Reproduction: Asexual vs. Sexual

• Reproduction can occur asexually or sexually.

Asexual Reproduction

• Asexual reproduction involves a single parent and creates genetically identical offspring (clones) without meiosis or fertilization.
• Methods of asexual reproduction include:
  • Unicellular organisms use binary fission (bacteria & mitosis).
  • Multicellular organisms use budding, where buds grow and detach.
  • Multicellular organisms use fragmentation, where the organism breaks into pieces.

Sexual Reproduction

• Sexual reproduction involves the fertilization (fusion) of two reproductive (sex, gametes) cells to form a zygote.
• Both parents make gametes by meiosis.
• Offspring show traits in combination of parents.

Chromosomes

• Chromosome number differs between species but is the same within a species.
• Prokaryotes have 1 chromosome and a plasmid.
• Eukaryotic chromosome number:
  • Diploid Cells (2n): homologous pairs (one from each parent) are the same size, shape & gene order.
    • Somatic Cells (body cells) are diploid.
    • Cells formed by mitosis are diploid.
  • Haploid Cells (n): no homologous pairs and are reproductive cells (gametes: sperm, egg).
    • Cells formed by meiosis are haploid.
• Meiosis reduces chromosome number, and fertilization restores it.

Meiosis

• Meiosis is a nuclear division that reduces the chromosome number by half to produce gametes.
• Meiosis divides cells twice:
  • Reduction Division (meiosis I): separates homologous pairs, reduces chromosome number by half, and chromatids remain attached.
  • Normal Division (meiosis II): separates chromatids, and there's no change in chromosome number.
• Meiosis produces 4 gametes (sperm, egg, pollen, etc.) that aren't genetically identical because of genetic recombination by mixing genetic material and producing new combinations of genes.
  • Crossing over: pieces of homologous chromosomes are exchanged, creating new forms of chromosomes (different combinations of genes).
  • Independent Assortment: when forming gametes, mom's chromosomes don't always stay together but are randomly combined with dad's chromosomes.
• Replicated chromosomes have sister chromatids.
• Homologous chromosomes replicate into tetrads with 4 homologous chromatids or 2 homologous chromosomes
• Interphase:
  • Cells are in a state of no cell division.
  • Before cell begins division the chromosomes are all replicated (make an exact copy).
  • A diploid cell
• Prophase I:
  • Chromosomes coil, spindle forms.
  • Nuclear membrane breaks down.
  • Homologous pairs meet
  • Crossing over may occur
• Metaphase I:
  • Homologous pairs line up next to each other at the equator.
• Anaphase I:
  • Homologous pairs separate and move to opposite poles of the cell.
• Telophase I:
  • A cleavage furrow forms resulting in cytokinesis.
  • Two new cells are formed that are genetically different and haploid with replicated chromosomes.
• Metaphase II:
  • Replicated chromosomes line up on the equator.
• Anaphase II:
  • Spindle fibers pull chromatids apart, towards opposite ends of the cell.
• Telophase II:
  • A cleavage furrow forms resulting in cytokinesis.
  • The end result is 4 haploid cells that are all genetically different.

Gametes

• Spermatogenesis occurs in the testes.
• Oogenesis occurs in the ovaries.
• Oogenesis:
  • Oogonium (2N) undergoes mitosis, then growth & differentiation to become a primary oocyte (2N) before birth after birth.
  • A primary oocyte completes meiosis I, forming a 1st polar body (N) and a secondary oocyte (N) that completes meiosis II to produce an ovum(N)
• Primary oocytes are found in primordial, primary, and secondary follicles
• Mature follicles contain primary oocytes until hours before ovulation when meiosis I is completed.

Male Reproductive System Components

• Urinary Bladder: stores urine.
• Pelvic bone: provides structural support.
• Vas Deferens: transports sperm.
• Seminal Vesicle: produces semen, a fluid that contains nutrients for sperm and an alkaline fluid to neutralize vaginal acidity.
• Urethra: passage way for both sperm and urine, a muscle near the prostate gland contracts so urine can't flow during ejaculation
• Prostate Gland: adds a slightly alkaline fluid to semen to help neutralize vaginal acidity.
• Cowper's Gland (Bulbourethral): produces alkaline mucus before ejaculation to protect sperm from the acidic vagina.
• Penis: male sexual organ.
  • Shaft and glans (tip) are covered with foreskin unless removed by circumcision.
• Epididymis: long coiled tubes that store sperm until maturity.
• Testicle: produce sperm.
• Scrotum: a loose sac of skin and muscle that holds testes in position to regulate temperature for sperm production and changes position in response to body temperature.
• Glans
• Anus
• Testes (Gonads): male gonads that produce sperm.
• Semen: white fluid that carries and nourishes sperm.
• Ejaculation: expels semen during orgasm with 15mL semen containing 300 million sperm.
• Sperm: male gametes, smaller than egg, form in testes at puberty.
  • Sperm development takes ~74 days, and millions are made per day, with few surviving to reach the ovum.
  • The sperm head contains genetic material that fuses with the egg to make a zygote.
  • Mitochondria provide energy for swimming, with the tail providing motion.

Female Reproductive System Components

• Oviduct or Fallopian tube: transports egg to uterus.
• Ovary: produces eggs.
  • Gametes, born with ~2 million (7 million before birth), only ~400 will mature and begin maturing in ovaries at puberty.
  • There are 2 ovaries that release ovum 1 per month
  • Ovum bursts from ovaries and enter oviducts
• Uterus: a thick-walled, pear-shaped organ located between the bladder and rectum.
• Urinary bladder: stores urine.
• Cervix: opening of the uterus to the vagina.
• Pelvic bone: provides structural support.
• Rectum: stores solid waste.
• Urethra: passage way for urine.
• Anus: excretes solid waste.
• Clitoris: small elongated organ in front of the vulva.
• Vagina: connects the uterus to external reproductive organs, a muscular tube, a cavity penetrated during vaginal intercourse and an exit for menstrual flow.
• Vulva: external tissues that cover and protect reproductive organs.
• Labia: part of the vulva are folds of skin that protect vaginal and urinary openings.
• Labia Minor
• Labia Major

Menstrual Cycle

• Hormones (progesterone) levels are low after menstruation begins, and the hypothalamus detects low progesterone.
• A message (GnRH) is sent to the pituitary gland to release FSH (follicle stimulating hormone) and LH (luteinizing hormone).
• Follicle development begins, which holds the maturing egg.
• Follicle releases estrogen.
• Estrogen causes the lining in the uterus to thicken and LH to spike.
• LH spikes and causes ovulation, where the follicle releases the egg that moves to the oviduct.
• The follicle turns into the corpus luteum.
• The corpus luteum releases progesterone, which causes continued thickening of the uterine lining.

Fertilization vs. No Fertilization

• Fertilization:
  • Embryo implants.
  • Placenta develops.
  • Placenta secretes progesterone.
  • Progesterone maintains uterine lining.
  • The cycle doesn't start again because the hypothalamus detects high progesterone.
• No Fertilization:
  • Corpus luteum breaks down.
  • Progesterone level drops.
  • Lining sloughed off = menstruation.
  • Hypothalamus detects low progesterone, and the cycle repeats again.