Cell Development Flashcards
How are germ cells set apart during development?
Germline cells are specified and separated in the early embryo.
• Functional sperm and eggs are only produced in adults.
• In mice, primordial germ cells (PGCs) are first seen in the epiblast before gastrulation, migrate to the posterior end during gastrulation, and finally move to the gonads in the 4th week in humans.
What is the significance of germ cell migration to the posterior end during development?
A:
• It ensures only the healthiest PGCs migrate back in.
• Protects germ cells during early development.
• Controlled by chemical signals and involves unique gene expression for pluripotency, adhesion, and migration.
What occurs during prophase of meiosis?
Homologous chromosomes pair up.
• Recombination occurs: DNA sequences are exchanged between maternal and paternal chromosomes, creating new alleles.
How does meiosis ensure haploid gametes are formed?
Chromosomes are replicated once but undergo two divisions.
• This reduces the chromosome number from diploid (2n) to haploid (n).
What is oogenesis, and how does it progress?
Germ cells divide mitotically while migrating to the ovaries.
• Meiosis begins but arrests in prophase I until ovulation.
• First division completes at ovulation; second division occurs after fertilization, forming a haploid egg and polar bodies.
What is the function of polar bodies in oogenesis?
Polar bodies are small cells formed during meiosis to discard extra genetic material.
• They are not involved in embryonic development.
What is spermatogenesis, and how does it progress?
Germ cells in embryonic testes arrest at G1 phase.
• After birth, they divide by mitosis to form stem cells (spermatogonia).
• At sexual maturity, spermatocytes differentiate and undergo meiosis to form genetically diverse mature sperm.
Why do human oocyte numbers decline with age?
At birth, ~1 million oocytes are present, but only ~400 are ovulated during a woman’s lifetime.
• Possible reasons include apoptosis and failure of meiotic recombination, leading to germ-cell death.
What mechanisms prevent polyspermy during fertilization?
Fertilization triggers a calcium wave in the egg, leading to cortical granule exocytosis.
• Cortical granule contents harden the vitelline envelope and release bound sperm, making the egg impermeable to additional sperm.
What are the steps of fertilization in mammals?
Sperm undergoes capacitation (membrane remodeling and removal of inhibitory factors).
- Sperm penetrates the hyaluronic acid layer and zona pellucida.
- Sperm plasma membrane fuses with egg plasma membrane, and the sperm nucleus enters the egg cytoplasm.
- Cortical granule exocytosis prevents polyspermy.
How does asexual reproduction occur in Hydra?
Hydra reproduce by budding.
• Epidermal and intestinal cells divide mitotically to form a bud.
• The bud grows while attached to the parent, then constricts at the base and separates.
What is parthenogenesis?
Development of an embryo from an unfertilized egg cell.
• Examples:
• Worker bees (female) have non-functional ovaries.
• Drone bees (male) arise from unfertilized haploid eggs.
• Queen bees (female) are produced sexually from fertilized diploid cells.
How is ovary growth suppressed in worker bees?
Worker bees are fed phenolic acid, which suppresses ovary growth.
