Sexual Reproduction

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Sexual Reproduction

• Meiosis
• Gametogenesis
• Fertilization

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Meiosis

• Reduction division process
• Also called gametogenesis

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Meiosis Another kind of cell division

• Remember Somatic cells 2N DIPLOID
• Gametes (eggs, sperm) N HAPLOID
• Sexual reproduction Egg Sperm fuse to form
  zygote (new individual cell)
• Each gamete provides 1 copy of all genes for the
  species
• Zygote 2N has PAIRS of all genes

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Meiosis

• 2 divisions
• Preceded by DNA replication
• 1st Division process
• divides homologous chromosomes
• 2nd Division process
• divides sister chromatids
• Produces Gametes with 1 chromosome of each type

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Meiosis

• Division stages similar to Mitosis
• Meiosis I and Meiosis II for 2 divisions
• Prophase I and Prophase II
• Metaphase I and Metaphase II
• Anaphase I and Anaphase II
• Telophase I and Telophase II
• Cytokinesis

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PROPHASE I

• 1. Nuclear membrane disappears
• 2. Nucleolus disappears
• 3. Chromatin coils to form chromosomes as sister
  chromatids
• 4. Spindle fibers begin forming

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MEIOSIS ISEPARATES HOMOLOGOUS CHROMOSOMES
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PROPHASE I

• SYNAPSIS - the pairing and bonding together of
  homologous chromosomes to form tetrads
• Homologous chromosomes consist of one maternal
  and one paternal chromosome

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HOMOLOGOUS CHROMOSOMES

• Homologous chromosomes consist of one maternal
  and one paternal chromosome
• They are the same SIZE, SHAPE and have the same
  GENES on them.

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GENES

• Genes are known to occur in different forms
  called ALLELES
• The eye color gene in humans for example occurs
  in dark alleles or light alleles
• Two chromosomes can thus have the same genes but
  different alleles of those genes

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PROPHASE I

• CROSSING-OVER
• Exchange of chromatid segments within tetrads to
  produce new allele combinations
• Nonsisters cross over, break, and rejoin
• Increases genetic variety

VIEW OF NUCLEUS AND TETRADS
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CROSSING -OVER

• Notice the exchange of segments that has happened
  in two places on the larger homologous pair
• This is crossing over

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METAPHASE I

• Tetrads line up in the middle of the cell in a
  straight line at equator
• Spindle fibers attach to only 1 kinetochore of
  each centromere

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ANAPHASE I

• Tetrads are separated as homologous chromosomes
  are pulled to opposite poles of the cell
• (Remember the chromosomes are still actually
  sister chromatids)

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TELOPHASE I

• Cytokinesis may occur and two HAPLOID cells
  containing sister chromatids are produced
• A brief period of interkinesis occurs

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INTERKINESIS
A short interphase like stage between
divisions
TELOPHASE I
PROPHASE II
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MEIOSIS IISEPARATES SISTER CHROMATIDS
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PROPHASE II

• Nuclear membranes disappear
• Nucleoli disappear
• Spindle fibers form
• Chromatin coils to form chromosomes
• What does this remind you of ?
• Prophase II is basically the same as mitosis
  prophase x 2

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METAPHASE II

• In metaphase II the two cells line up their
  chromosomes in the middle of the cell in a
  straight line at equator
• Again this is the same as mitosis metaphase -
  only two cells are doing it
• Each cell is doing it with only 1 chromosome of
  each kind, not pairs

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ANAPHASE II

• During this phase the centromeres of the
  chromosomes in the two cells divide and sister
  chromatids move to opposite poles of the cells
• Very mitosis-like again

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TELOPHASE II

• Final act of meiosis
• Nuclear membranes reappear
• Nucleoli reappear
• Chromosomes uncoil to chromatin
• Spindle fibers disappear
• Cytokinesis occurs to produce a total of 4
  haploid cells (N)

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MEIOSIS II
PROPHASE II
MEIOSIS II
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Gametogenesis and Human Reproduction
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Cells produced by Meiosis

• Some organisms cells continue to divide by
  mitosis and become haploid body cells (some
  fungi, protists )
• More often specialize into gametes
• Egg OOGENESIS is egg cell formation
• Sperm SPERMATOGENESIS is the formation of
  sperm cells

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SPERMATOGENESIS

• Male spermatogenesis most closely follows the
  general scheme of meiosis
• Spermatogenesis occurs within the testes in
  microscopic tubes called SEMINIFEROUS TUBULES
• There are approximately 300-500 of these tubules
  per testis and each is 1-3 feet long - this
  means that males have about a quarter mile of
  sperm-producing tubules

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SPERMATOGENESIS

• The testes are located within the SCROTUM - a
  thin pouch of skin which allows the testes to
  cool down to a temperature of 94-95 degrees
• At temperatures above about 96 degrees sperm
  production drops off sharply
• This can cause temporary sterility

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MEIOSIS IN THE TUBULE
SPERMATOGONIUM
(Pl. Spermatogonia)
PRIMARY SPERMATOCYTE
SECONDARY SPERMATOCYTES
SPERMATIDS
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Names of cells formed in Spermatogenesis

• Spermatogonia are diploid (2N) cells may undergo
  spermatogenesis to produce sperm OR may undergo
  mitosis and make more spermatogonia
• Primary spermatocytes are 2N cells which have
  begun meiosis I
• Secondary spermatocytes are N cells which have
  begun meiosis II
• Notice that one primary forms two secondary
  spermatocytes

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SPERMATIDS

• Spermatids have completed meiosis and become
  haploid (N) cells
• They are very near to the inside edge of the
  seminiferous tubules
• They are NOT yet capable of functioning as sperm
  cells
• They must undergo a second process --
  SPERMIOGENESIS

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Spermatogenesis is the MEIOSIS process to produce
sperm cells, but...SPERMIOGENESIS is the process
of maturation of a spermatid into a sperm
cellSPERMIOGENESIS

• In this process spermatids mature into sperm
  cells
• A matter of arranging the cytoplasm and
  organelles differently
• And growing the sperm flagellum for swimming

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Spermiogenesis

• The spermatid on the left develops into the sperm
  on the right by rearranging its contents
• HEAD - composed of acrosome and nucleus
• ACROSOME - golgi bodies with hyaluronidase
• NUCLEUS - the DNA - the only part to enter egg
• MIDPIECE or Neck - many mitochondria - energy
• TAIL - the flagellum for swimming

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Male Reproductive Anatomy

• Testes - contain seminiferous tubules
• Epididymis - sperm storage
• Vas deferens - sperm duct for delivery
• Urethra - exit passageway
• Penis - penetrates female
• Glands to produce semen
• Seminal vesicles
• Prostate gland
• Bulbourethral glands

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Male Hormones

• FSH (Follicle Stimulating Hormone)
• Secreted by pituitary
• Stimulates development of testes
• ICSH (Interstitial Cell Stimulating Hormone)
• Secreted by pituitary
• Stimulates Interstitial cells to secrete
  testosterone
• Testosterone - Primary male hormone
• Stimulates sperm development
• Secondary male characteristics

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OOGENESIS
Thats Oh Oh !! NOT Ooooh! NOT Uh Oh

• Female gametogenesis is more complicated than in
  males
• Not only must females produce and release an egg
  but ...
• They must prepare the body for pregnancy via the
  menstrual cycle

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OOGENESIS (Summary)
Oogonium (2N)
Primary Oocyte 2N
1st Polar Body 1N
Secondary Oocyte 1N
2nd Polar Body 1N
Ovum or Egg
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OOGENESIS

• Oogenesis occurs in the ovaries within structures
  called FOLLICLES
• 3 kinds of follicles in different stages of
  development
• PRIMORDIAL (Primary oocyte)
• IMMATURE (Primary oocyte)
• MATURE released at ovulation (Secondary oocyte
  meiosis completed only if ovum is fertilized)
• In humans all 2 million ova already in prophase
  I at birth!

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OOTID (OVUM)

• Upon penetration by a sperm cell the secondary
  oocyte completes Meiosis II
• At telophase II the oocyte divides into a 2nd
  polar body (again by unequal cytokinesis) and an
  ootid or ovum
• The ootid is the equivalent of the spermatid in
  males but no process of further development must
  occur so this is the same as an ovum

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Female Reproductive Anatomy

• Ovaries
• produce ova within follicle
• Corpus luteum forms after ovulation
• Fallopian tubes / Oviducts - deliver egg and site
  of fertilization
• Uterus (endometrium lining) - houses egg
• Cervix - opens into uterus
• Vagina - receives penis and sperm/semen
• External Genitalia (vulva)

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Hormone Interactions

• Follicle-Stimulating Hormone FSH
• follicle, estradiol
• Luteinizing Hormone LH
• ovulation, corpus luteum, estradiol
• Estradiol
• uterus lining, sex characteristics
• Progesterone
• uterine lining, sex characteristics
• Oxytocin (uterine contraction, milk ejection)
• Prolactin (milk production)

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MENSTRUAL CYCLE
The Menses or period usually occupies the first
5 or 6 days of each cycle
Day 1 of the period is the first day of the
cycle - NOT THE LAST DAY OF THE PERIOD
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ALL TOGETHER
LH
Progesterone
FSH
Estrogen
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OVARY
Three types of follicles are found in the ovary
of a woman in her reproductive years (about 12-50
yrs)
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MATURE FOLLICLE
PRIMORDIAL FOLLICLE
IMMATURE FOLLICLE
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PRIMORDIAL FOLLICLES

• One layer of follicle cells surrounding one
  PRIMARY OOCYTE
• It is 2N and is in Meiosis I

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IMMATURE FOLLICLES (some begin development each
month at sexual maturity)

• Surrounded by several layers of follicle cells
  and have a larger primary oocyte in the center
• The oocyte is larger because it has almost
  completed meiosis I

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MATURE FOLLICLES

• Mature follicles develop each menstrual cycle
  about two weeks from the end
• They have a large space called the antrum filled
  with a fluid - liquor folliculi
• One large secondary oocyte and a tiny 1st Polar
  Body are found within mature follicles

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SECONDARY OOCYTES

• Secondary oocytes correspond to secondary
  spermatocytes
• They are 1N cells that will finish meiosis II
  after penetration by a sperm cell
• 1st Polar bodies are tiny 1N cells formed by
  unequal cytokinesis in Telophase I to concentrate
  cytoplasm in the future egg

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GENERAL DEVELOPMENT
Zygote
2 Cell
4 Cell
8 16 Cell
Fertilized egg

• Early development consists of repeated mitotic
  divisions which increase cell numbers but do NOT
  involve an increase in size
• The early simple clusters are called morula
  embryos

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DEVELOPMENT
Gastrula
Blastula

• The blastula is an embryonic stage that consists
  of a hollow ball of cells with a cavity called a
  blastocoel
• The gastrula is the stage where a gut called
  the archenteron begins to form - the opening to
  the archenteron is the blastopore

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HUMAN DEVELOPMENT

• Human development is similar to this general
  pattern but more complex
• We have several important membranes around our
  embryos