كتاب الاحياء حادي عشر متقدم فصل اول امارات 2026 pdf

Calculated Risks Genetic counselors, who advise people about potential genetic disorders, may hold several types of positions. Some genetic counselors work one-on-one with families, and others work in genetic testing labs, interpreting test results and writing reports for doctors and patients. The goal of genetic counselors is to provide people with information so that they can make informed decisions about their healthcare. Genetic Counseling People who are concerned about their risk of having a genetic disorder or who are considering getting tested for a genetic disorder often consult with genetic counselors. The counselors ask clients about their family history of a disease, disorder, or condition. They provide information about genetic disorders and the types of tests that can detect them, including the tests' accuracy. Genetic counselors discuss with clients the pros and cons of being tested for a disorder. The advantages include being able to take action to protect one's health. The disadvantages include the possibility of a false positive or an inconclusive test.

FOCUS QUESTION Which experiments led to the discovery of DNA, and which led to the structure of DNA? Discovery of DNA Once Mendel's work was rediscovered in the 1900s, scientists began to search for the molecule involved in inheritance. Scientists knew that genetic information was carried on the chromosomes in eukaryotic cells, and that the two main components of chromosomes are DNA and protein. For many years, scientists tried to determine which of these macromolecules-nucleic acid (DNA) or proteins-was the source of genetic information. Griffith The first major experiment that led to the discovery of DNA as the genetic material was performed by Frederick Griffith in 1928, Griffith studied two strains of the bacteria Streptococcus pneumoniae. He found that one strain could be transformed, or changed, into the other form. Of the two strains he studied, one had a sugar coat and one did not. Both strains are shown in Figure 1.

Unwinding DNA helicase, an enzyme, unwinds and unzips the double helix. The enzyme breaks the hydrogen bonds between the base pairs and the double helix unzips leaving single strands of DNA. Then, proteins called single-stranded binding proteins associate with the DNA to keep the strands separate. As the helix unwinds, another enzyme, RNA primase, adds a short segment of RNA, called an RNA primer, on each DNA strand. Base pairing The enzyme DNA polymerase catalyzes the addition of nucleotides to the new DNA strand. The nucleotides are added to the 3' end of the new strand, as illustrated in Figure 10. DNA polymerase continues adding nucleotides to the chain by adding to the 3' end of the new DNA strand. Recall that each base binds only to its complement-A binds to Tand C binds to G. In this way, the templates allow identical copies of the original double-stranded DNA to be produced. Notice in Figure 10 that the two strands are made in a slightly different manner. The leading strand is elongated as the DNA unwinds. This strand is built continuously by the addition of nucleotides to the 3' end in the 5' to 3' direction. The lagging strand elongates away from the replication fork. It is synthesized discontinuously into small segments, called Okazaki fragments, by DNA polymerase in the 3' to 5' direction. These fragments are later connected by the enzyme DNA ligase. Each Okazaki fragment is about 100-200 nucleotides long in eukaryotes. Because one strand is synthesized continuously and the other is synthesized discontinuously, DNA replication is semidiscontinuous as well as semiconservative.