Cell division

Mitosis cell division

 

 

Cell biology  The cell cycle  

http://www.sumanasinc.com/webcontent/animations/content/meiosis.html 

Mitosis tutorial & animation

The Mammalian Cell Cycle
Using a microscope to watch a mammalian cell reproduce culminates with two visually stunning events: when the nucleus divides (termed mitosis) and when the cell physically splits in two (termed cytokinesis) [2,5].
From start to finish these two events take approximately one hour to complete and together are considered one phase of the mammalian cell cycle called the M phase (M = mitosis). Although M phase is visually dramatic, there is quite a long period between one M phase and another (on average 23 hours), a time that under the microscope appears to be an intermission where the cell merely grows in size.
This period of visual inactivity is called interphase—which on the molecular level is, in fact, quite an active time for the cell. Interphase is divided into the three remaining phases of the cell cycle. During interphase, a cell replicates its nuclear DNA, ensuring that when it does divide at the end of M phase each of the newly created daughter cells will contain a full set of genes. The period in which this is accomplished is called S phase (S = synthesis).
Before DNA synthesis begins and after it is completed, are the two remaining phases of the cell cycle. Since these periods occur between M and S phases they are designated gap phases in which the cell continues to grow. The interval between the completion of M phase and the beginning of S phase is the G1 phase (G = gap), while the G2 phase is the interval between the end of S phase and the beginning of M phase. Gap phases constitute a decision making period for the cell [6] It is within these phases that molecular events within the cell will determine whether to initiate M phase (during G2), S phase (during G1), or to extend the gap for a longer period and allow further cell growth.
Mitosis (Nuclear division)
Separation of the cell’s chromosomes is the key requirement to producing two genetically identical daughter cells, a requirement that is accomplished during M phase of the cell cycle [5].
Chromosomes are normally diffuse strands of DNA that begin to become visible under the light microscope as M phase is about to start. At this stage, the chromosomes, which have been replicated during an earlier S phase, appear as long threads that gradually shrink and thicken so that they are physically easier to separate during mitosis. As the chromosomes condense, a cytoskeletal structure called the mitotic spindle is assembled to carry out their separation (see Figure 2). It is composed of long molecular tubes (called microtubules) that attach to the chromosomes and physically pull them into separate daughter cells.





Figure 2: Chromosomes condense. This occurs in a continuous fashion that has traditionally been divided into five stages (the names of each stage are included below—see Figure 3) [2,5]:
1. The replicated chromosomes condense and the mitotic spindle begins to assemble outside the nucleus (prophase).
2. The membrane surrounding the nucleus (nuclear envelope) breaks down and allows the mitotic spindle to contact the chromosomes (prometaphase).
3. All the chromosomes are gathered at the center of the mitotic spindle (metaphase).
4. The chromosomes are split apart and pulled to opposite sides of the cell (anaphase).
5. The nuclear envelope reassembles around the two new sets of separated chromosomes to form two nuclei (telophase).





Figure 3. Steps in Mitosis. Once the two new nuclei are formed, the chromosomes de-condense and gene transcription can resume (i.e. genes can begin to function). All that remains is for the cell to divide in two.
Cytokinesis (Cell Division)
The second feature of M phase is the time in which the other components of the cell— membranes, cytoskeleton, organelles, and soluble proteins—are distributed to the two daughter cells through a process called cytokinesis. This is the final task that a cell must complete to finish its reproduction (see Figure 4).





Figure 4. Cytokinesis An inward wrinkling of the cell membrane is the first sign of cytokinesis. This is caused by a second cytoskeletal structure called the contractile ring [2], which begins to tighten in that area of the cell. The wrinkle occurs perpendicular to the mitotic spindle that is pulling the chromosomes to opposite sides of the cell and thus ensures cellular division will result in one nucleus in each of the daughter cells.
Once assembled, the contractile ring will tighten further and further until the cell is pinched in two. The force required to do this is provided by the molecular architecture of the contractile ring, which is composed of two types of filaments (called actin and myosin filaments) that slide over each other to create a tightening power [2]. In fact, the contractile ring is capable of exerting a force strong enough to bend a fine glass needle inserted into the cell. This is a transient structure that gradually becomes smaller as cytokinesis proceeds and disassembles completely once the cell is cleaved in two.
The Cell Cycle And Continuation of Life
Once cytokinesis is completed, the cell has successfully gone through one turn of the cell cycle and produced two cells from a single precursor. For bacteria or yeast, which are single-celled organisms, this cell division will produce a new and complete organism. In a multicellular organism (like human beings), a fertilized single-celled egg requires many cell divisions to make a new individual. In either case it is the completion of the cell cycle that produces new organisms, a process that can go on throughout life. In an adult human, for instance, adult nerve and muscle cells do not divide at all. Liver cells divide once a year. More than one division a day occurs in blood cell precursors in the bone marrow and in the cells lining the gut. Our survival requires a production of millions of cells a second. This fact was illustrated by the exposure of organisms to massive doses of x-rays that stop all cell division and cause an individual to die within a few days. In the end, it is the cell cycle that ensures life will always be able to produce more life in an organized fashion.

 Reference:         Cell cycle

Meiosis - reproductive cell division

The cell

Cell structure and processes

-cell structure and function, membrane structure, transport across membranes

Eukaryopolis - The City of Animal Cells: Biology #4

 http://www.youtube.co Animal cell crashcourse 
A fast and funny narration of the cell structure and function.

                                                    Cell structure and functions simplified.

Identify the organelles inside an animal cell by looking at the video 

Cell division

Eukaryotic cell cycle

Prokaryotes, Eukaryotes, & Viruses Tutorial

Eukaryotes

Basic structure

The basic eukaryotic cell contains the following: plasma membrane glycocalyx (components external to the plasma membrane) cytoplasm (semifluid) cytoskeleton - microfilaments and microtubules that suspend organelles, give shape, and allow motion presence of characteristic membrane enclosed subcellular organelles

Characteristic biomembranes and organelles

	Plasma Membrane A lipid/protein/carbohydrate complex, providing a barrier and containing transport and signaling systems.
	Nucleus Double membrane surrounding the chromosomes and the nucleolus. Pores allow specific communication with the cytoplasm. The nucleolus is a site for synthesis of RNA making up the ribosome.
	Mitochondria Surrounded by a double membrane with a series of folds called cristae. Functions in energy production through metabolism. Contains its own DNA, and is believed to have originated as a captured bacterium.
	Chloroplasts (plastids) Surrounded by a double membrane, containing stacked thylakoid membranes. Responsible for photosynthesis, the trapping of light energy for the synthesis of sugars. Contains DNA, and like mitochondria is believed to have originated as a captured bacterium.
	Rough endoplasmic reticulum (RER) A network of interconnected membranes forming channels within the cell. Covered with ribosomes (causing the "rough" appearance) which are in the process of synthesizing proteins for secretion or localization in membranes. Ribosomes Protein and RNA complex responsible for protein synthesis.
	Smooth endoplasmic reticulum (SER) A network of interconnected membranes forming channels within the cell. A site for synthesis and metabolism of lipids. Also contains enzymes for detoxifying chemicals including drugs and pesticides.
	Golgi apparatus A series of stacked membranes. Vesicles (small membrane surrounded bags) carry materials from the RER to the Golgi apparatus. Vesicles move between the stacks while the proteins are "processed" to a mature form. Vesicles then carry newly formed membrane and secreted proteins to their final destinations including secretion or membrane localization.
	Lysosymes A membrane bound organelle that is responsible for degrading proteins and membranes in the cell, and also helps degrade materials ingested by the cell.
	Vacuoles Membrane surrounded "bags" that contain water and storage materials in plants.
	Peroxisomes or Microbodies Produce and degrade hydrogen peroxide, a toxic compound that can be produced during metabolism.
	Cell wall Plants have a rigid cell wall in addition to their cell membranes.

Endangered Cricket frog

Northern Cricket Frog

Features: Northern Cricket Frog (Acris crepitanis) is a small, rough-skinned member of the Tree Frog Family. The toe pads which are a characteristic of tree frogs are not well developed in this subspecies. It is normally brown or gray in colour and has a distinctive V-shaped marking between the eyes. The call is said to resemble the sound of "pebbles clicking together." This subspecies is more highly aquatic than other North American tree frogs. After the breeding season, it remains in shoreline areas of marshes, ponds and streams, and can often be found in emergent aquatic plants bordering these sources of permanent water.
Status: Endangered Provincially and Nationally
Range: This subspecies has an extensive distribution in its United States range, which extends from Michigan, Ohio and Kentucky west to South Dakota, and south through Texas into northern Mexico. Although considered abundant in parts of this range, it has declined in some areas. The subspecies is likely extirpated in Ontario. Confirmed records for the province exist from Point Pelee, Essex County, where the first Ontario specimen was taken in 1913, and from Pelee Island. Range Maps
Threats: Habitat loss resulting from drainage, dredging and landfill was responsible, in part, for the decline of Northern Cricket Frog in Ontario. Natural flooding which occurred over the winter of 1972 was likely responsible for the disappearance of Northern Cricket Frogs at one site, and may have affected other sites on the Island. In addition, introduced, non-native Carp (Cyprinus carpio) which eat Cricket Frog larvae gained access to the frogs' breeding ponds during periods of high water. Since the apparent disappearance of the Cricket Frog on Pelee, the highly predatory Bullfrog (Rana catesbeiana) has increased dramatically at the last known Cricket Frog site, which decreases chances of a recovery, should a small population remain.
Protection: Northern Cricket Frog is listed under Ontario's Endangered Species Act, 2007, which protects the species and its habitat. A recovery plan, sponsored by the Ministry of Natural Resources, has been developed.

 




 Biodiversity, short for biological diversity, is the term used to describe the variety of life found on Earth and all of the natural processes. This includes ecosystem, genetic and cultural diversity, and the connections between these and all species. The different aspects of biodiversity all have a very strong influence on each other. We have only just started to understand the relationships between living things and their environments. It is helpful to think of an ecosystem as a woven carpet; if you pull on a loose thread it might only affect the thread and those closest to it or it might unravel the whole carpet.
Biodiversity also helps us in our day-to-day lives. Unfortunately, the greenhouse gases produced by human activities are building up in the atmosphere and causing climate change. Climate change is a major threat to biodiversity.

 The diversity of species on Earth is incredible. Right now, there are more than 1.7 million species that have been discovered and named while scientists think that there are millions more we don’t know about. The different plants, animals and other forms of life interact with each other in so many ways that we are still figuring them out.
Over time, many species of plants and animals have evolved to depend on one another. For example, some plants can only be pollinated by a certain kind of bird or insect. In relationships like these, if one species becomes extinct, the other species could too.
The complex relationships between species are often disturbed when organisms are transported to new places or a new element is introduced (for example, chemicals from pesticides). This can be by accident or on purpose. Sometimes they compete with species that are already there or they prey on them. It is also possible that they may have no effect on the native species.
Here are a couple of tips
• Never introduce an invasive plant species into an area it does not naturally exist
• Never release your exotic pets, including fish or turtles, into the wild
• Plant only native species
• Avoid using pesticides or chemicals on your lawns or in your garden.