Gametogenesis, Genetics, and Density

Gametogenesis

• Gametogenesis is the process where cells undergo meiosis to produce mature haploid (n) gametes.
• There are two types:
  • Spermatogenesis (male)
  • Oogenesis (female)

Spermatogenesis

• Occurs in the wall of the seminiferous tubule.
• Begins with germ line cells undergoing mitosis.
• These cells produce other germ line cells, some of which are ready to undergo meiosis.
• Spermatogonia replicate their chromosomes and become primary spermatocytes.
• Primary spermatocytes undergo Meiosis I, creating two secondary spermatocytes (n).
• Secondary spermatocytes go through Meiosis II to produce four spermatids.
• Spermatids reach the lumen of the tubule and grow a flagellum for movement.
• Spermatids undergo differentiation to become spermatozoa (mature sperm cells).
• A spermatid becomes a sperm when the Golgi apparatus and centriole form an acrosome (head) containing a nucleus.
• The midpiece contains mitochondria for energy to move.
• The tail is made of inverted microtubules and helps push the sperm forward.

Spermatogenesis Overview

• Spermatogenesis is the production of spermatozoa (sperm) in the seminiferous tubules of the testes.
• The process begins when spermatogonia undergoes mitosis and cell growth.
• After this, they become spermatocytes (primary and secondary).
• The spermatocytes undergo meiosis I and II to form four haploid daughter cells (spermatids).
• The spermatids then undertake a process of differentiation to become functional sperm cells (spermatozoa).

Spermatogenesis Stages

• Spermatogonia (2n) undergo mitosis.
• Primary spermatocyte (2n) undergoes meiosis I.
• Secondary spermatocytes (n) undergo meiosis II.
• Spermatids (n) undergo spermiogenesis.
• Spermatozoa (sperm cells) are produced.

Oogenesis

• Starts with a germline cell called an oogonium.
• This cell undergoes mitosis.
• The cell starting meiosis is called a primary oocyte.
• This cell will begin the first meiotic division but be arrested in the prophase stage.
• Once puberty begins, the primary oocyte finishes Meiosis I and produces two unequal cells:
  • A secondary oocyte
  • The first polar body
• The cell divides unequally, with most of the cytoplasm going to the secondary oocyte.
• Cytoplasm is divided unequally because the ovum must maximize its nutrients to nourish the embryo during early development stages.
• During meiosis II, the secondary oocyte divides into two parts unequally.
• More cytoplasm goes toward the ootid, which then differentiates into an ovum, ready for fertilization.
• Less cytoplasm goes to a second polar body that will eventually disintegrate.
• If the ovum is fertilized by the sperm, then it will create a zygote (2n).

Oogenesis Overview

• Process begins in a female before birth (in the uterus) and eventually creates a female gamete - ovum (egg cell).
• Begins with a germline cell that becomes an oogonium.
• Oogonium goes through mitosis and becomes a primary oocyte that will stay in Prophase I until the female reaches puberty - it will also create the first polar body.
• Once puberty is reached, the primary oocyte will go through meiosis I and create a secondary oocyte along with the second polar body.
• An ootid is formed once that is over and eventually becomes a mature ovum which can be fertilized by the sperm cell.
• If fertilized then it will create a diploid zygote.

Oogenesis Stages

• Oogonium (2n) undergoes mitosis.
• Primary oocyte remains in Meiosis I - Prophase I until puberty.

Spermatogenesis vs Oogenesis

• Spermatogonium vs. Oogonium
• Primary spermatocyte vs. Primary oocyte
• Secondary spermatocytes vs. Secondary oocyte
• Spermatids vs. First polar body
• Spermatozoa vs. Ovum

Fertilization

• Egg (Ovum) with 23 chromosomes
• Sperm with 23 chromosomes
• Fertilization creates a Zygote with 46 chromosomes in 23 pairs
• Embryo has 46 chromosomes in 23 pairs

Sex Determination

• All gametes (sex cells) are haploid (n).
• All egg cells will carry an X chromosome, so all humans will have at least one.
• Sperm cells have a 50% chance of carrying X or Y.
• Whichever type of sperm fertilizes the egg determines the sex of the fetus.
• When fertilization occurs, the zygote will be diploid (2n).

Nondisjunction

• A failed separation of a pair of homologous chromosomes or sister chromatids during meiosis.
• The excess or deficit chromosomes alters cell function and may be lethal.
• Nondisjunction can occur in Meiosis I or Meiosis II, leading to gametes with an abnormal number of chromosomes (n+1 or n-1).

Sex-Linked Traits

• Genetic disorder caused by or linked to gene(s) located in the sex chromosomes.
• Passed down through families through X or Y chromosomes.
• Term “sex-linked” often refers to X-linked (affects the X chromosome).

Chromosomal and Sex-Linked Disorders

• Down Syndrome (Trisomy 21)
• Edward’s Syndrome (Trisomy 18)
• Hemophilia
• Sickle Cell Anemia
• Cystic Fibrosis
• Huntington Disease
• Duchenne Muscular Dystrophy
• Color blindness

Density

• Used to describe how much space an object or substance takes up (in volume) in relation to the amount of matter in that object (its mass).
• Low density: If particles are loosely packed together with plenty of space between them.
• High density: If particles are tightly packed together with no or very little space between the particles.