Flashcards in Lecture 1 Deck (22):
What is the difference between meiosis and mitosis?
Mitosis is a process where cells undergo one set division moving from 4n to 2n
Meiosis is a process used in generation of germ cells where the cell undergoes two rounds of cell division going from 4n to 1n and contributes to the generation of genetic diversity
What are the three major functions of germ cells?
The preservation of genetic integrity of the germ line
The generation of genetic diversity
the transmission of genetic information to the next generation
How do germ cells preserve the genetic integrity of the germ line?
They are typically produced from cells set aside early in embryonic development where they remain ageless
What is the process of oogenesis?
During embryonic development primordial germ cells migrate and enter the ovary where mitotic divisions are stopped and the cell is frozen in the first phase of mieosis
During puberty hormonal stimulation gives rise to ovulation allowing the meiosis to be completed, each meiotic division is uneven with production of polar bodies which are broken down allowing the oocyte to preserve its size
What is the process of spermatogenesis?
Primordial germ cells multiply and migrate and are then arrested in G1 phase until after birth where mitosis is resumed by spermatogonia (stem cells) which can in turn generate spermatocytes which undergo meiosis to form spermatids which will differentiat into immature sperm
What are the structural features of sperm cells?
Almost all cytoplasm is removed during the process of differentiation
Contains a haploid nucleus, a propulsion system (flagellum), many mitochondria to provide the ATP required for dynein which is attached to microtubules and drives movement
An acrosome which contains all the enzymes required to lyse the outer coverings of the oocyte
Many species have G actin between the acrosome and nucleus which can polymerize to F-actin aiding in penetration of the egg
What are the structural features of the oocyte?
As all the material required for the beginning of growth and development must be stored in the egg it is rich in proteins, ribosomes, tRNA, mRNA, morphogenetic factors and protective chemicals such as DNA repair enzymes
What is capacitation?
It is maturation of the sperm so that it is able to fertilize an egg, for mammals this occurs in the female reproductive tract and involves he removal of inhibitory molecules from the head of the sperm
What are the coverings outside the egg?
Outs the egg cell membrane is the vitelline envelope which is important for species specific sperm binding
In mammals the vitelline envelope is called the zona pellucida
What is the corona radiata?
a layer of cells just inside the zona pellucida
Where is the cortex of the egg located and what is its function?
It is immediately beneath the egg cell membrane and rich in G actin which polymerize during fertilization to pucker the egg surface forming microvilli which creates a rough surface which is easy for the sperm to bind to and enter the egg
It also contains cortical granules which have enzymes and mucopolysaccharides to prevent polyspermy
What are the four major events in fertilization?
Contact and recognition between sperm and egg
Regulation of sperm entry into egg
Fusion of genetic material of the sperm and egg
Activation of Egg metabolism to start development
How are sperm cells attracted to the egg?
Chemotactic gradients, Mammals also use thermotactic and chemotactic gradients
What is the chemotactic and thermotactic gradient in humans?
Cumulus cells release progesterone fro mthe egg whcih creates the chemotactic gradient allowing the sperm to swim faster and harder to the egg
After spending time in the storage site the sperm undergo capacitation becoming very temperature sensitive responding to the ~2 degrees gradient between the storage and fertilization site
What are the physical barriers the sperm must pass through to penetrate the egg?
The cumulus cells embedded in a layer of hyaluronic acid followed by the zona pellucida
The first layer is penetrated through use of a hyaluronidase the second layer is penetrated by the sperm through attachment via SED1 (applied to sperm in the epididymis)
What triggers the acrosomal process of the sperm?
When the sperm galactosyltransferase binds to the N-acetylglucosamine residues on ZP3
ZP2 is also critical for recognition
How does membrane fusion between the egg and sperm occur?
It involves the protein CD9 Tetraspanin on the egg cell membrane in mice and Izumo1 on sperm but the mechanism itself is unclear
What occurs in the fusion of the sperm and egg membranes?
It allows entry of the sprem pronucleus antriole into the egg along with a small amount of cytosol with mRNA and protein
Sperm mitochondria do enter the egg but are degraded so only maternally derived sperm will persist in the embryo
How is polyspermy prevented?
In many organisms there are two phases to this process there is a fast block (not found in mammals) which involves depolarization of the membrane of the egg from -70mV to +20mV
This is followed by a slow block where as the membrane potential returns to normal an impenetrable membrane (the fertilization membrane)
What triggers the formation of the fertilization membrane?
The slow block is triggered by a wave of Ca2+ release from the endoplasmic recticulum triggered by sperm entry, this Ca2+ then triggers the release of the contents of cortical granules via exocytosis
How is the fertilized egg made metabolically active?
On fusion of the egg and sperm the egg completes meiosis intiated by an influx of calcium, the pronuclei fuse and the fertilized egg enters mitosis