Reproduction

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Reproduction

• BSCS Ch 12

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Reproduction

• The ability to reproduce is one of the unifying
  characteristics of all living things.
• Sexual reproduction produces offspring that are
  genetically different from their parents.
• Asexual reproduction produces offspring
  genetically identical to their parent.

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Asexual Reproduction

• Fission, budding, and fragmentation are some of
  the mechanisms that allow organisms to reproduce
  asexually.
• Asexual reproduction allows an organism to
  rapidly produce many offspring without the time
  and resources committed to courtship, finding a
  mate, and mating.

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Sexual Reproduction

• In sexual reproduction new individuals are
  produced by the fusion of gametes to form a
  zygote.
• Sperm are male gametes.
• Ova (ovum singular) are female gametes.
• Sexual reproduction offers the benefit of
  generating genetic variation among offspring,
  which enhances the chances of the population's
  survival.
• Costs of this process include the need for two
  individuals to mate, courtship rituals, as well
  as a number of basic mechanisms described later.

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Human Male Reproductive System

• Human reproduction employs internal
  fertilization.
• Gonads are sex organs that produce gametes.
• Male gonads are the testes, which produce sperm
  and male sex hormones.

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• Testes are suspended outside the abdominal cavity
  by the scrotum, a pouch of skin that keeps the
  testes away from the body to be at an optimal
  temperature for sperm development.
• Seminiferous tubules are inside each testis, and
  are where sperm are produced.
• Sperm production begins at puberty with several
  hundred million sperm being produced each day.
• Once sperm form they move into the epididymis,
  where they mature and are stored.

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• Sperm pass through the vas deferens which
  connects to a short ejaculatory duct that
  connects to the urethra.
• The urethra passes through the penis and opens to
  the outside.
• Secretions from the seminal vesicles add fructose
  and prostaglandins to sperm as they pass.
• The prostate gland secretes a milky alkaline
  fluid.
• The bulbourethral gland secretes a mucus-like
  fluid that provides lubrication for intercourse.
• Sperm and secretions make up semen.

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Human Female Reproductive System

• Female gonads are the ovaries, which produce eggs
  (ova) and female sex hormones.
• The ovary contains many follicles composed of a
  developing egg surrounded by an outer layer of
  follicle cells. Each egg begins oogenesis as a
  primary oocyte.

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• A developing egg (secondary oocyte) is released
  each month from puberty until menopause, a total
  of 400-500 eggs.
• After puberty the ovary cycles between a
  follicular phase (maturing follicles) and a
  luteal phase (presence of the corpus luteum).
• The ovarian cycle lasts usually 28 days. During
  the first phase, the oocyte matures within a
  follicle. At midpoint of the cycle, the oocyte is
  released from the ovary in a process known as
  ovulation.

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• The secondary oocyte passes into the oviduct
  (fallopian tube or uterine tube). The oviduct is
  connected to the uterus.
• The uterus has an inner layer, the endometrium,
  in which a fertilized egg implants.
• At the lower end of the uterus the cervix
  connects the uterus to the vagina.
• The vagina is the birth canal.

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The Menstrual Cycle

• Menstrual cycles vary from between 15 and 31
  days.
• The first day of the cycle is the first day of
  blood flow (day 0) known as menstruation.
• During menstruation the uterine lining is broken
  down and shed as menstrual flow.

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• Around day 14 (midcycle) hormones trigger the
  maturation and ovulation of an egg.
• If pregnancy does not occur, the drop in hormones
  causes the sloughing off of the inner lining of
  the uterus by a series of muscle contractions of
  the uterus.

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Chromosome Numbers

• All animals have a characteristic number of
  chromosomes in their body cells called the
  diploid (or 2n) number.
• These occur as homologous pairs, one member of
  each pair having been acquired from the gamete of
  one of the two parents of the individual whose
  cells are being examined.
• The gametes contain the haploid number (n) of
  chromosomes.

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Chromosome Numbers in Some Species (Diploid)

• Buffalo 60
• Cat 38
• Human 46
• Dog 78
• Horse 64
• Apple 34
• Potato 48

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Gamete Formation Meiosis

• During the formation of gametes, the number of
  chromosomes is reduced by half (haploid), and
  returned to the full amount (diploid) when the
  two gametes fuse during fertilization.
• Chromosomes that carry the same genes are termed
  homologous chromosomes.

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Meiosis

• Meiosis is a special type of nuclear division
  which segregates one copy of each homologous
  chromosome into each new "gamete".
• Two successive nuclear divisions occur, Meiosis I
  (Reduction) and Meiosis II (Division). Meiosis
  produces 4 haploid cells.

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Meiosis I

• Interphase Before meiosis begins, genetic
  material is duplicated.
• First division of meiosis
• Prophase 1 Duplicated chromatin condenses. Each
  chromosome consists of two, closely associated
  sister chromatids. Crossing-over can occur during
  the latter part of this stage.

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Crossing Over

• When chromosomes pair in the early prophase of
  the first division of meiosis (Meiosis I), a
  crossover occurs between two non-sister
  chromatids.
• This results in an exchange of genetic material
  between the maternal and paternal chromosomes.
• The drawing provided shows a crossover between
  the non-sister chromatids of two, homologous
  chromosomes.

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Crossing Over Animation
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• 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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Meiosis Animation
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Meiosis and Spermatogenesis

• During meiosis, 4 sperm are produced by the two
  nuclear divisions.
• These four sperm each have the haploid number of
  chromosomes.
• The sperm are equal in size.

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Meiosis and Oogenesis

• Only one egg is produced by meiosis.
• The ova need to contain a lot of nutrients (to
  get the embryo through its first set of
  divisions), so these cells need to be big. 
• So, the cytoplasm is not divided equally between
  the 4 resulting cells.
• The other cells are called polar bodies and they
  break down and disintegrate.

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Prokaryotic Reproduction

• Prokaryotes are much simpler in their
  organization than are eukaryotes.
• The usual method of prokaryote cell division is
  termed binary fission.
• The prokaryotic chromosome is a single DNA
  molecule that first replicates, then attaches
  each copy to a different part of the cell
  membrane. When the cell begins to pull apart, the
  replicate and original chromosomes are separated.
  
• Following cell splitting there are then two cells
  of identical genetic composition

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Binary Fission
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Sexual Reproduction in Plants

• Many plants reproduce sexually.
• The advantage to the plant is that its offspring
  have a selection of genes from two parents, so
  each individual's genes are different.
• The offspring are not identical, and there is
  variety in the species.

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Plant Reproductive Structures

• Flowering plant's sexual organs consist of
• Male the stamen, or male sex structure,
  consisting of a filament and a pollen-bearing
  anther at the tip
• Female the pistil or female sex structure,
  consisting of ovary and ovule, style, and stigma
  at the tip. (The pistil is also sometimes called
  the carpel.)

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Pollination

• An insect or the wind carries pollen grains from
  the anther of another flower.
• The pollen grains land on the stigma and a pollen
  tube grows down through the style to the ovary.
• The nucleus of the pollen grain passes down the
  tube. It fertilizes the egg cell inside the
  ovule.
• The fertilized egg cell develops into an embryo.
  The ovary becomes the fruit and the ovule becomes
  a seed - from which (once dispersed) the
  offspring plant will grow.

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