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Lesson 6.2 Introduction to the chapter

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Introduction to the lesson

 All of life is dependent on two molecules: DNA and RNA. In this lesson we will begin to study the structure of these two molecules. Understanding their structure is important for understanding their functions which will be the focus of the rest of the chapter.

Chemical factories of the future

Read the section “Chemical factories of the future” in your Biology course book on p.111.

Thinking it through

  • How do you respond to the genetic engineering described in this article?
  • What ethical issues, if any, do you think could be raised with regard to this type of genetic engineering?
  • What advantages could there be in this type of genetic engineering?

Introduction to genetic control

Read the untitled section on p.111 of your text book.

Thinking it through

  • What are the two essential features for the “genetic molecule”?
  • If you are interested, you may want to research how DNA was discovered.
  • Do some research into the differences and similarities between DNA and proteins.

The structure of DNA and RNA

Read the section “The Structure of DNA and RNA” on p.111-113 of your text book. Take note that DNA and RNA are macromolecules – made up of smaller molecules called nucleotides.

Nucleotides

  1. Study Figure 6.2 on p.112 of the text book.
    1. Notice that a nucleotide is made up of three parts:
      1. a nitrogenous base (a.k.a. a nitrogen-containing base)---these are cytosine, adenine, guanine, thymine (in DNA), and uracil (in RNA).
      2. a pentose sugar (5-carbon sugar)
      3. a phosphate group
  2. Study Figure 6.3 also on p.112 showing the chemical structure of the different parts of the nucleic acid.
    1. Notice the phosphate group is always the same.
    2. Look at the difference between the ribose and the deoxyribose sugars. As its name suggests, deoxyribose sugar has one less oxygen atom than ribose sugar---hence, the structural difference between deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
    3. Look at the bases – notice that adenine and guanine are double ringed structures and are therefore classified as purine bases, while thymine, uracil and cytosine are single ringed bases and are classified as pyrimidine bases. [Note: you are not expected to know the chemical structure of the bases but you must know which bases are purine bases and which are pyrimidine bases and what the main difference between the two groups is.]

Learning Activity 1

Answer the following questions in your exercise book.

  1. Draw sketch of a nucleotide showing the three basic parts.
  2. List the four nitrogen-containing bases found in DNA.
  3. List the four nitrogen-containing bases found in RNA.
  4. What is the difference in the nitrogen-containing bases of DNA and RNA?
  5. Notice the shapes of the bases drawn in Fig. 6.2 (p.112). Copy these shapes and the names of the bases associated with each shape. You should notice that the shapes are complementary -- thymine/uracile match with adenine, while cytosine matches with guanine (T/U-A, C-G). Explain the difference in the pentose sugar in DNA and RNA.
  6. Draw simple sketch, using the same symbols as are used in Fig. 6.2, of the following strand of DNA (don't worry about joining each base pair to its complementary pair; you are only drawing one strand): AUG AGA GAA CGA UAA

ATP

Learning Activity 2

Answer the following questions in your exercise book. Refer to Fig. 6.4 on p.114 of your text book.

  1. Which organelle that you learned about in chapter one produces ATP?
  2. In what ways is ATP similar to DNA and RNA?
  3. In what ways is ATP different from DNA and RNA?
  4. What is the difference between AMP, ADP and ATP?

Polynucleotides

  1. Note that there is a covalent sugar-phosphate bond (phosphodiester bond) linking the 5-carbon of one sugar molecule to the 3-carbon of the next. Therefore, the first DNA/RNA strand is always 5'3' and the complementary strand is always 3'5'.
  2. Notice that in Fig. 6.5b the two polynucleotides run in opposite directions.
  3. Note that the nucleotides in the DNA/RNA molecules are joined by the bases.
  4. The bases are held together by a hydrogen bond. Notice in the diagram in Fig. 6.5b that there is a double hydrogen bond between A and T but a triple hydrogen bond between C and G. (The hydrogen bonds are indicated by the dotted lines joining the bases). This is important to remember as it is a simple way to distinguish an A-T/U bond from a C-G bond even if you are not given the names of the bases.
  5. Again notice the complementary base pairing – A always binds with T (in DNA and with U in RNA) and C always binds with G.

Learning Activity 3

Answer the following questions in your exercise book. Refer to Fig. 6.5 on p.114 of your text book.

  1. What is the role of the phosphate and the pentose sugar in the polynucleotide?
  2. Copy Fig. 6.5b in your exercise book. Make sure that you label each strand 5'3' and 3'5'.

Definitions and review

Definitions

  1. DNA
  2. RNA
  3. Nucleotides
  4. Polynucleotide

Mindmap

Start a new mindmap for chapter six and add this section to it.