All organisms are made up of cells, which are the basic unit of organisms.
Cells are made up of may types of smaller structures, or organelles.
Let's take a look at the structure of animal and plant cells
Cell Membrane - flexible structure that holds the cell together, helps control what enters and leaves the cell
Cell Wall - provides suport and protection (only found in plant cells)
Chloroplasts - captures the energy from sunlight and uses it to convert carbon dioxide and water into food (plant cells)
Central Vocuole- stores food, water, minerals, and waste
Cytoplasam - is a gelatinlike substance that contains many chemicals that the cell needs
Cytoskeleton - is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion
Gogli Bodies - organelles that package cellular materials and transport them within the cell or out of the cell
Ribosomes - small cytoplasmic structure on which cells make their own protien
Nucleus - controls most of the cells activities
Nucleolus - is a non-membrane bond structure composed of protiens and nucleic acids found in the nucleus. Ribosomal RNA is transcribed and assembled in this structure.
Lysomes - spherical organelles that contain enzymes. They break up food so it is easier to digest.
Mitochondrion - converts food energy into a form that the cell can use
Smooth Endoplasmic Reticulum - cytoplasmic organalle that movies materials around in the cell and is made up of a complex series of folded membranes without attached ribosomes
Rough Endoplasmic Reticulum - cytoplasmic organalle that movies materials around in the cell and is made up of a complex series of folded membranes with attached ribosomes
Cells that lack a membrane-bound nucleus are called prokaryotes (from the Greek meaning before nuclei). These cells have few internal structures that are distinguishable under a microscope. Cells in the monera kingdom such as bacteria and cyanobacteria (also known as blue-green algae) are prokaryotes.
Prokaryotic cells differ significantly from eukaryotic cells. They don't have a membrane-bound nucleus and instead of having chromosomal DNA, their genetic information is in a circular loop called a plasmid. Bacterial cells are very small, roughly the size of an animal mitochondrion (about 1-2µm in diameter and 10 µm long). Prokaryotic cells feature three major shapes: rod shaped, spherical, and spiral. Instead of going through elaborate replication processes like eukaryotes, bacterial cells divide by binary fission.
Eukaryotic cells (from the Greek meaning truly nuclear) comprise all of the life kingdoms except monera. They can be easily distinguished through a membrane-bound nucleus.
Eukaryotic cells also contain many internal membrane-bound structures called organelles. These organelles such as the mitochondrion or chloroplast serve to perform metabolic functions and energy conversion. Other organelles like intracellular filaments provide structural support and cellular motility.
Another important member of the eukaryote family is the plant cell. They function essentially in the same manner as other eukaryotic cells, but there are three unique structures which set them apart. Plastids, cell walls, and vacuoles are present only in plant cells.
Like many things, cells wear out and die. If an organism is to live and grow it must reproduce. Therefore cell division serves an important role in an organism's health and growth. Cell division occurs rapidly in living organisms. For example, in an adult human, millions of cells divide each second to maintain homeostasis (the proper balance in cells).
Cell division begins with interphase, when the cell replicates all of its genomic and cytoplasmic material and prepares for division. After preparation is complete, the cell enters the 4-phased mitosis. In mitosis, the cell sequentially goes through prophase, metaphase, anaphase, and telophase. Immediately after the completion of telophase, cytokenesis is initiated to end cell division by literally separating the cell in two.
Before a cell can enter cell division, it needs to prepare itself by replicating its genetic information and all of the organelles. All of the preparations are done during the interphase. Interphase proceeds in three stages, G1, S, and G2. Cell division operates in a cycle. Therefore, interphase is preceded by the previous cycle of mitosis and cytokenesis.
[Diagram showing the cell cycle. The size of the arrows show the relative length of each stage of the cell cycle. Notice mitosis (M) is quite short.]
After mitosis is complete the new daughter cell begins to accelerate its biochemical processes which were slowed down by mitosis. The length of the G1 phase creates the difference between fast dividing cells and slowly dividing cells. The G1 phase can be slowed by reducing the nutrients available in a system - thus the cell will take longer to build up the resources necessary for cell division. If there is a severe depletion in nutrients the cells can virtually stop growing. It is interesting to note that cells that aren't growing are always stopped in the G1 phase, being mitotically arrested. This suggests that once the cell enters the S phase, it is committed to cell division, regardless of the external cell conditions.
[Animal cell interphase. The DNA has been replicated. Also, notice the increased cell size as the cytoplasm has been enlarged.]
The S phase begins with the replication of the cellular DNA. This is described in further detail in DNA replication. When the cellular DNA has been duplicated, leaving the cell with twice as many chromosomes (each chromosome is made up of two identical chromatids), the cell moves onto the G2 phase.
During this phase proteins, such as kinase (which catalyzes protein phosphorylation), which are necessary for cell division are synthesized at this time. The chromosome begins to condense and the proteins necessary for construction of the mitotic spindle also are synthesized. When the chromosomes become visible the cell enters the first stage of mitosis, prophase.
During prophase the chromosomes are identical chromatids connected at the center by a centromere, forming a X-shaped object. The distinguishing feature of prophase is the setup of the mitotic spindle, which is used to maneuver the chromosomes about the cell. The spindle is formed by excess parts from the dismantled cytoskeleton. The spindle is initially setup outside the nucleus.
The cell's centioles are duplicated to form two pairs of centrioles. Each pair becomes the part of the mitotic center which forms the focus for an array of microtubules, called the aster. The two asters lie side by side close the the nuclear envelope. Near the end of prophase the asters pull apart and the spindle is formed.
[Animal cell in prophase. Notice that the DNA has been condensed into chromosomes.]
The prometaphase provides a transition from prophase to metaphase. In prometaphase the nuclear envelope, which surrounds the nucleus, breaks up. The spindle now can move into the center of the cell. Kinetochores develop, which are attached to kinetochore fibers, which are linked to the chromosomes. The kinetochores then control the movements of the chromosomes. During this period the kinetochores are wildly oscillating as they try to attach themselves to one of the polar fibers. When they manage to do so the chromosome settles down.
In Metaphase the kinetochores that are responsible for moving the chromosomes jump begin to orientate the chromosomes. The chromosomes are orientated so that 1. each kinetochore faces the pole and 2. it moves each chromosome into a plane at the center of the spindle so that each chromosome tail is facing each other.
[Animal cell metaphase. Notice the chromosomes are lined up along the equator of the cell. Two poles representing each daughter cell is also formed by the spindle fibers.]
In anaphase two events occur. First the kinetochores begin to move towards the poles. Then the polar fibers elongate, spreading the poles farther apart from each other.
[Animal cell anaphase. Notice the spindle fibers begin to pull the two sets of chromosome toward the opposite poles.]
By telophase there are two separate groups of chromosomes at each pole. A nuclear envelope begins to form around each set of chromosomes to form two nuclei, that are temporarily in one cell. After the envelope reassembles RNA synthesis begins to break down the chromosomes, causing the nucleolus to reappear.
[Animal cell telophase. Notice the two poles begin to condense to form two nuclei. The cells begin to build the cell membrane in between.]
Now there are two separate nuclei, but they are in the same cell. The cell now needs to be split in half now. Cytokenesis begins in anaphase and continues on through telophase. The first visible sign of cytokenesis is when the cell begins to pucker in, a process called furrowing. Furrowing tends to take place at right angles to the axis of the spindle (so that each nuclei is placed in a different cell of course!). The cytoskeleton is reused to build the next spindle for mitosis. Now the two cells will continue the cell cycle and begin their interphase again!
Meiosis starts with diploid cells, or cells that have two sets of chromosomes from their parents. Haploid cells only have one set of chromosome from their parents. In meiosis the diploid cell eventually forms four germ cells (also called 1n) that have half the chromosomes. In meiosis two sets of nuclear division occur. In Meiosis I the diploid cell is changed into two diploid cells. Then, in Meiosis II the two diploid cells are split into four haploid cells. Since meiosis involves two divisions and only only replication of DNA, it leaves half the amount of chromosomes. Thus it is also called reduction-division.
The purpose of meiosis is to increase the genetic variation. After meiosis there are four haploids, each with different sets of chromosomes. However, in mitosis the end result are two identical diploids. Meiosis is used in sexual reproduction, since to reproduce, an egg and a sperm have to come together for reproduction to occur. This further increases the genetic variation which allows for evolution and the adaptation of organisms to different environments.
In prophase I the chromosomes become visible. However, unlike prophase of mitosis, the two chromosomes combine or synapse to form tetrads. Tetrads are also known as bivalents because they contain two pairs of chromosomes. At this point the chromosomes cross over at points called chiasmata. Crossing over allows the chromosomes to exchange genetic material, allowing for more different combinations of genetic material. As in the prophase of mitosis the nuclear envelope disperses, the spindle moves into the center, and the tetrads become connected to the spindle fibers by kinetochores.
Diagram of prohpase
In metaphase I the tetrads are again arranged across the center by the movements of the kinetochores with the two centromeres opposite each other, but this time the sister chromatids will not be pulled apart as in mitosis.
Diagram of metaphase
In anaphase I the chromatids holding the chromosomes together loosen. The two homologous chromatids of each tetrad are separated into separate poles. Since the chromosomes from each parent can go into either pole this is another means to increase genetic diversity.
Diagram of anaphase
In this phase, like in mitosis the chromosomes are moved into opposite poles and the nuclear envelope reforms and the spindle is broken down. Remember that there are two chromosomes, not one as in mitosis.
Diagram of telophase
In meiosis the cell goes directly from telophase I to prophase II without the interphase. In prophase II the nuclear envelope is again dissolved and the spindle is set up again. Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes.
Again the chromosomes move into the center and line up. Now there are two chromosomes, instead of two tetrads, so that the chromatids will split off this time.
The kinetochores move towards the poles, splitting up the sister chromatids
In telophase II the chromatids concentrate in the poles and the nuclear envelope is reformed and the spindle again is dissolved. The cells divide for the last time, leaving a total of four haploid cells, which have half the chromosomes of a diploid cell. Unlike the daughter cells from mitosis, the daughter cells produced here cannot immediately cycle back to interphase.
Explination of parts
Onion Root Tip magnified by 100 microns
Rapidly dividing cells give us the opportunity to study the various stages of cell division. Notice that these plant cells, unlike animal cells, have cell walls. more images
Assignment Discovery Cells
Assignment Discovery Elements of a Cell
Cellupidia (2010). http://library.thinkquest.org/C004535/introduction.html
Dioni,W. (1995). Homage to the Onion Skin. http://www.microscopy-uk.org.uk/mag/artnov03/wdonion.html
How stuff works (2010). http://science.howstuffworks.com/
University of Kansas (2010) Medical Center http://www.kumc.edu/instruction/medicine/