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Lecture 4

BIOL 104 Lecture 4: Exam#4 Lecture Study Guide

5 Pages

BIOL - Biology
Course Code
BIOL 104
Dr. Major

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Biol 106 Human Genetics and Health Dr. Major Guide for Exam 4 Material (Spring 2018) Conjoined Twins In order to understand how conjoined twins come about, it is necessary that you first understand how an early embryo is formed. Not surprisingly, development occurs in stages. 1 Fertilization: sperm (haploid) and egg (haploid) nuclei fuse together to form zygote (diploid) 2 Cleavage: Zygote undergoes a series of rapid cell divisions to form a blastocyst 3 Gastrulation: Cells begin to move around to form three cell layers A sperm cell is structured in a way to promote the possibility of fertilization: A tail that contains microtubules and dynein motor protein that whip the sperm cell forward to the egg. A midpiece that contains a lot of mitochondria to produce the energy needed for the long migration to the egg. An acrosome that contains enzymes that will spill out to digest the jelly that surrounds the egg. This explosion is called the acrosome reaction The sperm cell is guided to the site of fertilization (ampulla) in the fallopian tube by three factors: 1 Contraction of the muscular uterus 2 The heat of the ampulla attracts the sperm (Thermotaxis) 3 Chemicals secreted by the egg attract the sperm (Chemotaxis) An egg cell is structured in a way to prevent fertilization by more than one sperm (polyspermy). A plasma membrane An outer zona pellucida A jelly coat When one sperm gets through these layers, the zona pellucida separates from the plasma membrane and poofs up and away to prevent a second sperm from getting through. Gastrulation The embryo forms a groove through which cells dive (Primitive streak). As cells migrate through, the embryo goes from 1 layer to 3 layers. Each layer will go on to become specific tissuesorgans in the embryo. At one end of the streak lies the Node. It is possible that at this early stage, the node is the most important region because it expresses genes that induce an entire second axis. This is why we call it The Organizer. Transplantation studies by Spemann and Mangold highlight the power of having a second node within the embryo. Conjoined Twins probably form through one of two possibilities: 1 Through the incomplete splitting of a single embryo to essentially form two identical twins that are still attached. 2 Through the movement of cells from an already produced identical twin to the other twin
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