cell in your body has about six billion base pairs, these combinations of As and Ts, and

Cs and Gs here. To put that in perspective, a credit card is about one millimeter thick.

If you stacked six billion credit cards in a straight line you could go from

San Francisco to past the North Pole. That's a lot of plastic though, so maybe

don't. What I find fascinating though is how good DNA is at making copies of itself.

As your cells are growing and replicating, your DNA is copying its genetic data into

those new cells. And copying errors do happen, but still, they only happen at about one in

ten thousand base pairs.

Even then, our genetic machinery has checkpoints in place to make sure those copying

errors get tossed out and don't harm us.

And all that is so we can make new cells. In today's final

episode of this season of Human, we're going to learn how the cell grows and replicates,

including how it copies that DNA, and we'll get into a little stem cell talk.

In the previous episode, we talked about how complex DNA is and how it transcripts and translates genetic

code into usable proteins. That's all great — our cells made stuff. I emphasize stuff

for a reason. The proteins that we make from DNA aren't alive and don't have any genetic

information of their own. They're just kinda nonliving stuff. It becomes a little trickier

when it comes to making new cells. And the way that adults make new skin cells or connective

tissue cells is different from how they make new sperm or egg cells. The difference between

them comes back to DNA, specifically, how we store DNA in those larger structures called

chromosomes. Big picture, the purpose of all cell division is to grow cells and replace the

old ones. So our bodies have a few ways of making replacements for those cells. One way

to do that is by making identical cells from some kind of parent cell. If we think of the

parent cell as a template for its offspring, it should be a super simple copying process,

right? Well, that's where we see the first asterisks of the episode. The copying process

depends on what we're making and what genetic information we'll copy into the new cell.

To understand that, we need to get deeper into chromosomes. Chromosomes are made of

chromatin, a combination of DNA and proteins called histones. The histones are there so

we can wrap those long strands of DNA around them and pack a lot of genetic information

into a small package. We have twenty three pairs of chromosomes, one from each parent,

for a total of forty six chromosomes. All of our cells, except sperm, egg cells, and

red blood cells, have twenty three pairs of chromosomes. That makes almost all of our

cells diploid, meaning they have two sets of each chromosome. Sperm and egg cells, what

we call gametes or germ cells, are an exception because they only have one set of

their chromosomes — they're haploid. Now, this wouldn't be a biology series if we

didn't go through the stages of mitosis. I'm not going to quiz you on the names of

the phases, but this process happens in phases. The first thing your cells do is make identical

copies of the genetic material during interphase. DNA replication could fill multiple videos

worth of information by itself, so we're dramatically oversimplifying here. But after

DNA replication, you have two identical copies of your DNA. These get assembled into identical

sister chromatids. At this point, each sister chromatid gets joined together by a little

structure called a centromere.

Now the cell needs to organize all those chromatids

and prep them for division, so it condenses them them into thick, tight bands. This is

prophase, and it's the first time during the cell cycle that you can see chromosomes with

a simple light microscope. This phase is also when important structures are being built

just outside of the nucleus that we'll need in the next phases of mitosis. During prometaphase,

the nucleus starts to dissolve, which gives those newly built structures access to the

chromosomes, allowing them to attach to each one. Now, some of those new structures are

called microtubules, and in this next phase, metaphase, those microtubules line the chromosomes

up in the middle of the cell. Then during anaphase, the chromosomes are pulled towards

opposite ends of the cell. At this point, each chromatid is a brand new chromosome.

The last phase is telophase, where these new chromosomes get wrapped in a new nucleus,

giving us two nuclei in one cell. Finally, the cell splits into two identical daughter

cells. Almost every cell in your body is the result of mitosis. That single cell that would

become the trillions in your body replicated and grew and eventually, your body became

a thing because of mitosis. So then why do we have a separate process for egg and sperm

cells? We need a separate way to make these cells, in this case, meiosis, because our

goal is to end up with a cell that has half of the genetic material as its parent. Then

when a sperm and egg cell combine, their single sets of chromosomes combine into a brand new

genome, making a new diploid cell. We're not trying to make a totally perfect clone,

we're trying to make something intended to be combined with another thing. In a lot

of ways, mitosis and meiosis are really similar. Namely, the parent needs to make copies of

its genetic information. But it differs in a few ways — in meiosis, there are two rounds

of cell division instead of one, and the chromosomes swap genetic material with each other to create

a totally unique cell. That recombination and first division happens during the first

branch of meiosis, or Meiosis 1. This step is crucial, it's what gives your offspring

some kind of genetic variation. At this point, you now have haploid cells. Then in its sequel,

or Meiosis 2: Meiosis Strikes Back, they divide again really similarly to mitosis. By the

end of meiosis, we end up with either four sperm cells or one egg cell. The process is

different for each type of cell, but both undergo meiosis.

The cells that eventually become egg

cells develop while you're still in the womb, but stop growing for a period of time

until puberty. Of the thousands of cells that could become mature egg cells, about four

hundred do. Speaking of how long cells can grow for, we need to talk about a special

type of cell. In the last few years, scientists have been researching different cells that

have their own interesting replication process, stem cells. We've mentioned them a few times

throughout this series, but stem cells, depending on specifics, can differentiate into other

types of cells, or it can keep making copies of stem cells. For context, your body started

from a single fertilized egg cell and through mitosis, grew into exponentially more cells.

But at some point, those cells had to differentiate into the many specialized cells you have now.

Those specialized cells came from stem cells. They can become any type of cell in the body

and have the ability to keep replicating throughout a person's life. Because of that, scientists

are interested in studying them for certain treatments, like growing some nerve cells

from stem cells and using them for degenerative diseases like Parkinson's. In the past,

scientists were extra interested in embryonic stem cells, or stem cells that came from embryos.

Using these types of cells comes with some ethical concerns which I'm not going

to talk about since it would ignite a dumpster fire in the comments section and nobody would

be happy. Luckily, these days we have less controversial ways of making stem cells, like

turning mature cells into stem cells. Back in 2006, scientists transformed mature fibroblast

cells from a mouse to create a stem-cell-like state by activating certain genes. They called them

induced pluripotent stem cells, or iPSCs, since they could induce the cell's ability

to become stem cells. And they're pluripotent because they can become any cell type in the

body. And these things are so useful, not only because it gets around some of the ethical

concerns, but because they come from someone's mature cells, scientists could develop new

tissues that are compatible with that person's immune system and are less likely to get rejected.

Plus, we can culture them to study different diseases as well. Many scientists do animal

studies on rodents as a way to test different treatments. But obviously there are some differences

between us and mice, so treatments don't always translate a hundred percent. By taking

someone's skin cells or fat cells and reprogramming them into stem cells, we could research how

real human tissue reacts to whatever we're trying to test. This technology has only been

around since earlier this century, and in the time since, we've already seen clinical

trials aiming to improve human lives. For instance, a few different research groups

have used iPSCs in treatment trials of age-related macular degeneration, or AMD, a common cause

of vision loss for the elderly. They take a few of the patient's cells, turn them

into stem cells, then turn them into the right kind of retinal cell, and implant them back

into the patient's eye.

Sometimes, it works and it restores the patient's vision! Sometimes

it doesn't work out though, and that's something to keep in mind. This is a cool

and new science, but it's still science, not a magic cure-all. And that's one of

the things I love about this field of study. We are constantly learning new things that

our bodies can do and how they interact with the world, both the big macro world and the

tiny chemical world. We hope you've enjoyed the season, I know I've loved writing and

hosting it. For more body related content, check out our ongoing series Sick, and follow

us on all our social media for even more content. We're @seeker on everything.

I'm Patrick Kelly, thanks for learning with us.