Molecular structure of RNA | Macromolecules | Biology | Khan Academy

Summary

This script explores the molecular differences between DNA and RNA, highlighting the transition from a double-stranded DNA to a single-stranded RNA during transcription. It explains the change from deoxyribose to ribose and the replacement of Thymine with Uracil in RNA, emphasizing the evolutionary significance and functional diversity of RNA, including its roles in protein synthesis and regulation.

Takeaways

• 🌟 DNA is a double helix structure composed of two strands, each with a backbone of deoxyribose sugar and phosphate groups, attached to nitrogenous bases.
• 🔍 The absence of a hydroxyl group on the 2' carbon of the sugar distinguishes deoxyribose from ribose, confirming the presence of DNA over RNA.
• 🌿 To conceptually convert DNA to RNA during transcription, replace deoxyribose with ribose by adding a hydroxyl group to the 2' carbon of the sugar.
• 🔄 RNA contains the same nitrogenous bases as DNA, except that uracil replaces thymine, making RNA less stable and more prone to errors than DNA.
• 🧬 The evolutionary significance of uracil in RNA is suggested to be due to its error-prone nature, which was suitable for the early, mutable stages of life.
• 🔬 Thymine in DNA provides greater stability, which is crucial for the accurate storage and transmission of genetic information.
• 📜 Messenger RNA (mRNA) carries genetic information from DNA to the ribosome for protein synthesis, with its orientation being 5' to 3'.
• 🔑 Transfer RNA (tRNA) plays a critical role in translation by carrying specific amino acids and recognizing codons on mRNA through anticodons.
• 🧬 Ribosomal RNA (rRNA) contributes to the structure of ribosomes, the cellular machinery where protein synthesis occurs.
• 🪢 MicroRNAs are short RNA molecules involved in the regulation of gene expression by controlling the translation of other RNA molecules.
• 🧬 The script suggests that RNA may have predated DNA, with the first life forms potentially being self-replicating RNA molecules, highlighting the primordial and ongoing importance of RNA in biology.

Q & A

What is the main difference between the sugar component of DNA and RNA?

The main difference is that DNA contains deoxyribose, which lacks a hydroxyl group on the 2' carbon, while RNA contains ribose, which has a hydroxyl group on the 2' carbon.

How does the presence of a hydroxyl group on the 2' carbon affect the stability of the nucleic acid?

The presence of a hydroxyl group on the 2' carbon in RNA makes it less stable compared to DNA, which has deoxyribose and is more stable for long-term storage of genetic information.

What are the nitrogenous bases found in DNA?

The nitrogenous bases found in DNA are Adenine, Guanine, Cytosine, and Thymine.

How does the nitrogenous base composition differ between DNA and RNA?

In RNA, Uracil replaces Thymine, while Adenine, Guanine, and Cytosine are common to both DNA and RNA.

Why might Uracil be more error-prone than Thymine?

Uracil is more error-prone because it can form less stable bonds and may interact with other molecules more readily than Thymine, which contributes to the instability of RNA.

What is the evolutionary significance of Uracil in RNA?

Uracil may have been suitable in the early stages of evolution when there was a need for more change and less stability. It allowed for a higher error rate and more variability.

What is the role of messenger RNA (mRNA) in the process of transcription?

Messenger RNA (mRNA) carries the genetic information from DNA to the ribosome, where it serves as a template for protein synthesis during translation.

What is the purpose of transfer RNA (tRNA) in the translation process?

Transfer RNA (tRNA) carries specific amino acids to the ribosome and pairs with the mRNA's codons through its anticodon, facilitating the assembly of proteins.

What is the structural difference between DNA and RNA strands during transcription?

During transcription, DNA is double-stranded with an antiparallel orientation, while the mRNA produced is single-stranded and has a 5' to 3' orientation.

Why might the instability of RNA be beneficial in certain cellular processes?

The instability of RNA can be beneficial because it allows for rapid turnover of molecules like mRNA, preventing them from accumulating and ensuring that only active and necessary proteins are being produced.

What are some other types of RNA molecules besides mRNA and tRNA?

Other types of RNA molecules include ribosomal RNA (rRNA), which forms part of the ribosome structure, and microRNA (miRNA), which can regulate the translation of other RNA molecules.

Outlines

00:00

🌟 DNA to RNA: The Molecular Transformation

This paragraph explains the molecular structure of DNA, highlighting the double helix formation with two strands made of deoxyribose sugar. It distinguishes DNA from RNA by the absence of a hydroxyl group on the 2' carbon of deoxyribose, which is present in ribose found in RNA. The transformation from DNA to RNA during transcription is illustrated by adding a hydroxyl group to the 2' carbon of the sugar backbone and replacing Thymine with Uracil in the nitrogenous bases. The differences in stability between Uracil and Thymine are discussed, suggesting that Uracil's error-prone nature might have been beneficial in the early stages of evolution when RNA predated DNA.

05:00

🔍 The Evolutionary Role of Uracil in RNA

The second paragraph delves into the evolutionary significance of Uracil in RNA, contrasting its instability with the stability of Thymine in DNA. It suggests that the less stable nature of Uracil was initially advantageous for the dynamic early RNA world, allowing for more variability and adaptation. As information transfer became more critical, Thymine emerged to stabilize DNA. The paragraph also ponders why Uracil persists in RNA, considering its role in the transient nature of messenger RNA (mRNA) and its importance in the cell beyond mere information transfer, such as in tRNA and other RNA types.

10:01

🧬 RNA: Beyond the Intermediary

The final paragraph emphasizes the importance of RNA beyond its role as an intermediary in protein synthesis. It discusses the various types of RNA, including messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA), and microRNA, each with distinct functions in the cell. The paragraph also touches on the hypothesis that RNA may have predated DNA, suggesting that the first life forms could have been self-replicating RNA molecules, and that DNA evolved from RNA, with RNA retaining its utility in various cellular processes.

Keywords

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. In the video, DNA is described as having a double helix structure composed of two strands, with a backbone made of deoxyribose sugar and phosphate groups, and nitrogenous bases. The script explains the transition from DNA to RNA during transcription, highlighting the molecular differences between the two.

💡Double Helix

The double helix is the iconic structure of DNA, resembling a twisted ladder. This structure is crucial for DNA's function, as it allows the two strands to separate during replication and transcription. The video script describes the double helix and its significance in the context of DNA's molecular structure.

💡Deoxyribose

Deoxyribose is a five-carbon sugar that is a fundamental component of DNA's backbone. It is distinguished from ribose by the absence of an oxygen atom on the 2' carbon. The script explains how the presence of deoxyribose indicates the molecule is DNA, not RNA.

💡Nitrogenous Bases

Nitrogenous bases are the building blocks of DNA and RNA, providing the genetic information. They include adenine, guanine, cytosine, and thymine (in DNA) or uracil (in RNA). The video script discusses the specific pairing of these bases and their role in the structure and function of DNA and RNA.

💡Transcription

Transcription is the process by which the genetic information in DNA is copied into RNA, specifically messenger RNA (mRNA). The video script describes how during transcription, one strand of DNA serves as a template for the creation of a complementary RNA strand.

💡Messenger RNA (mRNA)

mRNA is a type of RNA that carries the genetic information from DNA for protein synthesis. The video script illustrates the process of transcription, where mRNA is formed as a single-stranded copy of a DNA sequence, with uracil replacing thymine.

💡Ribose

Ribose is the sugar component found in RNA, differing from deoxyribose by the presence of an oxygen atom on the 2' carbon. The script explains the transformation of deoxyribose to ribose as a key step in the conversion from DNA to RNA.

💡Uracil

Uracil is a nitrogenous base unique to RNA, replacing thymine found in DNA. The video script discusses the substitution of thymine with uracil in the process of transcription, which is a key distinction between DNA and RNA.

💡Transfer RNA (tRNA)

tRNA is a type of RNA that plays a critical role in protein synthesis by carrying amino acids to the ribosome, where they are assembled into proteins based on the sequence of the mRNA. The script describes tRNA's structure and function, including its anticodon and how it pairs with mRNA codons.

💡Hydrogen Bonds

Hydrogen bonds are a type of chemical bond that plays a crucial role in the structure of nucleic acids. In the script, hydrogen bonds are described as the means by which nitrogenous bases pair in DNA and RNA, such as adenine with uracil or thymine.

💡Purines and Pyrimidines

Purines and pyrimidines are two types of nitrogenous bases. Purines, which include adenine and guanine, have a double-ring structure, while pyrimidines, such as cytosine, thymine, and uracil, have a single ring. The video script explains the distinction between these two types of bases and their pairing in DNA and RNA.

💡Ribosomal RNA (rRNA)

rRNA is a type of RNA that is a component of ribosomes, the cellular structures where protein synthesis occurs. The script mentions rRNA as part of the discussion on the various roles RNA plays in the cell, beyond being an intermediary for genetic information.

💡MicroRNA

MicroRNAs are small, non-coding RNA molecules that regulate gene expression. The video script briefly mentions microRNAs as an example of the diverse roles RNA can have, including the regulation of other RNA molecules' translation.

Highlights

DNA's molecular structure is depicted as a double helix formed by two strands.

The five-carbon sugar in DNA's backbone is deoxyribose, identified by the absence of a hydroxyl group on the 2' carbon.

To transform DNA into RNA during transcription, deoxyribose is modified to ribose by adding a hydroxyl group on the 2' carbon.

RNA contains different nitrogenous bases compared to DNA, specifically uracil replaces thymine.

Uracil is less stable than thymine, making RNA more error-prone and potentially beneficial for early evolutionary stages.

The instability of RNA with uracil may be advantageous for its transient roles in the cell.

RNA molecules, including mRNA, play a crucial role in transcribing information from DNA.

mRNA has a 5' to 3' orientation, opposite to the DNA strand from which it is transcribed.

Transfer RNA (tRNA) carries amino acids and has anticodons that pair with mRNA codons during translation.

tRNA's unique structure allows it to attach to specific amino acids and facilitate protein synthesis.

Ribosomal RNA (rRNA) provides structural support within ribosomes for the translation process.

MicroRNAs are short RNA chains that regulate the translation of other RNA molecules.

RNA's importance extends beyond being an intermediary, with potential roles in the origin of life.

The evolutionary history suggests that RNA may have predated DNA, with DNA evolving for increased stability.

The differences between DNA and RNA highlight the unique roles each plays in the molecular biology of cells.

Understanding the transition from DNA to RNA during transcription is fundamental to molecular biology.

The various types of RNA, including mRNA, tRNA, rRNA, and microRNA, each have distinct functions in cellular processes.

Transcripts

- [Voiceover] We've already payed a lot of attention

to the molecular structure of DNA.

In fact right depicted in front of us, we have two strands

of DNA forming a double helix,

and we can look at the telltale signs that this is DNA.

In particular, we can look at the five-carbon sugar

on it's backbone.

We see, and let's actually number the carbons.

This is 1', 2', 3', 4', 5'.

We can see on the 2' carbon

we don't have an oxygen attached to it.

We don't have a hydroxyl group attached to it,

and because of that, we know that this is not ribose.

This is deoxyribose. This right over here is deoxyribose.

And these two are also deoxyribose,

so that tells us that we have two strands

of DNA, deoxyribonucleic acid.

So let me write this down.

This part of the chain,

this is derived from a deoxyribose being attached

to phosphate groups and a nitrogenous base.

So deoxyribose.

So, what would we have to do if we wanted,

instead of viewing this

as two strands of DNA in a double helix formation,

how would we have to edit the left hand strand,

if instead we wanted to imagine

that the left hand strand is a messenger RNA

being generated during transcription

with a single strand of DNA here on the right?

Well, to turn this into RNA, or to make it look like RNA,

on the 2' carbon, well, we want to turn the deoxyribose

into just ribose, so we would want to add

a hydroxyl group right over here.

So I add a hydroxyl group over there,

actually do the hydrogens in white.

So add one hydroxyl group there,

and I want to do on all the sugars

on the left strand's backbone

if I want this to be a single strand of RNA,

and RNA tends to be single stranded.

So oxygen, and then a hydrogen.

So adding this hydroxyl group instead of

just having another hydrogen,

this tells us that this sugar is no long deoxyribose.

This is ribose.

So we now have ribose in our backbone,

which is a telltale sign that

at least now we have the backbone of RNA, ribonucleic acid,

versus DNA, deoxyribonucleic acid.

Now, you might think we're done, but we're not quite done,

because the nitrogenous bases on RNA are slightly different

than the nitrogenous bases on DNA.

On DNA, your nitrogenous bases are

Adenine, Guanine.

Adenine and Guanine are the two ringed nitrogenous bases.

Right over here, this is Adenine.

This is Guanine.

And you also have Cytosine

I'm gonna do these all in different colors.

Cytosine and Thymine.

And this right over is Cytosine,

and this is Thymine,

Cytosine and Thymine are single ringed nitrogenous bases.

We called them pyrimidines.

Adenine and Guanine, we call them purines.

This is a little bit of a review.

In RNA, you still have Adenine.

You still have Guanine.

You still have Cytosine,

but instead of Thymine, you have a very close relative,

and that is Uracil.

So the way that this is drawn right now,

this nitrogenous base, remember when we started this video,

it was double stranded DNA,

this nitrogenous base right over here is Thymine,

and it forms hydrogen bonds with Adenine right over here.

If I want to turn it to Uracil,

I just have to get rid of this methyl group right over here,

so if I just do this and replace it with a Hydrogen,

that is just implicitly bonded there,

well, now I'm dealing with Uracil.

So you see that Uracil and Thymine are very close molecules

or very similar nitrogenous bases,

and that's why they can play a very similar role.

And it's still the case, what Uracil pairs with,

it pairs with Adenine, the same thing Thymine pairs with.

And everything else is, of course, still the same.

An interesting question is why Uracil? Why not Thymine?

Or you can say why Thymine? Why not Uracil?

And based on what I've read, it actually turns out

that Uracil is a little bit more error prone.

It might be able to bond with other things.

When you're coating, it's a little less stable than Thymine.

So Uracil makes the RNA molecule,

or actually makes the machinery of information transfer,

it makes it less stable.

It's a less stable way to transfer information.

Based on what I've read, in evolutionary history,

RNA molecules, most people believe, predate DNA molecules.

So in the early stages, you had a lot of change,

and so Uracil molecules were just fine,

and there was a lot of errors and whatever else.

But then information needed to be a little more persistent

and a little less error prone, well then,

Thymine helped stabilize things.

There's also the view, "why has Uracil stuck around?"

Well, RNA molecules, they have all of these roles in cells.

Messenger RNA molecules are taking information

from the DNA and getting it transcribed

or getting it translated at the ribosome.

But they shouldn't hang out forever.

You actually want them to be somewhat unstable.

So it's an interesting question to think about.

Why do we have Uracil instead of Thymine,

or why do we have Thymine instead of Uracil?

But this is one of the telltale signs of,

that we are now dealing with an RNA molecule.

So now what we have on the left hand side,

Now, all of this business,

actually let me do this in a different color.

all of this business, this strand right over here,

we can now, the way it's drawn,

we can now consider this an RNA molecule,

and if we assume that this is happening during transcription

where a single strand of DNA would want

to replicate it's information,

then this over here would be mRNA, messenger RNA,

and so what's going on here?

Well, let's think about it.

The messenger RNA, the way it's oriented,

if we go, we have phosphate group,

then we go to 5' carbon, 4', 3', then phosphate group,

then 5', 4', 3', then phosphate group,

so this is oriented 5' on top, 3' on the bottom,

while these DNA molecules are oriented the other way.

This is a 5' carbon. This is a 3' carbon,

so we have phosphate, 3', 5', phosphate,

so we have 3' is on top, and 5' is on the bottom.

So if we wanted to think about what's happening,

maybe using the symbols for the nitrogenous bases,

we could say, all right we have our mRNA molecule here,

and this is it's 5' end, and this is it's 3' end,

and then the top nitrogenous base over here, this is Uracil.

And then the second one over here, this is Cytosine.

This is Cytosine.

This is Cytosine over here, and this is being transcribed

from this DNA molecule on the right hand side,

so this is DNA, and this DNA has an antiparallel orientation

It's parallel, but it's kinda flipped over.

The sugars are pointed in a different direction,

so this is going from the 3' end.

This is the 5' end.

And we see that the Uracil is hydrogen bonded to Adenine.

That is Adenine

And I'll draw dotted lines to show the hydrogen bonds.

And that the Cytosine is hydrogen bonded to Guanine.

So this right over here, that is Guanine.

Actually I'll do the hydrogen bonds in white.

Actually there's multiple hydrogen bonds going on here,

but just to be clear, this is mRNA,

and on the right, we have DNA.

This could be happening during transcription.

Now, what are the types of RNAs out there?

We've talked about this in other videos.

Well, you have messenger RNA, which has an important role

in taking information from DNA

and getting it eventually translated

with the help of tRNAs in ribosomes,

and though I've just mentioned another type of RNA,

and that's transfer RNA, so transfer RNA, tRNA.

And in the overview video on transcription and translation,

we talk about how tRNA does this,

but it has amino acids attached on one end,

and then it has anticodons on the other end

that essentially pair with codons on the mRNA,

and, then, thus allows it to construct proteins.

And actually, this right over here is a visualization

of a tRNA molecule.

So a lot of times when we think about DNA,

we think about, okay, mRNA or RNA is an intermediary

to be able to eventually translate it into proteins,

and that is often the case, but sometimes,

you also just want the RNA itself.

The RNA itself plays a role in the cell

beyond just transmitting information,

and that's an example here with tRNA.

And you can see it's an interesting configuration,

where the amino acid will attach roughly in that area,

and then you see the anticodon

right down here in the bottom right,

and different tRNA molecules will attach

to different amino acids,

and they'll have different anticodons here.

So this is another use for RNA,

and then others include ribosomal RNA,

and they actually play a structural role in ribosomes,

which is where translation occurs.

And you also have things called microRNA,

which are short chains of RNA,

which could be used to regulate the translation

of other RNA molecules.

So DNA gets a lot of the attention,

but RNA is really, really, really important,

and a lot of people believe that RNA came first,

and there's potential that the first life

or pseudo-life ever was just self-replicating RNA molecules,

and that DNA eventually evolved from RNA,

but RNA stuck around, because it's still very useful.