Gene expression and regulation | Inheritance and variation | High school biology | Khan Academy

By now, you are likely familiar with the idea that DNA (deoxyribonucleic acid) is the molecular basis of inheritance. You might also have a sense that it is somehow involved with chromosomes. In this video, I want to make sure we can connect the dots with all of these concepts that we have when we come to genetics.

What's a chromosome? How does it relate to DNA? How does that relate to genes? And then how do genes and DNA relate to protein or other things? Well, as you can see in this diagram from the National Human Genome Research Institute, they're showing us a blown-up cell. So this is the outer membrane of the cell that you see right over here.

In here, they are showing us the nucleus of the cell, and inside the nucleus of eukaryotes, you have your DNA. Now, the way that they have shown it, the DNA all looks like these X's over here. Now, it's important to realize that DNA isn't always in this condensed chromosome form. When DNA replicates, it is in its loose or uncondensed form.

When it condenses, maybe in preparation for, say, mitosis, then it takes on an X shape. But this X actually has two copies of the same chromosome. While they are connected, consider that to be one chromosome. But then once they separate, say during mitosis, then people would consider it to be two chromosomes.

Now, if we look at chromosomes under a microscope, you might see something like this. As you notice, they come in pairs, and this is actually what chromosomes would look like for the human genome. You have 23 pairs. The way we know that this is a biological male is by looking right over here. This short little chromosome right over here is the Y chromosome.

You can see each of these pairs have what are called homologous chromosomes, which we've talked about in other videos. It's the view that these two chromosomes code for the same genes, but they might have different versions of the genes on them. You get one of the homologous pair from your female parent and one of the homologous pair from your male parent, and we've gone into some depth in other videos.

Now, it's hard to see it at this resolution, but in this image, each chromosome has been replicated and is actually two copies connected at the central mirror. If we were to spread it out, it would have that X shape right over there. Now, the question is: How does a chromosome relate to DNA?

You can see it in this broader diagram right over here. A chromosome is actually just a very, very long strand of DNA that's all wrapped up, and there are some other molecules and proteins that are involved, like histones, like you see right over here, that help package the DNA. But that's all a chromosome really is.

In the entire human genome, we have 3.2 billion base pairs in our DNA. Just as a reminder, base pairs, you can view as each of the rungs of this ladder. Now, those 3.2 billion base pairs are divided into these 46 chromosomes in a complete human genome. So you're looking at the order of tens to hundreds of millions of base pairs per chromosome.

Now, you’re probably familiar with this idea that the genes on the DNA are actually what code for protein, and that is indeed the case. If you have a long strand of DNA right over here, not all of it is protein-coding, but you have protein-coding genes on it. If you look at the entire human genome, you're looking at about 20,000 plus protein-coding genes, each of which is made up of an average of about 3,000 base pairs. But it can vary a lot depending on the gene you're looking at.

The way that we go from these protein-coding genes, which are really just sections of the DNA on these chromosomes, to proteins, this idea is known as the central dogma of biology, or oftentimes the central dogma of molecular biology. So if this is a chain of DNA and a protein-coding gene, the process of transcription is what takes us from one half of this to an mRNA strand, which you can view as a cousin molecule of DNA.

Now, messenger RNA, in particular, then goes to the ribosomes. It goes outside of the nucleus of the cell, goes to the ribosomes. We talk about this in some depth in other videos. At the ribosomes, that information is used to actually construct proteins out of amino acids. Then those proteins, those amino acids, will interact with each other, and they'll form a shape of some protein that is valuable in the human body.

Now, what's interesting is that even though we most associate DNA with genes that code for proteins in this way, the reality is that only 1 to 2% of DNA codes for proteins. So a natural question would be: What is the other 98 to 99% of DNA doing? That's actually an active area of research, but we have some good ideas.

We know that some of that DNA helps regulate the coding of other DNA. What's interesting about the human genome, or actually the genome of any organism, is that nearly every cell in the human body has this same set of chromosomes and has all of the information necessary to code for all of the proteins that any cell might need.

The exceptions are red blood cells, which lose their genetic material, and gametes (sperm or egg cells), which have half of the genetic material. Almost every other cell has all the genetic material, but obviously, cells are different. Heart cells are different than neurons, which are different than skin cells.

So a lot of the other DNA is part of the regulatory mechanism of which genes to express, which genes should be coded into proteins, and which ones should not, as we have different types of cells doing different things. This is known as differential gene expression. Those parts of the DNA that aren't coding directly for proteins, we would call those regulatory sections of DNA.

Now, there are other stretches of DNA that, instead of producing mRNA, which then gets translated into polypeptides, could code for other types of RNA, which we would call functional RNA because that type of RNA is directly useful. It has a function; it's not just transmitting information.

For example, you have ribosomal RNA, which can be directly transcribed from DNA, and it's used to make up parts of the ribosome. You might have seen transfer RNA when we learned about translation. They are also involved in the construction of proteins.

So, I will leave you there. Hopefully, this connects the dots between the idea that most of your cells in the human body—and we're talking about tens of trillions of cells—have the entire complement of 3.2 billion base pairs that are in these 46 chromosomes that are organized in 23 pairs. They contain these 20,000 plus genes, which only make up 1 to 2% of the base pairs.

The rest, we're still exploring, but it can be involved in functional RNA and regulation, which helps different cells do different things at different times.