How do you make a human? I mean, after the fertilization part. How does your body know
exactly where to put each finger and how to orient the heart? That’s where our genetic
instructions, or DNA comes in. But to understand DNA, we need to talk about creme brulee.
I have no idea how to make one of these tasty morsels, so I check out a recipe site, scroll
through a bunch of ads and boring stories, and I’ll get an ingredients list and instructions.
Now, imagine instead of doing this with the big letters and words in an online recipe
list, you’re doing the same thing at the tiny, molecular level. You have cellular machinery
that reads the instructions, and machinery that copies it, and separate pieces that assemble
the ingredients into bigger pieces. The list of ingredients in the recipe that makes our
bodies is called DNA. Today we’re talking about the genetic instructions for your body,
the cookbook that makes the ingredients that make up you. We’ll talk about the structure
of DNA and how you go from gene to those beautiful brown eyes of yours.
So you’re not a delicious
dessert, sadly. The ingredients that go into you are a little more complicated. Remember
back to episode one when we talk about biological hierarchy — your body is a collection of
organ systems, which are collections of organs, which are made of tissues, which are made
of cells. That whole thing? Well those cells are made of nonliving things like water, lipids,
and proteins. And our cells make proteins to do different jobs around the body. These
proteins aren’t alive, they’re complex bundles of amino acids that serve different
purposes. And they’re not just for building muscles. The pigment that makes your eye
color is a protein. The keratin that builds your hair is a protein. But you also need
to build proteins for a normal physiology, so you have instructions for proteins like
antibodies and enzymes in your genes as well. We have about twenty to twenty five thousand
genes, or recipes, in our cookbook. Genes are the genetic instructions that tell your
cell to make a specific protein. So your genes don’t tell your body to make brown
eyes, they tell it to make a specific protein, which collectively look like brown eyes at
the large scale. It's like you inherited this cookbook from your parents. Or rather, a copy of your
mom’s cookbook, and a copy of your dad’s cookbook. The vast majority of recipes were
the same, but a few might be slightly different. Let’s say on page five hundred thirty two
in the cookbooks, there’s a section for some kind of crunchy tortilla based food.
In your dad’s cookbook is a recipe for tacos, in your mom’s cookbook is tostadas. You’ll
end up making one of those recipes. These are alleles, variations in the same gene.
In your body, this might be an allele for freckles from your dad but an allele for no
freckles from your mom. Two versions of the same gene, but your body either has freckles
or it doesn't. Both fulfill the same role but end up with slightly different results.
Each of those recipes has to be written in a language that the reader can understand,
and in this case, the recipe is written in DNA. DNA stands for Deoxyribonucleic Acid,
it’s this long molecule with the classic double helix shape. And DNA is made from smaller
molecules called nucleotides, which have a few important pieces. Its backbone is made
up of a sugar called deoxyribose, the D in DNA, and a phosphate. Together, these give
the DNA molecule some structure. Between these backbones are four nitrogen bases — adenine,
thymine, cytosine, and guanine, or A, T, C, and G which are held together with weak hydrogen
bonds. Because of their complementary shape, A will always hook up to T, and C will always
hook up with G. So if you know what one side of the DNA strand looks like, you can tell
what the other side will look like too. ACA on one side will be met with TGT on the other.
We call these A-T and C-G pairs base pairs. As a result of DNA’s structure, these opposing
strands of nucleotides have specific directions. And just like how we need to know if the language
in our recipe should be read left to right or right to left, our cellular machinery needs
to know the direction of DNA too. To tell which part is the front and which is the back,
we need to look at the sugars in the DNA backbone. One end is called the five prime while the
other is called three prime. If you were to dissect one of the sugar-phosphate backbones,
you’d see the carbons atoms in the sugar molecule arranged in a certain pattern. Biologists
assign them a number, so this one is one prime, this is two prime, all the way to five prime.
To determine DNA’s direction, you have to look at which end is leading the sequence.
One end will have the five prime sticking out, and the other will have the three prime.
DNA is usually written starting with five prime and ending with three prime. So you
could have a sequence that goes 5 prime TTAGGG 3 prime. Bonus points if anybody can tell
me what that sequence refers to in the comments. Example aside, all the base pairs in your
DNA make up a code. They’re the genes that tell your body what proteins to make. Some
genes are short , but one gene, the gene that codes for a protein called dystrophin, is
over two million base pairs long. But how do our cells read DNA and make proteins? First,
I want to show you what’s actually making the protein. It’s this little guy called
a ribosome. In our cookbook example, the ribosome is the chef who actually puts the
recipe together. It takes amino acids and assembles them into bigger proteins. Here’s
the thing — ribosomes read RNA, not DNA. So DNA goes through a process called transcription
where it’s rewritten as RNA or ribonucleic acid. It’s chemically similar to DNA but
instead of deoxyribose sugar, it’s made of ribose. Also, instead of the Thymine, or
T base, it has Uracil. But structurally they’re quite different. DNA has double strands, RNA
has single strands and can be tweaked into different shapes. Transcription starts when
enzymes called RNA polymerase move along the DNA strand from five prime to three prime,
unwinding the DNA into two pieces. Now, remember how each base pair can only pair with one other
specific base? A with T and C with G? That comes in handy again. We can use these newly
unwound strands as templates to create new RNA. So if you know what one side is, you
know the other side will be a sequence of opposite basepairs.
These new RNA strands are made in little sections at a time. So while we
might have DNA strands that are hundreds of millions of base pairs long, most RNAs top
out at a few thousand base pairs. Some of these RNA are the end product and are useful
in the cell but others, called messenger RNA, or mRNA, go onto the next step to code
for specific proteins. These things are going to take the message from the DNA in the nucleus
out to the cytoplasm. At this point we have mRNA that’s been transcribed from DNA. This
new structure is ready to be fed into those ribosomes in a process called translation
to generate some proteins. For those wondering, the whole ribonucleic acid and ribosome thing
isn’t a coincidence. Again, these ribosomes are the chefs in our cookbook analogy — they’re
where our cells actually make the proteins, and we have millions of them per cell. When
an mRNA enters a ribosome, the ribosome reads the mRNA three letters, or bases, at a time. Each
of those three letter combinations, or codons, codes for a new amino acid. Some amino acids
like tryptophan can only be coded by one combination of bases, in this case, UGG. But some amino
acids can be coded by multiple combinations. Like tyrosine can be coded by a sequence of
UAC or UAU. This process of translation and protein building repeats, grabbing new amino
acids to throw on the growing protein. It continues until the ribosome reads a codon
that tells it to stop. Then translation is over. Our cells made the protein and it’s
off to wherever it’s supposed to go. What I find so interesting is how universal this
genetic code is. That code of amino acids coded by codons is used by almost a hundred
percent of life. It doesn’t matter if it’s in our bodies, your cat, or an E Coli bacteria.
The same three letter codes tell ribosomes to make the same amino acids. This genetic
code was discovered back in the mid 1900s and we still don’t have an answer for why
it’s so common or how it came to be. Genetics is one of the coolest studies in biology and
is obviously much bigger than one video can contain. Next time, we’ll dive into how
DNA makes copies of itself so efficiently, and where that fits in with the growth and
copying of cells themselves. Thanks for watching this episode of Seeker Human, I’m Patrick Kelly.