martes, 16 de marzo de 2010

ORGANELLES OF THE CELL

MITOCHONDRY
Citoplasmatic organelles are equipped with double membrane found in most eukaryotic cells.

ENDOPLASMATIC RETICULUM
This is a series of conduits or channels that serve as conductive pathways for nutrients, the rough endoplasmatic reticulum is lined with nutrients
RIBOSOME
Produces the nucleolus of the cell, has the function of proteins poducir, is formed by 2 subunits that are: 60s and 40s.
They are lining the rough endoplasmaco reticle.

GOLGI APPARATUS
Vesiulas Presena producing lysosomes.
It is a warehouse or lipid and protein, frmado by a series of flattened sacs, secretion produces a substance which ls Encentro lysosomes.

LYSOSOME
They are produced by the golgi apparatus.
Its role is to produce substances to aid in the digestive processes of the cell. (Enzyme function)
They help destroy yl patgenos agents disappears. In white blood cells there are large numbers of lysosomes.

CENTROSOME
In every cell must have 2 centrosomes and that is of great importance. (THERE ARE 3 IN EACH TUBULE microtubules)
Form the wire spindle where chromosomes traveling.

CHROMOSOME
ADN: Doexy ribonucleic acid
The ADN give the genetic information by living things in chromosome shape.

jueves, 11 de marzo de 2010

MEIOSIS


Is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle. Two cell divisions separate the replicated chromosomes into four haploid gametes or spores.

Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.

During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.

History
Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849–1922). It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846–1910), in Ascaris worms' eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann (1834–1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911 the American geneticist Thomas Hunt Morgan (1866–1945) observed crossover in Drosophila melanogaster meiosis and provided the first genetic evidence that genes are transmitted on chromosomes,

Evolution
Meiosis is thought to have appeared 1.4 billion years ago. The only supergroup of eukaryotes which does not have meiosis in all organisms is excavata. The other five major supergroups, opisthokonts, amoebozoa, rhizaria, archaeplastida and chromalveolates all seem to have genes for meiosis universally present, even if not always functional. Some excavata species do have meiosis which is consistent with the hypothesis that this group is an ancient, paraphyletic grade. An example of a eukaryotic organism in which meiosis does not exist is euglenoid.

Occurrence of meiosis in eukaryotic life cycles

Gametic life cycle.
Zygotic life cycle.
Sporic life cycle.Main article: Biological life cycle
Meiosis occurs in eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. The organism will then produce the germ cells that continue in the life cycle. The rest of the cells, called somatic cells, function within the organism and will die with it.

Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (gametic or diploid life cycle), during the haploid state (zygotic or haploid life cycle), or both (sporic or haplodiploid life cycle, in which there two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phase(s).

In the gametic life cycle, of which humans are a part, the species is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism. Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell. The general principle is that mitosis creates somatic cells and meiosis creates germ cells.

In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa are members of the zygotic life cycle.

Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce gametes. The gametes proliferate by mitosis, growing into a haploid organism. The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become the diploid organism again. The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.

Process
Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle.

Interphase
is divided into three phases:

Growth 1 (G1) phase: This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1 stage each of the chromosomes consists of a single (very long) molecule of DNA. In humans, at this point cells are 46 chromosomes, 2N, identical to somatic cells.
Synthesis (S) phase: The genetic material is replicated: each of its chromosomes duplicates, producing 46 chromosomes each made up of two sister chromatids. The cell is still considered diploid because it still contains the same number of centromeres. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I
Growth 2 (G2) phase: G2 phase is absent in Meiosis
Interphase is followed by meiosis I and then meiosis II. Meiosis I consists of separating the pairs of homologous chromosome, each made up of two sister chromatids, into two cells. One entire haploid content of chromosomes is contained in each of the resulting daughter cells; the first meiotic division therefore reduces the ploidy of the original cell by a factor of 2.

Meiosis II
consists of decoupling each chromosome's sister strands (chromatids), and segregating the individual chromatids into haploid daughter cells. The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells. Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).

Meiosis generates genetic diversity in two ways: (1) independent alignment and subsequent separation of homologous chromosome pairs during the first meiotic division allows a random and independent selection of each chromosome segregates into each gamete; and (2) physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of DNA within chromosomes.


A diagram of the meiotic phases[edit] Phases Of Meiosis
Meiosis I
Separates homologous chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids it is only considered as being N, with 23 chromosomes, because later in anaphase I the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division.

Prophase I
During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma).

Leptotene
The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".[1] During this stage, individual chromosomes begin to condense into long strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.

Zygotene
The zygotene stage, also known as zygonema, from Greek words meaning "paired threads",[1] occurs as the chromosomes approximately line up with each other into homologous chromosomes. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place.

Pachytene
The pachytene stage, also known as pachynema, from Greek words meaning "thick threads",[1] contains the following chromosomal crossover. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology. Exchange takes place at sites where recombination nodules (the chiasmata) have formed. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.

Diplotene
During the diplotene stage, also known as diplonema, from Greek words meaning "two threads",[1] the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in Anaphase I.

In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. In males, only spermatogonia (spermatogenesis) exist until meiosis begins at puberty.

Diakinesis
Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through".[1] This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Synchronous processes
During these stages, two centrosomes, containing a pair of centrioles in animal cells, migrate to the two poles of the cell. These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, which are essentially cellular ropes and poles. The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. [2][3]

Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole: these are called nonkinetochore microtubules or polar microtubules. A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton.

Metaphase I
Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.

Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores[3], whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center.

Telophase I
The last meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.

Meiosis II
Is the second part of the meiotic process. Much of the process is similar to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I. The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and Telophase II.

Prophase II
takes an inversely proportional time compared to prophase I.[citation needed] In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.

In metaphase II,
the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.

This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.

The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now complete and ends up with four new daughter cells.

Significance
Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count. In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality [4]. Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.

Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

This is a cause of several medical conditions in humans (such as):

Down Syndrome - trisomy of chromosome 21
Patau Syndrome - trisomy of chromosome 13
Edward Syndrome - trisomy of chromosome 18
Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY
Turner Syndrome - lacking of one X chromosome in females - ie XO
Triple X syndrome - an extra X chromosome in females
XYY Syndrome - an extra Y chromosome in males
[edit] Meiosis in mammals
In females, meiosis occurs in cells known as oogonia (singular: oogonium). Each oogonium that initiates meiosis will divide twice to form a single oocyte and two polar bodies.[5] However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lie dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of centrosomes.

In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males occurs during spermatogenesis.

In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo, but in the males, meiosis begins years later at the time of puberty. It is retinoic acid, derived from the primitive kidney (mesonephros) that stimulates meiosis in ovarian oogonia. Tissues of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL.[6][7] Meoisis involves Spermatocytes.

miércoles, 3 de marzo de 2010

MITOSIS


Mitosis or mitosis, the process of splitting the nucleus of somatic cells (not sex) whose result is the exact division of the genetic information that previously had doubled during interphase of the cell cycle. Mitosis ensures that the number of chromosomes in the cells remains constant from generation to generation. The progenitor cell gives rise to two daughter cells, genetically identical to the mother, which contain a diploid number of chromosomes characteristic of the species.
Multicellular organisms need to thrive mitosis, cells grow and replace damaged tissues or organs. All these agencies come from a single cell. This cell is multiplied through successive cell divisions that include processes of mitosis (division of the nucleus) and cytokinesis (division of cytoplasm), which lead to the development of complex organisms composed of trillions of cells. These processes also remain active throughout the life of the organism, which allows cells to replace damaged, diseased or dead. In the human body produces about 25 million per second mitosis.
Single-celled organisms reproduce asexually, resulting in offspring that are identical to the parent body. In addition, certain multicellular organisms such as starfish or sea anemones, are dependent on mitosis, at certain stages of their life cycle, to reproduce asexually.
2 THE CELL CYCLE AND CELL DIVISION
The cell cycle comprises a series of ordered phases. During these phases, the cell duplicates its contents and divides into two daughter cells. The cell cycle of a eukaryotic cell, ie a cell with a nucleus, consists of two periods: interphase and mitotic phase (M).
Interphase is the longest period of the cycle and is a step prior to cell division. During interphase, the cell duplicates its deoxyribonucleic acid (DNA) and produces cytoplasmic organelles and other components. The interface comprises three stages: G1, S and G2.
During the mitotic or M phase the cell divides. The mitotic phase involves splitting the nucleus (mitosis proper) and cytoplasmic (cytokinesis) and involves the equal distribution of cellular material between the two daughter cells.
3 THE MITOSIS OR DIVISION OF THE NUCLEUS
Mitosis, or division of the nucleus is the process by which DNA, which has doubled during interphase, separated in two sets of chromosomes that are distributed in each of the nuclei of future daughter cells.
4 phases of mitosis
Although mitosis or division of the nucleus is a continuous process for study differ in several stages: prophase, metaphase, anaphase and telophase.
4.1 Prophase
At the beginning of prophase chromatin fibers (DNA and proteins) are condensed, shortened and visualized under a microscope as small rods that are called chromosomes. As a result of DNA replication during interphase, each chromosome containing a pair of identical chromatids joined by a region called the centromere. Then, the nucleolus disappears and the nuclear membrane begins to fragment. The centrioles, two structures located in the centrosome, split each in migrating an opposite pole of the cell, while the mitotic spindle form or achromatic. The mitotic bone is a structure formed by microtubules that are arranged in an ovoid shape and extending between the centrioles to push toward the poles to grow. The primary function of centrioles is the formation and organization of microtubules, proteins that form the mitotic spindle.
4.2 Metaphase
In metaphase, the chromatid pairs are aligned right in the center of the spindle apparatus (or equatorial plane of the metaphase) by action of the spindle fibers, which are joined together by the centromeres. Each chromosome is displayed with an X in the central plane of the cell.
4.3 Anaphase
During anaphase, the centromeres divide, allowing each of the identical chromatids pair is separated (chromosomes children) and are routed to the two poles of the cell carried by the spindle microtubules. In this shift, the chromosomes acquire a V as the centromere, pulled by the microtubules, moves the first. The set of chromosomes from one pole of the cell is identical to the other pole. Each one of them, form the chromosome of each of the two daughter cells.
4.4 Telophase
The final phase is telophase of mitosis. During telophase, each set of chromosomes is unrolled and converted back into chromatin. Each chromatin mass is surrounded by a nuclear membrane in each new nucleus is a nucleolus and the mitotic spindle disappears.
5 cytokinesis
Coinciding with the end of anaphase or early telophase also begins the cytokinesis or division of the cytoplasm.
In animal cells, cytokinesis begins with the appearance, up to half of the cell, a dividing groove perpendicular to the spindle apparatus. This groove progressive advances producing a bottleneck that eventually leads to the physical division of the cytoplasm and thereby to the formation of two daughter cells with their corresponding cores. In plants, cellulose and other materials transported to the midline of the cell, where they form a new cell wall (fragmoplasto) separating the two new cells. The daughter cells enter interphase, restarting the cell cycle.
In multicellular organisms, mitosis and cytokinesis are controlled primarily by cellular proteins. The duration of the mitotic phase and cytokinesis depends on cell type and can last two hours or even a few minutes. The process of mitosis ensures that all progenitor cell genes passed on to daughter cells. However, errors sometimes arise in DNA replication or mitosis itself, which can lead to changes or mutations in genes that have not been copied correctly or unequal distribution of chromosomes.