Genes, proteins, and cells | Genes, cells, and organisms | High school biology | Khan Academy

So when I was younger, around seven or eight years old, I used to have a betta fish named Bob, and he happened to be a blue colored fish. Now, I've always wondered how he got his color. For example, were his parents also blue? Did he have any siblings that were blue?

So today, let's try to answer in simple terms how Bob gets his blue color, and we'll start by defining genes.

Genes—what are genes? Well, genes are basic hereditary units that are, well, first of all, they are passed down from parent to offspring. So I'll write: passed down from parent to offspring. They also contain information about an organism's traits. They contain info about an organism's traits. So it would only make sense that since Bob is a blue colored fish, he must have a parent or ancestor who is also blue colored.

Now, a single kind of gene can have many different versions, and we call these versions alleles. Alleles are just different variations of a single gene. So for example, there may be a gene that provides information about beta fish coloration. So let me draw an arrow from Bob to, we'll say, color gene. One allele of this gene may lead to a blue colored fish, so I'll write here: blue color. Another allele may lead to green coloration, and I'll write here next to blue color: green color.

But physically, what are genes exactly? Well, genes are part of DNA, or deoxyribonucleic acid. And I know it's a really big word, so let me write it out for us: DNA, deoxyribonucleic acid. DNA is a macromolecule, or a really big and complex molecule. So when you hear "macro," just think big or complex molecule. You can also think of DNA like a giant cookbook of genetic information because that's what it is. It is a cookbook of genetic information.

So here is a close-up sketch of DNA, and what I'm showing here is that DNA consists of subunits called nucleotides. Nucleotides are represented by A's, T's, C's, and G's. Now, genes are like the individual recipes. You can think of them as the recipes found inside your DNA cookbook. So they are specific segments of nucleotides within the long DNA molecule. A gene could be from here to here, and we think of this as a recipe.

But what do these gene recipes make, anyways? Well, many genes encode proteins, which are made of long sequences or chains of amino acids. So I'll write here under proteins that proteins are made of amino acid chains, and genes provide instructions for how to create these chains.

So thinking back to how genes are like recipes, because each gene has a different set of quote-unquote ingredients, or in this case, a different nucleotide sequence, different genes would therefore encode different proteins. So proteins, in this example, if you want to continue on with this analogy, would be like the finished products of your recipes.

Remember how I mentioned that alleles are different versions of a specific gene? Well, different alleles have different nucleotide sequences and are therefore likely to also encode different proteins. So I'll write under here: different alleles and draw an arrow to different proteins. So different genes and different alleles can give us different proteins.

So a blue fish like Bob likely has a distinct allele that encodes proteins, which give him his natural blue color, or his natural blue shade. Now, an organism has many different genes, which means an organism has the capacity to produce many different kinds of proteins. Proteins serve all kinds of functions, some of which include growth, sending messages, oh, also catalyzing chemical reactions if you have heard of enzymes before. So I'll write: catalyzing chemical reactions and providing structure.

Ultimately, it's the activity of these different kinds of proteins that help determine an organism's physical traits, just like how proteins can determine the color of Bob the betta fish.

Now, you might be wondering too what genes do in the big picture. For example, not all of the cells in Bob the betta fish function in the same way, right? You could say that there are different types of cells that perform specific functions, and this is known as cell specialization.

So cell specialization is when different cells specialize in different functions, and each type of specialized cell contains a unique combination of proteins that give the cell its specialized function within an organism. So I'll write here that cell specialization comes from unique combos of proteins.

And it's important to note that every cell in an organism contains the same genes. So going back to Bob here, we can say that all his cells contain the same DNA and genetic information, whether it's on his tail or in his eye. But what makes their functions different is which subset of genes are expressed, or used, to build proteins in the different cell types. So I'll write here: all of the cells in Bob the betta fish have the same DNA and genes, just different protein combinations.

So today we learned about genes, which are passed down from parent to offspring, just like how Bob the blue betta fish got his color. And they contain information about organisms' traits. So genes are parts of longer DNA molecules, and they consist of specific segments of nucleotide bases.

So thinking back to our cookbook and recipe analogy, right? Genes can encode all kinds of different proteins, and it's this unique set of proteins within each cell that gives the cell its specific function within the organism, as we talked about with Bob the betta fish, and all the cells involve having the same DNA and genes, just different proteins.