lecture 5 Flashcards
what is the driving force behind meiosis?
avoiding Muller’s ratchet = gradual accumulation of deleterious mutations: deleted mutations will be passed on and can’t go back
Number of mutations can only increase and it is detrimental to the number of individuals
This is avoided by PROKARYOTES by LATERAL GENE TRANSFER / Recombination
when do oocytes become arrested?
they become arrested in metaphase II
why do females have fewer and larger gametes?
After formation of the primary oocyte, the egg cell does not undergo any further divisions that would reduce the cell’s size
Spermatocytes are formed throughout adult life from stem cells, while in females, meiosis is initiated only once and they become arrested in metaphase II
Oocyte will develop into 1 egg cell and 2 polar bodies, while 1 spermatocyte will develop into 4 sperm cells.
what are the steps through which fertilization occurs?
The sperm has to penetrate several physical barriers. The first one is a sticky layer of hyaluronic acid and embedded somatic follicle cells, called cummulus cells.
The sperm head has hyaluronidase activity that helps penetrate through this first layer.
The second layer that the sperm encounters is zona pellucida. The sperm is able to pass through this layer because of the acrosome, which releases some enzymes by exocytosis.
After this fusion of the plasma membrane of the egg and the sperm occurs and the sperm nucleus and contents get injected into the egg cytoplasm.
how is polyspermy blocked in different animals?
In birds, more than one sperm enters the cytoplasm of the eg but only one nucleus fuses with the egg’s nucleus. Other sperm nuclei are destroyed in the cytoplasm by DNAses.
In sea urchins: the rapid block of polyspermy is triggered by a transient depolarization of the egg plasma membrane, there is a rapid electrical block
In mammals: polyspermy is prevented by changes that occur to both the zona pellucida and the egg plasma membrane once the first sperm has entered the cell (and basically sperm can no longer bind to the egg plasma membrane due to some specific enzymes). Juno (the receptor for sperm-egg recognition disappears 40 minutes after fertilization.
when are the polar bodies in oogenesis released?
The first polar body is after ovulation, and the primary oocyte becomes a secondary oocyte which gets arrested in metaphase II
The second polar body is released right after fertilization.
what is zona reaction and what triggers it?
the zona reaction refers to alteration in the structure of zona pellucida catalyzed by proteases released by the cortical granules. It is triggered by a wave of Ca that travels through the cell in response to fertilization by a sperm cell.
it results in hardening of the zona pellucida and the sperm that has not yet reached it or traversed through it will be stopped in their tracks.
the sperm receptors, Juno, gets degraded about 40 minutes after fertilization and the sperm won’t be able to bind to the zona pellucida anymore.
