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Chapter 3
Development
3-1
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Stages of the Human Life Cycle
• Genes orchestrate our physiology after
conception through adulthood
• Development is the process of forming an
adult from a single-celled embryo
• In humans, new individuals form from the
union of sex cells or gametes
– Sperm from the male and oocyte from the
female form a zygote
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Male Reproductive Tract
vas deferens
bladder
seminal
vesicle
urethra
prostate
bulbourethral
gland
epididymis
testis
Figure 3.1
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Female Reproductive Tract
uterine tube
ovary
uterus
cervix
vagina
Figure 3.2
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Gametes
• Form from cell division of germline cells
• Meiosis is cell division to produce
gametes
• Meiosis has two divisions of the nucleus
(Meiosis I and Meiosis II) and produces
cells with half the number of
chromosomes (haploid)
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Meiosis
• Reduces the genetic material by half
• Why is this necessary?
from mother
from father
child
too
much!
meiosis reduces genetic content
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Homologous Chromosomes
• Carry the same genes
• Pair during Meiosis I
• Separate in the
formation of gametes
• One copy of each pair
is from the mother
and one is from the
father.
Figure 1.2
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Sexual Reproduction
• Meiosis and sexual reproduction
increases genetic diversity in a
population
• Variation is important in a changing
environment
• Evolution is the genetic change in a
population over time
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Comparison of Mitosis and Meiosis
Table 3.1
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Meiosis
Interphase precedes meiosis I
Meiosis I
Meiosis II
Prophase I
Prophase II
Metaphase I
Metaphase II
Anaphase I
Anaphase II
Telophase I
Telophase II Figure 2.13
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Meiosis I : the reduction division
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Figure 3.4
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Prophase I
Late prophase
Early prophase
• Chromosomes condense
• Homologs pair
• Spindle forms
• Crossing over occurs • Nuclear envelope
fragments
Figure 3.4
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Metaphase I
• Homolog pairs align
along the equator of
the cell
Figure 3.4
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Anaphase I
• Homologs separate
and move to opposite
poles
• Sister chromatids
remain attached at their
centromeres
Figure 3.4
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Telophase I
• Nuclear membrane
reforms
• Spindle disappears
• Cytokinesis divides cell
Figure 3.4
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Meiosis II : like mitosis; sister
chromatids separate
Prophase II
(haploid)
Figure 3.4
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
nonidentical
haploid
daughter cells
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Prophase II
• Nuclear envelope
fragments
• Spindle forms
Figure 3.4
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Metaphase II
• Chromosomes align
along equator of cell
Figure 3.4
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Anaphase II
• Centromeres divide
• Sister chromatids
separate
Figure 3.4
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Telophase II
• Nuclear envelopes reform
• Chromosomes decondense
• Spindle disappears
• Cytokinesis divides cells
Figure 3.4
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Results of Meiosis
Gametes
• Four haploid cells
• Contain one copy of
each chromosome and
one allele of each gene
• Each cell is unique
Figure 3.4
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Meiosis: Cell Division in Two Parts
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid
Haploid
Haploid
Figure 3.3
Result: one copy of each chromosome in a gamete.
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Table 3.1
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Recombination (crossing over)
• Occurs in prophase of
meiosis I
A
A
B
B
C
• Homologous
chromosomes exchange
genes
• Generates diversity
b
C
D D
E
F
E
F
a
a
e
f
c
b
c
d
d
e
f
Figure 3.5
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Recombination (crossing over)
A
a
B
• Exchange between
homologs
• Occurs in prophase I
C
C
c
D D
E
F
d
E
F
e
f
b
c
d
e
f
Figure 3.5
Letters denote genes and case denotes alleles
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Recombination (crossing over)
a
A
B
b
C
•Creates chromosomes
with new combinations of
alleles for genes A to F
D
E
F
A
a
B
c
b
c
d
d
C
D
E
F
e
f
e
f
Figure 3.5
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Chiasmata
In prophase I, crossing over or
recombination events create
chiasmata.
Figure 3.5
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Independent Assortment
The homolog of one chromosome can be inherited
with either homolog of a second chromosome.
Figure 3.6
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Spermatogenesis: sperm
formation
Figure 3.7
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3.3 Gamete Maturation
• The cells of the maturing male and female
proceed through similar stages but with
sex specific terminology and different time
tables
• Males begin manufacturing sperm at
puberty and continue throughout life.
• Females begin meiosis as a fetus and
complete meiosis only if a sperm fertilizes
and oocyte
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Spermatogenesis- formation of
sperm cells
• Begins in the diploid cellspermatogonium.
• The spermatogonium divides by
mitosis to yield two daughter cells.
– One daughter cell will specialize into
mature sperm.
– One daughter cell will remain a stem
cell.
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• As the mature spermatogonium
accumulate cytoplasm, replicate DNAbecome primary spermatocytes.
• Meiosis I- each primary spermatocyte
divides forming two equal sized haploid
cells called secondary spermatocytes.
• Meiosis II- each secondary spermatocyte
divides to yield two spermatids.
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• Each spermatid then develops the
characteristic tail-flagellum.
• The base of the tail has many
mitochondria that release ATP propelling
the sperm in the female tract.
• After spermatidid differentiation some of
the cytoplasm connecting the cells falls
away leaving mature tadpole shaped
speramtozoa or sperm.
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Spermatogenesis
• Stem cells in testes divide
mitotically to produce
spermatocytes
•. Spermatocytes divide by
meiosis to produce four
equal sized haploid
spermatids that mature into
four sperm
Figure 3.9
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• Each sperm has a tail, body or
midpiece, and a head region.
• The membrane covered front endacrosome- enzymes to penetrate the
oocyte.
• In the head DNA is wrapped around
proteins- inactive.
• Built in protections– Spermatogonia exposed to toxins do
not mature into sperm or cannot swim.
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Oogenesis
Figure 3.11
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Oogenesis: Ovum Formation
•
•
•
•
Cells of the ovary divide to form oocytes
Oocytes divide by meiosis
Unequal cytoplasmic division
A discontinuous process
– At birth, oocytes are arrested in prophase I
– At ovulation, an oocyte continues to
metaphase II
• The four meiotic products produce a
functional ovum and three polar bodies.
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Fertilization
The ovum
completes
meiosis II after
fertilization
Figure 3.13
• Fertilization is the union of sperm and
ovum.
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Fertilization
• Hundreds of millions of sperm are deposited
in the vagina during sexual intercourse. A
sperm cell can survive for up to 3 days but
the oocyte can only be fertilized 12-24 hrs.
after ovulation.
• Woman’s body helps sperm reach the oocyte.
–
–
–
–
Capacitation chemically activates sperm.
Oocyte release an attractant chemical.
Female muscle contractions assist
Moving sperm tails
• Only 200 sperm come near the oocyte.
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• The encounter of sperm and oocyte
is dramatic.
– Wave of electricity
– Physical and chemical changes occur
over the entire oocyte surface.
These chemical reactions prevent additional
sperm from entering the ovum.
Additional sperm can enter but there is too much
genetic material for development to follow.
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• Usually only the sperm’s head enter the
oocyte.
• The ovum’s nuclear membrane disappears
and the two sets of chromosomes called
pronuclei approach.
• Within each pronucleus, DNA replicates.
• Fertilization completes when the two genetic
packages merge .
• The fertilized ovum is called a zygote.
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Multiple Births
Dizygotic twins
• Form from two differ zygotes
• Two ova are fertilized
• Same genetic relationship as any
siblings
Monozygotic twins
• One ova is fertilized
• Developing embryo splits during early
development
• Genetically identical
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Figure 3.16
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Abnormal Chromosome Number
• Atypical chromosomes account for at
least 50 percent of spontaneous
abortions, yet only 0.65 percent of
newborns have them.
• Therefore, most embryos and fetuses with
atypical chromosomes stop developing
before birth.
• See Table 13.2
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Polyploidy -most extreme - an
entire extra set .
An individual whose cells have three copies of each
chromosome is a triploid (designated 3N, for three sets of
chromosomes). Two-thirds of all triploids result from fertilization
of an oocyte by two sperm. The other cases arise from formation
of a diploid gamete, such as when a normal haploid sperm
fertilizes a diploid oocyte. Triploids account for 17 percent of
spontaneous abortions (figure 13.11). Very rarely, an infant
survives as long as a few days, with defects in nearly all organs.
However, certain human cells may be polyploid. The liver, for
example, has some tetraploid (4N) and even octaploid (8N) cells.
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Aneuploidy
• Cells missing a single chromosome or having an
extra one.
• A normal chromosome number is euploid, which
means “good set.”
• Most autosomal aneuploids (with a missing or
extra non-sex chromosome) are spontaneously
aborted
• Intellectual disability is common in aneuploidy
because development of the brain is so complex
• Sex chromosome aneuploidy usually produces
milder symptoms.
• Most children born with the wrong number of
chromosomes have an extra chromosome (a
trisomy) rather than a missing one (a monosomy).
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Nondisjunction
• The meiotic error that causes aneuploidy is called
nondisjunction.
• In normal meiosis, homologs separate and each of the
resulting gametes receives only one member of each
chromosome pair.
• In nondisjunction, a chromosome pair fails to separate at
anaphase of either the first or second meiotic division. This
produces a sperm or oocyte that has two copies of a
particular chromosome, or none, rather than the normal
one copy.
• When such a gamete fuses with its partner at fertilization,
the zygote has either 45 or 47 chromosomes, instead of the
normal 46.
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Most of the 50 percent of spontaneous abortions
that result from extra or missing chromosomes
are 45, X individuals (missing an X
chromosome), triploids, or trisomy 16. About 9
percent of spontaneous abortions are trisomy
13, 18, or 21. More than 95 percent of newborns
with atypical chromosome numbers have an
extra 13, 18, or 21, or an extra or missing X or Y
chromosome. These conditions are all rare at
birth—together they affect only 0.1 percent of all
children. However, nondisjunction occurs in 5
percent of recognized pregnancies
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Stages of Development
Table 3.2
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Early Development:
Ovulation to Implantation
Figure 3.14
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Cleavage
• Mitotic cell division; a morula
• Cells are called blastomeres
• The developing embryo becomes a
blastocyst, a hollow ball of cells
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Blastocyst
• The inner cell mass
(ICM) develops into the
embryo
• Other cells become the
extraembryonic
membranes important
for implantation and
support of embryonic
growth
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Gastrulation
• Primary germ layers form
• Cells differentiate
• Supporting structures form
– Chorionic villi
– Yolk sac
– Allantois
– By 10 weeks the placenta is fully formed
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Germ Layers:
Endoderm, Mesoderm, and
Ectoderm
Figure 3.15
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Germ Layers
Ectoderm:
the outermost germ layer develops
skin
nervous system
eye lens
Mesoderm:
the middle germ layer develops
muscle
connective tissue
blood vessels
kidneys
Endoderm: the innermost germ layer develops
lining of GI tract
liver
pancreas
thymus
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Embryo Develops
Figure 3.18
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Critical Periods of Development
• Organs develop at different times: a critical
period
• During its critical period, an organ is
vulnerable to toxins, viruses, and genetic
abnormalities
• Altering the normal development may cause
birth defects
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Critical Periods of Development
Figure 3.20
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Teratogens
• Cause birth defects during development
• Examples
– Thalidomide
– Cocaine
– Cigarettes
– Alcohol
– Some nutrients
– Some viruses
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Figure 3.21
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Maturation and Aging
• Genes may impact health throughout life
• Single gene disorders are expressed early in
life and tend to be recessive
• Adult onset single gene traits are often
dominant
• Interaction between genes and environmental
factors
Example: malnutrition before birth
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Table 3.3
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Aging
• Segmental progeroid syndromes
• Increases the rate of aging associated
changes
• Inheritance of longevity
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Table 3.4
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