Mitosis, Meiosis, Gametogenesis, & Gonads

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Mitosis

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-Series of events that takes place during cell division
-Results in division of the nucleus
-Results in the formation of 2 daughter nuclei that are identical to the mother nucleus
-In humans, mother and daughter cells have 46 chromosomes

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Phases of Mitosis

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1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

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Prophase

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-Nuclear envelope breaks down
-Chromatin begins to condense into chromosomes
-Formation of centromeres: small body that connects sister chromatids
-Centrioles migrate towards opposite ends of the cell
-Mitotic Spindle, make from microtubules, grows out from centrioles

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Metaphase

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Chromosomes line up at equator of cell

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Anaphase

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Centromeres split and the chromatids are pulled to opposite ends of the cell

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Telophase

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-Chromosomes at opposite ends of the cell begin to decondense to chromatin
-Nuclear membranes form around chromatin

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Cytokenesis

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-Follows Mitosis
-Division of cell cytoplasm & pinching off into 2 daughter cells

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Meiosis

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-Specialized type of nuclear division that occurs only in reproductive organs
-Results in the formation of 4 daughter nuclei which differ in comparison to the mother nucleus
-Goal is to produce gametes, sperm/egg, that have 1/2 the # of chromosomes that non-gametes have
-Meiosis if 1st part of Gametogenesis, which also involves further development of gamets

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Phases of Meiosis

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1. Meiosis I
2. Meiosis II

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Meiosis I

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-Diploid cell produces 2 haploid cells, resultin in genetic diversity
-DNA is duplicated
-Homologous chromosomes pair up, and exchange genetic material via crossing over
-Homologous chromosomes separate, resulting in 2 haploid daughter cells, each differing from each other

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Meiosis II

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-Daughter cells do NOT duplicate their DNA before separating
-Results in 2 unique haploid cells from each of the meiosis I daughter cells
-4 daughter cells total

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Gametogenesis

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1. Spermatogenesis
2. Oogenesis

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Spermatogenesis

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-Process of gametogenesis seen in males
-Begins at puberty & continues throughout life
-Occurs withing the seminiferous tubules of the testes

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Spermatogonia

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-Stem cells that line seminiferous tubules which go through mitosis to yield spermatogonia stem cells
-Also yield the Primary Spermatocyte

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Primary Spermatocyte (2n)

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-Cell designed to undergo meiosis
-Undergoes Meiosis I to yield secondary spermatocytes that are haploid (n)

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Secondary Spermatocytes (n)

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2 haploid cells that enter Meiosis II to yield 4 spermatids (n)

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Spermatids

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-4 haploid cells that resulted from Meiosis II that will mature into sperm cells (spermatozoa)
-These spermatozoa then under "spermiogenesis"

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Spermiogenesis

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A maturation process that spermatozoa undergo where they acquire their final, 'torpedo' shape

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Spermatozoa

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Motile streamlined sperm (torpedo shape)

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Oogenesis

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-Process of gametogenesis seen in females
-Occurs in the ovaries
-Begins prior to birth of women
-The immature egg or ovum develops within a sac-like structure called the follicle
-Only produces 1 functional gamete

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Oogonia

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Undergoes mitosis thousands of times prior to birth to yield more oogonia as well as the Primary Oocyte

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Primary Oocytes (2n)

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Produces prior to burth but are arrested in prophase 1 of meiosis until puberty

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Puberty

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-FSH (Follicle Stimulating Hormone) stimulates follicle development/ovum maturation approximately every 28 days
-As the follicle matures, follicle cells change from squamous to cuboidal
-Now referred to as the "Primary Follicle"

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Primary Follicle

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-Completes Meiosis I while still in the ovary to yield 2 differently sized haploid daughter cells
-Secondary Oocyte (n) whish is then arrested in metaphase II of Meiosis
-1st Polar Body (n) which may divide into 2 secondary polar bodies and disappear
-Primary Follicle mature into the Graafian Follicle

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Ovulation

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-Secondary Oocyte is released form the Graafian Follicle
-Travels down the uterine/fallopian tube in hopes of meeting a sperm
-This secondary oocyte is still arrested in Metaphase II of Meoisis at this point
-Remains arrested unless it is penetrated by sperm during fertilization

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Testes

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-Covered by a dense CT capsule: Tunica Albuginea
-Extensions of this enter into the testis, dividing it into segments
-Segments contain seminiferous tubules; sperm is produced here
-Travel out to the epididymis: storage & maturation location
-Sperm exit testicle through the vas deferens

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Ovaries

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-Consist of cortex & medulla; somewhat different to distinguish
-Follicles, located in ovarian cortex, contain ovum (eggs)
-When mature, ovum is ejected from the ovary; follicle becomes corpus luteum that releases hormones