DEVBIO CHAP 1

• Gametogenesis is the process through which germ cells develop into male and female gametes. This intricate biological process involves the transformation of primordial germ cells into spermatozoa in males and ova in females.
• Understanding gametogenesis is crucial for comprehending the intricacies of human reproduction and fertility.
• Primordial germ cells (PGCs) are the precursors to sperm and egg cells. They are the earliest form of germ cells that eventually give rise to the gametes (sperm in males and eggs in females) responsible for reproduction.
• Gametes derived from primordial germ cells (PGCs). PGCs formed in the epiblast during the second week.
• PGCs move through the primitive streak during gastrulation in 3rd week. Primary germ layers (ectoderm, mesoderm, and endoderm)
• PGCs migrate to the wall of the yolk sac During the fourth week, PGCs begin to migrate from the yolk sac toward the developing gonads
• PGCs arrive at the developing gonads by the end of the fifth week. Throughout their migration and upon reaching the gonads, these germ cells undergo mitotic divisions, increasing their numbers.
• Germ cells undergo gametogenesis in preparation for fertilization.
• Gametogenesis includes meiosis to reduce the number of chromosomes. Cytodifferentiation completes the maturation of germ cells
• Traits are determined by genes on chromosomes inherited from both parents. Humans have approximately 23,000 genes on 46 chromosomes.
• Genes on the same chromosome are inherited together and are known as linked genes. In somatic cells, chromosomes appear as 23 homologous pairs to form the diploid number of 46.
• There are 22 pairs of matching chromosomes (autosomes) and one pair of sex chromosomes (XX for female, XY for male). One chromosome of each pair comes from the maternal gamete (oocyte), and one from the paternal gamete (sperm).Each gamete contains a haploid number of 23 chromosomes.
• The union of gametes at fertilization restores the diploid number of 46 chromosomes in the resulting zygote, which then develops into a new individual.
• Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell. Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its DNA.
• Prophase Stage

  ●Chromosomes begin to coil, contract, and condense.

  ●Each chromosome consists of two parallel subunits called chromatids joined at the centromere.

  ●Throughout prophase, chromosomes continue to condense, shorten, and thicken.
• Prometaphase Stage

  ●The chromatids become distinguishable.

  Microtubules extend from the centromere to the centriole, forming the mitotic spindle.
• Metaphase Stage

  ●Chromosomes line up in the equatorial plane, and their doubled structure is clearly visible.
• Anaphase Stage

  ●The centromere of each chromosome divides, marking the beginning of anaphase.

  ●Chromatids migrate to opposite poles of the spindle.
• Telophase Stage

  ●Chromosomes uncoil and lengthen.The nuclear envelope reforms, and the cytoplasm divides.
• Cytokinesis

  The cytoplasm divides, and two daughter cells are formed, each with a complete set of chromosomes.
• Meiosis is the cell division that takes place in the germ cells to generate male and female gametes, sperm and egg cells, respectively. Meiosis requires two cell divisions, meiosis I and meiosis II, to reduce the number of chromosomes to the haploid number of 23.
• Male and female germ cells replicate their DNA at the beginning of meiosis I, resulting in duplicated sister chromatids for each of the 46 chromosomes.
• In meiosis I, homologous chromosomes align themselves in pairs through a process called synapsis, except for the XY combination.The homologous pairs then separate into two daughter cells, reducing the chromosome number from diploid to haploid.
• Meiosis II separates sister chromatids, resulting in each gamete containing 23 chromosomes.
• Crossovers in Meiosis I:

  ●Interchange of chromatid segments between paired homologous chromosomes

  ●Genetic variability is enhanced through crossover, which redistributes genetic material

  ●Random distribution of homologous chromosomes to the daughter cells

  ●Each germ cell contains a haploid number of chromosomes, restoring the diploid number of 46 at fertilization
• Polar Bodies

  ●During meiosis, one primary oocyte gives rise to four daughter cells, but only one develops into a mature gamete (the oocyte), while the other three (polar bodies) receive little cytoplasm and degenerate●In contrast, one primary spermatocyte gives rise to four daughter cells, and all four develop into mature gametes
• FIRST MEIOTIC DIVISION

  ●Pairing of Chromosomes: Homologous chromosomes are paired up next to each other.
• FIRST MEIOTIC DIVISION

  Chiasma Formation: The paired chromosomes show points of crossing over, known as chiasmata, where they can exchange genetic material.
• FIRST MEIOTIC DIVISION

  Double-Stranded Chromatid: The chromosomes are depicted with their sister chromatids clearly attached at their centromeres.
• FIRST MEIOTIC DIVISION

  Anaphase of 1st Meiotic Division: The homologous chromosomes are being pulled apart to opposite poles of the cell.
• FIRST MEIOTIC DIVISION

  Telophase of 1st Meiotic Division and Cytokinesis: Two new cells are formed, each with a nucleus containing a set of chromosomes.
• SECOND MEIOTIC DIVISION

  During the second meiotic division, the double-structured chromosomes, also known as sister chromatids, split at the centromere
• SECOND MEIOTIC DIVISION

  At the completion of the division, each of the four daughter cells contains a unique set of chromosomes.
• Oogenesis

  Primary oocyte after DNA replication, containing doubled-stranded chromosomes. First meiotic division, producing a secondary oocyte and a first polar body
• Oogenesis

  Second meiotic division, resulting in the formation of a mature oocyte (22 + X) and a second polar body. The mature oocyte contains 23 single chromosomes, one of which is labeled as X. The process results in the production of one mature gamete and three polar bodies
• Spermatogenesis:

  Primary spermatocyte after DNA replication. First meiotic division, resulting in two secondary spermatocytes. Second meiotic division, leading to the formation of four spermatids. Each spermatid contains 23 single chromosomes, with one sex chromosome labeled as either X or Y. The spermatids develop into spermatozoa
• 1.Multiplication Phase - In this phase, the diploid germ cell divides by mitosis to form diploid oogonia in oogenesis & spermatogonia in spermatogenesis.
• 2.Growth Phase - This phase involves only growth in size. It is the longest phase. It results in the formation of the primary oocyte in oogenesis and primary spermatocyte in spermatogenesis.
• 3.Maturation Phase -involves meiotic division, resulting in the formation of haploid gametes from diploid primary oocytes/spermatocytes in both oogenesis and spermatogenesis.
• Oogenesis: is the process by which female organisms produce eggs (ova). It takes place in the ovaries and involves the development of oogonia into mature egg cells.