There are different types of unicellular organism, including:. You might be tempted to think that these organisms are very simple, but in fact they can be very complex. They have adaptations that make them very well suited for life in their environment.
Bacteria are tiny. A typical bacterial cell is just a few micrometres across a few thousandths of a millimetre. The structure of a bacterial cell is different to an animal or plant cell. For example, they do not have a nucleus but they may have a flagellum. This is a tail-like part of the cell that can spin, moving the cell along.
Nerve cells have appendages called dendrites and axons that connect with other nerve cells to move muscles, send signals to glands, or register sensory stimuli. Outer skin cells form flattened stacks that protect the body from the environment. Muscle cells are slender fibers that bundle together for muscle contraction. The cells of multicellular organisms may also look different according to the organelles needed inside of the cell. For example, muscle cells have more mitochondria than most other cells so that they can readily produce energy for movement; cells of the pancreas need to produce many proteins and have more ribosomes and rough endoplasmic reticula to meet this demand.
Although all cells have organelles in common, the number and types of organelles present reveal how the cell functions. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. National Geographic Society.
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Any interactives on this page can only be played while you are visiting our website. This is because it is here that the chromosome compliment is reduced from diploid --two copies-to haploid -one copy. Interphase in meiosis is identical to interphase in mitosis. At this stage, there is no way to determine what type of division the cell will undergo when it divides.
Meiotic division will only occur in cells associated with male or female sex organs. Prophase I is virtually identical to prophase in mitosis, involving the appearance of the chromosomes , the development of the spindle apparatus, and the breakdown of the nuclear membrane. Metaphase I is where the critical difference occurs between meiosis and mitosis.
In mitosis, all the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that the chromatid arms may overlap and temporarily fuse, resulting in what is called crossovers.
During Anaphase I , the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell. In Telophase I , a cleavage furrow typically forms, followed by cytokinesis --the changes that occur in the cytoplasm of a cell during nuclear division, but the nuclear membrane is usually not reformed and the chromosomes do not disappear.
At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell, that is, while the original cell was diploid, the daughter cells are now haploid. Meiosis II is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. At this stage, a new set of spindle fibers form and the chromosomes begin to move toward the equator of the cell.
During Metaphase II , all the chromosomes in the two cells align with the metaphase plate. In Anaphase II , the centromeres split and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell.
In Telophase II , a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane. The chromosomes begin to fadeand is replaced by the granular chromatin characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.
In the case of male structures , all four cells will eventually develop into sperm cells. In the case of the female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell which is usually much larger than a sperm cell. These are called spontaneous mutations and occur at a rate characteristic for that organism. Genetic recombination refers more to a large-scale rearrangement of a DNA molecule. This process involves pairing between complementary strands of two parental duplex, or double-stranded DNAs, and results from a physical exchange of chromosome material.
The position at which a gene is located on a chromosome is called a locus. In a given individual, one might find two different versions of this gene at a particular locus. These alternate gene forms are called " alleles. This may cause the strands to break apart at the crossover point and reconnect to the other chromosome, resulting in the exchange of part of the chromosome.
Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different.
This process explains why offspring from the same parents can look so different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination.
This theory of " independent assortment " of alleles is fundamental to genetic inheritance. However, having said that, there is an exception that requires further discussion.
First, the frequency of recombination is actually not the same for all gene combinations. This is because recombination is greatly influenced by the proximity of one gene to another. If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart.
Linkage describes the tendency of genes to be inherited together as a result of their location on the same chromosome. Linkage disequilibrium describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distance apart.
Scientists apply this concept when searching for a gene that may cause a particular disease. They do this by comparing the occurrence of a specific DNA sequence with the appearance of a disease.
When they find a high correlation between the two, they know they are getting closer to finding the appropriate gene sequence.
Binary Fission--How Bacteria Reproduce Bacteria reproduce through a fairly simple process called binary fission , or the reproduction of a living cell by division into two equal, or near-equal parts. As just noted, this type of asexual reproduction theoretically results in two identical cells. However, bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.
Like more complex organisms, bacteria also have mechanisms for exchanging genetic material. Though not equivalent to sexual reproduction, the end result is that a bacterium contains a combination of traits from two different " parental " cells.
Three different modes of exchange have thus far been identified in bacteria. Conjunction involves the direct joining of two bacteria, which allows their circular DNAs to undergo recombination. Bacteria can also undergo transformation by absorbing remnants of DNA from dead bacteria and integrating these fragments into their own DNA.
Lastly, bacteria can exchange genetic material through a process called transduction , in which genes are transported into and out of the cell by bacterial viruses--called bacteriophages --or by plasmids --an autonomous self-replicating extra-chromosmal circular DNA. Viral Reproduction Because viruses are acellular and do not use ATP, they must utilize the machinery and metabolism of a host cell to reproduce. For this reason, viruses are called obligate intracellular parasites.
Before a virus has entered a host cell, it is called a virion--a package of viral genetic material. Virions --infectious viral particles--can be passed from host to host either through direct contact or through a vector, or carrier.
Inside the organism, the virus can enter a cell in various ways. Bacteriophages --bacterial viruses--attach to the cell wall surface in specific places.
Once attached, enzymes make a small hole in the cell wall and the virus injects its DNA into the cell. Other viruses like HIV enter the host via endocytosis, the process whereby cells take in material from the external environment. After entering the cell, the virus's genetic material begins the destructive process of taking over the cell and forcing it to produce new viruses. Figure 5. Types of Viruses This illustration depicts three types of viruses: a bacterial virus, otherwise called a bacteriophage left center ; an animal virus top right ; and a retrovirus bottom right.
Viruses depend on the host cell that they infect to reproduce. When found outside of a host cell, viruses, in their simplest forms, consist only of genomic nucleic acid--either DNA or RNA depicted as blue surrounded by a protein coat, or capsid. The form of genetic material contained in the viral capsid --the protein coat that surrounds the nucleic acid--determines the exact replication process.
Then, there are two different replication processes for viruses containing RNA. A second group of RNA-containing viruses, called the retroviruses , use the enzyme reverse transcriptase to synthesize a complimentary strand of DNA so that the virus's genetic information is contained in a molecule of DNA rather than RNA. The viral DNA can then be further replicated using the host cell machinery. When the virus has taken over the cell, it immediately directs the host to begin manufacturing the proteins necessary for virus reproduction.
The host produces three kinds of proteins: early proteins --enzymes used in nucleic acid replication, late proteins --proteins used to construct the virus coat, and lytic proteins --enzymes used to break open the cell for viral exit. The final viral product is assembled spontaneously, that is, the parts are made separately by the host and are joined together by chance. This self-assembly is often aided by molecular chaperones , or proteins made by the host that help the capsid parts come together.
The new viruses then leave the cell either by exocytosis or by lysis. Envelope-bound animal viruses instruct the host's endoplasmic reticulum to make certain proteins, called glycoproteins , which then collect in clumps along the cell membrane.
The virus is then discharged from the cell at these exit sites, referred to as exocytosis. On the other hand, bacteriophages must break open, or lyse the cell in order to exit.
To do this, the phages have a gene that codes for an enzyme called lysozyme. This enzyme breaks down the cell wall, causing the cell to swell and burst. The new viruses are released into the environment, killing the host cell in the process. Why Study of Viruses? Viruses are important to the study of molecular and cellular biology because they provide simple systems that can be used to manipulate and investigate the functions of many cell types. We have just discussed how viral replication depends on the metabolism of the infected cell.
Therefore, the study of viruses can provide fundamental information about aspects of cell biology and metabolism. The rapid growth and small genome size of bacteria make them excellent tools for experiments in biology. Bacterial viruses have also further simplified the study of bacterial genetics and have deepened our understanding of the basic mechanisms of molecular genetics. Because of the complexity of an animal cell genome, viruses have been even more important in studies of animal cells than in studies of bacteria.
Numerous studies have demonstrated the utility of animal viruses as probes for investigating different activities of eukaryotic cells. Other examples in which animal viruses have provided important models for biological research of their host cells include studies of DNA replication , transcription , RNA processing , and protein transport.
Deriving New Cell Types Look closely at the human body and it is clear that not all cells are alike. For example, cells that make up our skin are certainly different from cells that make up our inner organs. Yet, all the different cell types in our body are all derived , or arise, from a single, fertilized egg cell through differentiation. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver or muscle cell.
During differentiation, certain genes are turned on, or become activated , while other genes are switched off, or inactivated. This process is intricately regulated.
As a result, a differentiated cell will develop specific structures and perform certain functions. Mammalian Cell Types Three basic categories of cells make up the mammalian body: germ cells , somatic cells , and stem cells. Each of the approximately ,,,, cells in an adult human has its own copy, or copies, of the genome. The only exception being certain cell types that lack nuclei in their fully differentiated state, such as red blood cells.
The majority of these cells are diploid, or have two copies of each chromosome. These cells are called somatic cells. This category of cells includes most of the cells that make up our body, such as skin and muscle cells. Germ line cells are any line of cells that give rise to gametes--eggs and sperm--and are continuous through the generations.
Stem cells , on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells. They are best described in the context of normal human development. Human development begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells.
Approximately four days after fertilization, and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells and inside this hollow sphere there is a cluster of cells called the inner cell mass. The inner cell mass cells will go on to form virtually all of the tissues of the human body. Although the inner cell mass cells can form virtually every type of cell found in the human body, they cannot form an organism.
Therefore, these cells are referred to as pluripotent , that is, they can give rise to many types of cells but not a whole organism. Pluripotent stem cells undergo further specialization into stem cells that are committed to give rise to cells that have a particular function.
Examples include blood stem cells that give rise to red blood cells, white blood cells and platelets and skin stem cells that give rise to the various types of skin cells.
These more specialized stem cells are called multipotent -capable of giving rise to several kinds of cells, tissues, or structures. Replication, like all cellular activities, requires specialized proteins for carrying out the job. In the first step of replication, a special protein, called a helicase , unwinds a portion of the parental DNA double helix. This newly synthesized strand is called the leading strand and is necessary for forming new nucleotides and reforming a double helix.
Because DNA synthesis can only occur in the 5' to 3' direction, a second DNA polymerase molecule is used to bind to the other template strand as the double helix opens. This molecule synthesizes discontinuous segments of polynucleotides, called Okazaki fragments. Another enzyme, called DNA ligase , is responsible for stitching these fragments together into what is called the lagging strand.
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