Unit A Reproduction

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Birth About nine months or 266 days after fertilization, the muscles of the mother’s uterus contract regularly (every 15 to 20 minutes, and lasting at least 40 seconds). The birth process, called labour, has begun. Hormones control the process, which is described in Figure 6. A The cervix begins to dilate, or open up. The membrane surrounding the baby is forced into the vagina (now called the birth canal). Did You KNOW? All Pregnancies Are Not Equal Pregnancy is also called gestation. The gestation period is often related to the size of the mammal. Here are some mammals and their gestation periods in weeks: • elephant—88 • horse—48 • grizzly bear—30 • lion—17 • wolf—9 • squirrel—3.5 B The amniotic membrane breaks, and amniotic fluid lubricates the canal. This is called the “breaking of the water.” LEARNING TIP Imagine the birth process (Figure 6) and the explanations (A to D) happening visually. Share your mental images with a partner. C Once the cervix has widened to 10 cm, uterine contractions push the baby’s head into the birth canal. The baby’s head is followed by the rest of the body. D When the head and shoulders are free of the birth canal, the rest of the baby slips out easily. The baby is born. A short while later, the placenta, often called the “afterbirth,” is pushed out of the uterus. Once the baby is breathing on his or her own, the umbilical cord is cut and tied off to prevent bleeding. The “belly button” is the scar from the umbilical cord. Figure 6 The birth process NEL 4.3 Pregnancy: Fertilization to Birth 119
Figure 7 The zygote has divided several times. This ball of cells contains stem cells, which are unspecialized. As these cells divide, they can differentiate into all the different types of cells in the body. Figure 8 About one in 250 births produce identical twins. 300X If you would like to learn more about stem cells, go to www.science.nelson.com GO Stem Cells and Differentiation The one-celled zygote has the instructions in its DNA to produce the more than 200 specialized types of cells in your body. Differentiation is the process of growing from unspecialized cells into many different specialized cells. After the zygote divides several times, it is a ball of roughly 150 unspecialized cells (Figure 7). This ball has stem cells in it. Stem cells are unspecialized cells that have the ability to reproduce themselves. When stem cells undergo cell division, the daughter cells will either remain stem cells or begin differentiating into one of many different types of specialized cells. As the embryo grows and develops into a fetus, and these stem cells divide, they lose their ability to grow into all the different types of cells. These can then only differentiate into certain families of tissues. As the fetus develops further, stem cells are found in only certain tissues. Stem cells in these tissues can only form into the types of cells found in these tissues. For example, the stem cells found in the bone marrow can differentiate into the more than a dozen types of blood cells. Stem cells are of great interest to scientists because of their ability to differentiate into different types of cells. Scientists might be able to use this property of stem cells to develop new technologies. For example, scientists might be able to grow new heart muscle cells to replace cells that have been damaged in heart attacks. In order to do research, however, scientists need a source of stem cells. Stem cells can be found in certain adult tissues, such as bone marrow, and in the blood in the umbilical cord of a newborn. Another possible source is the extra embryos produced after in vitro fertilization (You will learn about in vitro fertilization in humans in Section 4.7.) Many eggs are fertilized, but only a few embryos are implanted. The extra embryos are frozen for future use. There is much controversy around stem cell research, and legislation in Canada controls the use of extra embryos for research. GO Twins Occasionally, more than one fetus develops inside the uterus. There are two types of twins: identical twins and fraternal twins. Fraternal twins occur when two eggs are released at the same time, usually one from each ovary. Since there are plenty of sperm cells in the oviducts, both eggs are fertilized. Each zygote develops its own placenta. Fraternal twins have the same birth date, but they are as similar or different as any brothers and sisters. They have different genetic material. Identical twins (Figure 8) result from one fertilized egg that splits at the zygote or early embryo stage. Normally, the zygote undergoes mitosis to become a ball of cells, which stay together and become an embryo. Occasionally, when the zygote undergoes cell division, the daughter cells separate from each other and each daughter cell develops into an embryo. However, they share the same placenta. These twins contain identical genetic material because they came from the same egg and sperm. Identical twins are the same gender because they contain identical DNA. 120 <strong>Unit</strong> A <strong>Reproduction</strong> NEL
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CHAPTER 2 Cell Growth and Reproduct
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2.1 The Importance of Cell Division
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2.1 CHECK YOUR Understanding 1. Giv
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Figure 2 Plant cell structures cyto
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2.3 From DNA to Proteins You have l
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Did You KNOW? The Human Genome The
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LEARNING TIP Think about the sectio
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USING MITOCHONDRIAL DNA TO SOLVE MY
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centrioles nucleus chromosome spind
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LEARNING TIP Make connections to yo
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2.5 CHECK YOUR Understanding 1. Wha
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Binary fission allows for rapid pop
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2.6 CHECK YOUR Understanding 1. At
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could clone frogs, they were unable
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2A Investigation Observing Cell Div
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2B Investigation Determining the Ra
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Unit A Reproduction

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