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CONCEPlUAL LIFE SCIENCE
ReproductioD
LIFE CYCLES
Life begins withjertilization, which is the union ofmale and female sex cells
otherwise known as gametes. The female sex eell is known as the egg and the male sex
cen is caned a sperm cell. When the spenn ceU fertilizes the egg, the resultiDg fertiliad
egg is caned a zygote. The zygote divides by mitosis to produce a diploid (211) organism.
This process is known as development.
In order to reproduce, the adult organism produces gametes (sperms orqgs) that
contain the haploid (II) nmnber ofchromosomes. The ceUs that give rise to the gametes
undergo a special reduction division process known as meiosis.
In meiosis, there are two divisions called Division] and Division U. Division] is
the separation ofthe chromosome pairs, while Division U is the separation of chromatids.
By separating the chromosome pairs, meiosis accoinplisbes a Rduetion of the
chromosome Dmnber. The beginning cell has the 211 number. At the end of the process,
the resulting four cells each contain the II number.
May lower plats display alternation 0/generations. In these plants, meiosis
results in haploid ~res that produce separate male 8Dd female haploid plants. These
male and female haploid plats produce male and female gametes, respectively, which
UDite to form a zygote. The zygote gives rise to a diploid pI_ that will produce the
haploid male and female spores. This process is shown in Figure 17-1.
DIPLOID GENERATION (2n)
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DlelOld
Pant
HAPLOID GENERATION (n)
Figure 17-1. Alternation of generations in pl~ ·111 Iower pllall, such • the IDDSI pIInt,
fcnilizltioa producCa I DeW diploid pllm dw powsllld develops. Tbc adult p . )II'Oduc:es RlpI'OCIuc:tM
ceIJs that undcrIo reduc:tioD divisiaII (meiosis). As. result of meiosis, hIpIoid IpCII'CIII'C .pnNIaced dill
Jive rise to male IllClfemiIe blploid pIaats. Tbc
produce spams ad the female pIaaIs procIuCe
ega. 'Jbae Illite 10 PJOduce the diploid plaalllntClllre1IiIt Cl*ima the C)'dc.
male'"
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GAMETOGENESIS
Gametogenesis is the fonnation ofsex cells, each with the II, or haploid, nmnber
of chromosomes. The first step in gametogenesis is meiosis. The process ofmeiosis
consists oftwo divisions separated by a period oftime called i1llerlcinesis. The individual
chromosomes, however, separate only once.
Staga of lDeiosis
Meiosis Division I-separation ofchromosome pairs
Prophase I-tbe chromosomes get short and thick.
Metaphase l-the chromosome pairs line up in the oenter of the cell.
Anaphase l-tbe chromosome pairs separate. One set goes to one pole of the
cell while the other goes to the opposite pole.
Telophase J---two daughter nuclei form, each containing the " number of
chromosomes. Cytokinesis completes the process. The result is
two haploid cells.
lnterldnesis-a period between the two divisions. The length ofinterkinesis depends on
the organiSm involved. ·Some species hBve interkinesis periods that are very short. Other
organisms may have yeais between the two divisions.
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Meiosis Division ~OD of chromatids
Prophase II-the chromosomes shorten and thicken.
Metaphase D-tbe chromosomes line up in the center oftbe oeD.
Anaphase D-tbe chromatids separate. One set of chroJDatjds FeB to one pole
of the Cell while the other pair ofchromatids.loes to the opposite pole
of the cell.
Telophase D-tbe daughter nuclei form and cytokinesis occms.
Each of the haploid cells Bom Division I enters Division D. A total offour nuclei Rsult.
The fate of these nuclei depends upon whether the meiosis is part ofpmetogenesis in a
female or gametogenesis in amale.
Oogeaesis
Gametogenesis in females is called oogenesis. Oogenesis (pronounced oh-oh­
GENesis), results in the formation ofthe egg cell by meiosis. Each hmnan female
receives approximately 700,000 oogonia in her ovaries. During each menstrual cycle,
one oogomunfbecomes the primary oocyte (which is a 2" ceD) while the cells
smrounding it become the/o/1ide.
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17-3
The primary oocyte divides by Meiosis I to produce a secondary oocyte. The
other nucleus resulting from Division I is a throw-away nucleus known as apolar body.
A polar body consists of the chromosomes and nucleus JeSU1ting from meiotic division
but it receives almost no cytoplasm. All the cytoplasm is found in the secondary oocyte.
After interkinesis, the secondmy oocyte divides by Meiosis D, to produce 1be egg
cell and Bpolar body. The earlier polar body also divides to fonn two polar bodies. The
result is the egg ce)) and three polar bodies that are discarded;
Uneven cytoplasmic division in the formation ofthe egg cell is advantageous in
that the cytoplasm will serve to provide energy for the developing embryo until it can
implant in the utenJS. In other animals, such as the frog, there iSB supply offood
associated with the egg cell that will nourish the developing embryo. If the cytoplasmic
division were equal, then the egg cell would receive only one-quartel' ofthe cytoplasm
and the remaining three-quarters would be wasted.
SperaaatogeDesiI
The testes contain millions of speTlflDlogonia. Each of these cells can develop
into a primary spenn8tocyte. A primary spermatocyte wiD undergo Meiosis I to ~
two secondmy spermatocytes. Finally, each secondary spermatocyte underaoes Meiosis
n resulting in fOlD' sperm cells. Each sperm cell n:ceives the same amount of cytoplasm.
Spermatogenesis results in equal distribution ofthe cytoplasm in contrast to oogenesis
which has unequal cytoplasmic division.
FEMALE REPRODUC11VE SYSTEM
The female human bas two OWlries, each ofwhich is associated with an oviduct.
lbe oviducts connect with the lIte11lS. Sperm are iD1roduced into the uterus via the
'VQgillQ and the cervix. They swim up the uterus and into the oviducts where fertilization
takes place.
OwIation is the process in which the egg leaves the ovary. Ovulation is UDder the
control of two hormones. Follicle stimulating hormone and leutenizing hormone
stimulate the egg and its foDicle to develop.. During the menstrual cycle, the blood levels
ofthese two hmmones increase to a peak. Ovulation occurs just after the levels reach the
peak. The uterine lining prepares for nOlD'ishing 8Dembryo during this tiDie. Ifthe •
remains unfertilized,. the uterine lining breaks down and menstnlation occurs. Then the
whole cycle repeats itself the next month.
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MALE REPRODUCI1VE SYSTEM
Speno are produced in the testes, which are f01Dld outside the body. Upon
leaving the testes, the sperm travel up the vas deferens, and into the semina1 vesicle froDi
which they are released tluough the urethra.
Fertilization takes place most often in an oviduct The sex ofthe person is
determined when the gametes unite. Then the cleavage stages ofdevelopment begin.