BIOLOGY CHAPTER 11 - Nucleic Acids=DNA and RNA

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BIOLOGY CHAPTER 11 Nucleic Acids: DNA and RNA DNA: Chemical Structure of Nucleic Acids & Phosphodiester Bonds Chapter 11 / Lesson 1 Transcript n this lesson, you'll discover what nucleotides look like and how they come together to form polynucleotides. We'll also explore nucleic acids and focus on DNA in particular. In a cell, nucleic acids are represented by two separate yet equally important forms: the DNA that stores information in the nucleus and RNA that is used to translate that information into proteins. These are their stories. Nucleic Acids Miss Crimson: Good day, ladies and gentlemen of the jury. My name is Miss Crimson, and I'm here to prove to you without a shadow of a doubt that my client, Colonel Custard, is not guilty of the most heinous crime of murdering poor Mr. Bones. The host of the International Cookbook Writers Convention was cruelly murdered with a lead pipe in a spiral staircase while on his way to deliver his award-winning cookbook to the kitchen. However, the defendant, Colonel Custard, was not responsible. The testimony of my expert witness will not only clear my client of all wrong- doing but will also reveal the identity of the true killer of our poor departed Mr. Bones. Your honor, I call to the stand Professor Pear, an expert in DNA structure and function! Professor, could you please explain to us what DNA is? Professor Pear: Why, yes. I'd be happy to tell you about DNA. DNA stands for 'deoxyribonucleic acid,' and it's a fascinating molecule. It's one of two basic types of nucleic acids, the other being RNA, or 'ribonucleic acid.'Nucleic acids are the molecules that cells use to store, transfer and express genetic information. The Functions of DNA Miss Crimson: That's very nice, Professor, but could you elaborate on the DNA molecule? Professor Pear: Yes, yes, of course. It's the molecule that stores genetic information in an organism. It's essentially providing directions… like a recipe, if you will… for pretty much everything that makes us, well, us. If proteins are like the building blocks for structures, enzymes and other cool things in a cell, DNA is like a recipe that tells a cell how to create those building blocks. The three components of a nucleotide

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BIOLOGY CHAPTER 11 - Nucleic Acids=DNA and RNA

Transcript of BIOLOGY CHAPTER 11 - Nucleic Acids=DNA and RNA

Page 1: BIOLOGY CHAPTER 11 - Nucleic Acids=DNA and RNA

BIOLOGY CHAPTER 11 Nucleic Acids: DNA and RNA

DNA: Chemical Structure of Nucleic Acids &

Phosphodiester Bonds Chapter 11 / Lesson 1 Transcript

n this lesson, you'll discover what nucleotides look like and how they come together to form polynucleotides. We'll

also explore nucleic acids and focus on DNA in particular.

In a cell, nucleic acids are represented by two separate yet equally important forms: the DNA that stores information in the nucleus and RNA that is used to translate that information into proteins. These are their stories.

Nucleic Acids

Miss Crimson: Good day, ladies and gentlemen of the jury. My name is Miss Crimson, and I'm here to prove to you without a shadow of a doubt that my client, Colonel Custard, is not guilty of the most heinous crime of murdering poor Mr. Bones. The host of the International Cookbook Writers Convention was cruelly murdered with a lead pipe in a spiral staircase while on his way to deliver his award-winning cookbook to the kitchen. However, the defendant, Colonel Custard, was not responsible. The testimony of my expert witness will not only clear my client of all wrong-doing but will also reveal the identity of the true killer of our poor departed Mr. Bones.

Your honor, I call to the stand Professor Pear, an expert in DNA structure and function! Professor, could you please explain to us what DNA is?

Professor Pear: Why, yes. I'd be happy to tell you about DNA. DNA stands for 'deoxyribonucleic acid,' and it's a fascinating molecule. It's one of two basic types of nucleic acids, the other being RNA, or 'ribonucleic acid.'Nucleic acids are the molecules that cells use to store, transfer and express genetic information.

The Functions of DNA

Miss Crimson: That's very nice, Professor, but could you elaborate on the DNA molecule?

Professor Pear: Yes, yes, of course. It's the molecule that stores genetic information in an organism. It's essentially providing directions… like a recipe, if you will… for pretty much everything that makes us, well, us. If proteins are like the building blocks for structures, enzymes and other cool things in a cell, DNA is like a recipe that tells a cell how to create those building blocks.

The three components of a nucleotide

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Miss Crimson: So, you're saying that everyone has this DNA stuff inside his or her body?

The Structure of a Nucleotide

Professor Pear: Oh my, yes. DNA is an integral part of every organism, just like other types of organic molecules found in our body, such as carbohydrates, lipids and proteins.But the structure of DNA is very distinct from those other types of organic molecules. Deoxyribonucleic acid gets its name from the fact that DNA possesses a sugar called deoxyribose. That sounds like a fancy name, but really it's very easy to remember that it's a sugar like other sugars you may have encountered, such as sucrose and fructose. Just remember that all sugars end in '-ose.' A monomer of DNA is called a nucleotide.

Miss Crimson: Professor, can you please restate that definition in simpler terms?

Professor Pear: Oh, I'm sorry. A monomer is just the most basic subunit of a complex molecule. That makes the nucleotide the most basic subunit of DNA, or, more generally, of any nucleic acid. A nucleotide is composed of three things: a sugar (in the case of DNA, the deoxyribose I mentioned earlier), a phosphate group and a nitrogenous base. As you can see in this diagram, the phosphate group and nitrogenous base are both attached to the sugar. When a nucleotide is part of a polynucleotide chain, it is often referred to as a base.

Phosphodiester Bonds

Oh, um, did I forget to tell you what a polynucleotide is? A polynucleotide is simply a long chain of nucleotides. 'Poly-' means 'many', so 'polynucleotide' literally means 'many nucleotides.' Now, you're probably wondering how these nucleotides are held together. Nucleotides in a polynucleotide molecule are held together by the bond between the phosphate group of one nucleotide and the sugar of a second nucleotide. A hydroxyl group (or -OH) in the sugar and one of the oxygen atoms in the phosphate group form what's called a diester bond. For this reason, the bond between the phosphate group and the sugar in a polynucleotide molecule is called aphosphodiester bond.

A phosphodiester bond holds nucleotides together

Linking nucleotides together forms a structure that looks like half of a ladder that was cut down the middle with an axe. The phosphate groups and sugars form the backbone of the ladder, while the nitrogenous bases provide the rungs. There are four different types of nitrogenous bases, and the order of the bases in a polynucleotide is what tells a cell how to make a structural protein, an enzyme or any other number of important components of a healthy, functioning organism.

Lesson Summary

Miss Crimson: Okay, Professor Pear, let me stop you right there. You've been throwing around a lot of fancy jargon. Before you go any further, let me see if I can summarize for the jury what you've told us so far.

Nucleic acids are the molecules cells use to store, transfer and express genetic or hereditary information. Nucleic acids can be categorized into DNA, or deoxyribonucleic acid, and RNA, or ribonucleic acid. DNA is the molecule that stores genetic information in an organism.

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A single monomer subunit of a nucleic acid is called a nucleotide. A DNA nucleotide is composed of a sugar, called deoxyribose, a phosphate group and a nitrogenous base. A polynucleotide is a nucleic acid molecule consisting of a long chain of nucleotides. Nucleotides in a polynucleotide molecule are held together by a bond between the phosphate group of one nucleotide and the sugar of a second nucleotide, which is called aphosphodiester bond.

DNA: Adenine, Guanine, Cytosine, Thymine &

Complementary Base Pairing Chapter 11 / Lesson 2 Transcript

Learn the language of nucleotides as we look at the nitrogenous bases adenine, guanine, cytosine and thymine.

Armed with this knowledge, you'll also see why DNA strands must run in opposite directions.

Nucleic Acids Review

Previously on DNA & RNA:

Miss Crimson: The testimony of my expert witness will not only clear my client of all wrongdoing, but will also reveal the identity of the true killer of our poor departed Mr. Bones.

Professor Pear: Nucleic acids are the molecules that cells use to store, transfer and express genetic information. DNA stands for deoxyribonucleic acid. It's the molecule that stores genetic information in an organism. That makes the nucleotide the most basic subunit of DNA, or, more generally, of any nucleic acid.

The four nitrogenous bases in DNA

The Function of DNA

Miss Crimson: So, Professor, you told us that a DNA nucleotide consists of a phosphate group, a sugar and a nitrogenous base. Can you tell us how nucleotide structure pertains to the case at hand?

Professor Pear: Oh, yes. You see, you need to understand the chemistry behind DNA to fully appreciate the importance and function of the molecule. The phosphate group and sugar are the same in every nucleotide, but there are four differentnitrogenous bases: guanine, adenine, thymine and cytosine. They are often abbreviated by the first letter of each nitrogenous base: G, A, T and C.

They essentially function as a four-letter alphabet. Or, if I may make an analogy to the case at hand, the information in DNA is like a recipe in one of our poor victim's cookbooks. 'Reading' the DNA code ultimately tells a cell how to make proteins that it can use to perform various functions necessary for life. For instance, reading a specific sequence of DNA tells one cell how to make hemoglobin protein to carry oxygen molecules throughout the body. On the other hand, another cell might read a different recipe, which tells it how to make insulin protein to control blood sugar levels. Oh, and 'reading', or transcribing, DNA is really an intriguing process.

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Pyrimidines and Purines

Miss Crimson: Yes, Professor, I'm sure DNA transcription is very interesting, but let's stick to the basic characteristics of DNA that pertain to the trial at hand. You were telling us about the nitrogenous bases.

Cytosine bonds with guanine and adenine bonds with thymine

Professor Pear: You're quite right. The bases can be categorized into two different groups. The single-ring nitrogenous bases, thymine and cytosine, are calledpyrimidines, and the double-ring bases, adenine and guanine, are called purines. (Miss Crimson has a puzzled look.) I guess you might wonder how I can remember that, but it's really quite simple. 'All Gods are pure.' Adenine and guanine are purines. And, by process of elimination, that means cytosine and thymine have to be pyrimidines. See?

Miss Crimson: Yes, yes. That's a very nice mnemonic aid. Adenine and guanine are purines, but we're getting off track. You were telling us why the chemical structure of nucleotides is important.

Complementary Base Pairing

Professor Pear: Oh, yes. The chemistry of the nitrogenous bases is really the key to the function of DNA. It allows something called complementary base pairing. You see, cytosine can form three hydrogen bonds with guanine, and adenine can form two hydrogen bonds with thymine. Or, more simply, C bonds with G and Abonds with T. It's called complementary base pairing because each base can only bond with a specific base partner. The structures complement each other, in a way, like a lock and a key. C will only bond with G and Awill only bond with T in DNA. Because of complementary base pairing, the hydrogen-bonded nitrogenous bases are often referred to as base pairs.

DNA Strands are Antiparallel

The sugar and phosphate ends of a DNA strand are referred to by their

carbon numbers

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Remember how I said that DNA polynucleotides look like half of a ladder? Well, hydrogen bondingcompletes the ladder. Since the nitrogenous bases can hydrogen-bond, one polynucleotide can bond with another polynucleotide, making the nitrogenous bases the rungs of the ladder. Each polynucleotide participating in this ladder is often referred to as a strand. Because the bases can only fit together in a specific orientation, a parallel orientation between the strands won't work. The strands must be antiparallel, or upside-down, relative to one another.

Miss Crimson: What do you mean antiparallel?

Professor Pear: Well, remember that the backbone is made of phosphate groups and sugars. Therefore, each strand will always have a phosphate at one end and a sugar at the other end. Rather than having to refer to the phosphate or sugar end, scientists simply refer to the ends of the DNA by the closest carbon in the sugar ring. Since the carbons in the sugar are numbered one to five, the sugar end of the strand is called the 3' end and the phosphate end of the strand is called the 5' end. Remember that complementary base pairing works like a lock and key, so there's only one orientation in which hydrogen bonding will work. If you try to orient the two strands parallel to each other, the sugar ends of the polynucleotides are both at one end and the phosphate groups are at the other end. However, the nitrogenous bases can't hydrogen-bond in this orientation. The key can't fit into the lock.

DNA strands are antiparallel to one another to allow for hydrogen

bonding

For hydrogen bonding to work, the two DNA strands must run in opposite directions. The 3' end of one strand can hydrogen-bond with the 5' end of the other strand. If we represent the strands as arrows with the arrowhead at the 3' end of the stand, we can see that the strands in a DNA molecule are organized antiparallel relative to each other.

Lesson Summary

Miss Crimson: Okay. Let me stop you again, Professor, so I can summarize your testimony for the jury.

There are four nitrogenous bases found in DNA that are called guanine, adenine, thymine and cytosine. They are abbreviated by the first letter in their name, or G, A, T and C. The bases can be divided into two categories: Thymine and cytosine are called pyrimidines, and adenine and guanine are called purines. Each nucleotide base can hydrogen-bond with a specific partner base in a process known as complementary base pairing: Cytosine forms three hydrogen bonds with guanine, and adenine forms two hydrogen bonds with thymine. These hydrogen-bonded nitrogenous bases are often referred to as base pairs.

Because of the alternating nature of the phosphate groups and sugars in the backbone of nucleic acids, a nucleic acid strand has directionality. The end of a nucleic acid where the phosphate group is located is called the 5' end. The end of the nucleic acid where the sugar is located is called the 3' end. Finally, DNA strands are antiparallel, meaning that the strands in a DNA molecule are parallel, but are oriented in opposite directions. Essentially, the 5' end of one strand pairs with the 3' end of the other strand.

To be continued . . .

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DNA: Double Helix Structure and Hereditary Molecule Chapter 11 / Lesson 3 Transcript

This lesson will help you to navigate the twists and turns of DNA's structure. We'll also clue you in on the amazing

discoveries that put this nucleic acid in the limelight as the molecule of heredity.

Previously, on 'DNA and RNA':

'…but we'll also reveal the identity of the true killer of our poor departed Mr. Bones.'

'…but there are four different nitrogenous bases: guanine, adenine, thymine and cytosine.'

'You see, cytosine can form three hydrogen bonds with guanine, and adenine can form two hydrogen bonds with thymine.'

'If we represent the strands as arrows, with the arrowhead at the three prime end of the strand, we can see that the strands in a DNA molecule are organized antiparallel relative to each other.'

Discovery of the Transforming Principle

Miss Ivory: Objection! How do we even know that what he's saying is true? Professor, you say that deoxyribonucleic acid, or DNA, is the recipe for life, but how do we know you aren't just throwing around your fancy science word in a big smokescreen to clear the accused, Colonel Custard, of this crime? Can you offer proof that DNA is the molecule responsible for transmitting heritable traits in organisms? Answer me that?

Professor Pear: Oh, yes. I certainly can! There's lots of evidence, but let me just summarize the work of Frederick Griffith and Oswald Avery. Their labs provided strong evidence that DNA is the molecule of heredity. Griffith was studying the bacterium that causes pneumonia. He observed two variants of the same bacteria under the microscope. One had a smooth outer appearance. Let's abbreviate those bacteria as 'S bacteria.' The other had a rough appearance. Let's abbreviate those bacteria as 'R bacteria.'

If he injected a mouse with S bacteria, the mouse died. If he injected a mouse with R bacteria, the mouse lived. We would later discover that the S bacterium is smooth because it has a protective coating, which helped it survive in the mouse; whereas, the R bacterium lacked a coating and was susceptible to the mouse's immune system. If Griffith heated the S or R bacteria, it killed the bacteria. Not surprisingly, a mouse injected with the dead bacterial parts did not die. Oddly though, if he mixed dead cell parts from S bacteria with living R bacteria and injected a mouse, the mouse died! Isn't that peculiar? But that's not all.

When he examined the dead mouse more closely, he found S bacteria, not R, inside the corpse! Somehow the R bacteria had transformed into S bacteria. This transformation was permanent, meaning it was a trait that was inherited from generation to generation. Using the mysterious new S bacteria from the dead mouse in a new injection experiment also produced a dead mouse. That meant that the R bacteria had permanently been changed into S bacteria.

DNA is the Molecule of Heredity

Oswald Avery dubbed Griffith's mysterious substance that transformed the R bacteria into S bacteria as the 'transforming principle'. He decided he and his lab would determine the identity of this substance.

I should probably provide a little context for this time in scientific history. At the time of Avery's experiments, most scientists believed DNA to be uninteresting compared to proteins and less complex than even carbohydrates or lipids. For instance, there were twenty known amino acids but only four types of DNA nucleotides. Surely a molecule of such simple complexity couldn't be the molecule of heredity.

Avery spent many years purifying the transforming principle, and then tried to characterize which type of molecule was responsible for transforming R bacteria into S bacteria. To identify the molecule that could transform the R

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bacteria into S bacteria, Avery and his team devised a series of clever experiments. By spinning the purified sample very fast in a machine, known as a centrifuge, fats were eliminated from the sample. Sample treated in this manner could still transform R cells into S cells.

Shockingly, treating the sample with something that degrades protein did not affect the ability of the purified sample to turn R cells into S cells. Treating the sample with a substance that degrades RNA also did not affect the transforming principle. However, treating the purified sample with something that degraded DNA eliminated the R to S transformation.

Spinning the bacteria in a centrifuge did not stop transformation

Avery concluded that the transforming principle was DNA. Today, we know that permanently changing the characteristics of an organism can be accomplished by changing its DNA content.

Discovery of the Double-Helix Structure

Miss Ivory: Well, what about your assertion that you know the structure of DNA? You, yourself, said that it's a microscopic molecule found inside a cell. If that is true, how can you possibly know what it looks like?

Professor Pear: Oh, actually that's another fascinating story. 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. Examining X-ray images of DNA revealed that the molecule had a helical, or spiral, shape. Data from another lab indicated that there is a one-to-one ratio between adenine and thymine. The lab also demonstrated that there is a one-to-one ratio between guanine and cytosine. By using cardboard cutouts of the bases, Watson realized that two hydrogen bonds could form between A and T and three hydrogen bonds could form between G and C.

Interestingly, Watson originally predicted there were only two hydrogen bonds between G and C, but we know now that there are three. Even a Nobel Prize-winning scientist is wrong sometimes! In order to reconcile the X-ray data and Watson's model of the ratio between bases, Crick realized that the DNA strands had to be oriented antiparallel to one another. This led the two of them to postulate the famous double-helix structure of DNA.

A helix is a cylindrical spiral. A double-helix is basically just two cylindrical spirals. Picture DNA as a ladder with backbones made of phosphate groups and sugars and rungs of nitrogenous bases held together by hydrogen bonds. Then, twist the ladder around an imaginary central axis. The structure of the molecule looks a little like a spiral staircase, not unlike the one on which the body of the victim was discovered.

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The helix and double-helix DNA structures

James Watson, Francis Crick and another scientist named Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine for 'their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.'

DNA is a Four-Letter Language

Miss Crimson: Madam Prosecutor, if you're done attempting to rattle my witness, I'd like to continue. Now, Professor, you've provided us with quite a nice foundation about the structure and function of nucleic acids. In fact, I wouldn't be surprised if we'd all be able to pass a biology test after this. But let's return to the case at hand. How can DNA be used to vindicate the innocent and catch a criminal?

Professor Pear: Well, you can think of the different DNA bases as letters in an alphabet. The DNA alphabet is just a simple alphabet with only four letters. Letters can be combined to form different words, sentences and paragraphs. Likewise, the order of DNA base pairs determines what biological molecules can be produced and, in turn, the characteristics that makes each of us unique individuals. There are approximately three billion base pairs in each human cell.

With the exception of twins, the chance that a sequence of DNA for any one person is exactly the same as another person is pretty much impossible simply due to the variation present from person to person. Consider that there's a one-in-four chance that someone has the same base pair at a given point in a piece of DNA as another person. There's another one-in-four chance at the next base pair and so on. If you examine enough pieces of DNA, it becomes statistically impossible to mistake one person's DNA for another person's.

My lab compared DNA sequence from Colonel Custard, the blood found at the scene of the crime and the victim. We found no DNA from Colonel Custard at the scene of the crime. However, we did find DNA from the blood of another person attending the conference mixed with the victim's blood.

Miss Crimson: Thank you for your expert testimony, Professor.

Lesson Summary

Ladies and gentlemen of the jury, you can see that the work of Fredrick Griffith and Oswald Avery establishes DNA as the molecule responsible for transmitting heritable traits.

James Watson and Francis Crick developed the double-helix model for the structure of DNA. In short, DNA is organized as a twisted ladder with phosphate groups and sugars composing the backbone of the strands, and nitrogenous bases linked by hydrogen bonds make up the rungs. Each strand is oriented antiparallel to the other. With three billion bases in humans, each person has a unique DNA sequence.

Based on the sequence of the DNA found in the blood at the scene of the crime, I contest 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 a lead pipe!

To be continued…

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Differences Between RNA and DNA & Types of RNA

(mRNA, tRNA & rRNA) Chapter 11 / Lesson 4 Transcript

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.

RNA Structure vs. DNA Structure

The structural components of RNA

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 thisribonucleic 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,

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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.

The central dogma states that DNA creates RNA and RNA makes protein

The Function of RNA

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.

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Single-stranded mRNA is created from DNA and uses uracil bases

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?

Molecular Stability

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.

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.

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DNA is more stable than RNA due to its many hydrogen bonds

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.

Lesson Summary

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.