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In this lesson, you'll explore RNA structure and learn the central dogma of molecular biology. Along the way, you'll meet the three types of RNA and see how the cell uses them most effectively.
Previously, on 'DNA and RNA:'
Professor Pear: Today, we know that permanently changing the characteristics of an organism can be accomplished by changing its DNA content. James Watson and Francis Crick devised a model of the structure of DNA based on the evidence produced by several different laboratories at the time.
Miss Crimson: I can attest that my client is innocent of the murder. In fact, based on the DNA evidence, we have reason to believe that Mr. Teal murdered Mr. Bones in the staircase with the lead pipe.
Miss Ivory: Professor, you said that you found DNA evidence at the scene of the crime; however, you said nothing about RNA evidence. Didn't you say that there are two types of nucleic acids? What about this ribonucleic acid, or RNA? It seems like you've conspicuously avoided talking about RNA altogether. Is it because the lack of RNA evidence directly links Colonel Custard to the crime?
Professor Pear: That's an excellent question. I would be remiss to talk about nucleic acids and only talk about DNA. RNA is, in fact, the second of the two types of nucleic acids; however, there are a number of structural differences between the two.
First let's address the name. Like DNA, RNA is a nucleic acid composed of a sugar, a phosphate group and a nitrogenous base. One difference between DNA and RNA is the sugar. Whereas the sugar in DNA is deoxyribose, the sugar in RNA is ribose. Now, I won't dwell on the exact chemical difference between the two sugars, but ribose has one extra hydroxyl group compared to deoxyribose.
Second, there are four different nitrogenous bases found in DNA and RNA; however, there is one difference. The bases found in DNA are guanine, cytosine, adenine and thymine. The bases found in RNA are guanine, cytosine, adenine, and uracil. Uracil forms two hydrogen bonds with adenine and functions just like thymine does. It's simply used in RNA instead of thymine.
Finally, unlike DNA, which is double-stranded, RNA is single-stranded.
Miss Ivory: Please answer the question, Professor. Knowing that RNA is structurally different than DNA is interesting but not really relevant to this murder trial.
Professor Pear: But it is! It is! You see I needed to explain the structure of RNA, so you could better understand the function of RNA.
DNA, RNA, and protein are functionally linked together in a concept known as the central dogma.
Remember that DNA houses recipes to make different biological molecules; however, this information is not accessed directly from the DNA. Instead, a copy of the recipe is made in the form of RNA. This copy of the recipe can then be read to make a protein.
The central dogma states that DNA makes RNA, and RNA makes protein. At each step, a cell translates the information between the different molecular languages. That is, DNA language is transcribed into RNA language at the first step, and RNA language is translated into protein language at the second step.
Three major types of RNA play a role during the journey from DNA to protein. Although the functions of each type of RNA are different, one type of RNA is called messenger RNA, or simply mRNA. mRNA is created when the DNA recipe is copied in the first step of the central dogma. The information found in mRNA can be interpreted by using two other forms of RNA in the second step of the central dogma.
mRNA is translated into protein at a cellular structure known as the ribosome. A second type of RNA helps form the structure of a ribosome. This type of RNA is called ribosomal RNA, or rRNA.
Remember that DNA and RNA differ slightly at the nucleotide level. Therefore, the process of transcribing DNA into RNA not only changes the information from a double-stranded into a single-stranded molecule, but also changes all the thymine bases into uracil ones.
Proteins are made of amino acids, so the formation of any protein requires assembly of a chain of amino acids. Transfer RNA, or tRNA, molecules ferry amino acids to the ribosome for this assembly.
As you can see, there are many types of RNA performing all kinds of interesting jobs. In fact, why don't I tell you about why we believe RNA actually preceded DNA?
Miss Ivory: Professor, please get to the point. You've laid out very nicely that RNA plays a number of key roles in translating the information in DNA into protein, but you have yet to provide an adequate explanation to account for the absence of RNA evidence in your testimony.
Professor Pear: Oh, right. I'm sorry. Sometimes I just get carried away talking about nucleic acids. One of the major roles of RNA in a cell is to make proteins and proteins carry out many cellular functions in biology; however, it's inefficient for the cell to maintain a constant level of mRNA and protein.
Miss Ivory: Please, explain for the jury, Professor, what all this really means.
Professor Pear: Think of the electronic devices in your room. Let's say that I keep my computer on 24 hours a day because I want to be able to do an online search whenever I feel like it. There's a cost to that practice. First, I'm going to have to pay for the electricity to power the computer. Second, let's also say I've decided to also leave my monitor, printer, speakers, and a number of other devices plugged in as well, so many in fact, that I'm using up all the electrical outlets in the room.
Now, if I want to plug in a new electronic device, I will need to turn off one of my devices before I can plug in a new one. I may be able to surf the Internet faster, but it may be at the cost of the setup time to use another device, like say a hair dryer. If I use a lot of other electronic devices besides the computer, I might not be saving myself that much time in the long run if I constantly have to turn off the computer to plug other things in.
An alternative strategy would be to keep all of the electrical devices off both to conserve energy and to minimize the startup time for using any one electrical device.
The same conservation strategy applies to a cell.
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If RNA was a very stable molecule, it might tie up a lot of resources in a molecule that isn't being used. For instance, yeast consumes sugar for energy. Although there are many different types of sugar that a yeast may encounter in its environment, it makes sense to only express the RNA and protein to consume the available type of sugar rather than waste energy maintaining every RNA and protein required to break down every type of sugar.
For a variety of structural reasons, mRNA has a very short lifespan compared to DNA.
There are many hydrogen bonds holding a DNA molecule together. While the bases in a RNA molecule can hydrogen bond with each other, usually far fewer bonds can form compared to a DNA molecule. Fewer hydrogen bonds means a less stable structure.
Second, the extra hydroxyl group in the ribose sugar of RNA makes RNA more reactive than DNA. A reaction between that hydroxyl group and another molecule could destroy the RNA molecule.
Finally, proteins that degrade RNA are found everywhere.
If you consider all of these differences in stability between RNA and DNA, you can see why RNA is harder to isolate from a crime scene; however, even if we had isolated any RNA, consider the information we would gain from comparing mRNA samples to DNA samples from the same person.
According to the central dogma, mRNA is merely a temporary copy of the corresponding piece of DNA. Simple sequence analysis would yield the same results, albeit with uracil instead of thymine.
Miss Crimson: Ladies and gentlemen of the jury. I ask you to find my client, Colonel Custard, innocent of murder. The prosecution's main argument against the DNA evidence in this trial was the absence of RNA evidence.
I believe Professor Pear has satisfactorily demonstrated that while RNA, or ribonucleic acid, is a nucleic acid like DNA, it is both structurally and functionally distinct from DNA.
Ribose is the sugar found in RNA instead of deoxyribose like DNA. Uracil is the nitrogenous base found in RNA that bonds with adenine instead of thymine that is found in DNA. Like thymine, uracil forms two hydrogen bonds with adenine.
The central dogma tells us that DNA makes RNA and RNA makes protein.
Three major types of RNA are mRNA, or messenger RNA, that serve as temporary copies of the information found in DNA; rRNA, or ribosomal RNA, that serve as structural components of protein-making structures known as ribosomes; and finally, tRNA, or transfer RNA, that ferry amino acids to the ribosome to be assembled into proteins.
My witness has told you that RNA is inherently unstable and the central dogma states that mRNA is merely a copied version of DNA, so lack of RNA evidence is irrelevant. The absence of my client's DNA at the crime scene and the presence of Mr. Teal's should exonerate my client. Listen to the evidence and find my client not guilty. I rest my case.
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