Gametogenesis Flashcards
Meiosis
Produce gametes. 4 genetically unique haploid cells. Only in gonads (ovaries and testies). 2 divisions. Meiosos 1: Separation of tetrads. Meiosos 2: Seperation of homologous pairs(same as mitosis)
Prophase 1
Homologues pair up: Tetrads. Crossing over. Mitotic spindle begins to form. Nucleus and nuclear membrane disappear. Chromosome start to condense. Tetrads bind to microtubules from opposite poles.
Metaphase 1
Random alignment of tetrads on metaphase plate/equator.
Anaphase 1
Separation of tetrads into homologues. Random combination of paternal and maternal chromosomes in daughter cell.
Telophase 1
Mitotic spindle breaks.
Cytokinesis.
Two new haploid cells formed.
Crossing over
Exchange of corresponding portion of DNA between homologues chromosomes. Random. No chromatids are the same. New chromosomes with unique combination of genes.
Genetic variation
Each gamete is unique. Because of crossing over: Exchange of corresponding DNA between homologues chromosomes and random alignment on of tetrads on the equator.
Spermatogenesis
Production of sperm cells. Timing: Couple of months. Quantity: Millions per day. Location: Produced in the seminiferous tubules in between Sertoli cells. Production from puberty throughout life. Presence of stem cells: Spermatogonia Sperm cell: Spermatozoa. Size: Smallest human cell.
Meiosis of sperm cells
Meiosis 1
Primary spermatocytes split into secondary spermatocytes.
Meiosis 2
Secondary spermatocytes split into spermatids.
Spermatids transformed into spermatozoa by spermiogenesis.
Oogenesis
Production of eggs.
Timing: at least one decade.
One secondary oocyte is produced per month.
Eggs produced upon fertilization.
Production from puberty until menopause (no stem cells: Oogonia).
Size: Largest human cell.
Ovaries are female gonads, where female gamete is produced.
Meiosis of eggs
Takes at least a decade to perform.
Meiosis 1 starts in embryonic life and stops in prophase 1.
Before ovulation, meiosis 1 is completed and meiosis 2 stops in metaphase.
After fertilization meiosis 2 is completed.
Oocytes are kept and develop in ovaries inside follicles.
After ovulation
Remnants of follicles transform into corpus luteum.
Secrete progesterone
Avoid mensturation
Prepare endometrium for implantation.
Fertilization
Happens in distal portion of uterine tubes. During ovulation secondary oocyte is released, surrounded by corona radiata and underlying zona pellucida. 5 steps: 1. Capacitation of sperm cells. 2. Acrosome reaction. 3. Cortical reaction 4. Completion of meiosis 2 5. DNA replication 6. Fusion of pronuclei
- Capacitation of sperm cells
Sperm cells become competent for fertilization.
- Acrosome reaction
Sperm burrows through corona radiata. Contact with zona pellucida starts the reaction. The reaction being, making a hole in the zona pellucida.
- Cortical reaction
Secondary oocyte move receptor for sperm cells to avoid polyspermy (more than one sperm in egg). Triggered by fusion of sperm with oocyte membrane.
- Completion of meiosis 2
Formation of egg
- DNA replication
Male pronuclei - 23 chromosomes
Female pronuclei - 23 chromosomes
- Fusion of pronuclei
Male and female pronuclei fuse together. Zygote - single celled diploid.
Early embryogenesis
- Fertilization
- Cleavage
- Implantation
- Gastrulation
- Neurulation
Cleavage
Rapid mitotic division.
No increase in volume.
Zona pellucida still intact.
Cells smaller at each cleavage. Called Blastomeres.
Cells form compact mass through junctions called morula.
Implantation
6-8 days after fertilization.
Morula becomes blastocyst.
Penetration in the endometrium.
Blastocyst
Blastocoele - inner space
Trophoblast - cells surrounded by zona pellucida (syncitiontrophoblast, cytotrophoblast).
Zona pellucida - outer shell
Embryoblasts - former inner cell mass. totipotent embryonic stem cell.
Trophoblasts
Syncitiontrophoblasts: Secretes human chorionic gonadropin. Lacunae filled with blood: uteroplacental circulation (nutrient supply). No menstration.
Cytotrophoblast
