In our last instruction, we learned that all cells contain both DNA and RNA. You’ve probably heard about DNA in the movies or on television.

DNA is the genetic blueprint that determines the specific function of every cell or group of cells in every living thing on Earth. When something happens in our bodies, it’s DNA that sets the process in motion. Our DNA is determined at the moment of our conception. No two people on Earth have exactly the same DNA. That’s why DNA can be used to identify criminal suspects or dead bodies.

But what is DNA anyway? And why are we calling this “the role of RNA” instead of “the role of DNA”?

It’s because DNA (deoxyribonucleic acid) couldn’t do its job without RNA (ribonucleic acid). These nucleic acids are just fancy names for the chemicals that make up genes.

Nucleic Acids

DNA and RNA are both nucleic acids. These nucleic acids serve as a blueprint for proteins. Those proteins actually dictate all cell structures and functions. Obviously, these nucleic acids and the proteins they code for are of huge importance.

Protein

Proteins are the building blocks of life. There are an estimated 10,000 to 50,000 different proteins in the human body. Whenever a cell needs to do something, it makes a specific protein to do it -- like grow a tooth or digest your lunch. Proteins cannot be synthesized (built) without instructions from DNA. This process is called protein synthesis and we’ll talk more about it in a minute.

But first you need to know what proteins are made of.

Remember that we called proteins “the building blocks of life”?  Well, amino acids are the building blocks of protein. And it’s the order in which these amino acids go together – their “sequence” – that makes each protein unique. So protein synthesis is just a fancy name for making proteins.

Protein Synthesis / RNA

DNA is the director of the process of protein synthesis. But since DNA is confined within the nucleus of the cell, it needs a helper. That helper is RNA. There are four different kinds of RNA.

We’ll describe what they do as we describe the process of protein synthesis, but here’s the list:

• mRNA messenger RNA
   
• tRNA transfer RNA
   
• rRNA ribosomal RNA
   
• snRNA small-nuclear RNA

(RNA polymerase, an important enzyme in the synthesis of tRNA, is classified as snRNA)

Here’s a diagram of protein synthesis. It will help you to look at this diagram while you read how it works.

 

How It Happens

When a cell needs to make a protein to do something, here is what happens:

1. Within the nucleus of a cell, the cell’s DNA initiates the process. It makes a template (pattern) of the protein it needs by making a strand of mRNA (messenger RNA). This is called transcription.
    
2. The strand of mRNA leaves the nucleus to assemble (“sequence”) the correct amino acids to make the protein the DNA asked for. From this point
   on, the process is called translation.
    
3. The strand of mRNA attaches itself to the protein-making machinery of the cell, the ribosomes (rRNA – ribosomal RNA).
    
4. Now another form of RNA, tRNA (transfer RNA) begins to collect the right amino acids from the cell fluid (cytoplasm). There are many, many tRNAs. Each one of them carries its amino acid back to the strand of mRNA, which dictates the order (sequence) in which it should be attached.
    
   Scientists have assigned letters (A, B, C, etc.) to the various sites on the strand  where the amino acid is attached. Translation involves the movement of mRNA from what’s called the A site to what’s called the P site.
    
5. As the amino acids are lined up in the right sequence on the mRNA, an  enzyme bonds one amino acid after another to the growing protein strand.
    
   The strand is now getting longer and longer. This is called elongation. When all the correct amino acids have been attached, the completed protein is released to do whatever the DNA originally wanted it to do (like digest your lunch).
    
6. The tRNA is degraded and disappears. This is called termination.

It has taken a lot of words to describe this. But in a cell, 40 to 100 amino acids can be added to a growing protein strand in just one second.

If a genetic error alters the amino acid sequence, or if a mistake is made in copying the sequence, serious consequences can occur. If this happens early in the coding sequence (in what is called a “base deletion”) the result may be a “nonfunctional protein" which can make you sick.

But it can also make you well -- because scientists have learned how to manipulate the process. Many of the antibiotics we use to treat bacterial infections work just this way – by interrupting or stopping protein synthesis.

Protein synthesis has been called “the central dogma (belief) of molecular biology.”
This little illustration sums it up.

In Summary

Here, in words, is another brief summation of protein synthesis:

1. transcription DNA gives an instruction to RNA (initiation)
    
2. RNA carries out DNA’s instruction by synthesizing the proper amino acids, in the proper sequence, into a growing strand of whatever protein the DNA asked  for (elongation)
    
3. Protein is the final product and termination of the TRNA happens.
    
4. No RNA = No translation of DNA = No proteins = No life.
    

And you can’t really understand DNA without understanding RNA.



The central dogma of molecular biology.

In short, DNA makes RNA and RNA makes protein. This belief has been widely accepted for some time now. The usual route is DNA in the nucleus gives a code to RNA. That RNA leaves the nucleus with the code. It codes for the proteins in the ribosomes. It is supposed to be a one way trip. Some viruses don’t seem to know the rules.

Viruses like HIV, have an enzyme that allows them to reverse the process. If you recall earlier in this instruction, transcriptase is when DNA codes for RNA on a template. (Go ahead, check back if you don’t believe me.)

 The virus that causes AIDs actually does reverse transcriptase. That means that it copies its DNA into the RNA of the cell and puts its own DNA into the cell that it has invaded. As you know, AID’s is eventually deadly because the cells are no longer following the original blueprints. The proteins your cells are originally supposed to code for are no longer made.
 

For an animated explanation of protein synthesis, click:
http://www.lewport.wnyric.org/JWANAMAKER/animations/Protein%20Synthesis%20-%20long.html 
 

Experiments for Home and Classroom

This Instruction is about the RNA's vital role in carrying out DNA's instructions. In this experiment, students isolate DNA from wheat -- this can be done at home or in the classroom and it's a fascinating activity. Go to this web site and scroll down to "Isolating DNA." Click:
http://lappel.web.wesleyan.edu/expts.htm 

The science of identifying individuals through the use of DNA sequences is very clear -- and the probability of scientific error is very small. As a result, DNA evidence has been widely used to help identify perpetrators of crimes and to exonerate innocent people. In this activity, students are invited to "Catch a Criminal" by using DNA evidence to determine which of three possible suspects is actually guilty. This activity requires Flash -- which can be downloaded free. Click:
http://www.koshland-science-museum.org/exhibitdna/crim01.jsp 

In this interesting activity, students learn how to make a paper (origami) model of the famous DNA double helix. This activity requires Pdf. First, get the template (pattern) at:
http://www.dnai.org/teacherguide/pdf/ori_bw.pdf 
Then, for instructions, click:
http://www.dnai.org/teacherguide/pdf/origami_inst.pdf