Chapter 1
Basic
Mechanisms of
Sexual
Reproduction
• Sexual reproduction is the creation of an
offspring
– By the fusion of male and female gametes to
form a zygote.
• The female gamete is the egg.
• The male gamete is the sperm.
Male Reproductive Anatomy
• In most mammalian species
– The male’s external reproductive organs are the
scrotum and penis
• The internal organs
– Consist of the gonads, which produce sperm and
hormones, and accessory glands
Seminal
vesicle
(behind
bladder)
(Urinary
bladder)
Prostate gland
Urethra
Scrotum
Glans penis
Bulbourethral
gland
Erectile tissue
of penis
Vas deferens
Epididymis
Testis
(Urinary
bladder)
Seminal vesicle
(Rectum)
(Pubic bone)
Erectile
tissue of
penis
Vas deferens
Ejaculatory duct
Prostate gland
Bulbourethral gland
Urethra
Vas deferens
Epididymis
Glans penis
Testis
Scrotum
Prepuce
Testes
• The male gonads, or testes
– Consist of many highly coiled tubes surrounded by
several layers of connective tissue
• The tubes are seminiferous tubules
– Where sperm form
• Production of normal sperm
– Cannot occur at the body temperatures of most
mammals
• The testes of humans and many mammals
– Are held outside the abdominal cavity in the scrotum,
where the temperature is lower than in the abdominal
cavity
Ducts
• From the seminiferous tubules of a testis
– The sperm pass into the coiled tubules of the
epididymis.
• During ejaculation
– Sperm are propelled through the muscular vas
deferens, the ejaculatory duct, and exit the penis
through the urethra.
Glands
• Three sets of accessory glands
– Add secretions to the semen, the fluid that is ejaculated
A pair of seminal vesicles
– Contributes about 60% of the total volume of semen
The prostate gland
– Secretes its products (30% of the total volume of the
semen) directly into the urethra through several small
ducts
The bulbourethral gland
– Secretes a clear mucus before ejaculation that
neutralizes acidic urine remaining in the urethra
Questions
• Indicate the spermatozoa pathway through
the male reproductive tract from their
production to the site of ejaculation.
• What are the different glands that contribute
to the production of the semen?
• What is the importance of the seminal fluid?
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Female Reproductive Anatomy
Uterus
(Urinary bladder)
Reproductive anatomy of the
human
Oviduct
(Pubic bone)
Ovary
(Rectum)
Cervix
Vagina
Urethra
Shaft
Glans
Bartholin’s gland
Prepuce
Vaginal opening
Labia minora
Labia majora
Clitoris
Ovaries
Oviduct
Follicles
Uterus
Uterine wall
Endometrium
Corpus luteum
Cervix
Vagina
Ovaries
• The female gonads, the ovaries
– Lie in the abdominal cavity
• Each ovary
– Is enclosed in a tough protective capsule and
contains many follicles (400,000 follicle at puberty)
• A follicle
– Consists of one egg cell surrounded by one or more
layers of follicular cells
Ovulation
• The process of ovulation
– Expels an egg cell from the follicle
• The remaining follicular tissue then grows
within the ovary
– To form a solid mass called the corpus luteum or
yellow body, which secretes hormones, depending
on whether or not pregnancy occurs
Karyotype
• Karyotype:
– ordered display of an individual’s chromosomes.
– Collection of chromosomes from mitotic cells.
– Staining can reveal visible band patterns, gross
anomalies.
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Homologous Chromosomes
• Chromosomes exist in homologous pairs in
diploid cells.
Exception: Sex chromosomes (X, Y).
• All chromosomes are known as autosomes
except for sex chromosomes or gonosomes.
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In humans …
• 23 chromosomes donated by each parent (total =
46 or 23 pairs).
• Gametes (sperm/ova):
– Contain 22 autosomes and 1 sex chromosome.
– Are haploid (haploid number “n” = 23 in humans).
• Fertilization/syngamy results in zygote with 2
haploid sets of chromosomes - now diploid.
– Diploid cell; 2n = 46. (n=23 in humans)
• Most cells in the body produced by mitosis.
• Only gametes are produced by meiosis.
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Meiosis 1
First division of meiosis
• Prophase 1: Each chromosome dupicates and
remains closely associated. These are called sister
chromatids. Crossing-over can occur during the
latter part of this stage.
• Metaphase 1: Homologous chromosomes align
at the equatorial plate.
• Anaphase 1: Homologous pairs separate with
sister chromatids remaining together.
• Telophase 1: Two daughter cells are formed with
each daughter containing only one chromosome
of the homologous pair.
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Meiosis II
Second division of meiosis: Gamete formation
• Prophase 2: DNA does not replicate.
• Metaphase 2: Chromosomes align at the
equatorial plate.
• Anaphase 2: Centromeres divide and sister
chromatids migrate separately to each
pole.
• Telophase 2: Cell division is complete. Four
haploid daughter cells are obtained.
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Variation in DNA Content during
Meiosis
Mitosis vs. meiosis
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Meiosis creates genetic variation
• During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n to 2n)
• Meiosis results in genetic variation by shuffling of
maternal and paternal chromosomes and crossing
over.
No daughter cells formed during meiosis are
genetically identical to either mother or father
During sexual reproduction, fusion of the
unique haploid gametes produces truly unique
offspring.
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Independent assortment
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Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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In humans
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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Crossing over
Chiasmata – sites of crossing
over, occur in synapsis.
Exchange of genetic material
between non-sister chromatids.
Crossing over produces
recombinant chromosomes.
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Random fertilization
At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
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Gametogenesis
• The process whereby haploid gametes
(reproductive sex cells) are created.
• Spermatogenesis in males
• Oogenesis in females
Spermatogenesis
• Spermatogenesis: The production of sperm
using meiosis
• Occur in the seminiferous tubules in the testes
in males
Spermatogenesis
1.
Spermatogonia are the Germ-Line cells. They are diploid (2n). They
undergo mitosis to reproduce themselves.
2.
One of these spermatogonia grows and is called a primary spermatocyte.
It is diploid (2n).
3.
The primary spermatocyte undergoes MEIOSIS I to produce two
secondary spermatocytes. These are now haploid (n) but still contain two
chromatids per chromosome.
4.
Each of these secondary spermatocytes undergoes MEIOSIS II to each
produce two spermatids. Spermatids are haploid (n) and each contain 1
chromosome that was previously a chromatid. The result is 4 total
spermatids.
5.
Spermatids mature into the sperm we know by the process of
spermiogenesis, and these sperm cells are called spermatozoa. They are
then ready to leave the body and fertilize an egg.
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Spermiogenesis
• Development of a spermatid into a mature
sperm (spermatozoa).
Summary of Spermatogenesis
• Sertoli cells are in the seminiferous tubules and
help the process of spermatogenesis. They engulf
(take in) extra cytoplasm from the spermatids.
• SUMMARY of SPERMATOGENESIS:
– 4 haploid spermatozoa (mature sperm) are created via
meiosis from an original diploid spermatogonium
(germ line cell)
– Occurs in the testes in the seminiferous tubules
– The production of sperm is an ongoing process in the
seminiferous tubules
Oogenesis
• Oogenesis: creation of haploid egg cells using meiosis
• Occurs in the ovaries of females
• Follicles in the ovaries are structures that contain
primary oocytes. The germ line cells, oogonia,
produced all of the primary oocytes while the fetus
was developing. Therefore, when a baby female is
born all of her follicles contain primary oocytes! They
stay this way unless they undergo ovulation.
• Females are born with 1 million follicles in their
ovaries!
Oogenesis
• The primary oocytes that are diploid (2n) are arrested
(paused) at Prophase I of Meiosis I.
• When a female undergoes ovulation once a month, one of
these follicles with the primary oocyte matures. It fully
undergoes Meiosis I and results in two different structures:
– ONE haploid (n) secondary oocyte. It contains half the
number of chromosomes but still has sister chromatids.
– ONE haploid (n) structure called a polar body.
• The polar body does not go on to become an egg. Through
UNEQUAL CYTOKINESIS, the secondary oocyte gets most of
the cytoplasm and the polar body is left with little cytoplasm.
• After Meiosis I occurs, the secondary oocyte that is haploid (n)
is arrested (paused) at Metaphase II of Meiosis II. It stays in
this state until FERTILIZATION.
• Once fertilization occurs, the secondary oocyte undergoes
Meiosis II and the result again is two separate structure:
– ONE haploid ovum (egg)
– ONE polar body
• The other polar body from the previous step contained
chromatids, so it also undergoes Meiosis II and produces TWO
polar bodies.
Summary of Oogenesis
• SUMMARY of OOGENESIS:
– Results in three POLAR BODIES and one OVUM
(egg) that has extra cytoplasm
– Meiosis I occurs during ovulation
– Meiosis II occurs after fertilization
– Occurs in the ovaries (and fallopian tubes for
meiosis II) in females
Spermatogenesis vs. Oogenesis
Meiosis & sexual life cycles
• Life cycle = sequence of
stages in organisms
reproductive history;
conception to
reproduction.
• Somatic cells = any cell
other than gametes, most
of the cells in the body.
Generalized animal life cycle
• Gametes produced by
meiosis.
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