47 _ 18 Animal Development 2009

47 _ 18 Animal Development 2009 - Biol 61 Animal...

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Biol 61 – Animal Development 4/15/09-4/20/09 For this section we’re going to be talking about a little material from Chapter 18 in addition to Chapter 47. You should read pages 366-369, and Figs. 18.15, 16 and 19 from Chapter 18 (this most relates to the lecture material). For Chapter 47, you should read pages 1021-1033. Fertilization The first step in forming a vertebrate organism is fertilization, when a haploid (n) sperm fuses with a haploid (n) egg to form a diploid (2n) zygote. This zygote then divides to form the morula, blastula, gastrula, etc. of the developing embryo. OK, so sperm fuses with egg, the two nuclei fuse, and that’s it, right? Nope – fertilization is actually a little more complicated, in part because one of the worst things that can happen in fertilization is for multiple sperm to fuse with the egg at the same time. This could result in triploid (3n) or worse zygotic nuclei, which would disrupt both cell division (the process of meiosis especially) and embryonic development (because animal cells don’t like to be aneuploid, with weird chromosome numbers). Your book shows what happens with a sea urchin egg, but it really isn’t much different for any vertebrate (see Fig. 47.3). An egg is a very large cell (usually the largest in the body) that contains a haploid nucleus. It is covered by a protective layer of proteins and sugars outside its plasma membrane, and this is the jelly coat . A sperm is composed of a long flagella for swimming attached to a compact head that contains a haploid nucleus, a bunch of actin monomers, and an acrosomal vesicle (along with mitochondria to power its swimming). The acrosomal vesicle is a membrane-bound vesicle that contains enzymes that can digest through the outer glycoprotein jelly coat of the egg. The sperm head touches and binds to this jelly coat, and the contents of the acrosomal process begin digesting the coat proteins. At the same time, the actin monomers that were carried in the sperm head begin to form polymers of actin (basically, ‘microfilaments’ form as the single actin polypeptides bind together into a polymer that gets longer and longer). This actin rod pushes the plasma membrane of the sperm out like a little nose (think about Pinocchio lying and his nose extending, for those of you who saw any Disney movies as kids). The sperm head is pretty much stuck in place within the jelly coat, but this acrosomal process – this little nose that gets pushed out during actin polymerization – extends the plasma membrane of the sperm until it reaches the plasma membrane of the egg. If the sperm contains the correct proteins in its plasma membrane that fit, like a lock and key, with membrane proteins on the egg surface, then the plasma membrane of the sperm fuses with the plasma membrane of the egg and the sperm nucleus is sent inside. The biggest danger to the egg cell at this point is
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47 _ 18 Animal Development 2009 - Biol 61 Animal...

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