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Lecture 4

BIOL 104 Lecture 4: Exam#4 Lecture Study Guide
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by OneClass2384389 , Spring 2018
5 Pages
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Spring 2018

Department
Biology
Course Code
BIOL 104
Professor
Dr. Major
Lecture
4

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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 tissues/organs 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
Neural Tube Defects
During the early stages of human development, the embryo goes through a stage where it becomes
multilayered. The three layers, once formed, then further develop into specific tissues in the adult. The
three tissues are…
Ectoderm (the outer layer) becomes the epidermis (top layer of the skin) and nervous system
Mesoderm (the middle layer) becomes bone and muscles
Endoderm (the inner layer) becomes the digestive system and the respiratory system
For this lecture material, we are primarily interested in the ectoderm and the decision to become either
epidermis or nervous system. The notochord (a key structure in our own classification among similar
organisms) is a rod of tissue that lies underneath the ectoderm near the back of the embryo. It secretes
proteins that “tell” the overlying ectoderm to become neural, more specifically the neural plate. Changes
within the neural plate allow it to fold up into a tube (neural tube). This tube becomes the brain in the
anterior region of the embryo and the spinal cord in the posterior region of the embryo.
Therefore, the complete closure of the neural tube (neurulation) is an important event in the
development of the nervous system and failure to close properly is associated with neural tube defects
(NTDs). NTDs occur roughly in 1/1000 live births. This closure occurs at several points along the
anterior-posterior axis and towards the anterior (anterior neuropore) and posterior (posterior
neuropore).
When failure to close occurs near the head, anencephaly occurs because the forebrain is open to the
amniotic fluid and degenerates. Towards the posterior region, failure to close results in spina bifida and
total closure results in Craniorachischisis.
Causes of neural tube defects
-There are over 300 genes that normally contribute to proper neural tube closure. Therefore, it
isn’t a surprise that mutations in some of these genes lead to problems in humans and mice
models.
-Environmental factors also influence proper neural tube closure.
Drugs, diabetes, obesity, and toxins have been shown to be causes of NTDs.
Maternal dietary factors like cholesterol and folic acid (folate) have links to this process.
We discussed 4 different pieces of evidence for how folic acid contributes to lower the rate of NTDs in
newborns. At least one hypothesis is that it contributes epigenetically to control changes in gene
expression through histone/DNA methylation.
a. Adding folic acid to certain foods lowers the rate NTDs in the overall population and in animal
studies.
b. A binding protein for folic acid (Folic acid Binding Protein) is present in the region where the
neural tube fuses together before and during closure.
c. Mutations in the gene for Folic acid Binding Protein lead to neural tube defects in mice
d. Moms who give birth to children with NTDs have been found to produce antibodies to the Folic
acid Binding Protein while moms who do not give birth to NTD children do not produce
these antibodies.
We finished up by briefly talking about patterning and how tissues must be told where to be made in the
embryo. The neural tube must be patterned along several axes and we focused on the Dorsal-Ventral
axis. You will notice that sensory neurons (the neurons allowing us to receive sensory input from the
periphery like our fingertips) enter the spinal cord through the back region (dorsal) of the neural tube.
On the opposite ventral side motor neurons leave the neural tube in order to stimulate muscles in the
periphery (like the muscles in our hands and arms).
Therefore, the neurons along the dorsal-ventral axis are different. They become different because the
notochord (which is closest to the ventral side of the neural tube) secretes Sonic Hedgehog protein. As

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Description
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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