0:00:02 > 0:00:06It takes 120 trillion cells to make a human.
0:00:07 > 0:00:11They are the fundamental units of life,
0:00:11 > 0:00:16making up our brain, muscles, organs - every part of us.
0:00:21 > 0:00:22In the last decade,
0:00:22 > 0:00:26scientists have been able to witness what once seemed impossible -
0:00:26 > 0:00:29the world inside a human cell.
0:00:35 > 0:00:37When I was a student,
0:00:37 > 0:00:41the idea that we could burrow deep inside a living cell was unthinkable.
0:00:44 > 0:00:48Recent advances have made it so scientists can see inside cells
0:00:48 > 0:00:49like never before.
0:00:49 > 0:00:53We can see the parts of single cells and how they work together.
0:00:55 > 0:00:58The more we learn about the universe, the simpler it seems.
0:00:58 > 0:01:00But the cell isn't like that.
0:01:00 > 0:01:04The more we find out, the more complicated things get.
0:01:05 > 0:01:09But these beautiful worlds are also on the front line of the longest war
0:01:09 > 0:01:11in history.
0:01:11 > 0:01:15This is a battle that goes back into the depths of time,
0:01:15 > 0:01:19to a time when the earth was dominated by single cells and viruses.
0:01:19 > 0:01:23Every day, our cells confront these ancient virus enemies,
0:01:23 > 0:01:27tiny, ruthless machines that kill to reproduce.
0:01:27 > 0:01:31There is this whole mechanism inside cells that are taking out viruses
0:01:31 > 0:01:34that previously we just didn't know was there.
0:01:34 > 0:01:38It is a four-billion-year-old struggle that has changed the course
0:01:38 > 0:01:39of our evolution.
0:01:40 > 0:01:44This battle of these viruses against your cells,
0:01:44 > 0:01:47this amazing, epic science fiction movie,
0:01:47 > 0:01:50it's going on inside your body all the time,
0:01:50 > 0:01:52and you don't even know it.
0:02:16 > 0:02:22Cells are the basic building blocks of living tissue,
0:02:22 > 0:02:25and the smallest units of what makes us human.
0:02:27 > 0:02:29And yet...
0:02:30 > 0:02:36..beneath the surface of every one lies a world stranger
0:02:36 > 0:02:38than any in science fiction.
0:02:42 > 0:02:46A world in which a billion microscopic machines
0:02:46 > 0:02:50all play their part, working in concert through every second
0:02:50 > 0:02:52of our life.
0:02:53 > 0:02:57Every one of us in made of 120 trillion cells,
0:02:57 > 0:02:59and every one of those cells is different.
0:02:59 > 0:03:01But they contain the same instructions.
0:03:01 > 0:03:05Cells are a bit like babies. When they're born, they all look the same
0:03:05 > 0:03:06but they change very quickly.
0:03:06 > 0:03:09In different countries they learn to speak different languages,
0:03:09 > 0:03:12and our bodies are like that - some cells speak heart,
0:03:12 > 0:03:13and some cells speak liver.
0:03:17 > 0:03:21The workers of this incredible world are proteins,
0:03:21 > 0:03:24chains of complex chemicals
0:03:24 > 0:03:29that can lock together to transform into spectacular machines.
0:03:31 > 0:03:34Others work to create incredible structures,
0:03:34 > 0:03:38like the internal skeleton that holds the cell together.
0:03:38 > 0:03:43These great trusses are constantly adjusting to stresses and strains,
0:03:43 > 0:03:48building and rebuilding to give the cell its shape and strength.
0:03:51 > 0:03:54Then there are the motor-proteins, haulage workers that use
0:03:54 > 0:03:58the cell's skeleton as highways to deliver food, chemicals
0:03:58 > 0:04:02and the essential building materials of life to wherever they are needed.
0:04:06 > 0:04:10They are just one of the astonishing micro machines that keep
0:04:10 > 0:04:13this bustling community healthy.
0:04:13 > 0:04:16Scientists are asked all the time, how do things in a cell know
0:04:16 > 0:04:19how to get where they're supposed to go to do their job?
0:04:19 > 0:04:22And for sure cells are very chaotic and things are bumping
0:04:22 > 0:04:24into each other and most of that's just random.
0:04:24 > 0:04:28But enough things get where they're supposed to go
0:04:28 > 0:04:29that the entire system works.
0:04:32 > 0:04:36And powering all this activity are the cell's power stations.
0:04:38 > 0:04:42Inside these free-floating structures called mitochondria,
0:04:42 > 0:04:45turbines spin at over 1,000 times per minute...
0:04:48 > 0:04:52..recharging billions of tiny chemical batteries.
0:05:00 > 0:05:04Everything we do, every heartbeat,
0:05:04 > 0:05:08every movement, every thought,
0:05:08 > 0:05:12is powered by the batteries charged by these cellular power stations.
0:05:27 > 0:05:30And everything in this world works to a master plan.
0:05:34 > 0:05:37And the plan is protected deep in the heart of every cell.
0:05:39 > 0:05:43The nucleus is the vault containing the instruction manual for life.
0:05:46 > 0:05:48DNA.
0:05:52 > 0:05:56DNA is a chain of chemicals, organised into genes.
0:05:59 > 0:06:02Each gene holds the instructions to build a specific protein.
0:06:05 > 0:06:08The double helix contains over 20,000 instructions
0:06:08 > 0:06:12that tell our cells what to make and when,
0:06:12 > 0:06:17how to organise not just our cells, but our entire bodies.
0:06:19 > 0:06:23The double helix has become the icon of the 21st century,
0:06:23 > 0:06:25and it's pretty amazing stuff.
0:06:25 > 0:06:28There's six feet of DNA in every cell of the body.
0:06:28 > 0:06:32And if all of those bits were set out in a straight line,
0:06:32 > 0:06:35they'd reach to the moon and back thousands of times.
0:06:45 > 0:06:48But this crucial chain of chemicals would be useless
0:06:48 > 0:06:53without an army of microscopic machines that endlessly travel its length,
0:06:53 > 0:06:56repairing it and transcribing it,
0:06:56 > 0:07:01turning the DNA into instructions that the cell can understand.
0:07:08 > 0:07:11Once a gene has been copied,
0:07:11 > 0:07:14the instructions are carried outside the nucleus.
0:07:16 > 0:07:23Here, mobile factories read them and turn them into proteins.
0:07:29 > 0:07:32Up to two million different kinds,
0:07:32 > 0:07:36each with its own specific shape and purpose.
0:07:44 > 0:07:48And little goes to waste in the cell.
0:07:48 > 0:07:51Used and faulty proteins are tagged for recycling...
0:07:55 > 0:08:00..then chewed apart by powerful roving shredders called proteasomes,
0:08:00 > 0:08:04reducing them to tiny building blocks for new proteins.
0:08:07 > 0:08:11But each cell is also part of a wider neighbourhood of cells,
0:08:11 > 0:08:15all continually communicating with each other.
0:08:15 > 0:08:19Fragments of shredded proteins are constantly transported to the surface.
0:08:22 > 0:08:25Here, they are presented for inspection...
0:08:29 > 0:08:32..to be monitored by the guardians of our body's immune system...
0:08:36 > 0:08:38..our white blood cells.
0:08:43 > 0:08:47These roving soldiers check the protein fragments for signs
0:08:47 > 0:08:48of damage or infection.
0:08:57 > 0:09:00And for the moment, everything is in order.
0:09:23 > 0:09:26Every single human cell contains
0:09:26 > 0:09:29this world of breathtaking complexity,
0:09:29 > 0:09:32organised by the nuclear machines at its heart,
0:09:32 > 0:09:37ceaselessly working from instructions written down in our DNA.
0:09:50 > 0:09:53But our cells are under constant attack,
0:09:53 > 0:09:59and this cell is about to face an ancient enemy...
0:10:00 > 0:10:04..in an encounter that starts with an event so commonplace...
0:10:07 > 0:10:09..you seldom even notice it.
0:10:13 > 0:10:16Every day our bodies are constantly bombarded
0:10:16 > 0:10:18by these invisible critters,
0:10:18 > 0:10:20bacteria and viruses. But we have our skin,
0:10:20 > 0:10:24it's our first line of defence that keeps them out.
0:10:24 > 0:10:28But we have Achilles heels - we have openings to the outside world,
0:10:28 > 0:10:31our mouths, our noses, we touch things, we rub our lips,
0:10:31 > 0:10:35we rub our eyes or wipe our nose. They can get in.
0:10:35 > 0:10:37And once they're in, they're in.
0:10:50 > 0:10:52Inhaled from a sneeze,
0:10:52 > 0:10:54an alien army is being carried into our body.
0:10:58 > 0:11:01A million invaders, hellbent on destruction.
0:11:15 > 0:11:21This is one of our most common enemies - the adenovirus.
0:11:26 > 0:11:30It's a masterpiece of design,
0:11:30 > 0:11:33and each one has a single aim...
0:11:40 > 0:11:42..to breach a cell's defences...
0:11:44 > 0:11:46..and reach the nucleus.
0:11:48 > 0:11:53Once inside, any one of these viruses can take control of the cell...
0:11:56 > 0:11:59..and reproduce 10,000 times over.
0:12:01 > 0:12:03The result could be anything,
0:12:03 > 0:12:07from the common cold to pneumonia - even death.
0:12:15 > 0:12:17But our bodies are prepared.
0:12:28 > 0:12:31As the viruses approach the cell,
0:12:31 > 0:12:34they are met by a cloud of resistance.
0:12:39 > 0:12:45Antibodies, Y-shaped proteins that identify alien intruders
0:12:45 > 0:12:49patrol the space between our cells,
0:12:49 > 0:12:51looking for viruses.
0:12:54 > 0:12:59Recognising the invader, they lock to the virus's armour plating,
0:12:59 > 0:13:03shackling them together, making the viruses easy meat
0:13:03 > 0:13:07for the white blood cells that feed on alien invaders like these.
0:13:07 > 0:13:11Antibodies and white blood cells form the front line
0:13:11 > 0:13:12of our immune system.
0:13:12 > 0:13:16The immune system is certainly amazing,
0:13:16 > 0:13:19and it actually evolved to see invading microbes
0:13:19 > 0:13:22and get rid of them. But that's just one part of your body's defences.
0:13:22 > 0:13:26Our DNA encodes all these other features that help us
0:13:26 > 0:13:29to fight against virus at every single step.
0:13:39 > 0:13:41Despite the body's early immune response...
0:13:43 > 0:13:47..hundreds of thousands of viruses make it through to our cell.
0:13:50 > 0:13:54But at the surface, they face their next obstacle...
0:14:08 > 0:14:11..the cell's membrane, or skin.
0:14:13 > 0:14:16The surface of the cell is an amazingly complicated place.
0:14:16 > 0:14:20There are hundreds, maybe thousands of receptor proteins
0:14:20 > 0:14:23sticking out of the cell and they all have a unique function to play.
0:14:23 > 0:14:26Some of them will be just transporting information
0:14:26 > 0:14:28from outside of the cell into the cell.
0:14:28 > 0:14:30Other receptors can bring whole cargoes in.
0:14:34 > 0:14:38The surface of each cell is a living barrier,
0:14:38 > 0:14:41teeming with security proteins that constantly monitor molecules
0:14:41 > 0:14:43as they enter and leave.
0:14:51 > 0:14:53Small molecules like water
0:14:53 > 0:14:56and oxygen can simply seep through the membrane.
0:15:03 > 0:15:06Larger molecules, like sugar,
0:15:06 > 0:15:09are allowed entry through specialised pumps.
0:15:10 > 0:15:13But the largest deliveries require a special key
0:15:13 > 0:15:16before they are allowed into the cell.
0:15:18 > 0:15:23These protein keys are recognised by teams of mobile sentries
0:15:23 > 0:15:25that continually roam the surface.
0:15:29 > 0:15:33This sophisticated system is designed to keep harmful molecules
0:15:33 > 0:15:35out of the cell.
0:15:36 > 0:15:38But over billions of years of evolution,
0:15:38 > 0:15:43the adenovirus has evolved its very own key,
0:15:43 > 0:15:46etched into the end of these projecting fibres.
0:15:50 > 0:15:53Antibodies still cling to the some of these fibres,
0:15:53 > 0:15:58blocking many of the counterfeit keys - but not all.
0:15:58 > 0:16:02One by one, sentries all over the cell's surface are fooled.
0:16:10 > 0:16:13And the virus army quietly slips inside.
0:16:16 > 0:16:21In this ancient battle for the cell, it's round two to the virus.
0:16:30 > 0:16:33So, how far back does it go, this cat and mouse game,
0:16:33 > 0:16:36this battle between cells and viruses?
0:16:36 > 0:16:39Every indication suggests it goes right back
0:16:39 > 0:16:41to the origins of life on Earth.
0:16:42 > 0:16:46Wherever life started, very early on there was a divergence,
0:16:46 > 0:16:48two different strategies that life followed.
0:16:48 > 0:16:51One of them was to become more complex, to become cells,
0:16:51 > 0:16:54to become, ultimately, organisms like ourselves.
0:16:54 > 0:16:58The other way was to remain simple - to become viruses,
0:16:58 > 0:17:02and to exploit those cells to their own ends, to replicate themselves.
0:17:10 > 0:17:12Beneath the surface,
0:17:12 > 0:17:15the cell prepares to receive the deadly invaders.
0:17:16 > 0:17:20Fooled into thinking that the virus is an important nutrient,
0:17:20 > 0:17:24special proteins slot together to form a spherical mould.
0:17:30 > 0:17:33They pinch out a bubble of cellular membrane,
0:17:33 > 0:17:35wrapping the virus inside.
0:17:38 > 0:17:43Finally, a separate protein pinches the bubble free,
0:17:43 > 0:17:46delivering the virus into the cell's interior.
0:17:50 > 0:17:54Unwittingly, the cell has just taken a large step
0:17:54 > 0:17:55towards to its own downfall.
0:18:05 > 0:18:09Every single member of this invading virus army
0:18:09 > 0:18:12has the weaponry to ultimately destroy this cell.
0:18:13 > 0:18:17Its protein shell is a multi-layered cloak of deception,
0:18:17 > 0:18:20which has still more surprises in store.
0:18:22 > 0:18:27And at its heart, it carries a tiny string of DNA,
0:18:27 > 0:18:29its ultimate weapon.
0:18:29 > 0:18:32It's a masterpiece of evolution and design.
0:18:32 > 0:18:34And yet scientists still can't decide
0:18:34 > 0:18:36if it's actually alive or dead.
0:18:36 > 0:18:40At the level of large animals like ourselves,
0:18:40 > 0:18:43the difference between living things and non-living things
0:18:43 > 0:18:44is very obvious.
0:18:44 > 0:18:49Come down a level, though, to cells, and it becomes a bit more ambiguous.
0:18:49 > 0:18:52For our own cells, of course, you can still tell immediately
0:18:52 > 0:18:56that they are alive. Come down another level, though, to the virus,
0:18:56 > 0:19:00and it's no longer obviously alive. They don't look alive.
0:19:00 > 0:19:03Yet they behave perhaps as if they are.
0:19:03 > 0:19:06They behave with a sense of purpose.
0:19:08 > 0:19:10A virus isn't strictly alive,
0:19:10 > 0:19:13it can't make more of itself on its own.
0:19:13 > 0:19:18It only can replicate if it uses parts that it hijacks from a cell.
0:19:21 > 0:19:25But the cell still has a formidable array of defences
0:19:25 > 0:19:27to keep these killing machines at bay.
0:19:32 > 0:19:36Every delivery that the cell receives is taken
0:19:36 > 0:19:39to a sorting station, called an endosome.
0:19:41 > 0:19:45Endosomes process incoming supplies and decide where
0:19:45 > 0:19:47inside the cell they will be delivered.
0:19:50 > 0:19:54The first step of the process is to break them down.
0:19:57 > 0:20:00The virus army is about to be digested.
0:20:03 > 0:20:05The walls of the sorting stations are fitted
0:20:05 > 0:20:07with specialised protein pumps.
0:20:12 > 0:20:15The pumps draw in special atoms,
0:20:15 > 0:20:18turning the inside of the endosome into an acid bath.
0:20:25 > 0:20:30The acid breaks down large nutrients into smaller molecules
0:20:30 > 0:20:33that are easier for the cell to transport and use.
0:20:36 > 0:20:40And as the acid eats away at the virus's outer shell,
0:20:40 > 0:20:41it begins to break apart.
0:20:44 > 0:20:47This should spell disaster for the adenovirus.
0:20:50 > 0:20:53But the acid is part of its escape plan.
0:20:55 > 0:20:58The virus fibres are the first to break away.
0:21:01 > 0:21:04But their disintegration releases a special protein
0:21:04 > 0:21:06hidden inside the virus...
0:21:08 > 0:21:11..that targets the wall of the sorting station...
0:21:14 > 0:21:19..tearing the membrane apart and setting the virus free.
0:21:26 > 0:21:28But not every virus escapes.
0:21:30 > 0:21:34Many still carry antibodies locked to their surface.
0:21:34 > 0:21:37Their primary job was to alert the immune system to intruders,
0:21:37 > 0:21:40but their firm grip now ties the shell together.
0:21:42 > 0:21:44The fibres cannot break free,
0:21:44 > 0:21:48and the escape protein stays trapped inside the shell.
0:21:53 > 0:21:56Countless viruses are eaten away before they can escape.
0:22:05 > 0:22:07But enough are released.
0:22:07 > 0:22:09Now there is nothing between these viruses
0:22:09 > 0:22:12and the nucleus of the cell - their ultimate goal.
0:22:15 > 0:22:19Yet although they are just five micrometres from their target...
0:22:21 > 0:22:25..most might as well be a million miles away.
0:22:35 > 0:22:40For 90% of the army, the invasion will end here,
0:22:40 > 0:22:43floating helplessly beneath the surface.
0:22:52 > 0:22:56Although they are surrounded by the constant bustle of cellular activity,
0:22:56 > 0:22:59the inert invaders have no way of moving themselves.
0:23:05 > 0:23:08And they have no way of utilising the energy generated
0:23:08 > 0:23:10by the cells' floating power stations...
0:23:12 > 0:23:14..the mitochondria.
0:23:33 > 0:23:38Inside each mitochondrion, the food we eat and the air we breathe
0:23:38 > 0:23:42drives thousands of turbines that continually recharge billions
0:23:42 > 0:23:43of tiny batteries.
0:23:45 > 0:23:47But what is even more extraordinary
0:23:47 > 0:23:52is that scientists believe that mitochondria were once simple cells themselves.
0:23:54 > 0:23:56Then they one was swallowed by another cell,
0:23:56 > 0:24:00firing one of the greatest leaps in evolution -
0:24:00 > 0:24:01complex life.
0:24:03 > 0:24:07To be complex at all, you must have all this machinery,
0:24:07 > 0:24:09all these proteins encoded by genes.
0:24:09 > 0:24:14And to support all of that requires a tremendous amount of energy.
0:24:14 > 0:24:17All complex life share a single common ancestor,
0:24:17 > 0:24:21and that ancestor arose just once in four billion years
0:24:21 > 0:24:23of life on Earth.
0:24:23 > 0:24:26For two to three billion years it was bacteria and nothing else,
0:24:26 > 0:24:28and then this complex cell arose.
0:24:28 > 0:24:32One simple cell got inside another simple cell,
0:24:32 > 0:24:33it's a very rare event in itself.
0:24:33 > 0:24:37And once this happened, it transforms the energetic possibilities of life.
0:24:39 > 0:24:42Without that energy, evolution could never have produced
0:24:42 > 0:24:45the astonishing diversity of life that we see around us.
0:24:45 > 0:24:49Without that energy, we wouldn't see plants and animals,
0:24:49 > 0:24:54we wouldn't see ourselves. The world would be an almost sterile desert.
0:25:08 > 0:25:12Throughout each cell, hundreds of mitochondria feed energy to power
0:25:12 > 0:25:16the network of protein that make us the complex creatures that we are.
0:25:23 > 0:25:27The virus has evolved into a model of efficiency.
0:25:27 > 0:25:30But the simplicity of its design makes it useless
0:25:30 > 0:25:32without the machinery of complex life.
0:25:35 > 0:25:40But just beneath the surface, large numbers of motor proteins,
0:25:40 > 0:25:42molecular haulage workers,
0:25:42 > 0:25:46await nutrients processed for delivery by the endosomes.
0:26:01 > 0:26:04And in this billion-year arms race...
0:26:06 > 0:26:09..the virus has evolved the precise mechanism
0:26:09 > 0:26:12to attach to the cell's motor proteins.
0:26:23 > 0:26:26Now it can use the energy of the mitochondria.
0:26:31 > 0:26:33The virus is on its way.
0:26:39 > 0:26:42It has hijacked the cell's own transport system,
0:26:42 > 0:26:46and is being carried towards the nucleus and its ultimate prize,
0:26:46 > 0:26:50the DNA machines it needs to take control of the cell.
0:26:58 > 0:27:04These microscopic motorised legs are a wonder of the natural world.
0:27:04 > 0:27:07Slowed down to one-thirtieth of their normal speed,
0:27:07 > 0:27:10their movement is clearly visible.
0:27:10 > 0:27:14But at their actual speed, over 100 steps a second,
0:27:14 > 0:27:15they would appear a blur.
0:27:29 > 0:27:32But speed isn't everything.
0:27:32 > 0:27:35Cells are densely packed,
0:27:35 > 0:27:38and their internal highways are littered with obstacles.
0:27:41 > 0:27:45And these motor proteins can only move in one direction.
0:28:05 > 0:28:08For this virus, it seems to be the end of the road.
0:28:16 > 0:28:20But scientists have recently discovered the virus locks on
0:28:20 > 0:28:22to a second motor protein.
0:28:32 > 0:28:36And this one is built to move in the opposite direction.
0:28:52 > 0:28:55Together, the two motor proteins can navigate
0:28:55 > 0:28:57around almost any obstacles.
0:29:01 > 0:29:04And once again, the invader benefits.
0:29:22 > 0:29:25The virus is on the move again.
0:29:30 > 0:29:33And it leads an army of hundreds.
0:29:59 > 0:30:03It's been almost an hour since the adenovirus first attacked the cell.
0:30:06 > 0:30:09The nucleus is just one more hour away.
0:30:11 > 0:30:15Until recently, scientists thought that once the viral army
0:30:15 > 0:30:18was on the march, nothing could stop it.
0:30:18 > 0:30:23But then they found that the cell has its own internal immune system.
0:30:25 > 0:30:29There is this whole mechanism inside cells that are taking out viruses
0:30:29 > 0:30:31that previously we just didn't know was there.
0:30:31 > 0:30:34And I remember the day we published the paper about it,
0:30:34 > 0:30:37I woke up to hear it being announced on the national radio
0:30:37 > 0:30:40and then went into a shop to pick up the newspapers
0:30:40 > 0:30:42to discover it was on the front page.
0:31:18 > 0:31:21Dotted along the cell's highway system,
0:31:21 > 0:31:25a special protein searches for anything carrying antibodies
0:31:25 > 0:31:27from the surface.
0:31:27 > 0:31:30The clever thing about this protein is it uses systems
0:31:30 > 0:31:33that the cell already has in place.
0:31:33 > 0:31:35Once it's stuck to the antibody,
0:31:35 > 0:31:38it sends signals to a cellular machine called the proteasome.
0:31:38 > 0:31:42And the proteasome plays the role of recycling proteins in the cell.
0:31:42 > 0:31:45It gets brought along to the virus and it destroys the virus,
0:31:45 > 0:31:49breaking down all its parts into tiny fragments.
0:31:49 > 0:31:54Once attached, the defence protein initiates a chain reaction,
0:31:54 > 0:31:56attracting specialised tagging proteins.
0:31:58 > 0:32:01Together, they mark the virus for destruction.
0:32:11 > 0:32:13Then it's only a matter of time...
0:32:16 > 0:32:19..before the recyclers arrive.
0:32:27 > 0:32:30They rip the virus to shreds.
0:33:01 > 0:33:06Somewhere inside your body, this battle is raging right now.
0:33:15 > 0:33:18The discovery of TRIM21 provides potentially new ways
0:33:18 > 0:33:21of making therapeutics to fight viruses,
0:33:21 > 0:33:26and one way this could work is if we find ways of encouraging the immune system to make more TRIM21.
0:33:26 > 0:33:30So as soon as that virus enters into the cell, the TRIM21 is ready
0:33:30 > 0:33:33to recognise the antibodies and destroy the virus.
0:33:43 > 0:33:48By working together, the defence proteins and recycling shredders
0:33:48 > 0:33:51can destroy an army of viruses in just a few hours.
0:34:11 > 0:34:17But it only takes a single virus to take control of an entire cell...
0:34:18 > 0:34:21..spreading infection throughout the body.
0:34:24 > 0:34:26With no antibodies attached,
0:34:26 > 0:34:29this virus has evaded the cell's shredders.
0:34:39 > 0:34:43Nothing now stands between it and its target.
0:34:56 > 0:35:01The virus is now just one thousandth of a millimetre from the nucleus.
0:35:05 > 0:35:10But if it is to achieve its ultimate goal, it first has to get inside.
0:35:13 > 0:35:16Compared to the cell, the virus is tiny.
0:35:16 > 0:35:20But really they're just different versions of the same machine,
0:35:20 > 0:35:22and its only job is to copy itself.
0:35:22 > 0:35:26But the virus needs to take advantage of our cell mechanism
0:35:26 > 0:35:27for its own selfish ends.
0:35:31 > 0:35:35At the heart of every cell lies the nucleus,
0:35:35 > 0:35:37and it is a world all of its own.
0:35:43 > 0:35:46Its surface is made of the same molecules as the cell membrane.
0:35:48 > 0:35:52But entry into this world is governed by completely different gateways.
0:35:55 > 0:35:57Across the surface,
0:35:57 > 0:36:01protein arms search for molecules to draw inside nuclear pores.
0:36:06 > 0:36:09Through these gateways, billions of chemical messages
0:36:09 > 0:36:12and instructions pass between the DNA and the cell.
0:36:15 > 0:36:18But only if they are recognised by the protein arms.
0:36:25 > 0:36:29But once again, the viral shell carries a counterfeit pass.
0:36:34 > 0:36:38The arms lock on, but the virus is too large to be ferried inside.
0:36:44 > 0:36:47Thinking that they have hit an obstruction...
0:36:49 > 0:36:52..the motor proteins shunt the virus into reverse.
0:37:16 > 0:37:18Pulled in two directions...
0:37:23 > 0:37:25..the virus is ripped apart.
0:37:33 > 0:37:39But what looks like a catastrophe for the virus is, in fact, its masterstroke.
0:37:58 > 0:38:04Now the single strand of DNA it held inside is carried through the pore,
0:38:04 > 0:38:07and into the cell's control centre.
0:38:27 > 0:38:31Inside the human cell nucleus there are about 23,000 genes.
0:38:31 > 0:38:35They code for thousands and thousands of biochemical pathways.
0:38:35 > 0:38:38The virus has just got 40,
0:38:38 > 0:38:40but with those 40 it can do remarkable things.
0:38:42 > 0:38:45It's so tiny, just a piece of DNA,
0:38:45 > 0:38:49a couple of proteins to make its shell, and yet it can take over
0:38:49 > 0:38:53and wreak havoc in a huge human cell.
0:38:53 > 0:38:54It's brilliant.
0:39:06 > 0:39:09The adenovirus has proven itself a master of deception...
0:39:11 > 0:39:16..continually exploiting the cell's processes to further its own deadly aims.
0:39:17 > 0:39:20But its greatest trick is yet to come.
0:39:27 > 0:39:30The cell's DNA machines have no way of telling the difference
0:39:30 > 0:39:34between its own DNA and the DNA of the virus.
0:39:37 > 0:39:40Blindly, they set about converting its deadly code
0:39:40 > 0:39:44into thousands of instructions for the cell to act upon...
0:39:53 > 0:39:56..blueprints for the cell's own destruction.
0:40:26 > 0:40:30But the machines that turn the blueprints into proteins
0:40:30 > 0:40:32lie outside the nucleus.
0:40:36 > 0:40:40Out in the main body of the cell, the instructions are met
0:40:40 > 0:40:44by a squadron of mobile protein factories, called ribosomes.
0:40:47 > 0:40:50The ribosomes precisely follow the instruction
0:40:50 > 0:40:52and start to construct viral proteins.
0:40:53 > 0:40:56Each is carefully folded into a specific shape,
0:40:56 > 0:40:58with a unique job to do.
0:40:58 > 0:41:02These large cellular machines, ribosomes, are absolutely fundamental
0:41:02 > 0:41:05to life, and very similar forms of them are found in every type
0:41:05 > 0:41:07of living cell on the planet.
0:41:07 > 0:41:10They read the genetic information and they decode it,
0:41:10 > 0:41:12bringing in the building blocks that make up proteins
0:41:12 > 0:41:15and sticking them together to make these functional molecules
0:41:15 > 0:41:18that are going to work inside the living cell.
0:41:27 > 0:41:30Only these functional molecules
0:41:30 > 0:41:33are the kit of parts needed to build an enemy army.
0:41:55 > 0:41:57But the army will not be built out here.
0:42:08 > 0:42:13The raw material for the new army is drawn back inside the nucleus...
0:42:16 > 0:42:18..ready for construction.
0:42:33 > 0:42:35With its mission reaching its climax,
0:42:35 > 0:42:38the virus turns its attention to the cell's DNA,
0:42:38 > 0:42:41halting any process it doesn't need.
0:42:45 > 0:42:48The virus has taken complete control.
0:42:55 > 0:42:58And yet the cell still has a small window of opportunity.
0:43:01 > 0:43:04Before all normal activity stops,
0:43:04 > 0:43:08it has just enough time to send a message to the outside world.
0:43:27 > 0:43:30This parcel contains fragments of the viral army.
0:43:36 > 0:43:39The parcel merges with the cell membrane,
0:43:39 > 0:43:42and the enemy fragments are pushed to the surface,
0:43:42 > 0:43:46flags warning of the invasion that has taken place.
0:43:50 > 0:43:53If patrolling white blood cells spot the distress signal...
0:43:55 > 0:44:00..they will destroy the cell, along with the entire alien army inside.
0:44:00 > 0:44:07If not, the infection will spread from cell to cell, to cell.
0:44:16 > 0:44:19After just one day of occupation,
0:44:19 > 0:44:22the virus has complete control over the cell.
0:44:23 > 0:44:27With routine maintenance halted, the cell has started to decay.
0:44:28 > 0:44:34And all activity is now focused on building the brand new viral army
0:44:34 > 0:44:36inside the nucleus.
0:44:45 > 0:44:47The new army self-assembles.
0:44:51 > 0:44:55How do viruses know how to invade our cells, how to break
0:44:55 > 0:44:57and enter the nucleus itself?
0:44:57 > 0:45:03We know that viruses and cells co-evolved together over long periods of time, but it's more than that.
0:45:03 > 0:45:06We're actually surprisingly closely related.
0:45:06 > 0:45:09It turns out that the viruses that attack us
0:45:09 > 0:45:12are actually made from bits and pieces of our own cells.
0:45:12 > 0:45:14As our cells were evolving,
0:45:14 > 0:45:17as our nucleus itself was first coming to be,
0:45:17 > 0:45:20so these viruses were cobbled together from bits and pieces,
0:45:20 > 0:45:22and they can attack our nucleus
0:45:22 > 0:45:24because they're made of the same stuff.
0:45:26 > 0:45:29Already built into its surface are the binding sites
0:45:29 > 0:45:31for the cell's motorised legs.
0:45:36 > 0:45:38Fibres snap into place,
0:45:38 > 0:45:42arming each virus with the keys to enter other cells.
0:45:46 > 0:45:50But these shells are harmless without its instructions.
0:46:04 > 0:46:08The final component is loaded - identical copies
0:46:08 > 0:46:10of the virus's deadly DNA.
0:46:15 > 0:46:18Carried by powerful motors,
0:46:18 > 0:46:22long strands of DNA are fed into every single virus.
0:46:41 > 0:46:46All this is the result of one single virus
0:46:46 > 0:46:48getting through our cell's defences.
0:46:55 > 0:46:59It's been two days since the virus entered the body,
0:46:59 > 0:47:03and the nucleus, once the centre of cellular organisation,
0:47:03 > 0:47:06now harbours an army of 10,000 deadly viruses.
0:47:20 > 0:47:22But before it can begin its conquest,
0:47:22 > 0:47:24it has to overcome two barriers.
0:47:26 > 0:47:29The army is trapped inside the tough nuclear membrane,
0:47:29 > 0:47:32held at the centre of the cell itself.
0:47:34 > 0:47:37And then there is the skin of the cell itself.
0:47:40 > 0:47:43The protein factories outside the nucleus are instructed
0:47:43 > 0:47:45to build viral saboteurs.
0:47:55 > 0:47:57The first are released into the decaying cell
0:47:57 > 0:47:59and target its cytoskeleton.
0:48:05 > 0:48:07The effects are cataclysmic.
0:48:11 > 0:48:14Without support...
0:48:14 > 0:48:16the cell starts to collapse.
0:48:23 > 0:48:28Now the virus turns its attention to the nuclear membrane.
0:48:34 > 0:48:36A second protein is released.
0:48:40 > 0:48:44Called the Adenovirus Death Protein, it burrows into the membrane...
0:48:46 > 0:48:48..and weakens it.
0:49:01 > 0:49:05The nucleus can no longer contain the bulging army.
0:49:30 > 0:49:34Beyond the nucleus, the cell is a wasteland...
0:49:41 > 0:49:46..unrecognisable from the harmonious, buzzing community
0:49:46 > 0:49:47of just 48 hours ago.
0:49:55 > 0:50:00The cell is now completely helpless to stop the virus army
0:50:00 > 0:50:02from flooding into surrounding tissue...
0:50:06 > 0:50:10..attacking neighbouring cells
0:50:10 > 0:50:13and spreading infection throughout the body.
0:50:20 > 0:50:24The battle for this cell is over.
0:50:38 > 0:50:41But the war has only just begun.
0:50:56 > 0:50:59While the virus has been busy inside the cell,
0:50:59 > 0:51:02our antibodies have adapted and now come back in force,
0:51:02 > 0:51:07carrying new receptors, tailor-made to lock onto the escaping army.
0:51:09 > 0:51:14Yet even in these numbers, they cannot stop every virus.
0:51:17 > 0:51:19But they are not alone.
0:51:20 > 0:51:24The cell's dying message to the outside world was not in vain.
0:51:24 > 0:51:28Giant white blood cells flock to the stricken cell
0:51:28 > 0:51:31to devour the escaping hordes.
0:51:31 > 0:51:35They too are learning how to tackle this particular invader.
0:51:35 > 0:51:38Once the virus has been detected by the immune system,
0:51:38 > 0:51:41there's a heightened level of security inside your body,
0:51:41 > 0:51:44and one of the results of this is that the cells that make antibodies,
0:51:44 > 0:51:46and make the right antibody for that virus,
0:51:46 > 0:51:48will make lots of copies of themselves,
0:51:48 > 0:51:52and then they will start pumping out up to 5,000 antibodies per second
0:51:52 > 0:51:55to flood your bloodstream, the spaces between your cells,
0:51:55 > 0:51:57so as the viruses emerge from dying cells,
0:51:57 > 0:52:01they can get tagged by antibodies, then destroyed by white blood cells.
0:52:02 > 0:52:04Taking no chances,
0:52:04 > 0:52:08white blood cells engulf nearby cells that may have been infected.
0:52:13 > 0:52:18Meanwhile, surrounding healthy cells make the ultimate sacrifice,
0:52:18 > 0:52:22destroying themselves to stop the spread of the virus.
0:52:25 > 0:52:29It is only at this stage that we become aware of the battle
0:52:29 > 0:52:31taking place inside us.
0:52:31 > 0:52:35Increasing blood flow brings more white blood cells to the battleground,
0:52:35 > 0:52:38causing our nasal tissue to become inflamed.
0:52:38 > 0:52:42What we feel is a blocked nose is, in fact, the clearest sign
0:52:42 > 0:52:44of a viral onslaught.
0:52:46 > 0:52:47Once you've had an infection,
0:52:47 > 0:52:51one cell, that makes the antibody for that infection, will be kept
0:52:51 > 0:52:53inside your bone marrow for the rest of your life.
0:52:53 > 0:52:56So that if you ever get another infection with the same virus,
0:52:56 > 0:52:58the immune system already knows how to respond,
0:52:58 > 0:53:03it knows what antibody to make and it can respond very quickly and stop you getting sick.
0:53:06 > 0:53:10Working together, the body's immune system finally prevents
0:53:10 > 0:53:12the viral infection from spreading.
0:53:13 > 0:53:16It's one more battle in an unending war.
0:53:19 > 0:53:23The struggle between viruses and ourselves is evolution,
0:53:23 > 0:53:27but it's co-evolution - both sides have to change.
0:53:27 > 0:53:31It's a bit like an arms race - one party gets better weapons,
0:53:31 > 0:53:33the other party has to match them.
0:53:36 > 0:53:40Even though the individual cells are fighting this epic battle
0:53:40 > 0:53:43against viruses, remember, you have trillions of cells.
0:53:43 > 0:53:49And so even if one cell loses its war, most of the time the organism wins and we get better.
0:54:03 > 0:54:05The war is over.
0:54:07 > 0:54:09For now.
0:54:17 > 0:54:20Although many cells have been lost,
0:54:20 > 0:54:23there are many more healthy cells waiting to replace them.
0:54:31 > 0:54:35And at the heart of each one lies an identical copy of our DNA.
0:54:41 > 0:54:42Inherited from our parents,
0:54:42 > 0:54:45and their parents over countless generations,
0:54:45 > 0:54:50our DNA connects us to a family tree that stretches back
0:54:50 > 0:54:53over three billion years, to the very first cell...
0:54:55 > 0:55:00..a cell that existed long before humans, long before mammals,
0:55:00 > 0:55:01long before the dinosaurs.
0:55:04 > 0:55:09It's a lineage that connects us to every living creature and plant on Earth.
0:55:09 > 0:55:13We are all descended from a single prehistoric ancestor,
0:55:13 > 0:55:18a cell containing the single strand of DNA that started it all.
0:55:22 > 0:55:25But the virus is as old as we are.
0:55:25 > 0:55:28It has evolved alongside us, forcing us to adapt,
0:55:28 > 0:55:32to change or die in a deadly game of cat and mouse.
0:55:37 > 0:55:41This eternal arms race has driven our evolution
0:55:41 > 0:55:42and made us both stronger.
0:55:46 > 0:55:50We wouldn't be what we are today were it not for this battle
0:55:50 > 0:55:52with our ancient enemy.
0:55:55 > 0:55:59The story of the cell is a story of innovation and change,
0:55:59 > 0:56:03and because viruses continuously force cells to change,
0:56:03 > 0:56:08they actually aid their adaptation to different environments.
0:56:08 > 0:56:11And for that reason they've also helped shape us,
0:56:11 > 0:56:12they've made us who we are.
0:56:21 > 0:56:23Every minute of every day,
0:56:23 > 0:56:27this battle with the virus rages within seven billion of us.
0:56:32 > 0:56:37Though we are rarely aware of it, we fight each other,
0:56:37 > 0:56:39change each other,
0:56:39 > 0:56:41improve each other.
0:56:51 > 0:56:54Subtitles by Red Bee Media Ltd