Campbell Biology Chapter 46 Review: Animal Reproduction

46.1 Cloning or sex: how animals reproduce

pp. 996–999

On the AP exam? Background

The current course doesn't cover animal reproductive modes like budding or parthenogenesis. What carries over: asexual offspring come from mitosis and match the parent (Topic 4.5), sex creates variation (Topic 5.2), varied populations cope better with change (Topic 7.11), and outside cues time breeding (Topic 8.1).

In the course: Topic 4.5 Cell Cycle, Topic 5.2 Meiosis and Genetic Diversity, Topic 7.11 Variations in Populations, Topic 8.1 Responses to the Environment (notes, videos and more questions)

Key points

Sexual reproduction joins two haploid gametes into a diploid zygote. The egg is big and can't move on its own; the sperm is tiny and usually swims. Asexual reproduction makes offspring from one parent with no egg and sperm joining, usually by mitosis, so the young are genetic copies.
Many invertebrates reproduce asexually in one of three ways. In fission, one animal splits into two of about equal size. In budding, a new animal grows out of the old one. In fragmentation, the body breaks apart and each piece regrows what it's missing (regeneration).
In parthenogenesis, an egg develops without ever being fertilized. Aphids do it all summer, and it has turned up now and then in vertebrates, including some snakes, domestic turkeys and even California condors.
Sex is costly. In a sexual species, about half the young are males, who can't bear young themselves, while every offspring of an asexual female can. On paper, an asexual line should outgrow a sexual one, so sex must bring a real benefit to have lasted.
The leading explanation is variation. Meiosis and fertilization deal out new allele combinations every generation, so some offspring may cope with a change that would wipe out a line of clones, especially when parasites and pathogens keep evolving new ways to attack. When conditions stay steady and good, cloning comes out ahead, since a mother whose genes already work hands them on unchanged.
Most animals breed in cycles timed to the seasons, so young arrive when food is plentiful. Hormones control the timing, and the hormones in turn respond to outside signals: lengthening or shortening days, warming water, the start of a rainy season, even moonlight. If climate change shifts when food is available but not the signal an animal uses, breeding can fall out of step.
Some animals blur male and female. Hermaphrodites such as earthworms have both kinds of organs, so any two can mate and trade sperm, which helps animals that rarely meet. Others change sex during life: in a clownfish group, the largest fish is female, and if she dies, the breeding male becomes female.

Key terms (12)

sexual reproduction: Making offspring by joining two haploid gametes, usually from two parents, so each offspring gets a new mix of alleles.
asexual reproduction: Making offspring from a single parent with no egg and sperm joining. The young are usually genetic copies made by mitosis.
gamete: A haploid reproductive cell, an egg or a sperm, that joins with another gamete at fertilization.
zygote: The diploid cell formed when two gametes fuse. It's the very first cell of a new individual.
fission: Asexual reproduction in which one animal splits into two of roughly equal size.
budding: Asexual reproduction in which a new individual grows out of the parent's body, then breaks away or stays attached.
fragmentation: Asexual reproduction in which an animal’s body splits into pieces and each piece grows back into a whole animal.
regeneration: Regrowing body parts that were lost or damaged.
parthenogenesis: Development of a new individual from an egg that was never fertilized.
hermaphroditism: Having both male and female reproductive organs in one individual, as earthworms and garden snails do.
sex reversal: Switching from one sex to the other during life, as when a breeding male clownfish becomes the group's female.
reproductive cycle: A repeating pattern of breeding, often tied to the seasons and controlled by hormones that respond to cues like day length.

Check yourself: 46.1 Cloning or sex: how animals reproduce

4 questions on 46.1 Cloning or sex: how animals reproduce. Pick an answer to see if you got it, and why.

Question 1 of 4

A sea star loses an arm to a predator. The arm, which kept a piece of the central disc, grows into a complete new sea star while the original animal regrows its missing arm. Which kind of reproduction is this?

Question 2 of 4

A lab colony of an asexual planarian strain was started from a single worm. These worms reproduce only by pinching in two and regrowing the missing half. A year later there are 3,000 worms. Which best describes the genetic variation among them?

Question 3 of 4

Researchers studied a freshwater snail that can reproduce either sexually or asexually. In four lakes they measured how many snails carried a parasitic worm and what share of the snails reproduced sexually (invented data). Lake | Snails infected (%) | Snails reproducing sexually (%) 1 | 2 | 5 2 | 10 | 14 3 | 25 | 30 4 | 40 | 46 Which conclusion do the data best support?

Question 4 of 4

Pea aphids bear live daughters from unfertilized eggs through spring and summer, then switch to producing males and egg-laying females once autumn days get shorter. A line of these aphids is kept in a greenhouse with 16-hour days and fresh plants all year. Which outcome is most likely?

0 of 4 answered

46.2 Getting sperm and egg together

pp. 999–1002

On the AP exam? Background

External versus internal fertilization isn't tested in the current course. Related tested ideas: animals use signals like pheromones and courtship to find mates (Topic 8.1), mismatched gametes keep species apart (Topic 7.10), and eggshells and parental care are adaptations shaped by natural selection (Topic 7.2).

In the course: Topic 8.1 Responses to the Environment, Topic 7.10 Speciation, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

Fertilization is external when eggs and sperm meet outside the body, usually in water. It's internal when sperm are placed in or near the female's reproductive tract and meet the egg there.
Because the gametes are released unprotected, external fertilization nearly always happens in water or a very wet place: eggs and sperm would shrivel in dry air, and sperm need liquid to swim through. Timing is everything: many animals release gametes all at once (spawning), cued by temperature, day length, the moon or chemicals from neighbors. Reef corals spawning together a few nights after a spring full moon are a famous case.
Courtship helps a pair release gametes at the same moment and lets animals choose mates. Pheromones, chemicals that act on other members of the same species, can draw mates from far away; some male moths track a female's scent on the wind.
With internal fertilization, sperm travel inside the female’s body, so mating can happen far from water. It comes with a trade-off: a fish spawning in open water may release millions of eggs, while an animal with internal fertilization makes far fewer, but each one has a much better chance of surviving.
Many animals keep protecting embryos after fertilization. A placental mammal grows inside the uterus, fed by its mother through the placenta, while marsupial young are born tiny and finish growing in a pouch. Bird and reptile embryos develop in shelled eggs whose inner membranes stop them from drying out; fish and frog eggs get only a jelly coating. Parental care, from nest guarding to nursing, raises survival further.
Gametes come from a group of germ-line cells set aside early in the embryo. Some marine worms have no distinct gonads at all, while insects and vertebrates have gonads plus ducts and glands. Many female insects keep sperm in a storage sac, the spermatheca, and use it for months.
Most vertebrates other than mammals have a cloaca, one opening shared by the digestive, urinary and reproductive tracts. Because females of many species have several mates, sperm from different males can compete, and females can sway which sperm fertilize their eggs.

Key terms (12)

fertilization: The fusion of a sperm and an egg to make a zygote.
external fertilization: Fertilization outside the body, usually in water, after both parents release their gametes.
internal fertilization: Fertilization inside the female after sperm are placed in or near her reproductive tract.
spawning: Releasing eggs and sperm into the water, often by many animals at the same time.
courtship: Behaviors before mating that help animals choose a partner and release gametes at the right moment.
pheromone: A chemical signal one animal releases that changes the behavior or body of others of the same species.
gonad: An organ that makes gametes: an ovary or a testis.
spermatheca: A sac in many female insects that stores sperm, sometimes for months, until eggs are ready to be fertilized.
cloaca: A single opening used by the digestive, urinary and reproductive tracts in birds, reptiles, amphibians and many fish.
placenta: An organ made of both maternal and embryonic tissue that passes nutrients and oxygen to the embryo and carries its wastes away.
parental care: Anything parents do after fertilization to protect or feed their young, which raises the young's odds of surviving.
gametic isolation: A barrier between species in which their eggs and sperm can't fuse, often because the proteins on their surfaces don't match.

Check yourself: 46.2 Getting sperm and egg together

4 questions on 46.2 Getting sperm and egg together. Pick an answer to see if you got it, and why.

Question 1 of 4

Horseshoe crabs live in the ocean but come ashore on high spring tides to breed. A female digs a shallow nest in wet sand near the waterline and lays her eggs, and a male beside her releases sperm over them. Why do they nest in wet sand at the tide line rather than in dry sand higher up the beach?

Question 2 of 4Calculator allowed

Question 3 of 4

Biologists suspect that when one sea urchin spawns, a chemical it releases along with its gametes triggers nearby urchins to spawn too. Which experiment would best test this hypothesis?

Question 4 of 4

Two species of abalone (large sea snails) spawn on the same reef on the same nights. When their gametes mix, very few eggs of one species are fertilized by sperm of the other, because a protein on the sperm must bind a matching receptor in the egg’s coat. Which kind of reproductive barrier is this?

0 of 4 answered

46.3 Human reproductive organs and making gametes

pp. 1002–1008

On the AP exam? Background

You won't be asked to label reproductive organs. Gamete formation is where this links to tested ideas: meiosis halves the chromosome number (Topic 5.1), errors in meiosis cause nondisjunction (Topic 5.2), and sperm rely on mitochondria for ATP (Topic 2.1).

In the course: Topic 5.1 Meiosis, Topic 5.2 Meiosis and Genetic Diversity, Topic 2.1 Cell Structure and Function (notes, videos and more questions)

Key points

The ovaries hold thousands of follicles, each an immature egg (oocyte) wrapped in cells that feed it and make hormones. In a typical cycle one follicle releases its oocyte at ovulation, and what's left of the follicle becomes the corpus luteum, which makes progesterone and estradiol.
The released oocyte is swept into an oviduct (fallopian tube) by beating cilia and muscle contractions, and fertilization usually happens there. The uterus has a blood-rich lining, the endometrium, where an embryo can implant. The cervix connects the uterus to the vagina, which is also the birth canal.
The testes make sperm inside coiled seminiferous tubules, while Leydig cells between the tubules make testosterone. The testes hang in the scrotum, a few degrees cooler than the body core, because most mammals make sperm poorly at full body temperature.
Sperm mature and learn to swim in the epididymis, then travel through the vas deferens and ejaculatory duct to the urethra, the tube that also carries urine. The seminal vesicles, prostate and bulbourethral glands add most of the fluid in semen, including fructose for fuel and an alkaline pH that shields sperm from the vagina's acidity.
Spermatogenesis runs nonstop from puberty, making millions of sperm a day; each primary spermatocyte yields four sperm. A sperm is built for its job: a head holding the haploid nucleus, capped by an enzyme-filled acrosome; a midpiece packed with mitochondria; and a flagellum for swimming.
Making eggs is very different from making sperm. Each round of meiosis in oogenesis hands nearly all the cytoplasm to one cell, so a single egg forms and the tiny polar bodies break down. The process also stops and starts: oocytes enter meiosis I before birth, stay paused until the cycle in which one is released (anywhere from puberty to the 40s or 50s), and complete meiosis II only if a sperm gets in. The mainstream view is that a female’s whole stock of oocytes is set before she is born.
The egg is big because it stocks the nutrients, mRNA and organelles the early embryo uses before it can draw on the mother. The long pause in meiosis may help explain why nondisjunction becomes more common in eggs as people age.

Key terms (15)

ovary: The female gonad. It holds follicles, releases oocytes and makes estradiol and progesterone.
follicle: A small cluster of cells in the ovary surrounding one oocyte, feeding it and making estradiol.
oocyte: An egg cell that hasn't finished meiosis. Humans release a secondary oocyte at ovulation.
corpus luteum: The leftover follicle after ovulation, which turns into a hormone gland making progesterone and estradiol.
oviduct: The tube that carries an oocyte from near the ovary to the uterus; also called the fallopian tube. Fertilization usually happens here.
endometrium: The blood-rich layer covering the inside of the uterus, where an embryo implants.
testis: The male gonad, which makes sperm and testosterone. Plural: testes.
seminiferous tubule: One of the tightly coiled tubes inside a testis where sperm are made.
Leydig cell: A cell in the spaces between seminiferous tubules that makes testosterone when LH reaches it.
epididymis: A long coiled duct on each testis where sperm finish maturing and gain the ability to swim.
vas deferens: The muscular duct that carries sperm from the epididymis toward the urethra.
acrosome: A vesicle at the tip of the sperm head holding enzymes that help the sperm get through the coat around the egg.
polar body: A tiny cell made by unequal division in oogenesis. It gets a set of chromosomes but almost no cytoplasm and breaks down.
spermatogenesis: Making sperm. In adult men it runs continuously, and each cell that starts meiosis makes four sperm.
oogenesis: Making eggs. It starts before birth, pauses for years, and each cell that enters meiosis gives one egg plus polar bodies.

Check yourself: 46.3 Human reproductive organs and making gametes

4 questions on 46.3 Human reproductive organs and making gametes. Pick an answer to see if you got it, and why.

Question 1 of 4

Which sequence shows the route sperm take from where they are made to where they leave the body?

Question 2 of 4

A mutation leaves the mitochondria in a man's sperm unable to carry out oxidative phosphorylation, though the sperm are otherwise normal in shape and number. Which problem would you most expect?

Question 3 of 4

A chemical stops the vesicle at the tip of the sperm head from releasing its contents by exocytosis. Treated sperm swim normally and reach the egg. What would most likely happen next?

Question 4 of 4

Dogs have 78 chromosomes in their body cells (2n = 78). How many chromosomes are in each primary spermatocyte, each secondary spermatocyte and each spermatid?

0 of 4 answered

46.4 Hormones that run the reproductive cycles

pp. 1008–1011

On the AP exam? Background

The current course won't ask you to name GnRH, FSH or LH or the phases of the cycle. It's still a great Topic 4.4 example: negative feedback keeps hormone levels steady, and the LH surge before ovulation is positive feedback. Hormones reaching target cells through the blood is Topic 4.1.

In the course: Topic 4.4 Feedback, Topic 4.1 Cell Communication, Topic 4.3 Signal Transduction Pathways (notes, videos and more questions)

Key points

Reproduction in both sexes is run from three levels. The hypothalamus releases GnRH, which tells the anterior pituitary to release FSH and LH (the gonadotropins), which act on the ovaries or testes. The gonads then make the sex hormones.
The sex hormones (androgens such as testosterone, estrogens such as estradiol, and progesterone) are steroids. Before birth, androgens build the male reproductive organs. At puberty, sex hormones bring on secondary sex characteristics, like a deeper voice and facial hair, or breast growth and wider hips.
In males, LH tells Leydig cells to make testosterone, and FSH targets the Sertoli cells that nurse developing sperm. Testosterone slows GnRH and LH release, and Sertoli cells release inhibin, which slows FSH. These two negative feedback loops keep hormone levels and sperm production fairly steady.
A female cycle has two linked parts. The ovarian cycle runs from the follicular phase (a follicle grows) through ovulation to the luteal phase (the corpus luteum works). The uterine, or menstrual, cycle runs from menstrual flow, to the proliferative phase (the lining thickens), to the secretory phase (lining glands release nutrients). Cycles average about 28 days, but healthy lengths vary.
Early on, growing follicles make a little estradiol, and that low level keeps FSH and LH down by negative feedback. Once estradiol climbs high, its effect flips: it stimulates the hypothalamus and pituitary, setting off an LH surge (positive feedback). The surge makes the follicle rupture, releasing the oocyte about a day later.
After ovulation, the corpus luteum makes progesterone and estradiol, which keep the lining thick and hold FSH and LH down so no new follicle develops. If no embryo arrives, the corpus luteum breaks down, hormone levels fall, the lining is shed, and FSH rises to start the next cycle.
Most mammals other than humans and their close primate relatives don’t menstruate. If no pregnancy starts, their estrous cycle ends with the body breaking down the uterine lining and taking it back up, and the female will mate only around ovulation (‘in heat’). In humans, menopause, the end of ovulation and menstruation, comes when the ovaries run out of working follicles and stop responding to FSH and LH, usually around age 50.

Key terms (15)

GnRH: Gonadotropin-releasing hormone, sent from the hypothalamus to the anterior pituitary to trigger FSH and LH release.
FSH: Follicle-stimulating hormone, from the anterior pituitary. It drives follicle growth in females and acts on Sertoli cells in males.
LH: Luteinizing hormone, from the anterior pituitary. Its surge triggers ovulation, and in males it tells Leydig cells to make testosterone.
androgen: A steroid sex hormone, such as testosterone, that drives male development, sperm production and male traits.
estradiol: The main estrogen. Made by growing follicles, it thickens the uterine lining and controls FSH and LH by feedback.
progesterone: A steroid hormone from the corpus luteum, and later the placenta, that keeps the uterine lining ready for an embryo.
Sertoli cell: A cell lining the seminiferous tubules that feeds and supports developing sperm. It responds to FSH and makes inhibin.
inhibin: A hormone made in the gonads that slows FSH release from the pituitary, a negative feedback signal.
ovulation: The release of an oocyte when a mature follicle bursts open, triggered by the LH surge.
ovarian cycle: The monthly cycle in the ovary: follicle growth, ovulation, then the life and breakdown of the corpus luteum.
menstrual cycle: The monthly cycle of the uterine lining: shedding, rebuilding and getting ready for an embryo. Also called the uterine cycle.
follicular phase: The first part of the ovarian cycle, when follicles grow and estradiol rises, ending at ovulation.
luteal phase: The stretch of the ovarian cycle after ovulation, when the corpus luteum makes progesterone and estradiol.
estrous cycle: The reproductive cycle of most non-primate mammals. If no pregnancy occurs, the body takes back the uterine lining instead of shedding it, and the female will mate only around ovulation.
menopause: The time, usually around age 50, when ovulation and menstrual cycles stop because the ovaries have run out of working follicles.

Check yourself: 46.4 Hormones that run the reproductive cycles

4 questions on 46.4 Hormones that run the reproductive cycles. Pick an answer to see if you got it, and why.

Question 1 of 4

Early in the ovarian cycle, a modest rise in estradiol lowers FSH and LH release. Late in the follicular phase, high estradiol sharply raises LH release, which pushes the follicle to make even more estradiol. How should these two effects be classified?

Question 2 of 4

In male mammals, Sertoli cells release inhibin, which acts on the anterior pituitary. Researchers inject male rats with an antibody that binds and inactivates inhibin. Which result do you predict?

Question 3 of 4

Blood progesterone was measured in one woman over a cycle in which she did not become pregnant (invented data). Day of cycle | Progesterone (ng/mL) 3 | 0.5 10 | 0.6 15 | 2 18 | 9 22 | 14 27 | 1 What best explains the drop between day 22 and day 27?

Question 4 of 4

Suppose a drug blocked androgen receptors only in a man's hypothalamus and anterior pituitary, leaving his other tissues unaffected. What would most likely happen to his LH and testosterone levels?

0 of 4 answered

46.5 Pregnancy, birth and reproductive technology

pp. 1011–1018

On the AP exam? Background

Oxytocin's positive feedback during labor is a named Topic 4.4 example, so know it well. The rest of pregnancy, contraception and fertility treatment isn't tested, but it uses tested ideas: hormones and target cells (Topic 4.1), and surface area and membrane transport in the placenta (Topics 2.2 and 2.5).

In the course: Topic 4.4 Feedback, Topic 4.1 Cell Communication, Topic 2.2 Cell Size, Topic 2.5 Membrane Transport (notes, videos and more questions)

Key points

Fertilization usually happens in the oviduct. The zygote starts dividing (cleavage) on its way to the uterus. By about day 5 it has become a blastocyst, a fluid-filled sphere of cells, and roughly a week after fertilization it burrows into the endometrium.
The embryo has to keep the corpus luteum from breaking down. Its outer cells release hCG, a hormone that acts like LH and keeps progesterone coming, so the lining isn't shed. Pregnancy tests detect hCG in urine. By around week 10, the placenta makes enough progesterone on its own.
The placenta grows from embryonic and maternal tissue together. Fetal capillaries sit inside finger-like villi bathed in the mother's blood, but the two bloods normally don't mix. Oxygen, nutrients and wastes cross by diffusion and transport proteins, and the villi's huge surface area keeps exchange fast. Alcohol, nicotine and many drugs can cross too.
A pregnancy lasts about 38 weeks from fertilization, split into three trimesters. Nearly all organs start forming in the first trimester, and after 8 weeks the developing baby gets a new name: fetus. That makes the first trimester the riskiest time for damage from alcohol, some drugs or radiation. The later trimesters are mostly growth. Identical twins come from one early embryo splitting; fraternal twins come from two eggs fertilized separately.
Labor runs on positive feedback. Oxytocin makes the uterus contract, contractions trigger more oxytocin (and prostaglandins), and the contractions keep getting stronger until the baby and then the placenta are delivered. After birth, prolactin drives milk production, and oxytocin releases milk when the baby nurses.
Contraception works by blocking a step. Combined pills, patches and rings keep hormone levels steady, which prevents the LH surge and so stops ovulation. Condoms and diaphragms keep sperm from reaching the egg, tubal ligation and vasectomy close off the ducts, and IUDs mainly prevent fertilization. Condoms also give the best protection against sexually transmitted infections.
About 1 in 6 people face infertility at some point. In vitro fertilization (IVF) combines eggs and sperm in a dish and places early embryos in the uterus; in ICSI, one sperm is injected straight into an egg. Before birth, ultrasound, testing fetal cells, and screening DNA from the placenta in the mother's blood can detect many genetic conditions.

Key terms (14)

cleavage: The rapid cell divisions of the zygote that make an early embryo of many small cells without making it any bigger.
blastocyst: The embryo stage reached about 5 days after fertilization: an outer cell layer, which will help form the placenta, around a fluid-filled space and a small inner cluster of cells.
implantation: The embryo burrowing into the endometrium about a week after fertilization.
hCG: Human chorionic gonadotropin, a hormone from the embryo that keeps the corpus luteum making progesterone. Pregnancy tests detect it.
trimester: One of the three periods of about 3 months that a human pregnancy is divided into.
organogenesis: The formation of the body's organs, which happens mostly in the first trimester.
fetus: The developing human from about 9 weeks after fertilization until birth, once all the major organs have started to form.
umbilical cord: The cord carrying the fetus's blood vessels to and from the placenta.
labor: Strong, regular contractions of the uterus that open the cervix and push out the baby, then the placenta.
oxytocin: A hormone released by the posterior pituitary that makes the uterus contract during labor and makes milk flow during nursing.
prolactin: A hormone from the anterior pituitary that tells the mammary glands to make milk.
contraception: Deliberately preventing pregnancy, for example by stopping ovulation, keeping sperm and egg apart, or blocking the ducts they travel through.
in vitro fertilization (IVF): Combining eggs and sperm in a lab dish, then placing an early embryo in the uterus.
ectopic pregnancy: A pregnancy in which the embryo implants outside the uterus, usually in an oviduct. It can't continue and can be dangerous.

Check yourself: 46.5 Pregnancy, birth and reproductive technology

4 questions on 46.5 Pregnancy, birth and reproductive technology. Pick an answer to see if you got it, and why.

Question 1 of 4

During labor, uterine contractions trigger oxytocin release, and oxytocin makes the contractions stronger. Which other process works by the same kind of feedback?

Question 2 of 4

Question 3 of 4

In an experiment with a nonhuman primate, pregnant females received either an antibody that neutralizes the embryo's hCG or a harmless control injection, a few days after implantation. Blood progesterone was then tracked (invented data). Days after injection | Control (ng/mL) | Antibody (ng/mL) 0 | 20 | 20 4 | 22 | 8 8 | 25 | 2 What most likely explains the antibody group's results?

Question 4 of 4

A woman gives birth to twins, a boy and a girl. Assuming no unusual chromosome conditions, how did these twins most likely form?

0 of 4 answered