Cell - Cell division and growth | serii.info
Arbuscular mycorrhiza is a mutualistic plant-fungus interaction that confers great .. These results indicate that the genes involved in GA biosynthesis (GA20ox1 the mitosis and growth of the arbuscular mycorrhizal fungus Gigaspora rosea . In contrast, many plants, animals, and fungi produce offspring through sexual rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. and the gametophyte and the relationship between them vary among species. Mitosis is a form of cell division that results in two daughter cells that are through mitosis, but all bacteria, some unicellular protists and some fungi may When an animal, plant or other multicellular organism grows, it makes more cells .
Skin cells, for example, are constantly being sloughed off and replaced; in this case, the mature differentiated cells do not divide, but their population is renewed by division of immature stem cells. In certain other cells, such as those of the livermature cells remain capable of division to allow growth or regeneration after injury.
In contrast to these patterns, other types of cells either cannot divide or are prevented from dividing by certain molecules produced by nearby cells. As a result, in the adult organism, some tissues have a greatly reduced capacity to renew damaged or diseased cells. Examples of such tissues include heart muscle, nerve cells of the central nervous systemand lens cells in mammals.
Maintenance and repair of these cells is limited to replacing intracellular components rather than replacing entire cells.
Duplication of the genetic material Before a cell can divide, it must accurately and completely duplicate the genetic information encoded in its DNA in order for its progeny cells to function and survive. This is a complex problem because of the great length of DNA molecules. Each human chromosome consists of a long double spiral, or helix, each strand of which consists of more than million nucleotides see above The nucleus.
These progress along the moleculereading the sequences of nucleotides that are linked together to make DNA chains. Each strand of the DNA double helix, therefore, acts as a template specifying the nucleotide structure of a new growing chain.
In order for DNA to replicate, the two strands must be unwound from each other. Enzymes called helicases unwind the two DNA strands, and additional proteins bind to the separated strands to stabilize them and prevent them from pairing again. In addition, a remarkable class of enzyme called DNA topoisomerase removes the helical twists by cutting either one or both strands and then resealing the cut.
These enzymes can also untangle and unknot DNA when it is tightly coiled into a chromatin fibre. In the circular DNA of prokaryotesreplication starts at a unique site called the origin of replication and then proceeds in both directions around the molecule until the two processes meet, producing two daughter molecules. In rapidly growing prokaryotes, a second round of replication can start before the first has finished.
The situation in eukaryotes is more complicated, as replication moves more slowly than in prokaryotes. At to 5, nucleotides per minute versusnucleotides per minute in prokaryotesit would take a human chromosome about a month to replicate if started at a single site. Actually, replication begins at many sites on the long chromosomes of animalsplantsand fungi.
Distances between adjacent initiation sites are not always the same; for example, they are closer in the rapidly dividing embryonic cells of frogs or flies than in adult cells of the same species. Accurate DNA replication is crucial to ensure that daughter cells have exact copies of the genetic information for synthesizing proteins. It can erase its own errors and then synthesize anew. There are also repair systems that correct genetic damage to DNA.
For example, the incorporation of an incorrect nucleotide, or damage caused by mutagenic agentscan be corrected by cutting out a section of the daughter strand and recopying the parental strand. Cell division Mitosis and cytokinesis In eukaryotes the processes of DNA replication and cell division occur at different times of the cell division cycle. During cell division, DNA condenses to form short, tightly coiled, rodlike chromosomes.
Each chromosome then splits longitudinally, forming two identical chromatids. Each pair of chromatids is divided between the two daughter cells during mitosisor division of the nucleusa process in which the chromosomes are propelled by attachment to a bundle of microtubules called the mitotic spindle. Mitosis can be divided into five phases.
In prophase the mitotic spindle forms and the chromosomes condense. In prometaphase the nuclear envelope breaks down in many but not all eukaryotes and the chromosomes attach to the mitotic spindle.
Both chromatids of each chromosome attach to the spindle at a specialized chromosomal region called the kinetochore. In metaphase the condensed chromosomes align in a plane across the equator of the mitotic spindle. Anaphase follows as the separated chromatids move abruptly toward opposite spindle poles.
Finally, in telophase a new nuclear envelope forms around each set of unraveling chromatids. An essential feature of mitosis is the attachment of the chromatids to opposite poles of the mitotic spindle. This ensures that each of the daughter cells will receive a complete set of chromosomes.
The mitotic spindle is composed of microtubules, each of which is a tubular assembly of molecules of the protein tubulin see above The cytoskeleton.
Sexual life cycles
Some microtubules extend from one spindle pole to the other, while a second class extends from one spindle pole to a chromatid. Microtubules can grow or shrink by the addition or removal of tubulin molecules. The shortening of spindle microtubules at anaphase propels attached chromatids to the spindle poles, where they unravel to form new nuclei.
The two poles of the mitotic spindle are occupied by centrosomes, which organize the microtubule arrays. In animal cells each centrosome contains a pair of cylindrical centrioles, which are themselves composed of complex arrays of microtubules. Centrioles duplicate at a precise time in the cell division cycle, usually close to the start of DNA replication. After mitosis comes cytokinesisthe division of the cytoplasm. This is another process in which animal and plant cells differ.
In animal cells cytokinesis is achieved through the constriction of the cell by a ring of contractile microfilaments consisting of actin and myosin, the proteins involved in muscle contraction and other forms of cell movement.
In plant cells the cytoplasm is divided by the formation of a new cell wall, called the cell plate, between the two daughter cells. The cell plate arises from small Golgi-derived vesicles that coalesce in a plane across the equator of the late telophase spindle to form a disk-shaped structure. In this process, each vesicle contributes its membrane to the forming cell membranes and its matrix contents to the forming cell wall.
A second set of vesicles extends the edge of the cell plate until it reaches and fuses with the sides of the parent cell, thereby completely separating the two new daughter cells. At this point, cellulose synthesis commences, and the cell plate becomes a primary cell wall see above The plant cell wall.
Meiosis A specialized division of chromosomes called meiosis occurs during the formation of the reproductive cells, or gametesof sexually reproducing organisms. Gametes such as ovaspermand pollen begin as germ cells, which, like other types of cells, have two copies of each gene in their nuclei.
The chromosomes composed of these matching genes are called homologs. During DNA replication, each chromosome duplicates into two attached chromatids.
The homologous chromosomes are then separated to opposite poles of the meiotic spindle by microtubules similar to those of the mitotic spindle. At this stage in the meiosis of germ cells, there is a crucial difference from the mitosis of other cells.Mitosis-Updated
In meiosis the two chromatids making up each chromosome remain together, so that whole chromosomes are separated from their homologous partners. Cell division then occurs, followed by a second division that resembles mitosis more closely in that it separates the two chromatids of each remaining chromosome.
In this way, when meiosis is complete, each mature gamete receives only one copy of each gene instead of the two copies present in other cells. The formation of gametes sex cells occurs during the process of meiosis. The cell division cycle In prokaryotesDNA synthesis can take place uninterrupted between cell divisionsand new cycles of DNA synthesis can begin before previous cycles have finished.
In contrast, eukaryotes duplicate their DNA exactly once during a discrete period between cell divisions. This period is called the S for synthetic phase. The four periods G1, S, G2, and M for mitosis make up the cell division cycle.
The cell cycle characteristically lasts between 10 and 20 hours in rapidly proliferating adult cells, but it can be arrested for weeks or months in quiescent cells or for a lifetime in neurons of the brain. Prolonged arrest of this type usually occurs during the G1 phase and is sometimes referred to as G0. In contrast, some embryonic cells, such as those of fruit flies vinegar fliescan complete entire cycles and divide in only 11 minutes.
Animal, Plant, and Fungi Phylogeny: A Surprising Relationship in Eukaryota Phylogeny
In addition, the duration of the S phase varies dramatically. The fruit fly embryo takes only four minutes to replicate its DNA, compared with several hours in adult cells of the same species. Controlled proliferation Several studies have identified the transition from the G1 to the S phase as a crucial control point of the cell cycle.
Stimuli are known to cause resting cells to proliferate by inducing them to leave G1 and begin DNA synthesis.
These stimuli, called growth factors, are naturally occurring proteins specific to certain groups of cells in the body. They include nerve growth factorepidermal growth factor, and platelet-derived growth factor. Such factors may have important roles in the healing of wounds as well as in the maintenance and growth of normal tissues. Many growth factors are known to act on the external membrane of the cell, by interacting with specialized protein receptor molecules.
These respond by triggering further cellular changes, including an increase in calcium levels that makes the cell interior more alkaline and the addition of phosphate groups to the amino acid tyrosine in proteins. The complex response of cells to growth factors is of fundamental importance to the control of cell proliferation.
Failure of proliferation control Cancer can arise when the controlling factors over cell growth fail and allow a cell and its descendants to keep dividing at the expense of the organism.
Studies of viruses that transform cultured cells and thus lead to the loss of control of cell growth have provided insight into the mechanisms that drive the formation of tumours. Transformed cells may differ from their normal progenitors by continuing to proliferate at very high densities, in the absence of growth factors, or in the absence of a solid substrate for support.
An acutely transforming retrovirus shown at topwhich produces tumours within weeks of infection, incorporates genetic material from a host cell into its own genome upon infection, forming a viral oncogene.
When the viral oncogene infects another cell, an enzyme called reverse transcriptase copies the single-stranded genetic material into double-stranded DNA, which is then integrated into the cellular genome. Because they were formed through meiosis, each spore has a unique combination of genetic material. The spores germinate and divide by mitosis to make new, multicellular haploid fungi. Alternation of generations The third type of life cycle, alternation of generations, is a blend of the haploid-dominant and diploid-dominant extremes.
This life cycle is found in some algae and all plants. Species with alternation of generations have both haploid and diploid multicellular stages. The haploid multicellular plants or algae are called gametophytes, because they make gametes using specialized cells.
Meiosis is not directly involved in making the gametes in this case, because the organism is already a haploid. Fertilization between the haploid gametes forms a diploid zygote.
The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores.
The spores will then develop into the multicellular gametophytes. Example of alternation of generations: Haploid 1n spores germinate and undergo mitosis to produce a multicellular gametophyte 1n. Specialized cells of the gametophyte undergo mitosis to produce sperm and egg cells 1nwhich combine in fertilization to make a zygote 2n.
The zygote undergoes mitosis to form a multicellular, diploid sporophyte, the frond-bearing structure that we usually think of as a fern.
Animal, Plant, and Fungi Phylogeny: A Surprising Relationship in Eukaryota Phylogeny
On the sporophyte, specialized structures called sporangia form, and inside of them, haploid cells spores, 1n are formed by meiosis. The spores are released and can germinate, starting the cycle over again. Although all sexually reproducing plants go through some version of alternation of generations, the relative sizes of the sporophyte and the gametophyte and the relationship between them vary among species. In plants such as moss, the gametophyte is a free-living, relatively large plant, while the sporophyte is small and dependent on the gametophyte.
In other plants, such as ferns, both the gametophyte and sporophyte are free-living; however, the sporophyte is much larger, and is what we normally think of as a fern. In seed plants, such as magnolia trees and daisies, the sporophyte is much larger than the gametophyte: The gametophyte is made up of just a few cells and, in the case of the female gametophyte, is completely contained inside of the sporophyte within a flower.
Why is sexual reproduction widespread? In some ways, asexual reproduction, which makes offspring that are genetic clones of the parent, seems like a simpler and more efficient system than sexual reproduction. In addition, asexual reproduction only calls for one individual, removing the problem of finding a mate and making it possible for an isolated organism to reproduce. Despite all this, few multicellular organisms are completely asexual.
Why, then, is sexual reproduction so common? This question has been hotly debated, and there is still disagreement about the exact answer. The processes that generate genetic variation in all sexual life cycles are: Why is this genetic variation a good thing? Sexual reproduction continually makes new, random combinations of gene variants. This makes it more likely that one or more members of a sexually reproducing population will happen to have a combination that allows survival under the new conditions e.
Over generations, beneficial gene variants can spread through the population, allowing it to survive as a group under the new conditions.
This article is a modified derivative of the following articles: Download the original article for free at http: Retrieved October 15, from Wikipedia: Retrieved July 24, from Wikipedia: In Campbell biology 10th ed. Retrieved September 15, from Wikipedia: In Developmental biology 6th ed.
Chromosomes, the cell cycle, and cell division. In Biology 10th ed. Meiosis and sexual life cycles.