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Sexual reproduction involves the fusion of genetic material (gametes) from two parental organisms
To ensure the proper chromosomal numbers in the zygote (fertilized egg), each gamete must have haploid, or half (N, or one set of chromosomes), of the original diploid (2N, or two sets of chromosomes) amount of DNA
Meiosis: The process by which the chromosome number is reduced by half, resulting in new genetic combinations in the gametes
ORGANISMAL REPRODUCTION & MEIOSIS
The Two Stages of Meiosis
Meiosis is preceded by interphase; many meiotic events are similar to those of mitosis; the following points note the differences
Meiosis I
Prophase I: Chromosomes condense and organize; matched, or homologous, chromosomes (one maternal and one paternal in each pair) are physically paired; segments of chromatids can cross over at breakage points called chiasmata within each chromosome pair
Crossing Over
Metaphase I: Homologues line up at equator
Anaphase I: Homologues are separated into two groups, with each group having a mixture of maternal and paternal chromosomes
Telophase I: New haploid nuclei form for two new daughter cells
Interkinesis: No replication of DNA occurs because each chromosome is still duplicated and consists of two chromatids (although crossing over results in some chromatids with maternal and paternal segments)
Meiosis II
ORGANISMAL REPRODUCTION & MEIOSIS
Faunal & Floral Gametogenesis
In animals, meiosis occurs in germinal tissues and is called spermatogenesis in males and oogenesis in females; each results in gametes
In plants, the process is similar, except that more mitotic divisions may follow meiosis to produce gametes
Gametogenesis
ORGANISMAL REPRODUCTION & MEIOSIS
Reproductive Cycles & Hormones in Animals
Many animals, including humans, regulate the production of gametes and sexual behavior based on the presence of and complex interplay between circulating hormones that cycle based on monthly and seasonal rhythms
Reproductive Control in the Human Female
Hormones
At puberty, the hypothalamus releases gonadotropin-releasing hormone (GnRH); this stimulates the anterior pituitary to release follicle-stimulatinghormone (FSH) and luteinizinghormone (LH)
FSH and LH (but primarily FSH) trigger ovarian structures called follicles to grow and produce an oocyte (potential egg), as well as estrogen and progesterone
Estrogen and progesterone have major effects on the morphology and physiology of the uterus in anticipation that ovulation and subsequent sexual intercourse will result in a successful fertilization
Female Reproductive Hormone Pathway
Ovarian cycle
Under the stimulation of FSH and LH, some (but not all) follicles grow at different rates (during the follicular phase), with the Graafian follicle maturing the fastest; as the follicles grow, they also produce estrogen, with the Graafian follicle producing the most
Estrogen has a negative feedback role on the brain (both the hypothalamus and anterior pituitary) for most of the monthly cycle, thereby keeping FSH and LH levels relatively low; however, continually rising levels of estrogen from growing follicles trigger a positive feedback role that causes a temporary surge in FSH and LH, with the latter hormone spiking the most
This surge in LH triggers ovulation, in which the Graafian follicle bursts at the surface of the ovary and releases a secondary oocyte covered in associated follicular tissues (which will later become a barrier through which sperm must pass to contact the oocyte)
The remnants of the Graafian follicle now function as a corpus luteum (during the luteal phase), partly to prevent any additional follicles from developing and releasing potential eggs by combining estrogen production with the additional hormone progesterone (many birth control pills use this same hormonal strategy to prevent unwanted pregnancies) E. The corpus luteum will eventually disintegrate, unless fertilization and implantation subsequently occur
Uterine and menstrual cycle
Estrogen and progesterone from the ovaries stimulate the uterus to prepare for a potential embryo; specifically, the internal lining of the uterus (endometrium) is partially shed at the beginning of each cycle (menstrual flow phase or âperiodâ) if there is no successful fertilization and subsequent implantation; increasing estrogen (from growing follicles) stimulates a thickening of the endometrium (proliferative phase)
After ovulation, increasing progesterone levels additionally cause the endometrium to become highly vascularized and to secrete nutrients (secretoryphase) for embryo development; the disintegrating corpus luteum, and the concomitant drop in estrogen and progesterone, causes vasoconstriction of endometrial blood vessels, eventually leading to a shedding of ti...
