40 Fun Facts About Meiosis

Which fruit, sold mostly as the Cavendish variety, is seedless because its three chromosome sets jam meiosis?

Banana plants sold in shops carry every chromosome in triplicate, so some are left without a partner and seeds almost never form. Growers have to clone every plant instead.

Which hybrid animal is usually sterile because its 63 chromosomes cannot form matching pairs in meiosis?

Mule cells hold 32 chromosomes from a horse mother and 31 from a donkey father, so one is always left over. A few female mules have still managed to produce offspring.

Human egg cells begin meiosis at which point in a woman's life, then pause partway for years or decades?

Before birth is the surprising answer: every egg a woman will ever release started dividing while she was still a fetus. Each one resumes only around the time it is ovulated, which may be decades later.

Which condition, the commonest human chromosome disorder, arises when meiosis fails to split a chromosome 21 pair?

Down syndrome affects about 1 in 1,000 babies worldwide. In nearly nine cases in ten, the extra chromosome arrives in the egg rather than the sperm.

Which fact about a mother most sharply raises the odds that meiosis mis-sorts the chromosomes in her eggs?

Her age matters because eggs sit paused in mid-division for decades, and the ties holding their chromosomes together slowly weaken. Sperm, by contrast, are made continuously from puberty onward.

Which monk's pea-plant laws of heredity were later explained by how chromosomes separate in meiosis?

Gregor Mendel never saw a chromosome, and his results were ignored for more than three decades. Meiosis later supplied the mechanism: his paired 'factors' travel on chromosome pairs.

Which insect's males grow from eggs that never met a sperm, so every sperm they make is genetically identical?

Honeybee drones carry only one set of chromosomes, so their sperm form without the halving and shuffling of ordinary meiosis. A single drone makes some 10 million sperm this way.

One cell that completes both divisions of meiosis in a man's body gives rise to how many sperm?

Four sperm come from each starting cell, but in women the same two divisions yield a single egg. The other three products are tiny cells that are simply discarded.

A human sperm or egg leaves meiosis carrying how many chromosomes, half the count of a body cell?

23 is one complete set, so when sperm meets egg the embryo is back to 46. Without the halving, the chromosome count would double with every generation.

Which process in meiosis swaps matching segments between partner chromosomes, so no two eggs are alike?

Crossing over leaves each chromosome a patchwork of DNA from both of a person's parents. At least one swap happens on every chromosome pair.

Which collective name, from a Greek word for 'wife', covers the sperm and egg cells that meiosis produces?

Gametes carry half the chromosomes of a body cell, so they must fuse in pairs to start a new individual. The name shares its root with the Greek word for marriage.

Where in a man's body, held slightly cooler than his core temperature, does meiosis turn out sperm?

Testes sit outside the abdomen because sperm production runs best a few degrees below core body temperature. The work happens inside tiny tubules packed within them.

Which event restores the full chromosome number that meiosis halved, producing a single cell called a zygote?

Fertilization in humans usually happens in a fallopian tube, where the chromosomes of one sperm join those of the egg. The cell that results is genetically unlike any before it.

Which sex chromosome does meiosis place in about half of a man's sperm, where it makes a baby male?

Y sperm and X sperm are made in equal numbers, because the two chromosomes pair up and then part company in the first division. Every egg carries an X.

Compared with cloning, reproducing by meiosis gives a species which advantage for natural selection to work on?

Genetic variation is the payoff: the new DNA combinations made in meiosis are, alongside mutation, a major cause of the differences between individuals. Cloning is quicker but copies the parent.

Which other type of cell division, the kind that yields two identical cells, does meiosis II closely resemble?

Mitosis and meiosis II use the same machinery, but meiosis II starts with half the chromosomes. Its products are therefore sex cells rather than new body cells.

Which term, written as n rather than 2n, describes a sperm or egg carrying a single set of chromosomes?

Haploid cells are the everyday state for mosses and most fungi, which spend the main stage of their lives with one set. Most animals reserve it for sperm and eggs.

Which microbes, lacking a nucleus, never undergo meiosis and instead multiply by simple binary fission?

Bacteria still manage a kind of sex without it, passing stretches of DNA directly from one cell to another. Their distant cousins the archaea do the same.

The word meiosis, coined in 1905 by Farmer and Moore, comes from a Greek word meaning what?

Lessening refers to the cut in chromosome count. The two coiners actually spelled their new word 'maiosis', and other biologists corrected it within a year.

Which structure, built from protein fibers called microtubules, drags chromosomes apart in both divisions of meiosis?

Spindle fibers in human eggs assemble without centrosomes, the organizing centers that most animal cells rely on. The same is true of the eggs of most other animals.

Which stage of meiosis, by far the longest, is when paired chromosomes trade segments of DNA?

Prophase I takes up 13 of the 14 days that meiosis lasts in a mouse. In a human egg it can stretch across decades.

Which name is given to the two matching chromosomes, one from each parent, that pair up during meiosis?

Homologs, short for homologous chromosomes, carry the same genes at the same positions, though often in different versions. That match is what lets them find and line up with each other.

In which stage of meiosis do chromosome pairs line up across the middle of a cell, each facing a random way?

Metaphase I is where chance steps in: each pair can face either pole, so chromosomes from a person's mother and father are dealt out at random.

In which stage of meiosis are partner chromosomes pulled to opposite poles while each one's two chromatids stay joined?

Anaphase I is when the chromosome count is halved, as whole chromosomes head for each pole. A protein named from the Japanese for 'guardian spirit' keeps each one's two chromatids together.

Which law of heredity, found by tracking two traits at once, reflects chromosome pairs lining up at random in meiosis?

Independent assortment has many exceptions: genes that sit close together on one chromosome are linked and tend to be inherited as a block.

Which tiny cells, cast off as a human egg forms, let the egg keep nearly all of the cytoplasm?

Polar bodies receive a full share of chromosomes but only a scrap of the cell's contents, and most soon die. The lopsided split leaves one egg stocked with enough supplies to feed an embryo.

Which term names the alternative versions of a single gene that meiosis deals out one per sperm or egg?

Alleles is short for 'allelomorphs', a word coined in the early 1900s to mean 'other forms'. The gene behind the ABO blood groups alone has six common ones.

Which lineage, set aside early in an embryo, is the only one in the body able to enter meiosis?

Germ cells travel through the gut of the early embryo to reach the organs where eggs or sperm will one day form. Every other cell in the body divides only by copying itself.

In ferns and mosses, meiosis produces which single cells that can grow into a new plant entirely on their own?

Spores from a fern grow into a tiny separate plant, and only that plant goes on to make the sperm and eggs. Fungi use meiosis to make theirs too.

Which condition, the only survivable loss of a whole human chromosome, can arise when faulty meiosis leaves a single X?

Turner syndrome affects roughly one girl in every 2,000 to 5,000, and short stature is a common sign. It is named after the American endocrinologist who described it in 1938.

Which American geneticist, breeding fruit flies, showed in 1911 that chromosomes swap pieces during meiosis?

Thomas Hunt Morgan won the 1933 Nobel Prize for showing the role that chromosomes play in heredity. His laboratory at Columbia University became famous as the Fly Room.

By shuffling whole chromosomes alone, meiosis lets one person make roughly how many genetically different sperm or eggs?

About 8 million is what you get by doubling the options once for every chromosome pair a person has. The swapping of segments between partners then pushes the real figure far higher.

In which stage of meiosis do sister chromatids finally come apart, each heading for a different sex cell?

Anaphase II can begin only once the protein shielding each centromere is gone, which lets an enzyme cut the last ties between the chromatids.

Which X-shaped links, first described by Frans Janssens in 1909, show where paired chromosomes have swapped segments?

Chiasmata do a mechanical job too, holding each chromosome pair together until it is time to part. A pair that lacks one is usually dealt out wrongly.

Which name is given to the rest period some cells take after meiosis I, during which no DNA is copied?

Interkinesis is not universal: only some species pause here at all, while others run straight from one division into the next.

A human egg released at ovulation is frozen in which stage of meiosis until a sperm arrives?

Metaphase II is the egg's second long pause, and only the arrival of a sperm releases it. An egg that no sperm reaches never completes meiosis at all.

Which German biologist reasoned that sex cells must halve their chromosomes to keep the count steady across generations?

August Weismann also cut the tails off 68 mice over five generations to test whether injuries are inherited. Not one pup was born without a tail, or even with a shorter one.

Which term names the close side-by-side pairing of matching chromosomes along their whole length early in meiosis?

Synapsis is thought to close up like a zipper, drawing partner chromosomes to within about 100 nanometres of each other.

Which fruit's seedless form, first bred in Japan in 1939, has three chromosome sets and so cannot finish meiosis?

Watermelon without seeds grew to nearly 85% of US sales by 2014. Its pollen is sterile, so growers have to plant rows of ordinary vines alongside to pollinate it.

Which giant lizard can hatch young with no mate when an egg made by meiosis doubles its own chromosome set?

Komodo dragon mothers that breed this way produce only sons. One female at Chester Zoo in England laid a clutch on her own from which seven sons hatched.