<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] NARRATOR: People do all sorts of things to get attention.

And why?

It may be the last thing on his mind but this man's body is working toward this.

Whether we're thinking about it or not our bodies want to make babies.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] And our bodies are very good at it.

Around the world, about 365,000 new babies get made every day.

But as ordinary as it seems creating a new human being is no simple feat.

Just think of it.

No matter who you are, once upon a time, you looked like this.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] From a single cell, you built a body that has 100 trillion cells.

You made hundreds of different kinds of tissues and dozens of organs including a brain that allows you to do remarkable things.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] How did you do it?

Today, we can look closer than ever before-- into the womb... into a cell... <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] into the essence of life itself.

Not only can we see what's happening but now, we're beginning to see how it happens-- the forces that build the embryo; the molecules that drive this remarkable change.

We're uncovering the most intimate details of how life is created-- <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the secrets behind "Life's Greatest Miracle."

[Captioning sponsored by PARK FOUNDATION NORTHWESTERN MUTUAL FOUNDATION, SPRINT PCS THE CORPORATION FOR PUBLIC BROADCASTING <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] and VIEWERS LIKE YOU] LITHGOW: You might think all the people on this beach are just working on their suntans.

But beneath all that sunscreen, under the skin <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] there's a frenzy of activity.

Without even thinking about it almost all the adults here are busy trying to reproduce.

They can't help themselves.

The urge to procreate is a fundamental part of life not just for us, but for all life.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ( purring ) ( clicking ) Why is this urge so universal?

At least some blame can probably go to this-- DNA, the molecule that carries our genes; <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the chemical instructions for building our bodies and keeping us alive all wrapped up in a tiny winding staircase.

DNA has run the show for more than four billion years for one main reason: It's very good at making copies of itself.

The copies can get passed to a new generation in a couple of ways.

If you're a bacterium, you might be into cloning-- <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] making exact replicas of yourself.

All your descendants have the same DNA and, except for an occasional mutant, are just like you.

It's simple, it works and genetically, it's extremely boring.

It can also be dangerous.

If humans were all clones everyone would have the exact same immune system <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] and one successful parasite could wipe us all out.

Fortunately, there's sex-- the method of choice for 99.9% of the organisms on Earth more complex than bacteria.

With sexual reproduction two individuals each provide some DNA.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Most animals put it into sperm or eggs.

If the two can get together, a new being will be created-- one that's different from its parents and everybody else.

( penguins squawking ) Where there's sex, there's variety.

And when it comes to survival of the fittest variety has a definite advantage.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] All this comes at a price.

Sexual reproduction may be popular but it's also quite tricky.

To get an idea of how tricky just take a peek inside a man's testicle.

It's packed with tiny tubes, coiled into bundles.

Stretched out, they could cover half a mile.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Inside all this tubing, the average man is churning out a thousand new sperm every second.

That's about a hundred million new sperm every day and more than two trillion over a lifetime.

And here's the tricky part: Each and every sperm is one-of-a-kind-- carrying a unique genetic package.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] How is this possible?

How can one person produce so many different combinations of genes?

The answer lies in the very special way we make sperm and eggs-- a process called "meiosis."

In almost every cell of your body you have 30,000 or more different genes spread out on very long strands of DNA, called "chromosomes."

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Most cells have two versions of every gene on a total of 46 chromosomes.

Exactly half of those, 23, came from your mom and 23 came from your dad.

They come in pairs, where the partners are very similar but not quite the same.

The only time they get together is during meiosis.

Here's how it works inside a testicle that's making sperm.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] First, each chromosome makes an exact copy of itself keeping it attached at one point.

They condense, creating an X-shape.

Now, the chromosome partners get together and the two, or actually four, will embrace.

They cling so closely <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] big chunks, carrying whole bunches of genes get exchanged between the partners.

The cell then divides twice each time pulling the pairs apart.

<http://www.pbs.org/nova/miracle/divide.html>[type:PROGRAM][name:how cells divide][17BF] The final result is a sperm or an egg cell with 23 chromosomes, half the normal number.

By itself, the cell is incomplete but it still holds incredible promise because every chromosome now carries a combination of genes that has never existed before.

All this gene-shuffling means that within a single species there can be an enormous amount of diversity.

And the more diversity, the better the odds are that someone will survive to create a new generation.

( elephants roaring ) WOMAN: This is my mom and dad on their wedding day.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] MAN: My mom and dad on their wedding days.

You definitely have your mom's eyes.

You can see I definitely have my dad's eyebrows.

You do have your dad's eyebrows.

LITHGOW: Melinda Tate Iruegas and her husband, Sergio are expecting their first baby.

SERGIO: Here's Mom and Dad with me and my brother.

MELINDA: Oh, yeah.

My sister hadn't come along yet but this is what our little boy might look like.

LITHGOW: Their unborn child carries a mixture of genes <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] not just from them, but from all their ancestors.

Looks like the spitting image.

Mm-hmm.

You look so much like your mother here.

LITHGOW: But which genes got passed on from whom right now is anybody's guess.

...because here you are and this is what our little girl might look like.

SERGIO: I wonder if the baby will have the characteristic eyebrows that comes from my father's side of the family.

We call them the Iruegas eyebrows.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Or that it won't have my dad's nose.

Your nose.

( laughs ) It's a beautiful evening.

MELINDA: We talked about having children a lot.

He's been saying, "Five, six," you know and I was, like, "Well, let's start with one."

Yeah.

"You know, two, maybe three."

...and the baby likes the water.

Yeah.

LITHGOW: In their efforts to pass on their genes Melinda and Sergio pursue dramatically different strategies.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] I can't wait for the baby to come.

LITHGOW: Like most men Sergio has been constantly producing sperm since puberty.

But Melinda created all her eggs when she looked like this-- a fetus in her mother's womb.

<http://www.pbs.org/nova/miracle/program.html>[type:PROGRAM][name:watch this program again online][C9CD] Within a couple of months, she created several million eggs.

And then, the eggs began to die.

At the age of 31, Melinda may only have a few thousand left.

But that's okay, because inside an ovary as opposed to a testicle it's quality, not quantity, that counts.

Every month, one of a woman's two ovaries selects an immature egg cell to lavish with attention.

Hundreds of support cells tend the egg feeding it until it grows fat.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] When it's ready, the whole entourage-- the egg along with its helpers-- oozes out of the ovary.

Waiting for them is the open end of the Fallopian tube which leads to the uterus.

Its tentacles capture the egg and pull it inside.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] The egg is swept along by muscular contractions of the tube as well as the constant swaying of tiny cilia.

The egg has everything it needs to start a new life except for one thing, DNA from a sperm.

And it has to get it fast.

If the egg is not fertilized within a few hours, it will die.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] With sex, there will always be pressure to meet and impress a mate.

When it comes to actually choosing a partner there's a lot to consider.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] For us, it might be somewhat more complicated than picking the one that smells best.

But there's no doubt that the process can be heavily influenced by chemistry-- natural drugs that flood the brain.

WOMAN: ♫ Mi amor, mi amor, mi amor... ♫ ♫ Mi amor, mi amor, mi amor... ♫ <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ♫ Mi amor, mi amor, mi amor!

♫ LITHGOW: When love is in the air the body can undergo some dramatic changes.

Signals from the brain speed up the metabolism of glucose.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] As a result, body temperature rises, skin sweats heartbeat and breathing get faster.

In a man, hormones cue blood vessels to relax allowing the spongy tissue in the penis to fill with blood.

At the height of sexual excitement millions of sperm are squeezed out of storage and swept up by fluid gushing from several glands including the prostate.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] The flood carries them into a 15-inch-long tube looping into the abdomen and then out through the penis.

It's only about a teaspoon of liquid but it typically contains about 300 million sperm.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] They are immediately in peril.

The vagina is acidic, so the sperm must escape or die.

They start to swim... at least some of them.

Even in a healthy man 60% of the sperm can be less than perfect like this one with two tails.

For these guys, the journey is over.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But what about the rest?

What are the chances that one tiny sperm will reach and fertilize an egg?

Sperm are often portrayed as brave little warriors forging their way through hostile terrain to conquer the egg.

Nothing could be further from the truth.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] For every challenge the sperm face success is, to a great extent, controlled by the woman's body and even the egg itself.

Take the sperm's first obstacle, the cervix-- passageway to the uterus.

Most of the time, it's locked shut plugged with mucus that keeps bacteria and sperm out.

But for just a few days a month around ovulation <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the mucus becomes watery and forms tiny channels that guide the sperm through.

Arriving inside the uterus the sperm are still about six inches away from their goal-- at least a two-day swim.

But undulations of the uterine muscles propel the sperm into the Fallopian tube within 30 minutes.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Even a sperm that reaches the tube in record time has no guarantee of fertilizing an egg.

There may be no egg there.

Ovulation could still be days away.

It's the slowpokes-- caught up in the cilia lining the tube-- who may have a better chance.

It's probably here that chemicals in the woman's body <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] alter the sperm's outer coating.

Only those sperm that are altered can get a date with the egg.

The sperm are released gradually over the course of a few days, so at any given time only a couple hundred sperm will move on.

If all goes well then farther up the tube they'll find the egg.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But it's heavily chaperoned by support cells and the chaperones are picky.

Only some of the sperm are let through.

Those who make it will face yet another challenge.

Underneath the cloud of cells the egg itself is encased in a thick protein shell called the zona.

To fertilize the egg, the sperm must break through the zona; <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] but even the strongest can't do it by brute force alone.

The egg demands a proper introduction.

Proteins protruding from the sperm's cap must hook up precisely with a set of proteins on the egg's surface.

If they match, the sperm is held fast and undergoes a dramatic transformation.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] It sheds its outer coating, releasing powerful enzymes that dissolve a hole in the zona allowing the sperm to push its way through.

The final hurdle passed the sperm still does not thrust its way into the egg itself.

Rather, the membranes of the two cells fuse and the egg draws the entire contents of the sperm inside.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] I don't know.

We weren't being as careful as we should have been and October came around and I was a day late and actually I was having some other problems, with my wrist.

And we went to the doctor and the doctor had asked me was, like, "Well, are you pregnant?"

you know, because he wanted to do an X ray of my wrist.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] And I said, "No."

And then I thought about it and I was, like, "Well, I don't know."

I decided that I better check this out.

And sure enough, it was positive.

And when he came home, I was like...

I could tell she had something to tell me.

( chuckles ) And, um... so, I was, like "Well, you'd better sit down."

You just...

It was something that we had discussed but hadn't anticipated until about two more years down the road.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] So when she told me, yeah, I was ecstatic.

We were ready.

We were definitely ready even if it was a little early.

LITHGOW: Ready or not, once sperm and egg get together they have their own agenda, to create a viable embryo.

Their chances aren't great.

It's estimated that more than 50% of all fertilized eggs fail to develop.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] If it's going to survive, the egg has a lot of work to do.

First, it orders the zona to lock out all other sperm.

And then the egg must finish meiosis expelling half of its chromosomes into this tiny pouch, called a polar body.

With the door closed behind it the single sperm already inside releases its precious cargo.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] The sperm's 23 chromosomes stretch out in the roomy, welcoming egg.

The chromosomes of sperm and egg approach each other and then the cell divides.

Since the moment the sperm entered the egg 24 hours have passed.

All this time the fertilized egg is moving down the Fallopian tube <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] toward the uterus.

Every few hours, the cells divide... Four... eight... sixteen.... gradually creating the building blocks needed to construct an embryo.

On rare occasions the tiny cluster of cells splits into two groups and creates two embryos, identical twins.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But most of the time, the cells stick together.

They must complete just the right number of cell divisions before they arrive in the uterus about five days after fertilization.

What started as a large, single cell has divided into just over a hundred much smaller cells.

But they're still trapped within the hard shell of the zona.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Now called a blastocyst the bundle of cells must do two things to survive: break out of the zona and find a source of nourishment.

At the beginning of the sixth day, it orchestrates an escape.

It releases an enzyme that eats through the zona and the ball of cells squeezes out.

Free at last, the blastocyst lands on the blood-rich lining of the mother's uterus.

<http://www.pbs.org/nova/miracle/stemcells.html>[type:PROGRAM][name:the stem cell debate][F142] It has just passed one hurdle but is immediately presented with another.

For, in fact, it is now in very grave danger.

Stripped of its protective coating the blastocyst could be attacked by the mother's immune system as a foreign invader.

White blood cells would swarm in to devour it.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] In its own self-defense the ball of cells produces several chemicals that suppress the mother's immune system inside the uterus in effect, convincing the mother to treat it like a welcome guest.

Then it is free to get to work.

Searching for food and oxygen cells from the blastocyst reach down and burrow into the surrounding tissue.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Eventually, they pull the entire bundle down into the uterine lining... And sooner or later, the mother will notice.

Even brushing my teeth would make me... the minty flavor was just, like, gross and make me feel nauseous.

And I would get up and I would try to eat something and if anything smelt... off slightly then it was... it made me nauseous.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] My mother has told me stories of how my father had gone through morning sickness.

And, of course, that never really registered until the first time it started happening to me.

He literally got...

He would get really, really nauseous and upset and actually get physically ill sometimes.

There was a couple of times when that... well, more than a couple of times when that actually happened.

LITHGOW: Not everybody gets morning sickness.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Sometimes months can go by before the mother gets any sense of the drama unfolding within her body.

One milestone event takes place just two weeks after conception when the blastocyst is about the size of a poppy seed.

This is the moment when the cells start to organize themselves into an embryo.

The process is called gastrulation.

With animals like frogs <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] whose embryos develop inside transparent eggs it's easy to see it in action.

After the egg becomes a hollow ball of many cells some cells dive into the center, forming layers which will go on to develop into different organs.

In humans, gastrulation happens deep inside the mother's uterine lining so it can't be photographed.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But we think it works something like this.

The blastocyst creates two oblong bubbles one on top of the other.

Sandwiched between them is a thin layer of cells.

These are the cells which one day may become a baby.

At the beginning of gastrulation some cells begin moving toward the center.

Then they dive downwards, creating a new, lower layer.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] More cells plunge through squeezing in between, forming a third.

The cells in the three layers may not look different but for each layer, a very different future lies ahead.

The lower cells are destined to form structures like the lungs, liver and the lining of the digestive tract.

The middle layer will form the heart, muscles, bones and blood.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] And the top layer will create the nervous system including the spinal cord and the brain as well as an outer covering of skin, and eventually hair.

This is a human embryo three weeks after fertilization.

Less than a tenth of an inch long its neural tube, the beginning of the nervous system is already in place.

A couple of days layer, the top of the tube is bulging outwards <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] on its way to becoming a brain.

With the primitive brain cells exposed we can see some are sending feelers making connections to their neighbors.

As the days pass, changes proceed at a rapid-fire pace throughout the embryo.

Everywhere, cells are multiplying.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] And they're on the move.

Some reach out to one another, forming blood vessels.

A heart begins to beat.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] As the embryo lengthens, the precursor to the backbone forms.

Groups of cells bulge out on the sides-- the beginnings of arms and legs.

This is the embryo 4½½ weeks after fertilization.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] It is only about a fifth of an inch long.

The primitive backbone now curls into a tail which will disappear in a few weeks.

A large brain is developing and on the side of the head, an eye.

How does this happen?

How does the embryo transform itself from a blob of cells into different tissues and organs and finally into a fully functional baby?

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] The secret, of course, lies in your genes, in your DNA.

Inside most every cell in your body you have the same 46 chromosomes carrying the same genes.

But not all the cells in your body are the same.

Nerve cells... blood cells... <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] cells lining your intestine... they all look different and they do different jobs.

That's because in each of these cells different groups of genes are turned on and when a gene is turned on it tells the cell to construct a particular protein.

Proteins are the molecules that build your body-- like collagen, a fiber that makes up much of your skin, tendons and bones.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Or keratin in your hair.

Crystallin is the protein that helps make the lens of your eye clear.

Some proteins do work; actin and myosin move muscle fibers.

Hemoglobin in the blood carries oxygen from the lungs to the rest of the body.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] So when the embryo is developing how does a cell turn on the right set of genes and create the right proteins?

Part of the answer seems to be... location.

Once the basic body plan is established-- with a head on one end, back and front <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] and left and right sides-- cells seem to know exactly where they are and what they are supposed to become.

This is because cells talk to each other in the form of chemical messages.

Chemicals in one cell can trigger a reaction in the cell next door that can spread to the cell's nucleus and turn genes on or off.

But what's really going on in there?

How does a gene get turned on?

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] If all the DNA in a single cell were stretched out it would be about six feet long but it's all wound up very tightly coiled around balls of protein.

For a gene to be turned on something has to come in and loosen up the right section.

Then, the cell's machinery can latch on and read the DNA the first step on the long road to building a protein.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Those molecules that can turn genes on play a key role in every aspect of development including the process that transforms the embryo into a boy or a girl.

<http://www.pbs.org/nova/miracle/determined.html>[type:PROGRAM][name:how sex is determined][FF42] SERGIO: We didn't want to know.

We wanted to do it, I guess, the old-fashioned way.

MELINDA: Well, you kind of wanted to know.

We did a wedding- ring test where you took a piece of your hair and the wedding band, and you hold it over the belly and if it moves one way in a circle, then it's a girl if it moves in a straight line it's a boy.

And that said it was a girl.

( baby's heart beating over microphone ) MELINDA: And there was a point when we went into the ultrasound where I was waffling.

It was, like, "Well, we could look.

<http://www.pbs.org/nova/miracle/windows.html>[type:PROGRAM][name:windows on the womb][E92D] "At this very moment we could look and we could find out," and I didn't say anything.

See, but I was trying to be strong because she was very adamant about not...

I said, "No, no, no."

LITHGOW: By the time most ultrasounds are done around 18 weeks or so, doctors can sometimes make out the sex.

But in the early weeks, it's impossible.

Take a look at a seven-week-old embryo.

Try to guess what sex it is.

Think it's a boy?

Believe it or not, this is not a penis-- at least not yet.

It might become one but it could just as easily turn into a clitoris <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the female sex organ.

At this stage, boys and girls look exactly alike.

And not just on the outside.

Inside, there are two gonads which could become testicles or ovaries.

And there are two sets of tubes: one in case it's a boy, the other for a girl.

Of course, there is one way to tell the difference-- <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] look at the chromosomes in a cell from the embryo.

One pair among the 23 determines sex.

An embryo with two X chromosomes usually becomes a girl.

If one of those Xs is a Y, it will most likely be a boy.

Recently scientists came up with a good idea of how this works.

There are only about 30 genes on the Y chromosome.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] One of them is called SRY.

This gene seems to function just once in a lifetime late in the sixth week of embryonic development and only in one place: the gonad.

SRY turns on for a day or two and the cells churn out its protein.

But in that short time SRY sets off a chemical chain reaction turning on other genes eventually turning the gonads into testicles <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] which begin to make testosterone.

Testosterone travels throughout the body.

If it reaches the genitals then the cells here will build a penis.

But if there are two X chromosomes and no Y different genes get turned on, and the gonads become ovaries.

The embryo becomes a baby girl.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] This is the power of genes-- creating cascades of chemical reactions defining the form and function of all the cells in your body.

Sometimes genes send the message to multiply and grow as with the arm and leg buds.

Sometimes the message is to die as it is a few days later, to the cells between the fingers.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] As the weeks pass the embryo's genes send billions of individual messages constructing new kinds of cells and building organs and limbs.

Two months after fertilization the embryo is now called a fetus.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Almost all its organs are in place though they're not working yet.

The whole fetus is just over an inch long and weighs less than a third of an ounce.

Over the next 6½½ months it will grow almost 400 times larger and prepare for birth.

All of this demands a constant supply of nutrients.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Serge was a little frustrated because he thought he was going to be able to go out and get me whatever I craved and whatever I wanted and I had the problem of I didn't want anything or crave anything until I smelled it and I had to smell my food before I would eat it.

I could cook a whole meal and if it didn't smell right when I was done with it, you know just because I put the wrong spice in there or something then I couldn't eat it.

Well, what about this place here?

( sniffs ) Let's check out the menu.

Mmm... no.

No?

No.

SERGIO: We would go out on walks sometimes just around in the square.

<http://www.pbs.org/nova/miracle/journal.html>[type:PROGRAM][name:a woman's journal][BC51] What do you think?

( inhales deeply ) No, It's not going to work.

SERGIO: She would have to smell it first.

Just see what caught her fancy at that time.

Yeah.

Yeah?

SERGIO: As soon as it did then that's where we would go.

MELINDA: And that lasted throughout my entire pregnancy.

How is it?

Garlicky, yum.

Baby likes it?

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Yeah, I'm pretty hungry.

MELINDA: I'm still like that.

I still really want to smell my food and if I smell something and I'm just, like "Oh, I have to have that, and I have to have it now."

LITHGOW: It's no surprise that Melinda might be especially hungry.

The fetus she's carrying has only one source for all the raw materials it needs to grow into a baby... Melinda's blood-- which is systematically raided with the help of the placenta.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] The placenta began to form as soon as the blastocyst burrowed into the mother's uterus and in the early weeks, it dwarfed the embryo.

The underside of the placenta is covered with thousands of tiny projections, called villi which lie in pools of the mother's blood.

Without ever mixing the blood of mother and child <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the villi grab oxygen and nutrients.

The enriched blood flows about a foot and a half through the umbilical cord, back to the fetus whose heart beats about twice as fast as an adult's.

The heart is one of the few organs that actually work during the earliest weeks of development.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But with other organs, function comes later.

With the eye although the retina and lens are well formed by the ninth week the fetus doesn't respond to light until the fifth or sixth month.

And the same for the ear.

The outer ear quickly takes shape but the fetus can't hear yet.

Sound conduction relies on the tiny bones of the inner ear <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] and most of the bones in the fetus start out as cartilage.

By the fourth month, hard bone can be seen forming in the hand and the leg.

Finally, after five months the process is complete in the inner ear.

And then, the fetus begins to hear sound.

SERGIO: I would sing songs right on her belly <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] just so that it could hear my voice and get to know my voice, but there was... And what else?

And make whale noises.

( chuckling ): Yeah.

( humming in high pitch ) ( laughing ) SERGIO: One of the first times I did that the baby seemed to move its hand across her belly, and kind of touch my lips or at least I like to think it was a hand saying hello or something.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] And I even play music.

You know, I wanted to see what would happen to what different kinds of music.

And, you know, Mozart, it, you know, was mellow kind of made some movements.

And then I put salsa music in and it just started kicking, almost in rhythm.

So, it was great.

Got a particular beat that it likes.

Yeah.

There's three of us dancing.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ( laughs ) This is the baby moving.

LITHGOW: Inside Melinda's belly a remarkable transformation has taken place starting with the moment egg and sperm met.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A]<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A]<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Inside the womb, the first weeks are the most dramatic.

Later in pregnancy when the mother's body seems to be changing the most <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] life in the womb can appear, well, a bit uneventful.

All the organ systems are in place so during the last trimester, the fetus's main job is to grow.

But a few crucial events are unfolding beneath the skin.

Fat deposits are forming building reserves the baby will rely on after birth.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] But even more importantly fat is getting laid down in the brain.

In the sixth month, genes in the brain order the manufacture of a fatty substance called myelin which wraps around the long connections between brain cells.

This fatty covering allows nerve impulses to travel up to 100 times faster, greatly enhancing brain power.

The process will continue for years after the baby is born.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] INSTRUCTOR: Inhale... come up.

LITHGOW: The brain's hunger for fat in the last trimester puts an enormous strain on the mother.

INSTRUCTOR: Inhale.

LITHGOW: Over the course of the pregnancy her body has increased its own blood supply by about 50%-- all for the sake of the rapidly growing baby.

But late in pregnancy <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] the baby's need for fat becomes so great the mother can't keep up.

If it stays inside, the baby will begin to starve.

Somehow, it's got to get out.

( breathing rapidly ) It's climbing back up now.

( breathing rapidly ) <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] MELINDA: I've only had, like, one anxiety attack and it was the moment I was in the bathroom and I just had the thought of, like how's this baby going to get out?

I just don't think he's going to make it out.

And I hadn't really thought about it up until that very moment, where I was just, like, "No."

( chuckles ) <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] I love you.

LITHGOW: Giving birth is one of the most amazing experiences a woman can have.

It can also be one of the most painful.

( breathing rapidly ) ( moans ) ( machine humming ) Yeah, it's starting to go down.

Remember?

Think about being in that garden.

LITHGOW: Again and again, the uterus contracts <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] as the cervix opens up.

( breathing rapidly ) The tiny passageway that once allowed the entrance of a single file of sperm must now widen to about four inches to accommodate a baby's head.

( moaning while breathing rapidly ) LITHGOW: Human births are far more dangerous than those of other mammals, or even other primates.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ( Melinda moaning ) LITHGOW: The human brain is three to four times bigger than an ape's brain.

And the pelvis is narrower, to allow us to walk upright.

A human baby has to go through considerable contortions to make it through the narrow opening.

Sometimes, there is simply not enough room.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] If that happens today Melinda's baby can be delivered by Cesarean section.

But not long ago, before the rise of modern surgery death was a common outcome for the baby and the mother.

SERGIO: I can't help but feel a little guilty that I'm responsible for this but it's part of the natural cycle of life and I just want to be there in any way that I can <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] to support her through this whole process.

LITHGOW: Because of the pain and danger of human labor, we regularly give birth in the presence of others.

Today, at 4:25 a.m., Melinda's parents, along with Sergio will have the privilege of witnessing firsthand this extraordinary event-- life's greatest miracle.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Good job, good job.

Good job.

Can you get another breath?

Terrific.

( Melinda groans ) Yeah, grab it again.

Oh, that's excellent.

One more time.

Quick breath and push it right down some more.

( exhales audibly ) Oh, good, good, good.

( Melinda yells ) <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] Good job.

( Melinda yells ) Okay, stop.

( yelling ) ( Melinda yelling ) MR. TATE: Here we come!

A life!

A new life!

Here we go!

NURSE: 4:25.

All right!

( crying ) A little boy!

( crying ) MR. TATE: Hey, we have a new sound!

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ( baby crying ) Look.

Look at our little boy.

Hi, sweet pea.

Hi.

NURSE: Isn't he adorable?

How are you?

Look how handsome you are.

<http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] You're so handsome, mm-hmm.

SERGIO: We've been wondering who you were.

Yeah... yeah, we've been playing with you.

( chuckling ) <http://www.pbs.org/nova/miracle>[type:PROGRAM][name:NOVA: "Life's Greatest Miracle"][D17A] ( Melinda and Sergio cooing ) Hi.

Oh, there you go.

( laughing ): I love that little yawn.

Wake up.

"Life'eaacle is available on vssetDVD.

#]rder PVi 1[!Y- Next time onNO new story be e poon t. We'rtalkina so in the fut @éever existed in the We'rg with iéented numbers.

Watco-houray iatation: in the Balanc NOVA is a production of WGBH Boston.