120 Fun Facts About Cell

Who coined the term "cell" in 1665 after examining thin slices of cork under a microscope?

Robert Hooke never knew what he had named: all his cork showed was the empty walls the cells had left behind, and he had no idea such boxes were the basic unit of every living thing.

Which 1665 book first used the word "cell" in biology, for the pores in a sliver of cork?

Micrographia was a best-seller in its day: the diarist Samuel Pepys sat up until two in the morning with it and called it the most ingenious book he had ever read.

The word "cell" comes from the Latin cella, which means what?

Small room: the man who named the cell was looking at a slice of cork, and its tiny pores are said to have reminded him of the rooms monks lived in.

Roughly how many cells make up a typical adult human body?

30 trillion comes from a 2016 recount that also overturned a textbook claim: the bacteria living in you do not outnumber your own cells ten to one but roughly match them, at just under 40 trillion.

Your nerve cells and liver cells carry the same DNA, so what makes them differ?

Which genes are switched on: nearly every cell keeps the full genome and reads only part of it, which is why Dolly the sheep could be cloned in 1996 from a single adult udder cell.

Which botanist shares credit with Theodor Schwann for founding cell theory in the late 1830s?

Matthias Schleiden brought up the nucleus of plant cells over dinner in 1837; Schwann realised he had seen the same structure in animal tissue, and the two checked it together under a microscope.

Rudolf Virchow's 1855 maxim that every cell comes from another cell rejected which old idea?

Spontaneous generation, the belief that life can arise from lifeless matter, was the target, yet the insight was borrowed: Robert Remak had shown that cells come from dividing cells three years earlier, and Virchow gave him no credit.

Which organelle is nicknamed the "powerhouse of the cell"?

Mitochondrion: it carries a small genome of its own, 37 genes handed down from the mother, which is why geneticists use it to trace a maternal line far back in time.

The nickname "powerhouse of the cell" was the headline of a 1957 article in which magazine?

Scientific American ran it in July 1957, over an article by Philip Siekevitz, when exactly how the organelle makes its energy was still unknown: Peter Mitchell's now-accepted answer came only in 1961.

Which human cell throws out its own nucleus as it matures?

A red blood cell sheds its nucleus and its energy-producing organelles too, so the cell that carries your oxygen cannot use any of it and lives by fermenting sugar instead.

Which tiny blood components clump together to plug a wound and start a clot?

Platelets are not whole cells at all: they are scraps of cytoplasm pinched off giant cells called megakaryocytes, so they have no nucleus of their own.

A healthy adult makes millions of new red blood cells every second, in which tissue?

Bone marrow takes its orders from the kidneys: when they sense too little oxygen in the blood they release the hormone EPO, which tells the marrow to speed up.

About how long does a human red blood cell circulate before the body retires it?

About 120 days is all it gets: by then its membrane has aged, and macrophages in the spleen and liver swallow the worn-out cell and recycle its parts.

Which Delft draper first observed bacteria through tiny single-lens microscopes he made himself?

Antonie van Leeuwenhoek made more than 500 lenses in his lifetime and kept his best methods secret, so exactly how he made them stayed a mystery for centuries.

What name did 17th-century science give the tiny swimming creatures seen by the first man to observe bacteria?

Animalcules, Latin for "little animals", caused such a stir when his letter was read to London's Royal Society that it asked him to reveal his methods so that others could confirm them; he declined.

A ribosome builds a protein by reading the code carried on which molecule?

Messenger RNA is a working copy of a gene, read three letters at a time; the COVID-19 mRNA vaccines exploit this by handing our cells the instructions for one viral protein.

Which Romanian-American cell biologist first observed ribosomes in the mid-1950s?

George Palade spotted them as dense specks in electron-microscope pictures of cells, and for a few years they were known simply as Palade granules, until the word ribosome replaced his name.

A ribosome's 80S size measures how fast it does what?

Sinks in a centrifuge: the rate depends on shape as well as mass, so Svedberg values do not simply add, which is why a 40S and a 60S subunit make an 80S ribosome, not a 100S one.

Which Belgian scientist discovered lysosomes and shared the 1974 Nobel in Medicine?

Christian de Duve set out to learn how insulin works, but a chance oddity in a liver enzyme sidetracked him into finding the lysosome; by his own account he never did solve insulin.

A lysosome's digestive enzymes work only in which conditions, shutting down if they leak into the rest of the cell?

Acidic conditions, around pH 5, are kept up by proton pumps in the lysosome's membrane; the rest of the cell sits near a neutral pH 7.2, so enzymes that escape mostly stall instead of digesting it.

Which Italian physician identified the cell's protein-packaging 'apparatus' in 1898?

Camillo Golgi's find was written off by some as a mere artefact of his metal stain, and it took the electron microscope, in the 1950s, to prove the structure was real.

The stacked, flattened membrane disks that make up the cell's protein-dispatch organelle are called what?

Cisternae work like a production line: a protein enters at the cis face, is trimmed and tagged as it crosses the stack, and leaves from the trans face packed in a vesicle.

Which organelle carries out photosynthesis in plant and algal cells?

Chloroplast is a name for something surprisingly mobile: in dim light they spread across the cell to catch more of it, and in harsh sun they retreat to the side walls to avoid light damage.

The green pigment in a plant cell is held in stacks of disk-like sacs called what?

Thylakoids stack like coins into a granum, yet they only capture the light: the sugar itself is assembled in the fluid around the stacks, the stroma.

Lynn Margulis's 1967 paper argued that mitochondria and chloroplasts were once what?

Free-living bacteria swallowed by bigger cells, she argued: the paper was reportedly turned down by more than a dozen journals before one printed it, and the idea went on to become textbook biology.

Walther Flemming named mitosis in 1882 after the Greek word for what, describing how chromosomes looked under his dyes?

Thread is what he saw: newly invented aniline dyes picked out stringy material in the nucleus that shortened and split in two, and the threads were only later renamed chromosomes.

During which stage of mitosis do chromosomes line up along the middle of the cell?

The imaginary line they gather on is the metaphase plate; the checkpoint here makes sure every chromosome is attached before the cell pulls them apart.

What is the splitting of the cytoplasm into two daughter cells after nuclear division called?

Cytokinesis is, strictly, not a phase of mitosis at all: animal cells pinch in two with a contractile ring, while plant cells build a new wall, the cell plate, down the middle.

After both rounds of meiosis, how many haploid cells has a single starting cell produced?

Four, but in women only one of them becomes an egg: it keeps nearly all the cytoplasm, while the others are tiny polar bodies that soon break down.

During meiosis, paired chromosomes swap matching stretches of DNA in a gene-shuffling step known as what?

Crossing over means each chromosome a parent passes on is a fresh patchwork of the pair that parent inherited, which helps explain why no two eggs or sperm carry the same mix.

Which model of the cell membrane did Singer and Nicolson propose in 1972?

Fluid mosaic model: it treats the membrane as a two-dimensional liquid in which proteins drift sideways, overturning the older picture of a sandwich with a sheet of protein on either side.

A cell membrane is a double layer built mostly of which type of molecule?

Phospholipids each have a water-loving head and two water-hating tails, so dropped into water they line up tail to tail and form the double layer entirely on their own.

Which cell line, grown from a Baltimore cancer patient's tumour in 1951, became the first immortal human cell line?

HeLa is short for Henrietta Lacks, whose tumour cells were taken without her knowledge; where earlier human samples soon died in the dish, hers kept multiplying and have never stopped.

The first immortal human cell line was mass-produced in the 1950s so that whose vaccine could be tested?

Jonas Salk's polio vaccine had to be checked on a vast scale, so in 1953 the National Foundation for Infantile Paralysis set up a cell factory at Tuskegee Institute to supply the labs testing it.

What is the main job of the nucleolus?

Assembling ribosomes on such a scale makes it the most prominent structure in the nucleus, yet it has no membrane of its own: it is simply the dense region that forms around the ribosomal RNA genes.

Which botanist, whose name is on the jitter of tiny particles, named the cell nucleus?

Robert Brown was not the first to see one, but while studying how orchids are fertilised he noticed the same spot in cell after cell and called it the nucleus, and his word stuck.

Fungal cell walls are built mainly from which polymer?

Chitin is the same tough material as insect exoskeletons and crab shells, so a mushroom's walls have more in common with a beetle's armour than with a leaf.

Bacterial cell walls are made of which material, also called murein?

Peptidoglycan is the mesh penicillin attacks: the drug blocks the enzymes that cross-link it, so a growing bacterium's wall gives way, while human cells, which make no such mesh, are left alone.

Which human cell, about a tenth of a millimetre across, is just visible without a microscope?

The ovum is a giant because it carries stockpiles of what an embryo needs for its first days, which makes it several times wider than an ordinary body cell.

Roughly how many neurons does the human brain contain?

86 billion comes from a 2009 count that dissolved brains into an even suspension and tallied the cell nuclei, trimming the round 100 billion that textbooks had long repeated.

Which 1906 Nobel laureate is famed for his drawings of nerve cells?

Santiago Ramón y Cajal shared the prize with the rival whose silver stain he had borrowed, and who used his own Nobel lecture to insist that the brain was one continuous net, not separate cells.

In 2006 Shinya Yamanaka's team showed adult tissue could be reprogrammed back into what?

Just four genes did the trick, and the resulting induced pluripotent stem cells earned him a Nobel Prize six years later.

About how many cells does an average adult human lose each day to apoptosis?

50 to 70 billion sounds like a disaster but is routine upkeep: each cell shrinks and packs itself up, and a neighbour swallows the remains before anything can leak out and harm the tissue.

The word "apoptosis" comes from the Greek for what?

Falling off, as petals drop from a flower or leaves from a tree: the three pathologists who named it in 1972 took the word on the advice of a professor of Greek at Aberdeen.

Brenner, Horvitz and Sulston shared the 2002 Nobel Prize for tracing programmed cell death in which see-through organism?

A nematode worm a millimetre long: a C. elegans hermaphrodite builds 1,090 body cells as it grows and kills off the same 131 every time, which let Horvitz find the genes that give the order.

In 1961 Leonard Hayflick upset accepted wisdom by showing that normal human cells grown in a dish eventually do what?

Stop dividing, after some 40 to 60 rounds: until then biologists had trusted a Nobel laureate's claim, built on a chick-heart culture he said he kept going for over twenty years, that cells in a dish were immortal.

Which enzyme lets most cancer cells go on dividing without limit?

Telomerase rebuilds the protective caps on the ends of chromosomes, which otherwise shorten a little with every division; most healthy adult cells keep the enzyme switched off.

Prokaryotes keep their main genome in a region of the cytosol called what?

The nucleoid has no membrane around it, only tightly folded DNA; extra genes often ride on small separate plasmids, which is how antibiotic resistance hops from one bacterium to another.

Which two domains of life are made up entirely of prokaryotes?

Bacteria and Archaea look so alike under a microscope that they were filed as one group until 1977, when Carl Woese compared their genes and found two separate lineages hiding under one name.

Which organelle can swell to fill most of a mature plant cell, pushing outward to keep the plant from wilting?

Vacuole pressure, known as turgor, is what keeps a non-woody plant stiff, and swelling a sac with water is also a cheap way to grow: the cell gets far bigger on little more than water.

Which organelle stores calcium ions inside muscle fibres and releases them on a nerve signal?

Sarcoplasmic reticulum is a muscle cell's own version of the endoplasmic reticulum, kept loaded with calcium: when a nerve impulse arrives it floods the fibre with the ions, and that is the trigger for contraction.

The cytoskeleton's widest fibres, hollow rods that pull chromosomes apart, are built from which protein?

Tubulin stacks into hollow rods about 25 nanometres across, which double as the tracks that motor proteins walk along as they haul cargo across the cell.

Paul Boyer and John Walker's 1997 Chemistry Nobel was for explaining how cells make what?

An adult burns through roughly their own body weight of it every day, recycling each molecule hundreds of times.

Which process draws water across a membrane toward whichever side holds more dissolved salt?

Osmosis is why a cell dropped into brine shrivels as its water leaves, while the same cell dropped into pure water swells and may burst.

Which type of white blood cell is the most numerous in human blood?

Neutrophils are among the first cells to reach an infection, where they swallow microbes and die in huge numbers: pus is largely made of their remains.

In 1956 Joe Hin Tjio and Albert Levan corrected the textbooks by counting how many chromosomes in a human cell?

46 overturned the 48 that had stood for 33 years and been confirmed by at least forty studies; a new way of spreading the chromosomes apart finally made them countable.

During which part of the cell cycle is the DNA replicated?

S phase takes a dividing human cell only about eight hours because copying starts at tens of thousands of places at once: a single copying machine would need about 800 hours for one average chromosome.

What is the resting phase called in which a cell has left the cell cycle and stopped dividing?

G0 is called a resting phase, yet most adult neurons, some of the busiest cells in the body, sit in it for good: resting here only means not dividing.

Tim Hunt shared the 2001 Nobel Prize for finding which proteins, whose rise and fall tells a cell when to divide?

Cyclins build up and are then destroyed once in every round of division, a rhythm Tim Hunt first noticed in sea urchin eggs; each one works by switching on a partner enzyme that drives the cell forward.

The primary wall of a land plant is built mostly from which polysaccharide?

Cellulose is the most abundant organic polymer on Earth and makes up about nine-tenths of a cotton shirt, yet humans cannot digest it at all: cows manage only because microbes in their gut do the work.

Which infectious agent is a mere packet of genetic material in a protein coat, far simpler than any cell?

Virus particles have no metabolism and cannot copy themselves: each must get inside a living cell and take over its machinery, which is why they are often described as sitting at the edge of life.

The largest known bacterium, described in 2022 from a Caribbean mangrove swamp, is about as long as what?

A human eyelash is a fair match in size and shape for Thiomargarita magnifica, about 5,000 times larger than most bacteria; it even keeps its DNA in membrane sacs, a feature bacteria were thought to lack.

The word 'eukaryote' derives from Greek meaning what?

True nut or kernel, from Greek eu, 'good' or 'true', and karyon, the kernel being the cell's nucleus; the word was coined in passing in a 1925 paper on an amoeba and went all but unused until 1962.

Magnetotactic bacteria steer along Earth's magnetic field using a chain of crystals rich in which metal?

Iron, as crystals of magnetite or the iron sulfide greigite: the chain works as a fixed compass needle that swings the whole cell into line with the field, helping it find the depth where oxygen levels suit it.

Which single cell, formed at fertilisation, goes on to produce every cell type in the human body?

Zygote is where every body starts: this one cell can make all its tissues and the placenta too, an all-purpose power its descendants lose within the first few rounds of division.

Which slimy outer layer helps some bacteria avoid being swallowed by white blood cells?

Capsule made the difference in a famous 1928 experiment: mice died from the coated strain of pneumonia bacteria but shrugged off its bare twin, and tracing how the trait passed between them led to proof that genes are DNA.

Which genus of bacteria possesses only a cell membrane and no cell wall?

Mycoplasma cells are among the smallest that can reproduce on their own, and in 2010 one became the first cell to be run entirely by a genome built from scratch in a lab.

Roughly how long ago did cells first emerge on Earth?

About four billion years ago makes life no latecomer: the planet itself is only about 4.5 billion years old, so the first cells were under way within its first several hundred million years.

Bacteria cling to host tissue with which short bristles, also called attachment pili?

Fimbriae is Latin for fringe, and the grip matters: the E. coli that cause bladder infections begin by latching the sticky tips of these bristles onto the bladder's lining.

Linear plasmids have been found in which spirochete genus, the cause of Lyme disease?

Borrelia burgdorferi has the most fragmented genome of any bacterium known: its main chromosome is a straight line rather than the usual loop, and it carries more than twenty plasmids, some circular, some linear.

The glowing green protein used to light up living cells was first taken from which sea creature?

A jellyfish, Aequorea victoria, was the source of green fluorescent protein, which glows with no extra ingredients from the animal. Its gene now works as a visible reporter inside other species' cells.

Which gel-forming substance, used to set jam, glues neighbouring plant cells together?

Pectin is the main ingredient of the middle lamella, the thin layer that cements one plant cell to the next. Fruit softens as it ripens because enzymes break that glue down.

Which protein, the most abundant in mammals, forms the tough fibres of the matrix between animal cells?

Collagen makes up 25% to 35% of a mammal's protein, twisted into three-stranded ropes outside the cells. The body cannot build it without vitamin C.

Which tough protein, the stuff of hair and fingernails, also fills the cells of the skin's outer layer?

Keratin assembles into intermediate filaments, the middle-sized cables of the cell skeleton. Skin cells cram themselves with it and then lose their nuclei, leaving a tough outer layer.

Which type of human cell is filled almost entirely by one large droplet that flattens the nucleus against the edge?

Fat cells hold a single lipid droplet so big that the nucleus is squashed to the rim. In adults they mostly swell or shrink with body weight rather than change in number.

If the DNA packed inside one human cell were stretched out in a line, about how long would it be?

About 2 metres of DNA fit inside a nucleus only six thousandths of a millimetre wide, because the strand is wound around protein spools and then coiled again and again.

Which instrument, pioneered by Ernst Ruska in the 1930s, shows cell structures too fine for light to resolve?

The electron microscope picks out details about 0.1 nanometres across, against roughly 200 for light, because electrons have a far shorter wavelength.

Which hair-like projections line human airways and sweep mucus and trapped dust away from the lungs?

Cilia stand about 200 to a cell on the airway lining and beat in coordinated waves to shift mucus. Smoking can reduce their numbers.

Which part of the human eye is built from cells that lose their nuclei and organelles to stay transparent?

The lens is packed with long fibre cells that have no organelles or nuclei. It keeps adding new ones around the outside throughout life, so its centre is its oldest part.

Which organelle shortens very long fatty acids and uses the enzyme catalase to mop up the toxic by-product?

Peroxisome enzymes make hydrogen peroxide as they work, and catalase at once breaks it down to harmless water and oxygen. The organelle is named after that poison.

Which organelle, named by Theodor Boveri in 1888, organises the spindle fibres when an animal cell divides?

The centrosome is copied only once per cell cycle, so each daughter cell inherits exactly one, built around a pair of barrel-shaped centrioles.

Which ribosome-studded organelle forms sheets that run on from the outer membrane around the nucleus?

Rough ER gets its bumpy look, and its name, from the protein-making ribosomes dotted over its outer face. Its smooth counterpart has none and makes lipids and steroid hormones instead.

DNA in the nucleus is wound around which spool-like proteins, forming a fibre often likened to beads on a string?

Histones carry a positive charge that clings to DNA's negatively charged backbone. Weakening that grip unwinds a stretch of DNA so the cell can use it.

Which protein forms the microfilaments of the cell skeleton and slides against myosin to make muscles contract?

Actin filaments can be rebuilt rapidly, which lets a cell change shape, crawl about and pinch itself in two when it divides.

Liver and muscle cells stockpile glucose as which branched molecule, the animal counterpart of plant starch?

Glycogen in the liver is released to keep blood sugar steady between meals, while each muscle keeps its own store as fuel for itself.

Which small membrane-wrapped bubbles ferry cargo between organelles and fuse with the cell surface to release it?

Vesicles are wrapped in the same kind of membrane as the cell itself, which is why they can merge with it and spill their contents outside.

By which process, named from the Greek for 'cell eating', does a white blood cell swallow a whole bacterium?

Phagocytosis was named by Élie Metchnikoff in the 1880s and was the first immune defence to be understood. The swallowed microbe is sealed in a bubble inside the cell and digested.

Which cable-like extension of a nerve cell can stretch a metre, from the base of the spine to the big toe?

Axons to the big toe run in the sciatic nerve and are the longest in the human body, yet a typical one is only about a thousandth of a millimetre wide.

Which magnesium-centred pigment makes plant cells look green by soaking up red and blue light instead?

Chlorophyll is a poor absorber of green light, so that colour bounces back to the eye. Its magnesium atom sits in a ring much like the one that holds the iron in haemoglobin.

Which small adaptor molecule carries amino acids to the ribosome, matching each one to a three-letter codon?

Transfer RNA folds into a cloverleaf and then an L shape, with a three-letter anticodon at one end to read the code and the matching amino acid at the other.

Which channels link neighbouring heart muscle cells so that an electrical signal spreads and they beat together?

Gap junctions are rings of connexin proteins that line up between two cells, letting ions pass straight from one cell's interior into the next.

Which microscopic channels tunnel through plant cell walls so that neighbours can trade sugars and signals?

Plasmodesmata form when strands of a dividing plant cell's internal membranes are trapped in the new wall, leaving the two daughter cells linked.

Which leaf cell type, working in pairs, swells with water to open a breathing pore and goes limp to close it?

Guard cells let carbon dioxide in through the open pore, but water vapour escapes the same way, so the plant shuts them when water runs short.

Yoshinori Ohsumi won a 2016 Nobel Prize for explaining which process, a cell digesting its own worn-out parts?

Autophagy, from the Greek for 'self-devouring', lets a starving cell break down its own components to keep its energy up. Ohsumi found the genes behind it in yeast.

The gut bacterium E. coli can reproduce every 20 minutes by which method, with no mitosis involved?

Binary fission splits one E. coli into two cells that look identical, yet the one that keeps the older end of the cell behaves like an ageing parent.

Which shape-shifting single-celled organism crawls and catches food by pushing out bulges called pseudopods?

Amoeba comes from the Greek for 'change', and biologists now use the word for any cell that moves in this flowing way.

Which tiny finger-like folds of a gut cell's membrane form the 'brush border' that soaks up digested food?

Microvilli number in the thousands on a single gut cell, multiplying the room it has for absorbing nutrients and for holding digestive enzymes.

Diatoms, single-celled algae that make a fifth or more of Earth's oxygen, build cell walls of which material?

Silica gives every diatom an intricate glass case made of two overlapping halves. Their fossil remains pile up as diatomaceous earth, a soft rock that crumbles to powder.

Which pigment do skin cells pile above the nucleus like a parasol, shielding DNA from ultraviolet rays?

Melanin is made by a few specialist cells that hand it on in packets to their neighbours. Ultraviolet light steps up the supply, which is what a tan is.

Which simple sea animals can rebuild themselves after being squeezed through a fine cloth into separate cells?

Sponges broken up into single cells will clump together again, re-form an outer layer and gradually rebuild their water channels.

Which small protein is chained onto a worn-out protein as a 'kiss of death' that sends it to be shredded?

Ubiquitin got its name because it turns up in almost every tissue, and the cell's shredder recycles whatever carries a chain of it. The discovery won the 2004 Chemistry Nobel.

The inner membrane of the cell's 'powerhouse' organelle is thrown into deep folds known by which name?

Cristae give the inner membrane its wrinkled look and a far larger area for the chemical reactions that release usable energy from food.

Which rigid polymer, a fifth to a third of the dry weight of wood, stiffens plant cell walls and resists rot?

Lignin lends cell walls their rigidity and takes its name from the Latin for wood. Paper mills strip it out, and paper that keeps some of it stays brownish.

Which dormant structures let the bacteria behind anthrax and tetanus survive heat, drying and disinfectants?

Endospores are stripped-down dormant forms that can wait for centuries, then turn back into ordinary growing bacteria when conditions improve.

Which 1884 dye test sorts bacteria into two groups according to how thick their cell walls are?

The Gram stain leaves thick-walled bacteria purple and thin-walled ones pink. Hans Christian Gram devised it while working in a Berlin hospital morgue.

Which membrane protein, found by Jens Christian Skou in 1957, uses about a third of a typical cell's energy?

The sodium-potassium pump pushes three sodium ions out of the cell for every two potassium ions it lets in. Nerve cells can spend up to 70% of their energy keeping it running.

By which passive process does oxygen cross the thin walls of the lungs' air sacs into the blood?

Diffusion costs the cell no energy, because oxygen simply spreads from where it is plentiful to where it is scarce. The lungs help with very thin walls and about 80 square metres of lining.

Plants cannot pinch their rigid walls inward to divide, so which new partition do they build across the middle instead?

The cell plate starts in the centre of the cell and grows outward until it meets the sides, then stiffens into the new wall between the two daughter cells.

Which fatty insulating layer wraps many nerve fibres in segments, letting impulses leap from gap to gap?

Myelin is made by helper cells that wrap themselves around a nerve fibre, and the jumping it allows carries signals far faster than a bare fibre could.

Which pinched-in region joins the two copies of a duplicated chromosome and divides it into short and long arms?

The centromere is where the spindle fibres that drag the two copies apart take hold. The short arm is labelled p, reportedly for the French 'petit'.

Which pigment-filled plastids give ripe fruits, flower petals and some roots their red, orange and yellow colours?

Chromoplasts fill with carotenoid pigments as a fruit ripens, and their likely purpose is to attract the animals that pollinate flowers or eat fruit and spread the seeds.

Biologists measure cells and bacteria in which unit, also used to grade wool by the width of its fibres?

Micrometres, also called microns, are each a millionth of a metre. A typical bacterium is 1 to 10 of them long, and a human hair 20 to 200 across.

Günter Blobel won the 1999 Nobel Prize for showing that newly made proteins carry built-in what?

Address tags, short signal sequences within a protein, steer it to its proper place in the cell. Blobel gave all of his prize money to help rebuild Dresden, including its Frauenkirche.

Which dense spot in the cell nuclei of female mammals is an X chromosome that has been shut down?

The Barr body, first seen by Murray Barr in 1949, keeps females from making twice as much of each X chromosome product as males, who have only one X.

Which water-carrying tissue of a tree is built from cells that are dead and hollow by the time they start work?

Xylem cells die as they mature and their contents disappear, leaving empty tubes that carry water up from the roots at no energy cost to the cells themselves.

Which light sensors in the retina can respond to a single photon but play little part in colour vision?

Rod cells are about 100 times more sensitive than their colour-detecting neighbours, which is why colours fade in dim light: only the rods are still working.

Which protein builds a soccer-ball-like cage of hexagons and pentagons to pull a patch of cell membrane inward?

Clathrin comes as three-legged units that link up into a rounded lattice, bending the membrane into a bud that pinches off and carries its cargo into the cell.

Scattered through the lining of the airways and gut, what makes the mucus that traps dust and eases food along?

Goblet cells are named for their shape: a wide cup swollen with mucus-filled granules, sitting on a narrow stem like a wine glass.

Which enzyme-filled cap on the head of a sperm cell digests a path through the outer coat of the egg?

The acrosome's enzymes break down the egg's outer layer, the zona pellucida, so that the nuclei of the two cells can come together.

Which complex of DNA and protein fills the nucleus between divisions, loose where it is read and tight where silenced?

Chromatin is kept loosely packed where the cell needs regular access to the DNA and tightly packed where it does not. The loose form is called euchromatin, the compact form heterochromatin.