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For more information, please see full course syllabus of Biology
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Lecture Comments (18)

1 answer

Last reply by: Bryan Cardella
Mon Oct 5, 2015 11:10 AM

Post by Syaza Yasirah on October 5, 2015

Hi Mr Cardella,

Sorry to ask another question but if the sequence of a DNA strand is 5'-AGCTGTC-3', would I write the mRNA strand as 3'-UCGACAG-5' or 5'-GACAGCU-3'? Sorry, I am getting really confused with the whole synthesizing of a DNA and reading the strand and how the complementary strand would be read as.

Thank you

2 answers

Last reply by: Syaza Yasirah
Tue Jun 23, 2015 11:58 PM

Post by Syaza Yasirah on June 13, 2015

Hi Mr Cardella,

Thank you for the great lecture! I was wondering if I were to remove a base from a DNA sequence like removing T (from TAC) from CGG TCG TAC AGG TGT CGC CAG, will the amino acid sequence be shortened by several amino acids or be shortened just by one amino acid?

Thank you

2 answers

Last reply by: David Gonzalez
Sat Feb 14, 2015 5:37 PM

Post by David Gonzalez on February 10, 2015

Hi Mr. Cardella.

I recently learned that DNA not only codes for proteins, but that various genes can also code for RNA only (instead of a protein). I've always thought that the sequence was this: nucleotide sequence (gene) --> RNA molecule (transcription) --> amino acid sequence (protein).

How is a nucleic acid formed as a byproduct of a gene?

Thank you.

2 answers

Last reply by: Kan-yun, Lee
Sun Oct 26, 2014 10:15 PM

Post by Kan-yun, Lee on October 26, 2014

I'm curious that how splicesome recognize introns and extrons?

1 answer

Last reply by: Bryan Cardella
Tue Jun 24, 2014 10:08 AM

Post by Enrique Salinas on June 23, 2014

Can you please explain the following problem: in a DNA strand with a base sequence of TCAGTA, what would be its DNA complement??

0 answers

Post by Rebecca Leece on May 26, 2014

These are great lectures--extremely clear and easy to follow.  Thank you!  I also like how you give ways to help remember terms and relationship, like "exons exit the nucleus."  Super helpful.  

3 answers

Last reply by: Bryan Cardella
Sat May 31, 2014 8:33 PM

Post by Laura Mejia on March 8, 2014

I was just wondering what is the difference between an exon and an intron?


Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • The Central Dogma 0:04
    • Transcription
    • Translation
  • RNA: Its Role and Characteristics 2:02
    • Ribonucleic Acid
    • How It Is Different From DNA
  • DNA and RNA Differences 5:00
  • Types of RNA 6:01
    • Messenger RNA
    • Ribosomal RNA
    • Transfer RNA
    • Others
  • Transcription 9:26
    • Process in Which RNA is Made From a Gene in DNA
    • How It's Done
    • Summary of Steps
  • Transcription Steps 11:54
    • Initiation
    • Elongation
    • Termination
  • RNA Processing 21:35
    • Pre-mRNA
    • Modifications
  • Translation 27:01
    • Process in Which mRNA Binds with a Ribosome and tRNA and rRNA Assist
    • Summary of Steps
  • Translation the mRNA Code 28:59
    • Every Codon in mRNA Gets Translated to an Amino Acid
    • Chart Providing the Resulting Translation
  • Translation Steps 32:20
    • Initiation
    • Elongation
    • Termination
  • Mutations 40:22
    • Code in DNA is Subject to Change
    • Why Mutations Happen
    • Point Mutation
    • Insertion / Deletion
    • Duplications

Transcription: RNA

Hi, welcome back to, this is the lesson on RNA.0000

Before we get into the details about RNA, we have to go over the central dogma for biology.0006

A dogma is any critical belief, important component that has to be true for the rest of it to makes sense.0011

Even different faiths have their own dogmas.0020

But for the branch of science biology, the central dogma is that DNA codes for RNA which codes for protein.0023

That has to be true, this is something that has been studied time and time again.0032

It makes all of cellular activity make sense.0038

This is a fundamental truth about how cells work, every single trait in cells comes about0041

because a protein was put together because of the code in RNA that came from the code in DNA, because of this, cells are cells.0047

Transcription is the name of the process that gets you from DNA to RNA.0056

Transcription is really this arrow right here, that is how you get from the coded DNA to RNA.0061

Translation, the next part of it, is how you get from RNA to protein.0071

When RNA responsible for making protein, that is known as translation.0077

Here is a little image illustrating how you actually get from RNA to protein.0081

This thing called a messenger, I will tell you more about that in a little bit.0086

You see sugar phosphate, that is the S and P, here your bases of RNA.0089

You may notice one difference, compared to DNA, if you just watch the DNA lesson.0095

There are U as the bases here instead of T or thymine.0099

It is because uracil is found in RNA as a base instead of thymine.0103

This thing that is shaped as a T is known as transfer RNA of tRNA .0108

Here is little bits of protein that you put together, these are individual amino acids.0113

This green chunk that is a ribosome, that is where translation takes place.0117

RNA, its role and characteristics.0124

RNA stands for ribonucleic acid not deoxyribonucleic acid like DNA was.0126

Ribonucleic acid and its sugars actually has one more oxygen atom than deoxyribonucleic acid.0137

RNA transmits the code from DNA to other parts of the cell.0144

When we look at the average eukaryotic cell t hat has a nucleus, you have got all this DNA in there that does not leave.0148

Unless we are talking about cell division, then DNA is pulled apart.0156

When you talk about just a cell existing and its normal interphase part of its life,0159

you got DNA that is housed in the nucleus and is protected there.0166

The way that you get the code out to the cell parts and to actually make sense of DNA is all because of RNA.0169

In a sense, it is like a photocopy of DNA.0176

How is it different from DNA?0179

It is not just that its role is different, structurally it is very different.0181

When we look at the strands, DNA was double stranded.0185

RNA typically has one strand, single stranded and DNA has two.0188

There are instances where RNA is double stranded.0205

For the purposes of this basic biology class, we will not get into it.0216

If you take a more advanced science class, you will get into double stranded RNA and its function on the cell.0220

Sugars, when we look at the sugars in RNA, it is ribose, that is part of why it has that name ribonucleic acid.0227

The phosphates are the same, the bases are all the same except one but the sugar is different.0237

It is ribose instead of deoxyribose, as you would see in DNA.0243

Finally the bases, you will see the full names in a little bit, once again.0261

The bases in RNA are A, G, C, and U for uracil, instead of thymine.0266

That is part of what differentiates RNA from DNA.0290

There are few different reasons that they are different.0293

One of the major ones is this base difference, uracil replaces thymine.0295

Here is a good diagram that shows you the visual difference.0301

Here on the left, you got RNA, ribonucleic acid, here is that single strand.0304

The backbone we are made of sugar, phosphate.0309

Remember from the DNA lesson, phosphate sugar base your DNA determines your face.0314

You can say the same thing for RNA, phosphate sugar base, your RNA determines your face.0320

There are double stranded DNA here on the right, when we look at the colors inside of it,0326

the only major difference is when you look at the red.0331

On this side you see oranges, here is uracil, that orange base, instead of thymine.0335

Structurally, they are very similar, they are both pyrimidines.0342

To see more about that particular explanation, look in the DNA lesson.0347

The larger bases are purines, the smaller bases are pyrimidines.0351

Here is a good example of how they are different.0357

When we look at the types of RNA, they are categorized and there are few difference names because of their different roles.0363

I’m going to tell you about the three major ones, there are others and I will give you an example of the other ones.0370

mRNA, you are going to hear a lot about that in this lesson, it is messenger RNA.0375

It is really is a messenger from the nucleus.0380

Messenger RNA is a compliment of a particular gene of DNA that will be translated into the amino sequence or protein.0382

Also known as a polypeptide, that is the smallest protein.0390

A messenger RNA, when you look at it, you really can see through the codons what the different amino acids are going to be.0396

There is a chart later on in this lesson that allows you to anticipate what that translation is going to be.0403

rRNA is ribosomal RNA, it is weaved through out of ribosome with protein.0409

Before I move on, I just want to give you a brief kind of image of what you are typically see with the look of these different RNA.0418

mRNA is depicted as just a single strand, this is made with respect to a gene in DNA.0427

You are going to see it look like that.0437

With rRNA, a lot of textbooks will depict it like this.0439

They will just show you a picture of a ribosome with its large subunit and small subunit,0444

and they will do this because rRNA is weaved through out of it.0450

The rest of the ribosome structure are various types of proteins.0458

But the rRNA inside of the ribosome helps catalyze the reactions that happen when translation occurs.0462

rRNA is ribosomal RNA.0469

Finally tRNA, it really does look like a T when you look at the images in a lot of different textbooks.0472

Does it actually look like that, like this perfect little T shape, almost like an interesting kind of looking clothespin.0482

No, it is typically depicted that way to remind us that is tRNA.0489

I will show you a more accurate depiction that shows you how this relates to the more realistic three dimensional structure.0494

tRNA is a transfer RNA that brings amino acids to ribosomes and makes contact with mRNA codons to translate the genetic code.0502

Speaking of amino acids attached, up here you would have an amino acid.0512

Here at the bottom, the other side, you would have what is known as an anti codon.0521

That is why it makes contact with the codons in mRNA.0528

More about that later on in this lesson.0532

A few others, there are multiple kinds of RNA that are not listed here.0534

One of the other ones is snRNA.0538

snRNA you would in spliceosomes, you are going to here what they do with respect to RNA and0545

what after transcription something else has to happen before the mRNA use the nucleus.0552

Other types of RNA, you would definitely have to know about in an advanced placement biology class or an advanced college biology course.0557

Transcription, how do you get from DNA to RNA?0567

The process to get from RNA to DNA involves a lot of enzymes.0570

Just like with getting front having one set of DNA to having two sets of DNA.0575

When we talked about DNA replication in the DNA lesson, a lot of enzymes catalyze those reactions to make that new DNA strand.0582

To make RNA from DNA, it requires enzymes.0590

What transcription does is it gets the genetic code out of the nucleus to a place where it can be translated into its functional form.0595

That functional form is proteinaseus.0601

DNA codes for RNA, it codes for protein at central dogma.0607

This transcription activity would take place in the nucleus of all eukaryotes via numerous enzymes, as I told you before.0611

If it is not eukaryotic, if we are talking about bacteria, prokaryotes, they have a nuclear region0618

where DNA is found, usually near the center and that is where it would happen in them.0624

We tend to focus on eukaryotes, there are a lot of different species of eukaryotes out there for sure.0629

Basic summary of the steps, initiation, elongation, termination.0634

These three you are going to also see with the summary of translation,0639

the next process that gets us from RNA to protein, you are going to see the same names.0645

It helps you simplify thinking about making RNA and then subsequently taking RNA and getting protein out of it.0650

Initiation, something gets started.0658

Elongation in the middle, how do you extend to this RNA from DNA.0660

Finally, termination, how does it end?0665

Initiation, elongation, termination are a good way to think about transcription more simply.0669

The name transcription is really good name for this because when you transcribe something, this is before photocopiers or scanners.0674

Back in the day, if you wanted to make a copy of a transcript of some kind of scroll, you had a scribe that would actually transcribe it.0685

They would be looking at, here is the original, I'm going to make a new copy.0697

It was a very arduous long process, transcription is very much like that.0701

There is a code in DNA, you want to get that DNA code to this other molecule back and leave the nucleus,0706

that is why it is called transcription.0712

Transcription step 1, initiation, as I told you in the previous slide.0716

RNA polymerase and this name says it all, polymerase.0721

Enzymes tend to end with ase, it makes a polymer of RNA by putting together monomers which are just nucleotides.0726

It docks just upstream of the gene, right before the gene begins, and unzips the DNA.0733

You do not need helicase here, in DNA replication you saw that DNA helicase actually unzipped the DNA and made those replication bubbles.0738

But here, as RNA polymerase goes along, it helps bring apart the two sides of DNA.0748

This is a really accurate depiction of the shape of RNA polymerase.0754

All of these, this huge thing with blue helixes and these little other units.0758

They are all different polypeptide chains put together.0765

You can see the DNA that is wrapping around.0768

Here is the portion where it is actually unzipped the DNA and it will move along0772

separating those hydrogen bonds between the bases, and helping to put together the RNA that is copy to one side of it.0778

It unzips the DNA and then it puts together the beginning of the RNA and most importantly, the start codon.0785

This term codon, it means a triplets of bases.0793

Codon equals triplet of bases, three in a row.0800

That is how the words of DNA are read in the sentences that we called genes.0811

I use that analogy in the DNA lesson before.0816

The triplet bases, when you think about all the different bases of RNA, the A, G, C, U, there are 64, combinations that you can have.0820

AAA, UUG, GUA, there are 64 different ones.0828

All of those triplets are known as codons.0834

This one AUG, the start code, the way that I remember that is AUG is short for August, the month of August.0837

Oftentimes, school starts in August.0847

The start codon AUG short for august, that is a good way to remember it.0851

This is the particular RNA you get from reading TAC in DNA.0856

Imagine that these are three bases attached to the backbone of DNA.0863

You have the phosphate sugar here.0868

Here is the few bases of DNA, and as RNA polymerase goes along, it would put a new G.0872

If you are looking at the other side of DNA, you would see a T here.0882

Remember, uracil replaces T when we look at that.0886

Do not think that uracil is complementary to T, it is not.0890

Uracil is complementary to A just as T would, if we are looking at DNA.0896

If it is TAC here, you would see AUG.0901

If we continue to the next codon, which we are going to here more about in the next slide.0903

It might be TCG, and what do we get in mRNA, you will get AGC, and so on.0907

That is how we get started.0926

I said, most importantly the start codon because there is another region that can be put together before that AUG, it is called the UTR.0927

You are going to hear more about that, it is the translated region.0937

It is the beginning of a gene that does not code for protein and ribosome.0942

Most biology courses, the focus on the fact that the start codon exists in the beginning of that RNA,0947

when you make this RNA from the gene in DNA.0953

Elongation, this mRNA after you have the start codon being put together, it is going to continue to be made,0959

as the enzyme goes along the genes.0965

RNA polymerase continues to attach complementary RNA nucleotides corresponding to the DNA template.0968

This DNA template term is very important.0974

There are two sides of DNA, and RNA tends to be single stranded.0978

It is only making it with respect to one side.0982

The other side is not the template, the other side you can call the coding side or the coding strand.0985

The reason why I use that term is template is what it is copy from, you get the RNA from that template, that original.0993

The coding side, you can see the code of RNA in their because if RNA is complementary to one side of DNA,1000

the other side of DNA is going to look identical except for the T's instead of u’s.1008

There is another term down here in this picture, they call the template the antisense strand,1013

meaning it is the opposite sense of what the RNA is going to be.1022

The RNA is going to be the compliment of everything you see here in the antisense.1026

Down here, they call this other side of DNA decent strand because you can see the code for the resulting RNA.1030

If you compare this RNA that is being transcribed, UAC, TAC, UGC, TGC, it is all the same except for the T and U.1038

I like to use the term template encoding strand but either one works.1048

You could see RNA polymerase moving along, putting together these RNA nucleotides.1055

If you are wondering, where are these RNA nucleotides coming from?1060

Is RNA polymerase physically making them?1062

No, the RNA nucleotides are available in the nucleus.1066

RNA polymerase just has to take them and connect them, just make the bond between the neighboring nucleotides.1070

It is just putting together the compliment as it goes along.1077

You got to picture these RNA nucleotides floating around and the enzyme grabs them, and does it very fast.1081

Transcription step 3, however long the gene is, that enzyme is going to go along, put together those complimentary RNA bases.1091

Eventually, it is going to get to the end of the gene.1099

RNA polymerase eventually gets to the end of the gene where it forms the stop codon.1102

There are actually three stop codons.1108

Not just one like we saw with the start.1111

UGA, UAG, or UAA, how do you remember those.1113

I have the another device for you to remember those.1121

They might sound silly but they helped me remember them for years.1126

UGA that can stand for University of Georgia, that is the stop codon.1129

UAG, on the streets of various areas in the United States, you might hear people saying you a G, short for gangster.1136

UAG, it is silly to say but I just picture you a G.1158

UAG that is one of the ways I remember.1164

UAA, it is like a sentence, like U-A-A, like trying to get someone’s attention.1168

University of Georgia, you a G, and U-A-A, those are the ways you can remember the stop codons.1177

Once that stop codon is made when you get to the end of the gene, typically you are going to get detached when it is done.1185

Like I said earlier, with just upstream with the start codon, you can get this region that is un-translated.1194

After the actual stop codon, you can have other RNA nucleotides being put together.1199

Once the stop codon is ready with ribosome, you are going to stop putting together the amino acids.1207

That will make more sense when we talk about how translation works.1213

The stop codon you are going to find that at the end of the gene.1216

Here is what is called your pre mRNA, we are going to see what happens to it to make it finalized.1220

Messenger RNA is just going to leave the nucleus.1229

Notice this 5 prime and 3 prime thing.1231

If you remember how DNA is read and how DNA was made in the previous lesson, DNA is always read from 3 prime to 5 prime.1234

It is no surprise that, if you are reading DNA from 3 prime to 5 prime, you would be making it from 5 prime to 3 prime.1240

Just like when you are making DNA, it is the same with making RNA.1253

Here is your RNA polymerase, it is going to go on and end up making other RNA.1257

It is a very busy molecule, it is constantly happening in the nucleus.1265

There is a lot of genes that need to be read.1268

It is the command to the parts of the cell, we need to make this protein, let us get it going.1271

Coding strand, template strand, like I mentioned earlier, instead of template, you might see the antisense and that sense term.1276

I prefer talking about the template strand which is what this was a compliment from.1283

The coding strand, you can see the bases of the resulting RNA on the coding strand side.1288

RNA processing, here is how you get from the pre mRNA to making that finalized one.1296

It is the RNA that results from transcription, as long as pre mRNA.1304

But then, you want to get it to the finalized version.1309

A few modifications happen, how do you get to this finalized version?1313

I mentioned this earlier with snRNA spliceosomes removed introns and puts axons together from the pre mRNA.1317

What are these introns and axons?1324

The regions within the pre mRNA.1327

Here is the pre mRNA that was made from the process of transcriptions.1330

This is just been let go from that template strand.1335

Here is the 5 prime UTR which should be the untranslated region.1339

It is there in the RNA but the ribosome, when it gets to translation will ignore it.1349

Over here, this is the 3 prime at the other end of the RNA untranslated region.1371

Here are those exons and introns.1376

An exon can be many nucleotides long, same with an intron.1380

Notice that, when you get to this step, this finalized mRNA, there is no more blues.1384

The introns have been clipped out, this and this, they go.1390

What does that, this thing called a spliceosome.1395

It looks like a giant enzyme, it comes down and it clips out these parts and allows the exons to be bonded together.1399

Here you have those three exons together, these resulting pieces are what is actually going to be translated.1410

The codons in these three exon regions will actually end up being part of that code for amino acids.1417

What happens to the introns, they can be cut up again and reuse because there are usable RNA nucleotides in there.1427

The amazing thing is, depending on the needs of the cell that time, one gene in DNA can code for a variety of proteins.1434

It all depends on what are the exons and introns at that particular moment.1444

In another instance, you could actually see this exon being an intron that is clipped out.1449

This intron region can be an exon another time.1455

This is how the one gene, one protein hypothesis is not true.1459

This has to do with what Gregor Mendel proposed, you can look at this in the genetics lesson.1474

He thought that for every gene that is inherited, you get one trait.1479

That is a nice thought and theory but whether you think that it is a trait or a protein,1483

saying that one gene always codes for that, just one kind of RNA and one kind of protein, it is not true.1488

There are approximately 25,000 genes in a human genome.1493

There are way more than 25,000 proteins in a human body, a lot more.1497

How do you get multiple kinds of proteins from a single gene,1503

it has to do with what exons are you leaving in, what introns are being clipped out.1506

Like I said, this stuff can be switched around.1512

The introns they clipped out, exons leave the nucleus.1515

The way that I remember it, exon sounds like exit, same prefix.1519

The exons exit the nucleus, the introns stay and they do not go with the ribosome.1528

That is how you get step 1 of RNA processing happening.1537

Also, in front of this 5 prime UTR, you will get a region that is a 5 prime cap.1540

I have read that there are some guanosine units in there, it has guanine in other parts.1553

The 5 prime cap just signals the ribosome that here is the part that gets inserted, this is the beginning of it.1562

At the back end, you have what is called a poly A tail.1570

Meaning, it is just a lot of adenines, AAAAAAAAA, just repetitive.1576

I have read that, that can be a hundred long, it might be 50 long but you can have a lot of these adenines being put in.1582

Why, what is the point?1592

One of the theories I have read is, as this exits the nucleus, if the back end of these gets snagged, gets damaged,1594

no big deal because it is repetitive region.1601

You are not going to be harming these parts in the center that are really important and means something.1604

These three events, spliceosomes clipping out the introns, the 5 prime cap being added,1610

and the poly A tail being added, are important to getting RNA finalized.1617

Translation is the process which RNA binds with the ribosome.1623

This mRNA left the nucleus, it is finalized, it comes down to ribosomes, this giant green thing.1627

That is very tiny in a cell, the tRNA and rRNA assist with assembling the amino acid chain also known as a polypeptide.1633

Remember, ribosomes have two units, there is a large subunit and small subunit.1648

Here you have got your mRNA, these little things here, these are tRNA.1653

Like I told you, they are not this perfect little T, cartoony T that you tend to see.1659

This is more faithful to their interesting shape.1666

These little balls here, these are amino acids.1669

The different colors represent different amino acids.1672

There are 20 amino acids in nature.1675

The blue ones is different from the orange and yellow.1678

You are going to see what is going on with these tRNA and how this is actually assembled together.1681

But like I said, this is important to getting those amino acids put together.1688

Without this, you would not have proteins.1695

It might seem meticulous to go through this, but cells exist and can stay alive because of this.1697

The ribosomes, they are not always on the ER, they tend to dock there because1704

after you make this amino acid chain, you can get wrapped up in the membranous vesicle from the ER.1709

But also, free ribosomes floating around the cytoplasm can do this.1715

Before, I told you that a way to remember the steps of transcription.1719

Initiation is something gets started, elongation something continues, the lengthening of it, and then it gets terminated, it gets ended.1723

We can think of the same three basic parts, initiation, elongation, and termination, with translation.1733

Before moving to initiation and so on, and how it actually get started, we got to look at this chart.1741

This is how you actually can keep track of the mRNA codons being translated into amino acid.1746

Every code on the mRNA ends up equaling an amino acid at the ribosome.1753

Here is a chart providing that result in translation.1759

On this side, you have got the first base of your codon, you can find it here.1763

Here is your second base, over here is your third base.1774

Because, we are talking about triplets of bases, codons.1784

Let us pick AUG, let us say our first was AUG, that make sense it is to start codon.1788

We found A here, second base U, G, there it is.1794

Then I put it in italics to differentiate it from this one here, the ile one.1801

AUG codes for met, methionine, right here is a start codon just to remind you.1807

Methionine is the amino acid that A UG always codes for.1813

Next up, let us say we have GCA, GCA codes for alanine.1818

Last one, let us look at CCU, CCU codes for proline.1830

You can see that all of the codons in mRNA that start with CC, regardless of that third base, whether it is UCG, they all code for proline.1841

You will see that in all of these boxes where you are going to have one amino acid that is being coded for from all four varieties.1851

That is wobbled, this process of wobble means that there is a little bit of leniency, sometimes with what that third base is in mRNA.1862

This relates to mutations, you will see this at the end of the lesson, that if the third base is mutated in DNA,1877

when you look at the triplet of bases, sometimes it is okay.1884

It is not a big problem if the third base is switched because you are still going to get proline being put there.1888

Sometimes it does make a difference.1896

As you can see here, UUC codes for phenylalanine but UUA codes for leucine.1898

I gave you italics here, UUA and UUG codes for leucine not phenylalanine.1906

We got the stop codons, I forgot right here.1911

This is also a stop codon.1918

UAA stop codon, UAG stop codon, UGA stop codon.1922

Those three, they do not code for amino acids.1929

Once those are rigged with ribosome, it is done.1932

It tells the ribosome stop putting amino acids together, we are done translating.1936

Translation step 1, it is initiation.1942

How is it going to initiate?1945

mRNA docks in a ribosome, the first tRNA binds to the start codon.1946

AUG gets it started, bringing in methionine into place.1951

Remember, A UG codes for met.1955

By the way, on that chart, I just gave you the three letter abbreviations.1959

They all have full names, a simple internet search could bring up that chart and a list of all the different amino acids full names.1964

Like I said, I was going to show you an image of the more accurate three dimensional shape of tRNA.1974

Here is another kind of cartoony one because you can see that, you got what I showed you before, that little look.1981

They have all these little bases here because the majority of it is just sugar phosphate base the whole way through.1994

It is just curved and bonded in this particular way, to get to this three dimensional shape.2005

Here is how the purple orange, bluish purple, and turquoise parts, this is how they correspond to the three dimensional structures.2010

This is really what it looks like in a three dimensional sense.2021

Down here, you get what is called an anticodon.2026

We will see more about that coming up later on this lesson.2032

Attached up here is an amino acid.2039

This is what docks on the mRNA and the ribosome.2050

That is what gets initiated.2054

mRNA has that start codon and this tRNA comes down.2056

You are going to see this on the next page.2061

If this is AUG, that start codon of mRNA, here is mRNA.2064

I’m going to do the more cartoony ones for simplicity, right now.2074

Here is that tRNA, the anticodon is complementary to this.2080

You would see UAC bonding to it.2086

The funny thing about that is AUG was a compliment to what came from DNA, TAC.2091

The compliment of the compliment, just gets you back to the original look.2098

Kind of like the flipside of tails on a coin is heads, the flipside of heads is tails.2101

TAC from DNA gave you AUG in mRNA, the tRNA you are going to have the compliment of that UAC, instead of TAC.2107

That is the anticodon.2121

Up here, you would have methionine, that is how it gets initiated.2123

Transition step number 2, elongation, how do you continue to read the mRNA and ribosome,2132

and actually put together this amino acid chain.2138

A second tRNA docks on the ribosome on the second codon, whatever comes after AUG.2141

There are 64 possibilities of whatever that codon is, depends on that gene that was coding for the mRNA.2147

The two amino acids from the first tRNA and the second, they bond together and they form what is called a peptide bond.2155

This is why an amino acid chain like this is known as a polypeptide, because it is many of these peptide bonds put together.2164

Once the two get connected, this is showing you what happens later on, I will get to that in a second.2171

Once the first two amino acids have connected, that first tRNA can leave.2177

It has done its job, it attached its amino acid to the second tRNA that came down,2182

it leaves and gets attached to another amino acid that corresponds to the anticodon on that particular molecule.2188

That anticodon corresponds to a codon and mRNA.2197

You got the second tRNA with two amino acids on it.2200

There is a shift, and the shift happens, they call it translocation.2205

There are actually three sites on a ribosome.2209

The site that is not being labeled here is known as the E site for exit.2212

That is what is happening here, this particular tRNA is exiting.2218

And that happens to the first tRNA that is AUG.2223

Here is the P site, here is the A site.2227

Once this one exits and you get the binding there, they switch and you get these moving there, this tRNA will exit.2230

The next one will come down and they will do their little binding together at the P and A site.2239

This just keeps happening over and over again.2246

You have a little bit of use of energy to get this happening, to get the movement going.2248

You have to use ATP to do that.2253

This keeps going until you get to termination, just coming up next.2257

This process continues with a new tRNA making contact with the mRNA, and the previous tRNA leaving.2263

You can actually count these and figure out how many tRNA have been there before.2271

This one, this particular amino acid, this is the next one in sequence, this is coming after it, etc.2276

This should be combining with this, all of these will be attached to this one, and then, it will shift.2283

But if we count them, this one just let go, this tRNA have that amino acid, this one have that.2291

How many came before it, 1, 2, 3, 4, 5, 6, 7, 8 tRNA’s.2299

This one, this is methionine because that came from that start codon that first tRNA came from.2304

By the time you get to termination, the end of this process, you can have amino acid chain that is 60 long, that is 100 long.2312

It really depends on the length of the gene.2320

Translation step number 3, termination.2325

After codon upon codon has had a tRNA dock on it, eventually the stop codon is reached.2329

And that stop codon, here is one of them, UAG.2335

Remember it is UAG, U-A-A, or UGA University of Georgia.2339

If any of these three are read, mRNA will stop.2348

There is no actual amino acid brought down here.2354

There is a particular kind of tRNA that kind of ends it.2358

It comes down without an amino acid, terminating it, and this is what you got, all of these amino acids.2363

That is it, that is the end of it.2372

Once again, you got that EPA, I know in the previous slide, they were flipped around.2374

We saw the exits on the right hand side, the reason why it does not matter is this is a three dimensional structure.2379

Ribosomes are three dimensional unit and we can be looking at the back side of this and will look more like the previous side.2386

This is a different way of representing it.2394

This top part of the ribosome also known as the 50S unit is the large part, 30S, the bottom part is the small unit of the ribosome.2396

That is the process, once that stop codon is reached, it is done.2408

This will get dislodged, the mRNA will be released from the ribosome.2413

You have ended translation.2419

Mutations, mutations definitely affect how you are going to get DNA being expressed.2424

If you have a change in DNA, it is going to impact the RNA which is going to impact the protein that is made.2434

Remember that, when translation is over, you have a resulting amino acid chain.2440

Then, the amino acid chain would be combined with other polypeptides to make a finalized protein.2445

You want it to have the right shape, you want it to have the right function,2450

whether it is an enzyme or membrane protein or structural protein, whatever it might be.2455

The code in DNA is subject to change, for whatever it is meant to code for.2460

Sometimes mutations actually have a positive affect in a protein.2464

That is evolution, if you have enough mutations in a species, it changes them for the better over time.2468

Oftentimes, when DNA gets changed, it would not code for something that is improved or better.2475

It can actually cause problems.2480

Mutations happen because of enzyme error, DNA polymerase was actually going along2483

and putting together those nucleotides, it is not perfect.2489

It is really good up here in the right ones but sometimes makes an error.2493

The DNA repair enzymes do not catch it and fix it.2498

Radiation can also do that, whether it is UV radiation, X-rays, gamma rays, etc.2501

Mutagens, those are any kind of chemical, any agent that physically mutates the DNA2508

and changes the base of what it is supposed to be.2514

Here are some mutation types.2517

What I’m going to give you is the normal DNA, where it is supposed to be or where it was originally supposed to have its base sequence.2519

We look at like, what happens when you have this mutations occurring?2528

Good, bad, neutral, it depends.2532

Let us say that our original DNA is in black.2535

Here we have got 1, 2, 3, 4 protons.2556

Just to keep it straight, I’m going to put a line between here, here, and here.2559

What I’m saying is TAC that is one codon, GDG one codon, ATA, CCT, just to keep it straight.2568

If you are going to do this with the really long sequence, it is wise to separate them.2574

You do not cut one on the bases twice or make a mistake,2578

and do not get that multiple of three that you are supposed to have with codons.2582

That is the original DNA, that is the way it is supposed to be.2585

The normal and what they call the wide type DNA, we are talking about genetics.2590

A point mutation means we changed one base.2594

Let us say, I’m going to do it in blue.2599

Here is the mutated DNA and this is called a point mutation.2603

There it is, that is the one difference.2621

What is going to result, we have to look at the RNA, the mRNA that results from this.2625

Let us do the mRNA in red.2630

Here is the resulting mRNA that you are going to get from this particular sequence.2633

Start codon, so far so good.2640

We will come back and see if that is good.2648

That is resulting mRNA.2657

I have this as the first example of the point mutation or single base substitution, because we substituted guanine for adenine.2659

Instead of getting what should have been CCC, that should have come from GGG, CCC, we have CCU.2669

If you go back and look at that mRNA amino acid chart, CCC and CCU actually both code for the same amino acid, it is proline.2678

In that particular mutation, this point mutation has no negative effect.2689

They were called a silent mutation because you cannot even tell it is there.2694

There is no change in the function of the protein or structure of the protein, same amino acid being put into place.2698

Everything else is just hunky dory, everything else has the same exact bases.2704

This one based got changed.2709

Now, I’m going to actually erase this example, give you another point mutation that is not a silent mutation.2712

This time around, here is the mutated DNA.2732

We are going to say that is the same but then this has changed to ATT.2746

That is your point mutation this time around.2754

Now, let us see what happens with the resulting mRNA.2756

As a result of transcription, this occurs.2767

Here is the problem, if the DNA did not get mutated, we would not have a stop codon, that is the problem.2778

Originally, ATA should have code for UAU.2787

UAU was not a stop codon, which means this at a ribosome would have kept going,2793

it would have kept translating everything else after it, until it gets to that stop codon.2799

However, UAA is a stop codon, that means everything after that, including this,2802

and whatever other codons existing mRNA after that point,2811

they will be no amino acid put into place during the process of translation, and that is a problem.2817

This is a point mutation that can cause severe issues with the proteins that are going to be made2822

because you are not putting together the right amount of amino acids.2826

Sometimes, a point mutation, even if it does not result in a stop codon mistakenly being put together,2830

sometimes in point mutation, just codon for a different amino acid can cause problems like sickle cell anemia,2838

it is a disease genetically inherited that results in a misshapen red blood cell.2845

Your hemoglobin is not in the proper form and the red blood cells look like a sickle shape.2851

That is just due to point mutation of the many amino acids that hemoglobin is made up of, just that one error,2856

that one wrong amino acid affects how the hemoglobin bonds together.2863

Point mutations is one of the ones that could result in no problems.2867

But, it can actually cause problems when you have that base substitution of a single point.2872

Insertions and deletions, tend to be worse, here is why.2878

Here is that same original DNA like we had above.2884

Insertion/ deletion results in a fail shift, I will explain what that means in a moment.2894

This is saying that when DNA polymerase goes through here and copies this,2898

it is mistakenly adding a base in the middle of this that should not be there or it is accidentally leaving one out.2904

That is the insertion or deletion part.2911

Here is the problem, you may have already guessed this.2913

If codons are being read in multiples of three, when you add one or delete one, you will throw off that multiple big time.2915

Let us say that a T is added there, now it will be this in the DNA.2923

Look what happens when I do this, instead of having TAC, GGG, ATA, CCT.2939

After TAC, it is TGG, GAT, ACC, and so on.2947

It will just throw everything off, what that is probably going to do is code for a different amino acid the whole way down.2953

That is major disruption to protein form and functionality.2961

You can imagine the same thing happening if you delete one.2966

It throws off the frame reading, resulting to frame shift, and cause major problems.2970

Bad mutations here typically.2977

I can think of one way this would not be so bad, sometimes, is if actually three different bases together were actually inserted or deleted,2980

you would still have the correct frame reading but you are throwing in an amino acid in the middle2990

or removing amino acid that does not belong there.2996

It still can be a bad thing, but typically insertion or deletion is very bad.2999

Duplications can be a bad thing, in the process of evolution, evidences has show that3003

this can actually occasionally result in good things happening.3009

Let me show you how duplication can happen.3013

A duplication means that one of these codons, one of the segments, just gets duplicated a whole bunch.3025

Maybe when the DNA is copied, it ends being this.3032

You can see we had some duplication there, GGGG, just a lot of g’s, a lot of them.3044

That is a lot of duplication, this, this, and this were all duplicated from this original codon of DNA.3052

That oftentimes can be a very bad thing, having too many amino acid in that polypeptide has an effect in the protein.3060

I have read theories that duplication in the past could have resulted in better abilities with a sense or3069

having a certain part of the body get elongated or increased in size due to duplication of sections of DNA.3079

It is hard to say for sure but duplications, insertion/ deletions, point mutations,3088

are all mutations that would tend to cause problems.3093

But that minority of the time, when you get the protein actually resulting in some kind of improved ability,3096

it does something better than what it did before.3104

That is fuel for evolution, and that is what has made the amazing diversity of life that exists today.3107

From out of mutations, you can get some good things happening.3114

Thank you for watching