Meiosis and Sexual Reproduction

Main notes

Meiosis is the specialized cell division that creates gametes, the sperm and egg cells that carry the genetic material to the next generation. It halves the chromosome number so that fertilization can restore it, and it shuffles the genes so that no two offspring are ever identical. This chapter builds directly on the chromosome behavior introduced in the mitosis chapter and prepares the ground for the inheritance patterns covered in the next topic.

Meiosis I and II

Meiosis is a two-part division that converts one diploid cell, which carries two complete sets of chromosomes and is abbreviated 2n, into four haploid cells, each with one set and abbreviated n. Human body cells are diploid with 46 chromosomes arranged in 23 homologous pairs, one member inherited from the mother and one from the father. Human gametes are haploid with 23 chromosomes, one from each pair. The halving works because the DNA is copied only once, during interphase before the process begins, while the nucleus divides twice. The two rounds are called meiosis I and meiosis II, and no DNA replication happens between them.

Meiosis I is the reductional division, because this is where the chromosome number is cut in half. Early in prophase I, the homologous chromosomes pair up gene by gene in a process called synapsis, forming a four-chromatid unit known as a tetrad. While they are paired, the homologs physically exchange segments through crossing over, which is covered below. At metaphase I, the tetrads line up along the equator, and at anaphase I the homologous chromosomes are pulled to opposite poles, not the sister chromatids. Each of the two daughter cells that form therefore receives one chromosome from every pair, so each cell has 23 chromosomes in humans, and each chromosome is still made of two joined sister chromatids. Prophase I is the longest phase of meiosis, because so much happens there: pairing, crossing over, and the dismantling of the nuclear envelope.

Meiosis II is the equational division, and mechanically it resembles mitosis. The sister chromatids of each chromosome line up on the equator and are separated at anaphase II, then pulled to opposite poles. When the second round of cytokinesis finishes, the process has produced four haploid daughter cells, each with 23 unduplicated chromosomes in humans. The only time sister chromatids separate during meiosis is in meiosis II; during meiosis I they travel together as one unit.

The table below summarizes how the two divisions differ.

Common Mistake: The chromosome number is halved in meiosis I, when the homologous pairs separate, not in meiosis II. Meiosis II only splits sister chromatids and leaves the count unchanged at n.

The stages of meiosis I run in a fixed order.

1. Prophase I. Chromosomes condense and homologous pairs synapse into tetrads.
2. Metaphase I. Tetrads line up along the spindle equator.
3. Anaphase I. Homologous chromosomes separate to opposite poles.
4. Telophase I and cytokinesis. Two haploid cells form, each holding duplicated chromosomes.

ELI-10

The whole process is like dealing a deck of cards into four hands. First you sort the matched pairs so every hand receives one card from each pair, which cuts the number of cards in each hand in half. Then each card, which is really two copies stuck together, is split so the copies go to different hands. Every hand ends up with the same total, and no copy is lost. The two splitting steps together are the whole trick.

Crossing Over

Crossing over is the physical exchange of chromosome segments between homologous chromosomes during prophase I. While the homologs are zipped together in a tetrad, nonsister chromatids break at matching points and swap the segments that follow, so each chromatid ends up carrying genetic material from both the mother and the father. The visible X-shaped connections where the exchange is still in progress are called chiasmata (singular chiasma). Besides performing the swap, the chiasmata hold the homologs together, which helps the pairs align correctly and separate cleanly at anaphase I.

The traded segments are the same length and carry the same genes, so the exchange neither loses nor duplicates any DNA. It only shuffles which versions of each gene, called alleles, travel together along one chromosome. Because of crossing over, a finished chromatid is a mosaic of maternal and paternal DNA, and the outcome is genetic recombination: each chromosome in a gamete is a blend that did not exist in either parent. Every human egg and sperm therefore carries a unique patchwork of the grandparents' sets of genes.

Common Mistake: Crossing over happens between nonsister chromatids, one from each homologous chromosome, never between the two sister chromatids of a single chromosome. Sister chromatids are identical copies, so swapping between them would change nothing.

ELI-10

You and a friend each own the same recipe book but with different notes written in the margins. You tear out the same page from both books, trade the pages, and tape each into the other book. Now your book holds your friend's notes on that page, and hers holds yours. The rest of the books stay untouched, so nothing is lost, but both books are now blends of the two owners.

Independent Assortment

Independent assortment is the random orientation of the homologous chromosome pairs at metaphase I. Each tetrad lines up on its own, and nothing decides which member faces which pole, so when anaphase I pulls the pairs apart, a daughter cell can receive the maternal chromosome from one pair and the paternal chromosome from the next. The sorting of every pair is completely independent of every other pair, which is why the process is called independent assortment.

Because the pairs sort independently, the number of chromosome combinations a single cell can produce by independent assortment alone is 2 to the power n, where n is the haploid number of chromosomes. For human cells, n equals 23, so one cell could generate 2 to the 23, which is over eight million, different assortments of chromosomes in its gametes. Crossing over multiplies the variety even further, because each chromosome is already a blend before the pairs are sorted. Together, independent assortment and crossing over are the two great engines of genetic variation, and they explain why full siblings, except identical twins, inherit different mixtures of their parents' chromosomes.

Common Mistake: Independent assortment happens in meiosis I, when whole homologous chromosomes are sorted into different cells. The separation of sister chromatids in meiosis II does not create new combinations of chromosomes, because the two chromatids are identical copies.

ELI-10

Picture a drawer with four pairs of socks, and every pair contains one red sock and one blue sock. You are told to reach in blindfolded and pull one sock from each pair into your hand. You might grab red, blue, blue, red, or any other mix, and each pair is chosen separately. Because the choice for one pair never affects the choice for the next, the possible mixes multiply quickly. With many pairs, the number of combinations becomes enormous.

Nondisjunction

Nondisjunction is the failure of chromosomes to separate properly during anaphase. If homologous chromosomes fail to separate in meiosis I, both members of the pair travel to the same pole, so two gametes receive an extra chromosome and the other two lose that chromosome completely. If sister chromatids fail to separate in meiosis II, the same imbalance appears in two of the four products, while the other two stay normal. Any cell that ends up with the wrong chromosome count shows aneuploidy.

A gamete with an extra chromosome carries n plus 1 chromosomes, and one that is missing a chromosome carries n minus 1. When such a gamete fuses with a normal gamete at fertilization, the embryo has 47 or 45 chromosomes instead of the usual 46. Three copies of one chromosome is called trisomy, and a single copy is called monosomy. Most aneuploid embryos cannot complete development, and about half of early miscarriages trace to an abnormal chromosome count. Some aneuploidies are compatible with life. Trisomy 21, in which chromosome 21 is present in three copies, causes Down syndrome and is the most common surviving trisomy in humans, and its frequency rises steeply with maternal age. Aneuploidy of the sex chromosomes is generally less damaging, and a single X chromosome with no partner, called Turner syndrome, is the only monosomy that survives to birth in humans.

Common Mistake: Nondisjunction does not always affect all four gametes. A mistake in meiosis I makes every product abnormal, but a mistake in meiosis II leaves two of the four products normal. Check which division failed before predicting the gametes.

ELI-10

Two friends split a bag of candy so that each takes the same number of pieces. If one friend accidentally grabs two pieces in a single handful, her bag ends up with an extra piece and his bag comes up one short. Chromosomes separate the same way during meiosis. A slip at the wrong moment leaves some cells with too many chromosomes and others with too few.

Gametogenesis

Gametogenesis is the making of gametes through meiosis, and it runs very differently in the two sexes. Spermatogenesis takes place in the testes and starts at puberty. Each diploid spermatogonium divides symmetrically through meiosis, so one cell yields four functional haploid sperm cells. The process is continuous, which is why the human testes can release hundreds of millions of sperm every day. Each sperm is built for travel, with a head that carries the nucleus, a midpiece packed with mitochondria for energy, and a long flagellum tail for swimming.

Oogenesis takes place in the ovaries and begins long before birth. Each oogonium grows into a primary oocyte, which starts meiosis I but pauses in prophase I, sometimes for decades. At each menstrual cycle, one primary oocyte resumes meiosis I, and the division is strikingly unequal: one daughter cell, the secondary oocyte, keeps nearly all the cytoplasm, while the other collapses into a small polar body that is later discarded. The secondary oocyte begins meiosis II but arrests again, this time in metaphase II, and only finishes the division if a sperm penetrates it. The final products of oogenesis are therefore one large egg (ovum) and up to three polar bodies, not four equivalent cells.

The unequal division is not an accident. The egg must carry the food reserves and organelles that sustain the early embryo until it implants in the uterus, so the cell that becomes the egg hoards nearly everything, and the polar bodies, which contain almost no cytoplasm, disintegrate. At fertilization, the haploid sperm, with 23 chromosomes, fuses with the haploid egg, also with 23 chromosomes, and the diploid number of 46 is restored in the zygote, the first cell of the new individual.

ELI-10

Think of baking one huge birthday cake and sharing it with friends. You cut it so that one friend receives almost the whole cake and the others get crumbs. The big slice is the egg, carrying all the supplies, and the crumbs are the polar bodies. A sperm is the opposite, a small delivery scooter carrying just the cargo and nothing extra. This way the single egg holds all the food that the new baby needs at the start.

High-Yield:

• Meiosis converts one diploid cell into four haploid cells, so in humans 46 chromosomes become 23 in every gamete.
• DNA replicates once, yet the nucleus divides twice, so no replication occurs between meiosis I and meiosis II.
• Crossing over at chiasmata during prophase I and independent assortment in meiosis I are the two great engines of genetic variation.
• Independent assortment alone yields 2 to the 23, over eight million, chromosome combinations in human gametes.
• Nondisjunction makes n plus 1 and n minus 1 gametes, and an extra chromosome 21 causes Down syndrome.

Quick Review

• Meiosis is two divisions that convert one diploid cell into four haploid cells, halving the chromosome number.
• Meiosis I separates homologous chromosomes in the reductional division; meiosis II separates sister chromatids in the equational division.
• Crossing over swaps segments between nonsister chromatids at chiasmata during prophase I and creates recombinant chromosomes.
• Independent assortment randomly sorts each homologous pair at metaphase I, producing 2 to the n chromosome combinations.
• Nondisjunction in meiosis I or meiosis II creates aneuploid gametes, including the extra chromosome 21 that causes Down syndrome.
• Spermatogenesis produces four functional sperm, while oogenesis produces one egg plus polar bodies that are discarded.
• Fertilization joins two haploid gametes and restores the diploid chromosome number in the zygote.

Key terms

Key terms are emphasized and defined within the main notes.

Important formulas or processes

See the formulas, procedures, and process blocks in the main notes where applicable.

Common mistakes

See the labeled common-mistake callouts in the main notes where present.