5 Steps to a 5 AP Biology, 2014-2015 Edition

208 ❯ STEP 4. Review the Knowledge You Need to Score High

are transplanted to a different place near the ectoderm, the neural plate will develop in the

new location. The cells from the notochord region act as “project directors,” telling the

ectoderm where to produce the neural tube and central nervous system.

Homeotic genesregulate or “direct” the body plan of organisms. For example, a fly’s

homeotic genes help determine how its segments will develop and which appendages

should grow from each segment. Scientists interfering with the development of these poor

creatures have found that mutations in these genes can lead to the production of too many

wings, legs in the wrong place, and other unfortunate abnormalities. The DNA sequence

of a homeotic gene that tells the cell where to put things is called the homeobox.It is sim-

ilar from organism to organism and has been found to exist in a variety of organisms—

birds, humans, fish, and frogs.

Factors in Cellular Differentiation

Cytoplasmic distribution Asymmetry contributes to differentiation, since different

areas have different amounts of cytoplasm, and thus perhaps

different organelles and cytoplasmic structures.

Induction One group of cells influences another group of cells through

physical contact or chemical signaling.

Homeotic genes Regulatory genes that determine how segments of an organism

will develop.

The Influence of Hormones

In Chapter 15 we discussed the hormones that will be included in the AP exam. A few of

those play a critical role in human sexual development and reproduction. The hormones

involved include LH, FSH, estrogen, progesterone, and testosterone. You do not need to

know every detail, just the big picture. As proper etiquette requires, ladies first. Let’s talk

about the hormones involved in the female reproductive system.

Estrogen and progesterone continually circulate in the female bloodstream, and the

hypothalamus monitors these levels to determine when to release certain hormones. For

example, when the concentrations of estrogen and progesterone are low, the hypothalamus

secretes GnRH, which travels to the anterior pituitary gland to induce the release of FSH

and LH. ( Just to remind you, FSH is follicle-stimulating hormone, LH is luteinizing hor-

mone, and GnRH is gonadotropin-releasing hormone.) FSH induces the development of

the follicle that contains the primary oocyte during its development. It also causes the fol-

licular cells to release estrogen, which triggers the hypothalamus to dump more GnRH into

the system. This GnRH acts on the anterior pituitary to produce the LH surgethat initi-

atesovulation—the release of a secondary oocyte from the ovary.

This LH surge causes further release of estrogen and progesterone from the follicular cells,

which have now become a structure called the corpus luteum. The corpus luteum induces the

thickening of the endometrium, the site of future fertilized egg attachment. At this point in

the cycle, the levels of estrogen and progesterone elevate enough to make the hypothalamus

cut off production of GnRH so that the LH and FSH levels drop back down. (This decrease

in production of LH and FSH, due to high levels of estrogen and progesterone, is called

negative feedback.) Here lies a fork in the road for the female reproductive system. If fer-

tilization has occurred in the fallopian tube, and if the blastocyst attaches successfully to the

uterine wall, hCG will be secreted, which works to keep the corpus luteum alive. As a result,

the levels of estrogen and progesterone remain high and keep theendometrium intact.

KEY IDEA

BIG IDEA 3.B.2
Signal transmission
by homeotic genes
mediates cell
function.

BIG IDEA 2.C.1
Organisms use feed-
back mechanisms.