DNA | Biomolecules | MCAT | Khan Academy

Summary

This script explores the history and science of genetic inheritance, starting with Gregor Mendel's foundational work in the 1800s. It delves into the discovery of DNA's structure by Watson, Crick, and others, highlighting its role as the genetic information carrier. The video explains the DNA molecule's double helix structure, the significance of base pairing (adenine with thymine, cytosine with guanine), and how this mechanism facilitates DNA replication. It also touches on the human genome's complexity, with approximately 3 billion base pairs across 46 chromosomes, emphasizing the genetic code's role in defining traits and the ongoing nature vs. nurture debate.

Takeaways

• 🧬 The concept of inherited traits has been observed for centuries, but it wasn't until the 1800s that Gregor Mendel began to study inheritance scientifically.
• 🔍 Even Mendel, known as the father of genetics, did not understand the molecular basis of inheritance during his time.
• 🧬 The molecular basis of inheritance became clearer in the mid-20th century with the discovery of DNA's structure by Watson and Crick, building on the work of others like Rosalind Franklin.
• 🧬 DNA, or deoxyribonucleic acid, was discovered in the mid-1800s and was considered a potential molecular basis for inheritance due to its ability to store, replicate, and express information.
• 🧬 The DNA molecule's structure is a double helix, resembling a twisted ladder, with the sides made of sugar and phosphate and the rungs consisting of base pairs.
• 🧬 The bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G), with A pairing with T and C pairing with G, forming the rungs of the DNA ladder.
• 🧬 The sequence of these base pairs in DNA encodes genetic information, determining traits and characteristics of an organism.
• 🧬 The human genome consists of approximately 3 billion base pairs, spread across 46 chromosomes, with each chromosome containing an average of over 100 million base pairs.
• 🧬 DNA replication occurs through a process where the double helix ladder splits, and each half reconstructs its complementary base pair sequence.
• 🧬 The understanding of DNA's structure and function is fundamental to grasping how genetic information is passed down and expressed across different species.

Q & A

Who is considered the father of genetics and why?

Gregor Mendel is considered the father of genetics because he was one of the first to scientifically study how traits are inherited by breeding different types of plants.

When was the structure of DNA discovered, and by whom?

The structure of DNA was discovered in 1953 by James Watson and Francis Crick, based on data from scientists like Rosalind Franklin and Maurice Wilkins.

What was DNA known as before its structure was identified?

DNA was known as a molecule inside the nucleus of cells, with scientists speculating it might be involved in inheritance, but its exact role was unclear until its structure was determined in 1953.

What is the significance of DNA’s double helix structure?

The double helix structure of DNA is significant because it helps explain how DNA stores genetic information, replicates, and expresses traits. The pairing of bases in the helix provides a mechanism for copying genetic information.

What does the term 'deoxyribonucleic acid' (DNA) mean?

'Deoxyribonucleic acid' refers to the sugar (deoxyribose) and phosphate backbone of the DNA molecule, along with the fact that DNA is an acid and was found in the nuclei of cells.

What are the four bases of DNA, and how do they pair?

The four bases of DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine always pairs with thymine, and cytosine always pairs with guanine.

How many base pairs does the human genome contain?

The human genome contains approximately 6 billion base pairs.

How many chromosomes are in the human genome, and how are they organized?

The human genome contains 46 chromosomes, organized into 23 pairs. Each chromosome contains hundreds of millions of base pairs.

How does DNA replication occur?

DNA replication occurs when the double helix splits in half, with each half serving as a template for constructing a complementary strand based on base pairing rules (A with T, C with G).

What is the approximate diameter of the DNA molecule, and why is this significant?

The diameter of the DNA molecule is approximately one nanometer. This small size allows DNA to store an immense amount of genetic information in a very compact form.

Outlines

00:00

🧬 Introduction to Inheritance and DNA

This paragraph introduces the concept of inherited traits, noting how people have long observed similarities between offspring and their parents. It discusses the historical shift from anecdotal observations to scientific study, highlighting Gregor Mendel's foundational work in genetics. The paragraph emphasizes the mystery of inheritance's molecular basis until the mid-20th century, when the structure of DNA was elucidated by Watson and Crick, with significant contributions from Rosalind Franklin and others. The DNA's double helix structure is identified as the key to understanding genetic information storage and replication.

05:00

🔬 DNA Structure and Genetic Code

The paragraph delves into the structure of DNA, likening it to a twisted ladder with sugar-phosphate backbones and bases forming the rungs. It explains the term 'deoxyribonucleic acid', breaking down the components of DNA's name and structure. The four bases—adenine, thymine, cytosine, and guanine—are introduced, detailing their pairing rules: adenine with thymine, and cytosine with guanine. The paragraph also touches on the genetic implications of these base sequences, hinting at the complexity and information density within DNA, and how these sequences code for traits and characteristics.

10:04

🧬 DNA Replication and Information Storage

This section explains the process of DNA replication, illustrating how the double helix can split into two single strands, each serving as a template for a new complementary strand. It uses the base pairing rules to demonstrate how new DNA molecules are constructed, ensuring genetic information is passed on accurately. The paragraph concludes with a visual aid—an animated gif—to reinforce the concept of the DNA double helix, emphasizing the molecule's compactness and the efficiency of its information storage and replication mechanisms.

Keywords

💡Inherited traits

Inherited traits refer to the physical or behavioral characteristics passed down from parents to their offspring. In the video, this concept is introduced as a fundamental observation that has been made throughout human history, such as similarities in physical appearance or mannerisms between family members. The script uses the phrase 'offspring tend to have traits in common with the parent' to illustrate this concept, setting the stage for a deeper exploration of genetics.

💡Gregor Mendel

Gregor Mendel is known as the father of genetics and is credited with pioneering the field through his work with pea plants in the 1800s. The video script mentions Mendel as the starting point for the scientific study of inheritance, emphasizing his role in understanding the mechanisms of trait transmission without yet knowing the molecular basis for it.

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for the development, functioning, and reproduction of all known living organisms. The video explains that DNA was discovered in the mid-1800s and later recognized as the molecule storing genetic information. The script describes DNA as a 'twisted ladder' structure, highlighting its role in storing, replicating, and expressing genetic information.

💡Double helix

The double helix is the structure of the DNA molecule, consisting of two strands coiled around each other. The video script uses the visual metaphor of a 'twisted ladder' to describe this structure, emphasizing how the rungs of the ladder, which are the base pairs, hold the genetic information. This structure is crucial for DNA's ability to replicate and pass on genetic information.

💡Base pairs

Base pairs are the rungs of the DNA ladder, consisting of pairs of nitrogenous bases that connect the two strands of the DNA helix. The video script identifies four types of bases: adenine (A), thymine (T), cytosine (C), and guanine (G), and explains that adenine always pairs with thymine, and cytosine pairs with guanine. These base pairs are essential for the storage and replication of genetic information.

💡Rosalind Franklin

Rosalind Franklin was a British scientist whose work on X-ray diffraction images of DNA was instrumental in uncovering its structure. The video script acknowledges her contribution by stating that she 'essentially provided the bulk of the data' for Watson and Crick's work, which established the double helix structure of DNA.

💡Human genome

The human genome refers to the complete set of genetic information in humans, which includes all of the DNA in the 23 pairs of chromosomes in the nucleus of human cells. The video script provides the number of base pairs in the human genome as 6,000,000,000, emphasizing the vast amount of genetic information that determines the traits and characteristics of an individual.

💡Chromosomes

Chromosomes are structures within cells that contain DNA and associated proteins. The video script mentions that in humans, the DNA is organized into 46 chromosomes, or 23 pairs, which are located in the nucleus of most cells. Chromosomes play a critical role in the organization and transmission of genetic information.

💡Genetic code

The genetic code is the set of rules by which information encoded in genetic material (DNA) is translated into proteins by living cells. The video script refers to the sequence of bases in DNA as a 'code' that determines the traits of an organism, such as hair color, and is the basis for the expression of inherited characteristics.

💡Replication

Replication in the context of DNA refers to the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. The video script explains that DNA replication occurs when the two strands of the DNA helix separate, and each strand serves as a template for the synthesis of a new complementary strand, ensuring the transmission of genetic information.

Highlights

The concept of inherited traits has been observed for centuries, but it wasn't until the 1800s that it began to be studied scientifically.

Gregor Mendel is known as the father of genetics, pioneering the study of inheritance.

Even Mendel did not understand the molecular basis of inheritance during his time.

The structure of DNA was a crucial discovery that led to understanding the molecular basis of inheritance.

The work of Watson and Crick, along with others like Rosalind Franklin, established the structure of DNA.

DNA was discovered in the mid-1800s, but its role in inheritance was not understood until much later.

The DNA molecule's structure is a double helix, resembling a twisted ladder.

The genetic information in DNA is stored in the sequence of bases along the rungs of the 'ladder'.

DNA is composed of a sugar-phosphate backbone and pairs of bases: adenine with thymine, and cytosine with guanine.

The term 'deoxyribonucleic acid' comes from the sugar deoxyribose, the acidic phosphate group, and its location in the cell nucleus.

The sequence of bases in DNA encodes the information that determines an organism's traits.

The nature versus nurture debate revolves around the extent to which genetic information (nature) influences traits compared to environmental factors (nurture).

Humans share a significant amount of genetic material with other humans, more so than with other species.

The human genome consists of approximately 3 billion base pairs, spread across 46 chromosomes.

DNA replication is possible due to the base pairing mechanism, where each base on one strand finds its complement on the new strand.

The DNA molecule is extremely compact, with a radius of about one nanometer.

DNA's structure allows for efficient replication and expression of genetic information.

An animated gif is used to visually represent the double helix structure of DNA, illustrating the sugar-phosphate backbone and base pairs.

Transcripts

- [Voiceover] As long as human beings have been around

I could imagine that they have noticed that

offspring tend to have traits in common

with the parent.

For example, someone might have told you,

"Hey, you walk kind of like your dad,"

or, "Your smile is kind of like your mom,"

or, "Your eyes are like one of your uncles

"or your grandparents."

And so there's always been this notion

of inherited traits.

But it wasn't until the 1800 that that started to be

studied in a more scientific way with Gregor Mendel

the father of genetics.

But even then, even Mendel

who was starting to understand the mechanisms

or he was trying to understand how inheritance happened,

then you even could start to breed

certain types of things.

Even he didn't know exactly

what was the molecular basis for inheritance.

And the answer to that question wasn't figured out

until fairly recent times,

until the mid 20th century.

Not until the structure of DNA

was established by Watson and Crick

and their work was based on the work of many others

especially folks like Rosalind Franklin

who essentially provided the bulk of the data

for Watson and Crick's work,

Maurice Wilkins and many, many, many other folks.

But it's really the structure of DNA

that made people say, "Hey, that looks like

"the molecule that's storing the information."

Just to be clear, DNA wasn't discovered in 1953.

DNA was discovered in the mid 1800s.

It was this kind of this molecule

that was inside of nuclei of cells.

And for some time people said,

"Maybe this could be a molecular basis of inheritance."

You could imagine what you would need

to be a molecular basis of inheritance.

It would have to be a molecule

or a series of molecules that could contain information,

that could be replicated,

that could be expressed in some way.

But it wasn't until 1953

wherein this double helix structure

of DNA was established.

The people said, "Hey, this looks like our molecule."

So first, let's just talk about the structure here

and then actually we'll talk about where this name, DNA,

deoxyribonucleic acid comes from.

And then we'll talk a little bit about

why this structure lends itself well

to something that stores information,

that can replicate its information

and that could express its information.

We might go in depth on the expression of information

in future videos.

So this structure right over here

and this is a visual depiction of a DNA molecule.

You can view this as kind of a twisted ladder.

It has these two, I guess you could say

sides of the ladder that are twister.

That is one side right over there

and then it is another side.

There is another side right over here.

And in between those two sides

or connecting those two sides of that twisted ladder

you have these rungs.

And these rungs are actually where

the information, the genetic information is

I guess you could say stored in some way.

Because these rungs it's a sequence of different bases.

And when I say bases, you're gonna say wait.

This says acid, why are you saying bases right over here?

Well, the word deoxyribonucleic acid

comes from the fact that this backbone

is made up of a combination of sugar and phosphate.

And the sugar that makes up the backbone

is deoxyribose.

So that's essentially the D in DNA.

And then the phosphate group is acidic

and that's now where you get the acid part of it.

And nucleic is, hey this was found

in nuclei of cells.

It is nucleic acid.

Deoxyribonucleic acid.

It is actually mildly acidic all in total

but for every acid it actually also has a base,

and those bases form the rung of the ladders.

And actually each rung is a pair of bases

and as I said, that's where the information

is actually stored.

Well what am I talking about?

Well let me talk about the four different bases

that make up the rungs of a DNA molecule.

So, you have adenine.

Adenine.

And so for example, this part right over here.

This section of that rung might be adenine.

Maybe this right over here is adenine.

This right over here.

Remember, each of these rungs are made up by

it's a pair of bases.

And that might be adenine.

Maybe this is adenine

and I could stop there,

I mean I'll do a little more adenine.

Maybe that's adenine right over there.

And adenine always pairs with the base thymine.

So let me write that down.

So adenine pairs with thymine.

Thymine.

So, if that's an adenine there

then this is going to be a thymine.

If this is an adenine

then this is going to be a thymine.

Or if I drew the thymine first,

well say, okay it's gonna pair with the adenine.

So this is going to be a thymine right over here.

This is going to be a thymine.

If I were to draw this,

this would be a thymine right over here.

Now the other two bases,

you have cytosine which pairs with guanine

or guanine that pairs with cytosine.

So guanine and we're not gonna go into

the molecular structure of these bases just yet,

although these are good names to know

because they show up a lot

and they really form kind of the code,

your genetic code.

Guanine.

Guanine pairs with cytosine.

Guanine and cytosine.

Cytosine.

So actually if this is,

let's say there's some cytosine there,

let's say cytosine right over here.

Maybe this is a cytosine, maybe this is cytosine,

maybe this is cytosine, this is cytosine

and maybe this is cytosine.

Then it always pairs with the guanine.

So, let's see, this is guanine then

and this will be guanine.

This is guanine, this is guanine.

I actually didn't draw stuff here.

This is guanine, I didn't say what these could be

but these would be maybe the pairs of

they could be adenine-thymine pairs

and it could be adenine on either side

or the thymine on either side,

and they could be made of guanine-cytosine pairs

where the guanine or the cytosine is on the other side.

Actually just to make it a little bit more complete

let me just color in the rungs here as best as I can.

So those are guanines

so they're gonna pair with cytosine.

Pair with cytosine, pair with cytosine.

When you straw in this way

you might start to see how this is essentially a code,

the order of which the bases are...

I guess the order in which we have these

or the sequence of these bases essentially in code

the information that make you, you,

and you could be.

Well how much of it is nature versus nurture

and when people say nature, you know,

it's literally genetic,

and that's an ongoing debate, an ongoing debate

but it does code for things like your hair color.

When you see that your smile

is similar to your parents

it is because that information to a large degree

is encoded genetically.

It affects a lot of what makes you you

and actually not even just within a species

but also across species.

Humans have more genetic material

in common with other humans

than they do with say a plant.

But all living creatures as we know them

have genetic information.

This is the basis by which they are

passing down their actual traits.

Now you might be saying

well, how much genetic information

does a human being have?

And the number will either disappoint you

or you might find it mind-boggling.

The human genome and every species

has a different number of base pairs

to large degree correlated with how complex they are

although not always.

But the human genome has 6,000,000.

Sorry, not 6,000,000, 6,000,000,000.

6,000,000 would be disappointing,

even billion might be disappointing.

6,000,000,000 base pairs.

6,000,000,000.

6,000,000,000 base pairs.

And when you have your full complement of chromosomes

and this is in most of the cells in your body

and outside of your sex cells,

the sperm or the egg cells.

This is going to be spread over 46 chromosomes.

46 chromosomes or I guess you could say

23 pair of chromosomes.

If you divide 6,000,000,000 by 46

you get a little over on average 100,000,000.

I think it's a 100 and something million

base pairs per chromosome.

And some chromosomes are longer,

actually the longest are over 200,000,000

and some might be shorter.

That's just on average.

Now this number might to some of you might be exciting.

You're like, "I thought I was a simple creature.

"I didn't know I was this complex."

6,000,000,000, that's a lot of base pairs.

That feels like a lot of information.

For others of you it might not feel so great.

You might say, "Hey, wait I could store

"this much information on a modern thumb drive

"or on a hard disk.

"I thought I was more unique than that."

And of course we all are special and unique.

You're gonna say 6,000,000,000 base pairs.

I thought I was, you know,

I was infinitely complex and whatever else.

There's some arguments for that

along some other directions,

but this is the approximate length I guess you could say

or the approximate size of the actual human genome.

And when we talk about chromosomes

and we'll talk about chromosomes in much more depth,

imagine taking this zoomed in thing

that you have right over here

and you know, over here, I don't know how many we have,

Like one, two, three, four, five,

six, seven, eight, nine, 10,

11, 12, 13, 14, 15, 16, 17, 18, 19.

We have about 20 base pairs depicted here.

Imagine if you had about 200,000,000 of these base pairs

and then you were to take this thing

and you were to kind of coil it up

into that thing is a chromosome.

It is a chromosome and you're saying, "Wait,

"I have that much information in

"most of the cells of my body.

"This thing must be incredible compact."

And if you said that I would say,

"Yes, you are correct."

This, the radius, the radius of the DNA molecule

is on the order of one nanometer.

One nanometer which is a billionth of a meter.

So you can start to assess

kind of the scale of this thing.

This is a very dense way

to actually store information.

But just to have an appreciation of

and you might have seen it when I was coloring in

on why the structure lends itself

to being able to replicate the information

or even to be able to translate or express the information.

Let's think about if you were to take this ladder

and you were to just kind of split all the base pairs.

So, you just have 1/2 of them.

So you essentially have half of the ladder.

And so if you only have half of the ladder,

you're able to construct the other half of the ladder.

Let's take an example, let's say

and I'll just use the first letter to abbreviate

for each of these bases.

Let's say you have some...

So let's say this is one of the,

this is the sugar phosphate backbone right over here.

So this could be one of the sides.

Let's say there's some adenine.

Actually we do in the right color.

So you got some adenine, adenine.

Maybe some adenine right over here

and maybe there's an adenine there.

And maybe you have some thymine, thymine,

maybe thymine right over here

and then you have some guanine,

guanine, guanine.

And then let's say you have some cytosine

and you have some cytosine.

So with just half of this ladder I guess you could say,

you're able to construct the other half,

and this is actually how DNA replicates.

This ladder splits and then each of those

two halves of that ladder

are able to construct versions of the other half,

or versions of the other half

are able to constructed on top of that,

on top of that half.

So how does that happen?

Well, it's based on how these bases pair.

Adenine always pairs with thymine

if we're talking about DNA.

So if you have an A there,

you're gonna have a T on this end, T on this end.

T's right all over here, T right over there.

If you have a T on that end

you're gonna have an A right over there.

A, A.

If you have a G, a guanine on this side,

you're gonna have a cytosine on the other side.

Cytosine, cytosine, cytosine.

And if you have a cytosine

you're gonna have a guanine on the other side.

Hopefully that gives you an appreciation

of how DNA can replicate itself.

And as we'll see also how this information can be

translated to other forms of either related molecules

but eventually to proteins.

And just to kind of round out this video,

to get a real visual sense

what the DNA molecule looks like

or I guess a different visual depiction from this.

I found this animated gif

that, you know, if you haven't fully digested

what a double helix looks like, this is it.

And you see here, you see your

sugar phosphate bases here.

You see kind of the sugars and phosphate,

the sugars and the phosphates

alternating along this backbone,

and then the rungs of the ladder are these base pairs.

So this is one of the bases,

that's the corresponding,

that's this corresponding, I guess you can say partner.

And you can see that along all the way up and down

in this molecule.

Very exciting.