Sexual vs. Asexual Reproduction

Comparing Sexual and Asexual Reproduction

• Reproduction is the process by which parents produce offspring.
• Sexual reproduction involves two parents and leads to genetic variation.
• Asexual reproduction involves one parent and results in offspring genetically identical to the parent.
• Asexual reproduction involves only mitosis.
• Sexual reproduction involves meiosis.
• Offspring in asexual reproduction are genetically identical to each other and to the parent.
• Offspring in sexual reproduction are genetically different from each other and from their parents.
• Asexual reproduction is advantageous in stable environments where well-adapted parents produce equally well-adapted offspring.
• Sexual reproduction allows species to evolve in changing environments, as offspring may be better adapted than their parents.

Role of Meiosis in Sexual Reproduction

• Meiosis and the fusion of gametes play different roles in sexual reproduction.
• Fusion of gametes, or fertilization, involves the joining of a male and female gamete to form a new individual, bringing alleles together and doubling the number of chromosomes.
• Meiosis is a process where one diploid nucleus divides to form four haploid nuclei, breaking up parental gene combinations and halving the number of chromosomes.
• Meiosis reverses the doubling of chromosomes caused by the fusion of gametes, preserving the chromosome number over generations in sexual life cycles.

Comparing Male and Female Gametes

• All plants and animals have different types of male and female gametes
• Male gametes are motile, relatively smaller, and produced in large numbers with less food reserves.
• Female gametes are non-motile, larger, and produced in fewer numbers with more food reserves for embryo development.

Structure of the Reproductive Systems

• Male Reproductive System:
  • Includes the penis, which penetrates the vagina for semen ejaculation near the cervix.
  • Semen is transferred through the urethra during ejaculation.
  • Testes produce sperm and testosterone.
  • Sperm is stored in the epididymis until ejaculation.
  • The scrotum holds the testes at a temperature lower than the core body temperature to promote sperm development.
  • The seminal vesicle and prostate gland secrete a fluid containing alkali, proteins, and fructose to mix with sperm and create semen.
  • The sperm duct transfers sperm during ejaculation.
• Female Reproductive System:
  • Ovaries produce eggs, oestradiol, and progesterone.
  • The oviduct collects eggs during ovulation, is the site of fertilization, and moves the embryo to the uterus.
  • The uterus provides for the needs of the embryo and foetus during pregnancy.
  • The cervix protects the foetus during pregnancy and dilates to provide a birth canal.
  • The vagina stimulates the penis to cause ejaculation and forms the birth canal.
  • The vulva protects the internal parts of the female reproductive system.

Hormonal Control of the Menstrual Cycle

• Menstrual Cycle: Takes about 28 days and comprises events in the ovaries and uterus.
• Ovarian Cycle: Follicles develop in the ovary, with one typically releasing an egg (ovulation) around Day 14.
• Uterine Cycle: Endometrium thickens with blood supply to prepare for embryo implantation.
• If no embryo is present, the corpus luteum breaks down (in the ovarian cycle) and the endometrium breaks down and menstruation occurs (in the uterine cycle).
• FSH (Follicle Stimulating Hormone):
  • Rises in the first 10 days, stimulating follicle development and oestradiol secretion by follicle walls.
• LH (Luteinizing Hormone):
  • Peaks sharply around Day 14, stimulating oocyte maturation and ovulation.
  • LH promotes the development of the corpus luteum which secretes oestradiol and progesterone.
• Oestradiol:
  • Peaks in the second week, stimulating endometrium repair and thickening.
  • Inhibits FSH secretion (negative feedback) and stimulates LH secretion (positive feedback).
• Progesterone:
  • Levels rise post-ovulation, promoting endometrium thickening and maintenance.
  • Inhibits FSH and LH secretion (negative feedback).

Fusion of a Sperm and Egg Cell - Fertilization

• Sperm: Detects chemicals released by eggs, allowing sperm to swim towards the egg.
• Acrosome Reaction: Sperm releases enzymes to digest a path through the glycoproteins and zona pellucida.
• Fusion: The sperm's plasma membrane binds to the egg cell's plasma membrane, their membranes fuse, and the sperm nucleus enters the egg cell.
• Cortical Reaction: The layer of glycoproteins hardens, preventing additional sperm from entering.
• The nuclei from the sperm and egg merge, combining 23 chromosome from each nucleus into 46.

Preventing Polyspermy

• Coat of glycoproteins (zona pellucida) surrounds and protects the egg.
• Acrosome Reaction: Enzymes from the acrosome digest a small region of zona pellucida, allowing sperm to reach the egg.
• Cortical Reaction: When the first sperm has fused with the egg, cortical granules release enzymes that toughen the zona pellucida, preventing more sperm from penetrating it.

In Vitro Fertilization (IVF) Treatment

• IVF Steps:
  • Down-regulation: Pausing the woman's cycle, allowing control of egg production.
  • FSH injections: Stimulating follicle development to produce multiple eggs (superovulation).
  • hCG injection: Stimulating maturation of oocytes.
  • Egg collection: Eggs are extracted from the follicles.
  • Fertilization: Eggs mixed with sperm in sterile conditions, incubated.
  • Embryo transfer: Transfer one or more embryos into the uterus.
• Afterward, the woman will maintain ensure uterus lining is maintained.

Pregnancy Testing

• Detects hCG in urine produced by trophoblast cells in blastocyst and embryonic stages.
• Test Mechanism:
  • Antibody A (with blue dye) binds to hCG molecules.
  • This antibody-hCG complex binds to immobilized antibody B, creating a blue band indicating pregnancy.
  • Unbound antibody A binds to IgG antibody, creating a control blue band, confirming the test is valid.

Production of Gametes

• Gametogenesis refers to the production of gametes (sperm and eggs).
• Spermatogenesis:
  • Germinal epithelium cells in the seminiferous tubules undergo mitosis, cell growth, meiosis, and differentiation to form sperm.
  • Meiosis produces four haploid cells that differentiate into sperm.
  • Nurse cells aid in sperm differentiation.
• Oogenesis:
  • Germinal epithelium cells in the ovaries undergo mitosis, cell growth, meiosis (arrested at prophase I until puberty).
  • At puberty, primary follicles develop, but usually only one completes development and releases an egg.
  • Meiosis produces one oocyte with a large amoutn of cytoplasm and two small polar bodies that degenerate.
• Spermatogenesis produces millions of sperm per day starting from puberty; released during ejacualtion.
• Oogenesis produces one egg per month released around Day 14; early stages of production started when the individual was a foetus.

Development of a Blastocyst

• Zygote formed during fertilization divides by mitosis to form a 2-cell embryo, then further rounds of number of cells double every 18 hours.
• Early rounds of division occur without cell growth, causing decrease in cell size.
• A hollow ball develops by day 6 or 7 resulting from unequal cell divisions and migration.
• Blastocyst has inner cell mass that becomes the embryo and a trophoblast that help with implantation
• blastocyst implants in uterus and receives nutrition. The embryo is absorbed lining of the uterus becoming implanted.

The Placenta

• The placenta is a disc-shaped structure that grows during pregnancy to match the foetus' demands
• The foetal tissue (placental villus) is exchanged with material in the maternal blood.
• Materials needed by tissue (glucose, amino acids, oxygen) pass from maternal to foetal and waste products (carbon dioxide, urea) from foetus to maternal.
• The foetus is connected to the body by the ambililical arteries and veints.

Hormonal Control of Puberty

• GnRH synthesized by neurons transported to pituitary gland
• In males, FSH stimulate testis growth and LH stimulates secretion of the steroid hormone testosterone by the testes, causing the development of secondary sexual characteristics.
• In females, FSH stimulates follicle, follicle wall secrete oestradiol, LH causes corpus luteum, which continue oestradiol. causing enlargement of breast, enlarging of uterous etc

Hormones of Pregnancy and Childbirth

• hCG stimulates secretion of oestradial and progesterone. Prevent degeneration of uterous.
• progesterone prevents myometrium by interupting oxytocin. Progestrone lowers and allows pituary gland to secrete oxytocin to start contractions

Self-incompatibility Mechanisms

• Self-incompatibility- evolved to have to reject pollen from other plants in many ways. With pollen grains failing to or stopping to grow. Always has a genetic bias where plants with the same self-incompatibility alleles