the replication of the chromosome. The new chromosome attaches itself to the plasma membrane and the two chromosomes migrate to opposite ends of the cell. The plasma membrane in the middle of the cell grows inward until it closes to separate the cell into two compartments, each with a full complement of genetic material. The cell then "fissions" at the center, forming two new daughter cells.

//taken from :http://hyperphysics.phy-astr.gsu.edu/hbase/biology/celrep.html //

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//** Cell Reproduction **//

Most human cells are frequently reproduced and replaced during the life of an individual. However, the process varies with the kind of cell. **Somatic**, or body cells, such as those that make up skin, hair, and muscle, are duplicated by **mitosis**. The ** sex cells **, sperm and ova, are produced by **meiosis** in special tissues of male testes and female ovaries. Since the vast majority of our cells are somatic, mitosis is the most common form of cell replication

takem from:http://anthro.palomar.edu/biobasis/bio_2.htm

mitosis

Mitosis is the process that facilitates the equal partitioning of replicated chromosomes into two identical groups. Before partitioning can occur, the chromosomes must become aligned so that the separation process can occur in an orderly fashion. The alignment of replicated chromosomes and their separation into two groups is a process that can be observed in virtually all eukaryotic cells. Both the alignment and separation processes are the consequence of the chromosomes interacting with filamentous proteinaceous structures, known as microtubules. The microtubules become organized into a biconical array known as a spindle, which forms early in mitosis, and then disassembles as mitosis nears completion. Mitotic spindles are visible in living cells with the polarizing light microscope. Some of the spindle microtubules become attached to the chromosomes at sites known as kinetochores. The kinetochores cannot be seen with the light microscope, but they reside near the place on the chromosome known as its centromere, which can be observed with the light microscope. There are two kinetochores on each replicated chromosome (one on each chromatid), and when the replicated chromosome splits apart at its centromere at the onset of anaphase, each daughter chromosome possesses one centromere and one kinetochore. The linkages between kinetochores and microtubules are thought to be central in controlling both the positioning of the replicated chromosome at the central portion of the spindle during the alignment phase, and in moving the daughter chromosomes apart after they split at their centromeres. The separation of daughter cells from each other is a process known as cytokinesis, and is separate from mitosis. In cytokinesis, animal and plant cells differ considerably from each other. These differences are the consequence of having or not having a cell wall. Cytokinesis in fungi reveals some similarities with plant cells, and exhibits other features unique to the group.

**Top**

The purpose of this page is to show interested individuals how mitosis occurs in a stamen hair cell of the spiderwort plant, //Tradescantia virginiana//. The stages of mitosis are highlighted in a single cell, and the time/date generator information on each image provides the viewer with a sense of how long each step actually takes.


 * The stages of mitosis: a tour with pictures. **
 * **Prophase** || During prophase, the replicated chromosomes undergo extensive condensation (//i.e.,// coiling). The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. Late in prophase, within about 6 min of nuclear envelope breakdown, the mitotic spindle begins to grow, and two triangular 'clear zones' become visible, with one on each side of the nucleus. In three dimensions, the clear zones are actually conical and the nucleus is spherical. With continued spindle expansion, the nucleus rapidly becomes compressed and appears eliptical in the optical section provided by differential interference contrast optics. Prophase ends with the sudden dispersion of the nuclear envelope (nuclear envelope breakdown), and the chromosomal mass is no longer occupying a discrete, spherically-shaped zone in the cell. Prophase in stamen hair cells can last for as long as several hours. It can also be observed by downloading the movie. ||
 * <span style="font-family: Arial,Helvetica,sans-serif;"> || <span style="font-family: Arial,Helvetica,sans-serif;"> Once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a (double) membrane boundary. The microtubules begin to interact with the chromosomes, and the chromosomes undergo what is known as congressional movement, where they ultimately end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed toward one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome The congression of chromosomes and the alignment of centromeres on the metaphase plate represent essential prerequisites for the orderly separation of the replicated genome into two equal parts. The mechanisms underlying congression are under intense scrutiny. ||


 * <span style="font-family: Arial,Helvetica,sans-serif;">**Metaphase** ||  || <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: justify;">The replicated chromosomes converge toward the center of the spindle, and once they get there, significant movements cease. On either side of each centromere are sites for microtubule attachment to the chromosome; electron microscopists called these plate-like structures kinetochores. The kinetochores are not visible with the light microscope. At several points during metaphase, the chromatid arms may unwind from each other. This unwinding is especially apparent late in metaphase, just 1 or 2 minutes before the chromatids will split apart at their centromeres, with each replicated chromosome giving rise to two daughter chromosomes. The interval between nuclear envelope breakdown and anaphase onset includes prometaphase and metaphase, and requires approximately 33 minutes. ||


 * <span style="font-family: Arial,Helvetica,sans-serif;">**Anaphase** ||  || <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: justify;">Anaphase commences with the initial splitting of sister chromatids at their centromeres. These daughter chromosomes then begin to separate from each other, each moving away from the metaphase plate and toward one of the two spindle pole regions. The rate of chromosome separation in stamen hair cells is approximately 1.4 micrometers/min. The mechanisms that control chromosome separation clearly involve the interactions between microtubules and components in or near the kinetochore. Anaphase chromosomal movement is a topic of intense interest in the Cell Biology research community. ||


 * <span style="font-family: Arial,Helvetica,sans-serif;">
 * 1) **<span style="font-family: Arial,Helvetica,sans-serif;">Cell Plate Vesicle Aggregation **
 * 2) **<span style="font-family: Arial,Helvetica,sans-serif;">Plate Coalescence ** || <span style="display: block; font-family: Arial,Helvetica,sans-serif; text-align: justify;">About 19 minutes after anaphase onset, the chromosomes have moved close to the spindle pole regions, and the spindle midzone begins to clear. In this middle region of the spindle, a thin line of vesicles begins to accumulate. The vesicle aggregation event is a harbinger to the assembly of a new cell wall that will be positioned midway along the length of the original cell. It will form the boundary between the newly separating daughter cells. Vesicles movement and aggregation in the spindle midzone is facilitated by a microtubule network known as a phragmoplast. This basket shaped structure forms in late anaphase or early telophase and disassembles about the time that the vesicles begin to coalesce ||

takem from:http://www.life.umd.edu/cbmg/faculty/wolniak/wolniakmitosis.html

takem from:http://www.rebeccastmartin.com/mnemonicdevices/cell-mitosis-reporoduction.gif

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meiosis


 * Legend:**

Illustration of the process by which a **single parent diploid cell** (Both homologous chromosomes) divides to produce **four daughter haploids cells** (One homologous chromosome of the pair). Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced. Meiosis involves a reduction in the amount of genetic material. Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division.
 * **Interphase:** Before meiosis begins, genetic material is duplicated.
 * **First division of meiosis**
 * **Prophase 1:** Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage.
 * **Metaphase 1**: Homologous chromosomes align at the equatorial plate.
 * **Anaphase 1**: Homologous pairs separate with sister chromatids remaining together.
 * **Telophase 1**: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different. Meiosis differs from mitosis primarily because there are two cell divisions in meiosis, resulting in cells with a haploid number of chromosomes.
 * **Second division of meiosis: Gamete formation**
 * **Prophase 2**: DNA does not replicate.
 * **Metaphase 2**: Chromosomes align at the equatorial plate.
 * **Anaphase 2**: Centromeres divide and sister chromatids migrate separately to each pole.
 * **Telophase 2**: Cell division is complete. Four haploid daughter cells are obtained.

takem from:http://www.accessexcellence.org/RC/VL/GG/meiosis.php

takem from:http://www.carisgamba.com/CTmeiosis/meiosis-big.gif