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the waste products of the embryo diffuse back through the placenta into the mothers blood. • Substances including drugs can also diffuse through the center. • If the mother and just any harmful substances embryo is also affected. • Pregnant woman who smokes may have premature births and underweight babies. • pregnant women who abuse alcohol especially during early pregnancy may have babies that suffer from fetal alcohol syndrome. • Fetal alcohol syndrome is a group of birth defects which may include facial deformation and severe mental behavioral and physical retardation. • Inner cells of the blastocyst form the three primary tissue layers of the embryo one endoderm to mesoderm and three ectoderms • by the end of the third week blood vessels in the digestive system begin to develop. • The embryo is about 2 mm long. In the first week arm and leg buds form. • By the end of the fourth week all of the major organs begin to form and the heart begins to beat and the embryo has more than doubled its length. • During the second month the final stages of embryonic development takes place. • The arms and legs take shape. • By the end of the second month the embryo is about 25 mm long and weighs about 1 g. • from the end of the eighth week after fertilization until childbirth the developing human is called the fetus. • By the end of the first trimester the sex of the fetus can be distinguished. • During the second trimester from 4 to 6 months the fetus grows to about 15 cm in length. • Bone development continues toenails and fingernails form. The brain develops and begins to control muscle responses. • By the end of the fifth month the fetus begins to move. • The fetus has a fast heartbeat. In the third trimester from 7 to 9 months the fetus gains about 2.5 kg. • The brain and nerves develop. • The fetus can hear smell and see light as well as lungs mature. • The fetus moves into a head down position during the ninth month and becomes less active. • By the end of third trimester the fetus is able to live outside the mothers body. • The fetus leaves the mothers body in a process called labor. • Labor usually lasts several hours. • during labor the walls of the uterus contract and expel the fetus from the uterus. • Further contractions of the uterus expose the placenta and umbilical cord. Ch.15 Sec.3 • molecular biologist spent many years developing tools and methods to manipulating genetic material. • The basic tools of DNA manipulation rely on the chemical nature of genetic material and or adapted from natural processes discovered in cells. • tools include: 1. restriction enzymes 2. polymorphism 3. gel electrophoresis 4. denaturation 5. hybridization. • First GMO‘s was made by using plasmid and enzymes that are naturally present in some bacterial cell. • First tools used to manipulate DNA were enzymes that were made by bacteria as a defense. • Restriction enzyme is an enzyme that cuts double-stranded DNA into fragments by recognizing specific nucleotide sequences and cutting the DNA at those sequences. • Restriction enzymes are useful in two ways: 1. different enzymes recognize different sequences so the enzymes can be used to cut up DNA sample in specific ways 2. the cuts of most restriction enzymes create sticky ends. • Sticky ends have a few bases on one strand that are unpaired but complementary to unpaired bases on other sticky ends. • Sticky ends with easily bind to one another. • DNA carries an electric charge. • Electric current can be used to push or pull DNA fragments. • DNA fragments are forced through a gel a semi solid that allows molecules move slowly through it. • When a current is applied shorted fragments will move faster through the gel then longer fragments well. • Result of electrophoreses is a lane of fragment sorted by size. • if the fragment separates clearly each lane is called a ladder. • If the fragments have overlapping sizes and do not separate clearly each laying is called a smear. • There are many types of electrophoresis. • Different kinds of gels are used to sort different sizes of DNA fragments and other methods are used to sort RNA or proteins. • Newer methods used tiny tubes of gel to sort tiny samples that can be read by a machine and analyzed by computer. • The major methods for working with genes use some combination of basic tools and mechanism of cellular machinery. • Major methods include one PCR to blotting three DNA sequencing for gene recombination. • Polymerase chain reaction other known as pcr process is widely used to clone DNA sequences for further study or manipulation. • Pcr imitates the normal process of DNA replication in cells. • using pcr is as simple as combining the right components in a test tube and then controlling the temperature. • PCR process is called a chain reaction because it is repeated over and over. • Gene recombination and cloning steps: step one: cut DNA samples step two: splice pieces together step three: place into host step four: replicate gene step five: screen for gene. • Cut DNA samples: • two sets of DNA are cut by the same kind of restriction enzyme so that all the fragments have matching sticky ends. • One set of DNA is from an organism containing a specific gene in this case the human insulin gene. • The other DNA is part of a vector such as a virus or a bacterial plasmid that can carry or move DNA between cells. • The vector would be replicated when placed in a host such as bacterial cell. • Splice pieces together: • DNA fragments from the organism are combined with the fragments from the vector. • An enzyme called DNA ligase is added to help bond the sticky ends and all the fragments together. • place into host: • Some plasmids are recombinant with the human DNA. Does meds are placed in a culture of bacteria some cells take up the plasmids. • The cells are allowed to replicate normally. • replicate gene: • each time that a bacterial cell divides its plasmids are copied many times • In a few generations the cells make millions of clones of recombinant plasmids. • Screen for gene: • only some of the bacterial cells contain the recumbent plasma. • One clever way to identify recombinant cells is to use vectors that contain another gene that is easily detected. • Before PCR this process of recombination was the main way to Clone genes for further research. • Another use for a PCR is simply to produce a protein such as insulin from a cloned gene. • Recombinant organisms are created for many applications from agriculture to medicine.