Lesson Eight Assignments

Chapter Nine

Thinking Through the Concepts1. Draw the general structure of a nucleotide. Which parts are identical in all nucleotides, and which can vary?The phosphate and the sugar called deoxyribose remain the same, and the nitrogen-containing base varies form (A) adenine, (G) guanine, (T) thymine, or (C) cytosine.

2. Name the four types of nitrogen-containing bases found in DNA. (A) Adenine, (G) guanine, (T) thymine, and (C) cytosine.

3. Which bases are complementary to one another? How are they held together in the double helix of DNA?Adenine is complementary of thymine, and guanine is complementary of cytosine. The two DNA strands are held together by hydrogen bonds that form between the protruding bases of the individual DNA strands.

4. Describe the structure of DNA. Where are the bases, sugars, and phosphates in the structure?DNA is in the shape of a double helix and has four nitrogen bases, (A) adenine, (G) guanine, (T) thymine, and (C) cytosine. The backbone of DNA is made up of phosphates and sugars. Attached to the backbone are the bases. The number of hydrogen bonds depends on the type of base (pyrimidine and purine).

5. Describe the process of DNA replication. First, DNA helicase separate the parental DNA strands. Then, DNA polymerase synthesises new DNA strands, and finally, segments of DNA are joined together by DNA ligase, an enzyme that ties DNA together.

6. How do mutations occur? Describe the principal types of mutations. In most cells, mutations are minimised by highly accurate DNA replication, proofreading the newly synthesized DNA, and repairing any changes in DNA that may occur even when the DNA is not being replicated. Mutations usually happen when bases are mismatched.

Applying the Concepts3. Today, scientific advances are being made at an astounding rate, and nowhere is this more evident than in our understanding of the biology of heredity. Using DNA as a starting point, do you believe that there are limits to the knowledge that people should acquire? Defend your answer. I dont really think that there are limits as to what we can do with our DNA. It will just take a very long time to get there. A few hundred years down the line, something that we think as weird or atrocious may seem as normal to people in the future as bread is to us.

Chapter Ten

Thinking Through the Concepts1. How does RNA differ from DNA?RNA is similar to DNA, but differs structurally in three respects: (1) RNA is normally single-stranded; (2) RNA has the sugar ribose (instead of deoxyribose) in its backbone; and (3) RNA has the base uracil instead of the base thymine found in DNA.

2. What are the three types of RNA? What is the function of each?There is messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). The base sequence of mRNA carries the information for the amino acid sequence of a protein. rRNA combines with proteins to form ribosomes. The small subunit binds mRNA. The large subunit binds tRNA and catalyses peptide bond formation between amino acids during protein synthesis. Each tRNA carries a specific amino acid to a ribosome during protein synthesis. The anticodon of tRNA pairs with a codon incorporated into the protein. DNA is transcribed into mRNA, then translated into polypeptides where each codon has a complementary anticodon on the tRNA.

3. Define the following terms: genetic code; codon; anticodon. What is the relationship among the bases in DNA, the codons, of mRNA, and the anticodons of tRNA?The genetic code translates the sequence of bases in nucleic acids into the sequence of amino acids in proteins. If a genetic code was deciphered by using these artificial mRNAs, it is usually written in terms of base triplets in mRNA (rather than DNA) that code for each amino acid. These mRNA triplets are called codons. The ability of tRNA to deliver the proper amino acid depends on specific base pairing between tRNA and mRNA. Each tRNA has three exposed bases called the anticodon, which form base pairs with the mRNA codon.

4. How is mRNA formed from a eukaryotic gene?The DNA of a eukaryotic cell is housed in the nucleus, but protein synthesis occurs on ribosomes in the cytoplasm. Therefore, DNA cannot directly guide protein synthesis. There must be an intermediary, a molecule that carries the information from DNA in the nucleus to the ribosomes in the cytoplasm. This molecule is RNA. 5. Diagram and describe protein synthesisDuring transcription, the nucleotide sequence in a gene specifies the nucleotide sequence in a complementary RNA molecule. For protein-encoding genes, the product is an mRNA molecule that exits from the nucleus and enters the cytoplasm. During translation, the sequence is an mRNA molecule specifies the amino acid sequence in a protein.

6. Explain how complementary base pairing is involved in both transcription and translation. In transcription, mRNA is formed by pairing complementary bases with a section of DNA. In translation, tRNA pairs complementary bases (anticodons) to mRNA (codons).

7. Describe some mechanisms of gene regulation.Cells can control the frequency at which an individual gene is transcribed. The same gene may be used to produce different mRNAs and protein products. Cells may control the stability and translation of messenger RNAs. Proteins may require modification before they can carry out their functions. The life span of a protein can be regulated.

8. Define mutation. Are most mutations likely to be beneficial or harmful? Explain your answer. Mutation is a change in the base sequence of DNA in a gene; normally refers to a genetic change significant enough to alter the appearance or function of the organism. The mutations in an organism depend on how the mutation affects the function of the protein encoded by a mutated gene.

Applying the Concepts1. As you have learned in this chapter, many factors influence gene expression, including hormones. The use of anabolic steroids and growth hormones among athletes has created controversy in recent years. Hormones certainly affect gene expression, but in the broadest sense, so do vitamins and foods. What do you think are appropriate guidelines for the use of hormones? Should athletes take steroids or growth hormones? Should children at risk of being unusually short be given growth hormones? Should parents be allowed to request growth hormones for a child of normal height in the hope of producing a future basketball player? I think that people should only take steroids or hormones if it is only completely necessary, for example: health reasons. As for athletes and parents who want their child taller for him or her to become a basketball player, they should not take steroids or hormones at all. Sports should allow athletes only because of pure talent.