Molecular Biology Mendelian Genetics & Foundational Experiments

Hello, and welcome back to www.educator.com.0000

Today, we are going to talk about Mendelian genetics and foundational experiments in molecular biology.0003

This is going to be an interesting unit because we are talking mostly about the history of how we started with Gregor Johann Mendel.0009

We are going to go all the way up until we are somewhere in the 20th century, at least.0023

We have to start any talk about molecular biology by starting off with Gregor Mendel.0031

We will then go through chromosomal theory of inheritance.0039

A large chunk of this talk is going to be on the discovery of the genetic material of the cell which we now know is DNA.0043

We will talk briefly on the discovery of RNA and we will mention how the genetic code was cracked.0052

As I said, talking about molecular biology cannot be done without first started talking on Gregor Johann Mendel.0061

Gregor Mendel within the 19th century, at the time he was considered a biologist and a botanist.0071

He did most of his work on plants.0078

He published his similar paper in 1866, hybridization and inheritance of the pea plant.0080

Unfortunately for Mendel, his results were criticized at the time of publication,0090

as only being about hybridization and not being about inheritance.0095

Unfortunately for him, he was not given the respect that he deserved in his time0100

because his results were not widely accepted until after he passed away.0106

That happened in about 1900’s, that is when his work was picked back up by other scientists.0110

However, looking back after the fact now, we consider Gregor Johann Mendel as the father of modern genetics.0116

He did all of this work while being an Augustinian follower, he was a monk.0126

This was his research that he did on the side.0133

Let us talk about Gregor Mendel’s laws.0137

He made two very important laws, based off of his research.0141

This is what he postulated, he showed that there is a principle of independent segregation.0146

We are talking about alleles.0152

His second law is the principle of independent assortment, speaking toward genes.0154

His first law, he started with true breeding lines for certain observable traits, he used his pea plant.0163

What a true breeding line means is that they are homozygous.0172

They always produce the same phenotypes.0177

If it is a green pea, then when you mate two green peas together, you get only green peas.0182

He referred that an individual’s phenotypes was determined by a pair of unit factors.0195

We call those nowadays, we call unit factors, we call these genes.0202

He said different versions of a single gene are called alleles.0214

They can vary just slightly, maybe even as small as a single nucleotide different, in an entire many thousand base pair gene.0219

He said that alleles can be dominate or recessive.0230

What that means is that any recessive gene, for example, if we say that green is dominant but it can also be yellow.0234

Green, if these are the two alleles, anything, if green is dominant, the only thing that two dominant alleles, big g’s, will be green.0250

Anything with even one dominant allele, the big G will be green.0271

Only something with two recessive alleles will be a different phenotype.0281

In this case, let us say it is a red.0292

Your observable phenotype which is this, is what is called a phenotype.0300

Your genetic composition which is this stuff, that would be your genotype.0316

Genotypes can be experimentally determined by mating and then analyzing the progeny.0338

You analyze the phenotypes, the progeny.0347

Remember, phenotype is the innate thing you can see, genotypes is the actual genetic makeup.0349

Individual alleles, he says (Mendel), would segregate independently into gametes or sex cells, like sperm and eggs.0355

With each gamete receiving one allele from each pair, one allele from each pair of alleles.0365

Using a Punnett square, draw out all our possible genotypes and0380

phenotypes of a cross between true breeding white flowers and true breeding purple flowers, with purple being the dominant.0386

Proceed through the F2 generation.0398

First of all, we need to now what all these means.0401

First of all, a Punnett square is a way to visualize those mating.0405

An F2 generation is what is called the filial 2 generation.0412

You start with your parents here, mom and dad.0416

They cross and produce, let us say you and your sister.0427

Right here, let us say you have a baby of your own.0438

When you meet your, for my example, let us say a wife, together you produce a baby.0445

This is what is consider the parental generation.0465

This is the filial, 1st filial generation or F1, this is the F2 generation.0471

Proceed through there using a Punnett square.0478

Remember, our Punnett square would be looking like an actual square, based on all of the genotypes that we get for a specific gene.0480

Let us try this solution.0490

What was dominant, if purple is dominant, let us write that there.0501

Before we start on this, let us go back to Mendel.0511

He mated plants, he mated pea plants.0519

He looked at their seed coat and seed color.0524

If we do an example of Mendel’s seed color or seed coat, then maybe we will be able to understand how to do the flowers.0527

He had true breeding lines, meaning they are homozygous.0538

He took true breeding round seeds and true breeding wrinkled seeds.0541

The round seeds are R, the wrinkled seeds are r and r.0551

This is parental.0559

When you mate them, you get all R and r.0566

Remember, if round at this point is dominant, all of these will be round.0573

Now what you are going to do, you mate the F1 generation to itself.0581

It is going to be another R and r.0589

What you can do, these are going to be all the gametes possible.0594

If these are the female gametes, these are the male gametes, you can make a Punnett square.0609

Our Punnett square is going to look as such.0625

You are going to have your female gametes.0630

All you have to do is fill in the box across from each other.0643

This one, we have a r crossing with a R, this is R and r.0648

These are two R, this right here is a R and r.0657

This right here is two r.0664

In this case, when we end this, by the way, is our F2 generation.0668

In this case, what we have here, remember dominant is round.0675

We have this is round, this is round, this is round, and here is our wrinkled one.0678

As we see here, we have a 3 to 1 ratio of dominant to recessive for phenotype.0687

We have two different phenotypes but we have three different genotypes because we have two of these.0706

Maybe this will help us get our flower one better.0729

Do you want to try?0733

Let us go ahead and give this a try.0735

We have our purple and we have our white.0739

We normally call our genes for the dominant.0742

We will call our purple P and P, we will call our white p and p.0746

This is purple.0759

When they mate, what do we get?0765

We get P and p.0771

That is still going to be what color, purple.0773

This is our F1.0780

What do we have to do, we need to mate this to another P and P.0786

What are our options, we have our p, P, P, p.0793

Let us say this is the mom and this is the dad.0807

If we follow this out, we can make our nice Punnett square with P and P, Pp, our P and P.0816

We have Pp, we have Pp, p and p.0834

What do we have here, we have this being purple, this being purple, this being purple.0842

This one is our lone white.0850

Just the same as this, we have a 3 to 1, in favor of purple for phenotype.0854

We have 3 different genotypes, PP, Pp, pp.0867

Remember we have two of these.0878

Hopefully, we found that helpful.0881

What is very important is that, as I have said before, we always find the phenotype of the heterozygote,0889

or the phenotype of the dominant allele, being seen in the heterozygote, these ones.0896

We are seeing the dominant phenotype in those, in heterozygote.0904

For many alleles, this does not always happen.0913

We do not necessarily have to have alleles that are completely dominant or completely recessive.0919

Then, we got really lucky in the fact his pea plants were true breeding lines in his way and0924

they were either dominant or recessive alleles.0931

Sometimes you can have something called incomplete dominance or co-dominant.0933

That is where, let us say, for example, you had a red flower and a white flower, and you mated those.0937

Let us say if red was dominant, instead of getting 3 red flowers and 1 white flower in the F2 generation,0945

you might get your dominant ones being red, your recessive homozygous ones being white,0954

but your heterozygous ones maybe a shade of pink.0961

That would be what is called incomplete dominance.0965

Mendel’s principle of independent segregation, he is talking about the alleles.0974

Individual genes will sort independently into a gamete.0980

Individual genes, let us say if your gene 1 and gene 2, do not necessarily always have to go into the same sperm or same egg.0985

They can go into different ones.1002

Each gamete receives one allele of each gene with a 50-50 chance.1005

We can use Punnett squares again, to visualize the possible genotypes and phenotypes.1012

What we can see is that, we see that genes will act independently to determine unrelated phenotypes.1018

This is important because we can have multiple genes making a single phenotype, that complicate things even more.1026

Here is an example that of a cat that is being mated with another cat.1037

What we see here is, what is dominant is having a short tail.1044

Short tail is dominant, that is why it is S and brown is dominant.1053

When you are looking at multiple genes, we are looking out here at two genes, it is the coat color and tail length.1060

Each of these genes has a different allele.1081

It can be for coat color, you can be white or brown.1084

For tail length, it can be short or long.1095

Remember this is parental.1103

When you mate true breeding, meaning homozygous parental generation, you are going to get all heterozygote.1107

All your heterozygote, remember are going to have the phenotype of the dominant allele.1119

The dominant allele, the dominant allele for coat color is brown and the dominant allele for tail length is short.1126

What we see here is that all of our F2 generation, all 4 are brown and have short tails.1149

You would mate this F1 generation to each other to produce your F2 genes, right, your F2 offspring.1170

What we see here is that we are no longer looking at our 3 to 1 ratios because now we have two genes involved.1182

We have 16 different possibilities.1189

What do we end up seeing?1192

We see a 9 to 3 to 3 to 1 ratio in the F2 generation.1194

What is it, it is going to be 9 in the double dominant.1203

9 are going to be brown and have short tails.1210

These next 3 are going to be the heterozygous looking ones,1224

to where you are going to be dominant in one thing and recessive in another trait.1228

3 are going to be brown with long tails.1232

We have another separated group, the heterozygous group, because we are independent segregation of these.1241

We are going to have 3 that are white and short.1248

And then, you are going to have one that is recessive for both traits and that is going to be white and long.1255

If we find that one, here is the white and long.1269

This is how we are going to be able to determine any of our true breeding lines, when we are looking at two different genes.1277

We are always going to see this 9 to 3 to 3 to 1 ratio.1285

For an example, let us try this again.1298

Using the Punnett square, draw out all possible genotypes and phenotypes of a cross between white flowers with long stems1300

and red flowers with short stems, with white flowers in short stems both being dominant.1310

Proceed all the way through the F2 generation.1318

White flowers in short stems are dominant.1321

We have a cross between white flowers with long stems and red flowers with short stems.1328

Let us write out what are our parents, what are the parents looks like?1337

We need one that is white, let us give them the W, we need it also to have long stems.1343

Let us give them the s.1353

Now we need one that is red, that is the w.1357

We need one on the short stems, it is a S.1361

This is white and long.1365

This is red and short.1374

If we mate these, what are we going to get?1380

We are going to get the full heterozygote.1385

We are going to get, here are our gametes.1389

I will just write that out so we can see it.1394

Gametes are only W and s, this is only w and S.1397

These crossed are going to give us only this, this is our F1.1404

What is that, what is this going to be, what is dominant?1427

White is dominant, white dominant short.1432

All of these are going to be white with short stems.1434

What are our possible gametes here?1444

It is going to be WS, ws, Ws, and ws.1454

Now, we are going to be able to figure out how to breakdown our Punnett square.1470

Here is going to be our F2.1478

We draw these out, this is going to be the same for both maternal and paternal.1482

Let us draw them out.1487

We have WS right there, WS over there, Ws.1489

Let us go through and figure all these out.1556

This is going to be WW and SS, that is the easy one and this is the easy one too, right.1557

Two w and two s, those are easy ones.1567

We just have to fill them in.1570

This is Ww Ss, this is Ww SS, WW Ss.1572

This is WW Ss, Ww SS, Ww Ss.1578

This is WW ss, this is Ww Ss.1590

This is ww SS, this is Ww Ss, this is Ww ss.1616

Over here, Ww ss, ww Ss.1650

Over here, ww Ss.1662

If we want to look and see, and prove to ourselves that this is in fact in a 9 to 3 to 3 to 1 ratio,1671

let us pick out the simple one first.1684

Here is our single one that has homozygous recessive for both, that is R1.1689

Let us find the 3 that have dominant one and recessive in the other, and the vice versa.1699

Over here, let us see.1711

We have green for this.1715

We will say this is dominant one not in the other.1721

Dominant one not in the other.1727

Dominant one not in the other.1729

This one is red and short.1730

This one was red and short.1741

What about this one?1759

We are looking for the ones that are white.1767

We want to see the ones that are white.1771

Let us see here, we have white and long, and white and long.1775

These are white and long.1788

The other 9 are going to be the ones that are left.1793

Those are going to be the white and short.1803

Can we find those, 1, 2, 3, 4, 5, 6, 7, 8, 9.1812

Each of these 9 have both a dominant allele for white, at least one for white and for short.1822

Let us go on to the next.1839

As I said before, Mendel was not appreciated in his own time.1844

The only reason that he is now appreciate is because other scientists picked up his work1851

and gave him credit for doing the initial work.1857

In 1903, this is about 20 years after Mendel died,1862

the American biologist Walter S. Sutton linked cytological studies with Mendel’s previous work.1866

This researcher Sutton emphasized the importance that diploid cells have two morphologically similar sets of chromosomes.1875

Each haploid gamete, received a single set.1886

He stated that genes are found on chromosomes with one allele on each homologous chromosome.1891

We are going to get more further into this as we move into the course.1898

To explain Mendel’s work, Sutton stated that the gene for seed color1904

was on a different chromosome than the gene for seed texture.1910

Mendel’s second law states that genes should assort independently.1917

But as Sutton was noticing, many genes did not behave that way.1926

These genes were said to be linked to each other.1932

What that means is that these two genes do not sort up independently at a normal 50 to 50 ratio.1936

They are more often seen together than they are separated.1945

Linkage occurs between genes because the genes are located within close proximity to each other1950

and usually on the same chromosome.1958

Linkage is never 100% complete and that is due to the fact that you can have crossing over in meiosis, in late prophase 1.1962

Linkage can be above the normal 50-50 which would be considered an unlinked gene.1973

Seven years later, American biologist Thomas Hunt Morgan and his colleagues1983

found a mutation in the eye color for drosophila melanogaster.1989

This mutation was linked to the X chromosome; flies also have X and Y chromosomes.1994

The linkage of the white gene to the chromosomes is strongly supported2002

Sutton's chromosomal theory of inheritance, proposed just 7 years earlier.2007

Linkage to the sex chromosome was really apparent2016

because the eye color of the progeny depended on the sex of the white eyed parent.2019

The Y chromosome does not carry or copy the white gene at all.2025

No white gene on the Y chromosome.2029

For X linked genes, the allele is expressed phenotypically by males.2036

Regardless of whether it is dominant or recessive because it is the only copy.2041

An example here, we have this being the female, this being the male.2050

The male is white eyed, the female is what we call wild type meaning it is normal, what we see in nature.2062

What we are seeing here is, we have a white wild type female, a white eyed male.2073

We have the F1 generation, this is parental.2082

The F1 generation both females, these are males.2089

Both females and males are wild type, regular brick red eyes is what they call them.2095

In the F2 generation, these guys are going to be mated to each other.2105

In the F2 generation, what we see is we get, this is a male, this is a male, and these are two females.2110

What we end up seeing is, we get 3 that have the brick red wild type eyes and 1 that has white eyes, and it is a male.2120

When the parent is, with the white eyes is a male, in the F2 generation,2137

the one that gets the white eyes is also going to be a male.2147

The mutation for white eyes, we noticed, when wild type is red and2155

we know that the mutation arose spontaneously but very infrequently.2159

What is important is that we know that males which are just like in humans, XY is a male and female is XX.2164

Males always donate their Y to any son and they always donate their X to any daughter.2185

We know that sons always receive their X from their mom, this is important.2205

Using this information, let us find out what the F1 and F2 generations would look like,2217

if the original parents were white eyed female and a wild type male.2224

The opposite of what we saw before.2229

Let us draw out our parental.2234

We have a female that is white eyed, we are going to call the ww.2241

Then we have a male that is a red eyed, W.2250

Instead of putting another W there, the Y chromosome does not have the gene for white on it,2255

we are just going to put Y for Y chromosome.2261

When they mate, they are going to produce the F1 generation which is what, what can we get?2265

We are only going to get, it is different now.2279

We now, the whole X and Y.2286

All of the females will get the X from their father, they get a big Y, this is going to be a red eyed.2291

Let us make this clear, this is red eye and this is white eyed.2307

The female is going to only be able to inherit that, right.2320

They will be able to get one of these from their mom.2332

Any males will be like this.2339

What we see here is that the females in the F1 generation will all be wild type which is red.2358

The males will all be mutant which is white.2369

If we mate these two together, what are our options?2376

What we can see here is that we have two boys, two girls, or two females and two males.2412

What do we see, let us look at this one.2422

What is this one going to be? Ww.2426

First of all, we know it does not have a Y, it is going to be a female.2431

We see that it does not have a W, this is going to be white.2434

Let us look at this one, it is w and Y, we know it is a male.2445

We know it does not have a W, it is also going to be white.2454

When we look at this one, Ww, we know it does not have Y, it is a female.2463

We have a W, it is wild type or red eyed.2472

The last one here, WY, it got a Y so we know it is a male and we have a W, therefore it is also red.2476

That is what we see here, we get two white eyed and two red.2489

But we get both a white female and a white eyed male.2493

We can start moving closer and closer to current time.2507

Now, we are in the 20th century, 15 years or so, after we are able to go through the chromosomal theory of inheritance,2511

we can move on to thinking about how we can discover or how we discovered the genetic material of the cell.2526

To preface this, at the time, scientists believe that the genetic material of the cell were proteins.2534

They did not think that it was DNA, because at the time, the proposed structure of DNA did not lend itself toward replication.2542

They thought that protein was definitely the genetic material of the cell because it had instamatic activity.2556

In 1928, we have British bacteriologist Frederick Griffith,2563

identifying what he termed the transforming principle, the genetic material of the cell.2568

He developed this extremely and genius experiment that took this substance that is in a virulent streptococcus pneumoniae,2575

bacteria, he was able to transform the non-virulent strain of this bacteria into a virulent strain.2585

Virulent means is able to cause a disease.2596

How he did this was, he took the non-virulent strain or the R strain, it was rough looking when it was plated out on Petri dishes.2598

He injected it in a mouse, he took the blood of the mouse and there were no live bacteria.2610

The mouse was perfectly fine and alive.2620

He took this smooth strain, the S strain, because it looks smooth and have this extra protein capsule on it.2623

This was virulent, he injected into the mice, the mouse, all the mice died.2630

He was able to extract from the blood live streptococcus pneumonia.2638

In his next experiment, he heat killed this virulent strain.2645

He killed the bacteria, kind of just like we do with vaccines today.2649

He killed it but still injected into the mouse.2655

The mouse lived and he was not able to pull any live bacteria out of it.2659

This is the most important step right here.2663

He took live R strain which is not virulent and he took heat killed or dead S strain.2670

He mixed those together, injected into the mouse.2684

What did he find?2687

He found that the mouse died and he found that he can recover live S strain.2690

He said, this has got to be the genetic material of the cell, this transforming principle that somehow was able to transform R and S.2699

But what was that, we do not know.2711

Fast forward a little longer, this is on the side before we jump back into the big picture.2717

In 1941, we have Beadle and Tatum proposing that genes throughout the synthesis of enzymes.2724

They did this by irradiating spores on blood mold.2730

They found that these were unable to grow on media without argenine which is amino acid.2734

This amino acid was produced just by a single enzyme.2740

They came up with this hypothesis that one gene makes this one enzyme, one gene one enzyme hypothesis.2744

What I want us to understand is that, this has later been amended to be a one gene one polypeptide theory2753

because we can make more than one enzyme from a single gene.2761

That can be done by alternative splicing that we are going to talk about much later in this course.2765

That was kind of in the side, now we jump back to the main story.2771

In 1944, now we have Maclyn McCarty at the Rockefeller institute, showing what a transforming principle was and they say that it is DNA.2776

How do they do this, they took this heat-killed S strain.2792

They fractionated it into DNA, RNA, and protein.2795

Each fraction was tested for their ability to transform the R strain into the S strain.2811

What they did was they separated each fraction and only gave the DNA from the S strain to the R strain.2819

They would do another sample where there is only the RNA from the S strain to the R.2827

Only the protein from the S to the R, separately.2831

What they found is that the only DNA fraction that could transform an R strain to an S strain was the DNA.2835

The RNA and the protein fraction did not.2844

However, this was still not widely accepted because as I said earlier, at the time,2853

DNA structure that was proposed by Phoebus Leven was not something that can be applied2860

to be able to transfer information, it would be replicated.2869

This is the reason why proteins were thought to be the genetic material.2875

Phoebus Leven, much earlier, in 1910, he proposed what is called the tetranucleotide hypothesis.2880

He predicted that the structure of DNA was planar.2889

An equal quantity of each nucleotides were in each plane.2892

It looked like this down here, we had a guanine and a cytosine, a thymine and adenine, in each plane.2897

They look like squares and they added one on top of each other, so on and so forth.2904

With the phosphate backbone in the middle and the bases on the outside.2909

Luckily, due to this planar prediction, where we had equal, all four in the same plane all the way up,2922

we had another researcher named Chargaff showed that this was not the case.2937

In 1949, we have an Austrian chemist Erwin Chargaff proposing that DNA of different species have different nucleotide composition.2943

It always follows this rule, the amounts of adenines always equal the amount of thymines.2955

The amounts of guanines always equal the amount of the cytosines.2962

The amount of A’s and G’s always equal the amount of C’s the T’s.2967

Importantly, A and T ratio, A + T does not have to equal G + C.2972

They can be varying between different species.2982

This is important and this is something that we know nowadays. We know that A base pairs with T and G base pairs with C.2985

Therefore, this makes a lot of sense.2994

At the time, nobody knew this.2998

Back to discovering the genetic material of the cell, a few years after Chargaff, gave the light toward the end of the tunnel.3002

We have the scientists named Hershey and Chase, this American bacteriologist,3016

a geneticists, conclusively demonstrating that DNA is the genetic material.3021

They did this in a really cool experiment. They took bacteriophage.3029

Bacteriophage is a type of virus that infects bacteria.3035

The bacteriophage is very simple, all it is consisted of is DNA that is inside the head and the rest of it is all protein.3044

The head, the coat, the tail, all these stuff is protein, only what is inside the head is DNA.3060

What Hershey and Chase did is they took radioactive isotopes of sulfur and phosphorus,3070

incorporated them into the growing bacteriophages.3076

You can specifically incorporate sulfur into protein.3081

Meaning, the sulfur was only going to be in the protein coat, on the outside.3091

They also took the phosphorus, this isotope, that could be specifically incorporated into DNA,3100

meaning it is only inside that group on that DNA.3107

What they did was, after growing the bacteriophages in these isotopes, they were allowed to affect the bacteria.3112

As we see here, we have two different types.3123

Ones that were just labeled with the sulfur, only the proteins.3127

Ones that were just labeled with the phosphorus, only the DNA.3140

What they see is, you let them infect.3145

They latch onto the bacteria and shoot their DNA into the bacteria.3147

The whole point they want to do is make new viruses and breakout.3153

They were allowed to infect the bacteria and then, they were thrown literally into this blender3158

It is like a blender you have on your counter.3165

It is actually still held at Cold Spring Harbor, in New York, the original blender that Hershey and Chase performed this experiment with.3168

They threw everything into this blender, heat blend, that basically shakes the bacteria off of everything.3176

It breaks everything open, it shakes the phages to everything.3186

What they found is that the phosphorus, which we know is associated with DNA, was found into the bacteria.3191

As well as the progeny phage but the S35, the sulfur that is in the protein coat, there was not inside the cell.3203

It was only associated with the protein ghosts. That is what they will call the ghosts, it is the empty phage.3214

This is really great, DNA is what is being passed on, not protein, this is huge.3221

In the same year, we have Rosalyn Franklin.3232

Rosalyn Franklin is an English chemist and one of the premier X-ray crystallographers of her time.3237

This in fact is a picture of hers, called photo 51, this shows the diffraction pattern of DNA.3246

If you can actually look at it, we can see that we have a double helical nature.3254

We can even see the minor groove and the major groove, each one in DNA.3259

This is an amazing picture, especially at the time.3267

Unfortunately for Rosalyn Franklin, someone in her laboratory showed this picture, photo 51,3271

to some competing scientist without her authorization.3283

Those competing scientists are very well known now.3290

The names of those competitors are James Watson and Francis Crick.3298

A year later after getting that picture, that photo 51 from Rosalyn Franklin, they deduced the double helical structure of DNA.3303

They published their article in the Nature journal, it was only one page long.3313

It is one of the highest, or if not the highest, cited paper from that journal.3320

What they said in their paper was that two DNA strands are held together by hydrogen bonds and3329

that is between the opposing strands.3338

The bases coming together, make those hydrogen bonds.3342

They said base pair, and they are specific, A pairs with T, G pairs with C only.3346

We have already talked about this, but at the time that was a noble insight.3351

They said that the sequence of one strand defines the sequence of the other strand,3358

meaning that it is complementary.3361

They also noted that it was anti parallel.3366

Remember, 5 prime down to 3 prime for one strand, 5 prime up to 3 prime for the other strand.3368

In that one of the last sentences, they stated that the specific base pairing suggests a possible copying mechanism.3376

This was the whole basis of why DNA could not be the genetic material, as seen before,3387

because it could not be copied.3395

Now, they are saying that the phosphate backbone is on the outside,3398

the bases are on the inside, hydrogen bonded to each other, they are complementary.3401

Therefore, we could probably see a situation where you can copy this pretty easily, although,3405

they did not propose the mechanism.3413

As an example based on Watson and Crick’s solution, fill in this sequence on the bottom strand of DNA following what they said.3417

Remember, A’s pair with T’s, G’s pair with T’s.3431

DNA strands are anti parallel.3435

Remember, we have the 5 prime to 3 prime on the top strand.3438

We have the 5 prime to 3 prime down to the opposite on the bottom strand.3444

If we want to make our bonds, T pairs with A, A pairs with T, C pairs with G, G pairs with C, C pairs with G.3450

Let us not forget, how many bonds?3463

A and T makes two hydrogen bonds, G and C make three hydrogen bonds.3467

This would be our phosphate backbone, right here, of one strand, this is the phosphate backbone of another strand.3477

A few years after Watson and Crick deduced the structure of DNA helix,3491

we had an American biochemist named Arthur Kornberg, he isolated this protein from a bacteria that he called DNA polymerase 1.3497

He showed that DNTP or deoxyribonucleotide triphosphate, they say that DNTP’S meaning it could be an A, G, T, or C.3507

He said that DNTP’s and a DNA molecule will require for DNA synthesis.3520

He showed using templates with different base composition, whether it is high AT or high GC ratio,3531

he showed that the newly synthesized DNA molecules had a similar base composition.3538

Confirming, Watson and Crick’s saying that there was the complimentary between the two strands.3543

Kornberg established that the strand of DNA served as a template for DNA synthesis.3550

But still, he could not way off a mechanism by which it happened.3557

There were three mechanisms that we are offered at the time.3564

Now, we had semi-conservative in which we have here, we started with our parent strand, all red.3569

The two new daughter helices, one of the strand would be completely parental and one of the strand would be completely new.3579

We have conservative, where it is just like a photocopy mechanisms of the two daughters strands.3589

One would be completely old DNA, one would be completely new DNA.3594

We have dispersive, in which case both of the daughter molecules would have new and old.3599

But we would have stretches where both strands were completely old3606

and stretches where both strands are completely new DNA sequence.3611

How do we discover what type of mechanism this is?3616

Two years later, we have Meselson and Stahl showing that DNA replication is semi-conservative,3622

meaning you have the original DNA.3629

When it is newly made, we have one strand being completely old and one strand being completely new.3642

How did they do this, which is a really ingenious experiment?3653

DNA was made denser or heavy, by adding heavy isotope of nitrogen, some nitrogen 15.3658

Then it was subjected to high speed centrifugation through this dense medium of chloride.3666

You either grew DNA in what is called light nitrogen, the normal N14, or the heavy nitrogen the N15.3673

You call that heavy or light DNA.3682

They do grew through 14 generations, so that there was no bias coming in.3686

They are all different.3692

You are either completely in 15 or completely in 14, that was what it was showing.3694

After one round of DNA replication, it opposes the light isotopes.3701

All the daughter molecules have an intermediate density or heavy light, from 100% heavy to 100% intermediate.3710

And then, what they did is they did one more replication.3724

At the point, they saw a reduction of the medium, the intermediate, down to 50%.3729

They saw 50% being fully light.3736

This demonstrates that DNA replication is semi-conservative because you were having the old parental strand being the heavy.3742

You grew it only in heavy to start with, then you started growing it in light.3751

You can only be adding light, you could not add more heavy.3758

By adding, by growing from completely heavy to intermediate, after one replication,3763

it has to be half and half, half old and half new.3771

This does not necessarily mean that, right there that breaks away conservative.3778

We can still have distributive or disperse, as well as semi-conservative.3785

What they did is, they did more and more generations and found that as you increase the generations,3790

you are not adding any more of that old stuff.3797

You are not distributing in between strands; its every new strand is going to be a light strand.3800

That is how they can figure that it was semi-conservative replication, that is a big deal here.3805

That is the end of our DNA, the discovery of the genetic material and how it is replicated.3814

This is a quick overview on the discovery of RNA, we will talk more about RNA later.3821

Ribosomal RNA is one of the three types of RNA.3828

rRNA was discovered in the 50’s by Albert Claude and George Palade.3832

Ribosomal RNA is important for the structure of ribosomes, just a structural purpose.3841

Transfer RNA is what is responsible for bringing in amino acids to the ribosome,3849

to match up with that particular anticodon in the mRNA.3858

MRNA was found in 1956 by Zamecnik and Hoagland.3863

Messenger RNA which is made from DNA and is responsible for dictating the sequence of amino acids3869

for protein synthesis at the ribosome, was discovered in 1960 by Brenner, once again from Francis Crick.3878

Now that we have learn about our DNA and RNA,3886

we should talk quickly about the central dogma in molecular biology, what is that really mean?3889

This is something that we are going to talk about many times throughout this course.3900

The simplest way to explain it is that DNA can make more DNA and that is called the process of replication.3905

DNA can be turned into RNA, this is the metabolic process of transcription.3926

Finally, RNA can be turned into protein via the process of translation.3942

This is what everything that we believe in molecular biology follows.3954

One thing that violates the central dogma is something that you might be familiar with, that is called reverse transcription.3961

This is turning RNA back into DNA, we see this in the type of enzyme, such as retroviruses.3982

They have a sort of enzyme called reverse transcriptase and that is able to turn RNA back into DNA.3991

This is an important thing biologists always have in mind, when we are thinking about the molecular biology central dogma.4001

RNA can be made into protein, how do we figure that out?4009

How do we figure out which RNA will turn into which amino acids?4013

Cracking the genetic code, this is important.4019

If you remember, the genetic code, if we have not learned yet,4022

the genetic code is what tells us what triplet will repeat found in an mRNA will give us a specific amino acid.4026

In 1961, we have an American biochemist and geneticist named Marshall Nirenberg.4036

He discovered the first DNA triplet that would go on to make an amino acid.4041

He did this by taking a bunch of nucleic acid, RNA, but only was made up of U’s.4046

He fed this through the ribosome, in a cell extract, and only phenylalanine amino acids were made.4056

He found out that the first triplet codon ever distinguished was UUU, that coded for phenylalanine.4065

It took about 5 more years and some other researchers helped, but the code was completely finished in 1966.4075

What they found is that they were 64 possibilities of triplet repeats or triplet codons.4082

Each one of these could code for an amino acid.4096

They thought, three of them they found, these are important,4100

were found to not actually add an amino acid and instead cause the release of the polypeptide chain from the ribosome.4106

The other 61, other than these 3, the 61 possible codons all coded for amino acids.4114

What is also very important is that, for example, let us look at this.4127

4 of these different codons will code for serine amino acid.4132

This is what we call the degeneracy of the genetic code, meaning that we can get the same amino acid from multiple different mRNA bases,4139

meaning also since we got the mRNA from the DNA, we can have different genetic sequences giving us the same amino acid sequences.4150

I hope you enjoyed the lesson, thank you very much for coming and please come back to www.educator.com.4161