In my biology textbook, I read that the primary oocyte gets arrested in the early stages of meiosis in prophase I (diplotene stage). I wanted to know why this is so. I searched google, and this phenomenon is known as dictyate. Furthermore the source was talking about translational arrest after transcription. Why does it occur during cell division? Moreover, what's so necessary about this phenomenon?
I am not sure if I can answer the question of why the primary oocyte gets arrested during meiosis (and finishes after fertilization), but the stage of growth the egg is arrested at does vary widely between species; it's not always primary oocyte. This might suggest that the reason is species-dependent.
- In sea urchins, it's the fully mature ovum that gets fertilized.
- In humans, it's the secondary oocyte (arrested during this stage of meiosis) that gets fertilized.
- In the majority of mammals (dogs, etc.), it's the primary oocyte that gets fertilized. In both of these last two, the oocyte finishes meiosis after fertilization & the entry of the sperm nucleus into the egg.
The translational arrest of the egg you mentioned is because the egg is in a "dormant" state until it is fertilized, and it is "activated" by fertilization. Until fertilization, the mRNA's are present in the cytoplasm, but cannot be translated because they are missing poly-A tails (translational arrest); free levels of calcium in the cytoplasm are low; and respiration and oxygen levels are low. This is because the egg is "primed" for the explosion of activity necessary during early development (taking place after fertilization); the mRNA's code for proteins that are used at that time, but they are not translated until fertilization.
Source: http://www.ncbi.nlm.nih.gov/books/NBK10029/ (see the second paragraph under "Late Responses")