AP Biology Notes Outline
Animal Reproduction
Chapter 46
Reproduction in Animals: Asexual v. Sexual Reproduction
Asexual is common in invertebrates.
• Individuals are formed from genes coming together from only one parent – no fusion of eggs and sperm.
• Advantages: no need to find and convince mate – offspring can be produced quickly.
• Disadvantages: offspring are identical (little variation).
Types of asexual reproduction:
• Fission – one parent separates into two or more approximately equal-sized individuals (i.e. protozoans)
• Budding – new individuals split from existing ones (i.e. Hydra)
• Gemmules – release of specialized cells that grow into new individuals (i.e. sponges)
• Fragmentation – breaking of body into several pieces; some or all of which develop into complete adults (i.e. plants)
• Parthenogenesis – development of an unfertilized egg into a completely new individual – offspring are clones of the mother
(i.e. many invertebrates; some reptiles and fishes)
Sexual reproduction occurs when offspring are produced after the fusion of two haploid gametes.
• Ovum (egg) from the female parent (usually large and non-motile) and Spermatatozoan (sperm) from the male parent
(usually small and motile).
• Main advantage is that it increases genetic variation among offspring.
• Disadvantages: it can be much slower and more costly to find and convince a mate to cooperate
Hermaphrodism
This is a form of sexual reproduction – a hermaphrodite can function reproductively as a female or male.
• Many can self-fertilize – but most do not if it can be avoided because self-fertilization does not encourage variation as much
as out-crossing (mating with another individual).
• Often, each individual donates sperm and also receives sperm during mating (this is called reciprocal mating) – results in
twice as many offspring as would have occurred if only one individual were pregnated.
Modes of Fertilizaton in Animals
External fertilization (spawning) is where sperm and eggs are released into the environment.
• Common in some invertebrates, fishes, and amphibians.
• Tends to produce many more zygotes than internal fertilization (but survival rate is lower).
• Does not require cooperative behavior of mate – but pheromones ensure that eggs and sperm are released at the same time.
• Requires moist or aquatic habitat
Internal fertilization is when sperm is released or deposited inside the female (requires specialized apparatus).
• Common in vertebrates (and some fishes).
• Usually produces fewer zygotes but survival rate is often high because of protection.
• Requires cooperative behavior.
Reproductive Systems in Animals
Many different systems are found in the diverse animal phylum.
• Least complex systems lack gonads (i.e. polychaete worms) – eggs and sperm develop from undifferentiated cells lining the
coelom (body cavity).
• Some otherwise simple invertebrates have complex reproductive systems (i.e. flatworms).
• Some species have a spermatheca (organ for storing sperm).
Vertebrate Reproductive Systems
The general plan is very similar across most vertebrates. There are some common differences between groups:
• Non-mammalian vertebrates generally have single, common opening for releasing wastes and for reproduction. The
digestive, excretory, and reproductive systems all open into the single opening called a cloaca.
• Mammals have separate opening for digestive and reproductive systems.
AP Biology Notes Outline
Animal Reproduction
Chapter 46
Gametogenesis
Gametogenesis is the production of gametes (eggs and sperm). In males, it is called spermatogenesis. In females, it is called
oogenesis. Sperm are produced in testes, eggs are produced in ovaries. BEFORE PROCEEDING – REVIEW FIGURES 46.10 &
46.11 IN THE TEXTBOOK. READ TEXT PAGES 1003-1006 AND UNDERSTAND THE REPRODUCTIVE ANATOMY OF
HUMANS!
Spermatatogenesis (Figure 46.12) is the process of sperm production in males. It is a continuous process that starts at puberty.
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It begins as luteinizing hormone (LH) induces the interstitial cells of the testes to produce testosterone.
Together with FSH, testosterone induces maturation of the seminiferous tubules and stimulates the beginning of sperm
production.
In the seminiferous tubules, each spermatogonium cell (2n) divides by mitosis to produce two primary spermatocytes (2n),
which each can undergo meiosis I to produce 2 secondary spermatocytes (n).
Each secondary spermatocyte then undergoes meiosis II, which yields 4 spermatids (n).
These spermatids differentiate and move to the epididymis where they become motile. During this process, the developing
sperm are nourished by the Sertoli cell.
Oogenesis (Figure 46.12) is the production of ova. It begins prior to birth in females.
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Within the embryo, an oogonium cell (2n) undergoes mitosis to produce primary oocytes (2n).
These remain quiescent within small follicles in the ovaries until puberty, when they become reactivated by hormones.
FSH periodically stimulates the follicles to complete meiosis I, producing secondary oocytes (n), which are released at
ovulation.
Meiosis II then stops again and does not continue until fertilization, when a sperm penetrates the secondary oocyte.
Oogenesis differs from sperm formation in three ways: (1) it is a stop-start process beginning prior to birth and is completed
upon fertilization; (2) cytokinesis divides the cytoplasm of the cell unequally, producing one large cell and two small polar
bodies which will disintegrate and be re-absorbed by the female; and (3) one primary oogonium cell produces on one active
egg cell.
Hormonal Control of the Testes
Review Figure 46.13 in textbook – know the role of FSH, LH, GnRH and the examples of negative feedback. Be sure you understand
where these hormones are produced and what the target cell is!
• FSH – stimulates production of ova and sperm…regulated by hypothalamus.
• LH – stimulates ovaries and testes…regulated by hypothalamus.
Female Reproductive Cycles
Most mammals have estrous cycles. Humans and some other primates have menstrual cycles. In both, ovulation occurs after the
endometrium (lining of the uterus) thickens and develops a rich blood supply to support the emplantation of an embryo. One key
difference between these is the fate of the endometrial layer:
• Estrous Cycle – endometrium is reabsorbed and there is some, but not much bleeding.
o Animals with estrous cycles are generally only receptive to mating near ovulation (time is called estrus). The
duration of the estrous cycle depends on the species (rats: every 5 days; dogs: once a year)
• Menstrual Cycle – endometrium is shed and there is relatively more bleeding.
o Animals with menstrual cycles have less pronounced behavioral changes associated with their cycle (i.e. human
females may be receptive to mating at any time during their cycle). Human menstrual cycles last about 28 days (but
can be variable among individuals).
AP Biology Notes Outline
Animal Reproduction
Chapter 46
Human Female Reproductive Cycle
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KNOW IT! Review textbook Figure 46.14! The menstrual cycle in female humans is associated with changes within the uterus (also
called the uterine cycle) – this cycle is associated with and controlled by events in the ovarian cycle. The human female reproductive
cycle is really two closely related cycles involving two organs: the uterus and the ovaries.
The Ovarian Cycle in Humans
Follicular Phase
• Before ovulation, ovaries are in the follicular phase. This phase involves development and maturation of follicles (follicles
are eggs surrounded by cells that nourish and protect the egg).
• The hypothalamus releases GnRH which stimulates the pituitary.
• The anterior pituitary releases small amounts of LH and FSH (FSH stimulates follicle growth; LH helps).
• The growing follicle cells start to make estrogen (several follicles grow, but only one matures). Low levels of estrogen
inhibit the pituitary hormones (causing LH and FSH levels to remain low).
• As follicles mature, estrogen levels rise sharply (this stimulates the secretion of GnRH in the hypothalamus).
• LH and FHS levels respond to GnRH and increase to high levels.
Ovulation Phase
• High LH levels stimulate the final maturation phase of the follicle (ovulation occurs when the follicle and adjacent wall of the
ovary rupture, releasing a secondary oocyte).
Luteal Phase
• After ovulation, the rest of the cycle is called the luteal phase. This phase involves development and maturation of the corpus
luteum (a glandular structure that secretes progesterone and estrogen) which inhibits secretion of LH and FSH...levels fall
quickly.
• The corpus luteum eventually disintegrates. FSH and LH inhibition is lost and levels of these hormones start to rise again
(rising FSH startes the cycle over again, encouraging new follicles to develop).
The Uterine Cycle in Humans
Menstrual Flow Phase
• The first day of the menstrual flow (bleeding) marks the beginning of a new cycle (Day 1) – lasts from 4-7 days.
• Fresh follicles are just starting to develop in the ovary during this phase (corresponds with the follicular phase in the ovarian
cycle).
Proliferative Phase
• Estrogen secreted by growing follicles stimulates the endometrium to thicken and prepare for an embryo (corresponds to the
follicular phase in the ovarian cycle).
Secretory Phase
• Estrogen and progesterone from the corpus luteum continue to stimulate the endometrium to develop (corresponds to the
luteal phase of the ovarian cycle).
• When the corpus luteum disintegrates, ovarian hormone levels drop. This causes spasms in the arteries of the endometrium
which deprive it of blood.
• The upper two-thirds of the endometrium is shed (menstruation/bleeding).
Menopause in Humans
On average, human females undergo menopause (the cessation of ovulation and menstruation) between ages 46 and 54. During these
years, the ovaries lose their responsiveness to the hormones FSH and LH from the pituitary – and menopause results in a decline in the
production of estrogens by the ovary.
AP Biology Notes Outline
Animal Reproduction
Chapter 46
EXAMPLE #1
Describe negative and positive feedback loops, and discuss how feedback mechanisms regulate each of the following:
a.
b.
The menstrual cycle in a nonpregnant human female.
Blood glucose levels in humans.
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EXAMPLE #2
Sexual reproduction requires that half of the chromosomes in a zygote come from one parent and the other half from the second
parent.
(a) Describe the process by which a germ cell’s complement of chromosomes is halved in the formation of gametes.
(b) Choose one organism or group of organisms that reproduce asexually. Describe the mode of asexual reproduction in that
organism and explain the advantages to the organism of asexual reproduction.
(c) Choose one organism or group of organisms that reproduce sexually. Describe the mode of sexual reproduction in that
organism and explain the advantages to the organism of sexual reproduction.
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EXAMPLE #3
Compare and contrast the processes of gametogenesis in mammalian males and females.
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