Viruses

A virus (from the Latin virus meaning toxin or poison) is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell.

Viruese are strange things that straddle the fence between living and non-living.Viruses are too small to be seen by the naked eye. On the one hand, if they're floating around in the air or sitting on a doorknob, they're inert. But if they come into contact with a suitable plant, animal or bacterial cell, they spring into action. They infect and take over the cell like pirates hijacking a ship.

A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses have an additional layer around this coat called an envelope. Viruses can’t metabolize nutrients, produce and excrete wastes, move around on their own, or even reproduce unless they are inside another organism’s cells. So, they have to invade a 'host' cell and take over its machinery in order to be able to make more virus particles.

They aren’t even cells.

The cells of the mucous membranes, such as those lining the respiratory passages that we breathe through, are particularly open to virus attacks because they are not covered by protective skin.


More than 5,000 types of virus have been discovered. Viruses vary in shape from simple helical and icosahedral shapes, to more complex structures. They are about 100 times smaller than bacteria. The origins of viruses are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—others may have evolved from bacteria.

Viruses spread in many ways; plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food or water. Rotaviruses are often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sex.

Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox, influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases. Even some types of cancer -- though definitely not all -- have been linked to viruses.

Cells

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The word cell comes from the Latin cellula, meaning, a small room. Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.

The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

Living cells are dived into two types - procaryote and eucaryote (sometimes spelled prokaryote and eukaryote). This division is based on internal complexity.

Eucaryote:

Animals, plants, fungi, and protists are eucaryotes, organisms whose cells are organized into complex structures enclosed within membranes. The presence of a nucleus gives these organisms their name, which comes from the Greek (eu), meaning "good/true," and karyon, "nut." Many eukaryotic cells contain other membrane-bound organelles such as mitochondria, chloroplasts and Golgi bodies. These cells tend to be larger than cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size. Use the interactive animation of plant and animal cells to learn about their respective organelles.
http://www.cellsalive.com/cells/cell_model.htm


Procaryote:

The prokaryotes are a group of organisms that lack a cell nucleus, or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles.The word prokaryote comes from the Greek (pro-) "before" + karyon "nut or kernel", referring to the cell nucleus.

The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea were recognized as a domain of life in 1990. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.

The Origin of Life on Earth

How did life begin on Earth? Though no one is ever likely to know the whole story, virtually everyone has wondered at one time or another, how life on Earth began.

There are at least three types of hypotheses which attempt to explain the origin of life on Earth. The first and oldest of these hypotheses suggest that life was created by a supreme being or spiritual force. Most cultures and religions have their own explanations of creation that are passed down from generation to generation. Because these ideas cannot be proved nor disproved, we consider them outside the boundaries of science. For that reason, they will not be pursued here and are left to each individual to decide.

The second set of hypotheses suggests that life began in another part of the universe and arrived on Earth by chance, such as with the crash of a comet or meteor.

The third, and most common hypothesis in the scientific community, is that life began approximately 3.5 billion years ago as the result of a complex sequence of chemical reactions that took place spontaneously in Earth's atmosphere.

The Sun and its planets formed between 5 and 4.6 billion years ago as matter in our solar system began to coalesce because of gravity. By about 3.9 billion years ago, the Earth had an atmosphere that contained the right mix of hydrogen, oxygen, carbon, and nitrogen to allow for the creation of life. Scientists believe that the energy from heat, lightning, or radioactive elements caused the formation of complex proteins and nucleic acids into strands of replicating genetic code. These molecules then organized and evolved to form the first simple forms of life. At 3.8 billion years ago, conditions became right for the fossilization of the Earth's early cellular life forms. These fossilized cells resemble present day cyanobacteria. Such cells are known as prokaryotes. Prokaryote cells are very simple, containing few specialized cellular structures and their DNA is not surrounded by a membranous envelope. The more complex cells of animals and plants, known as eukaryotes, first showed up about 2.1 billion year ago. Eukaryotes have a membrane-bound nucleus and many specialized structures located within their cell boundary. By 680 million years ago, eukaryotic cells were beginning to organize themselves into multicellular organisms. Starting at about 570 million years ago an enormous diversification of multicellular life occurred known as the Cambrian explosion. During this period all but one modern phylum of animal life made its first appearance on the Earth. Figure below describes the approximate time of origin of the Earth' s major groups of plants and animals.

What Is Life?

What is life? Does this sound like a strange question to you? Of course we all know what is meant by the word "life", but how would you define it?

Do all living things move? Do they all eat and breathe? Even though we all seem to know what is meant by saying something is "alive", it's not very easy to describe what "life" is. It's almost as hard as describing where life came from.

Even the biologists (people who study life) have a tough time describing what life is! But after many years of studying living things, from the mold on your old tuna sandwich to monkeys in the rainforest, biologists have determined that all living things do share some things in common:

1. Living things must be able to move by using their own energy
2. Living things need to take in energy
3. Living things get rid of waste
4. Living things grow and develop
5. Living things respond to their environment (e.g., toward food, away from heat, etc.)
6. Living things reproduce and pass their traits onto their offspring
7. Over time, living things evolve (change slowly) in response to their environment

Therefore, in order for something to be considered to "have life" as we know it, it must possess these characteristics.

Nitrogen Cycle

Please click on the URL below and then click on the play button to see the process of nitrogen cycle.

http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/animations/ch29/1_nitrogen_cycle.swf

Recombinant DNA Technology Video Clip

The flollowing URL shows you how recombinant DNA is taking place. It would be better if you have headset so that you can listen to it while watching. Click on the URL below and Enjoy.

http://present.smith.udel.edu/biotech/rDNA.html