2.6 (Structure of DNA and RNA)

Question 1

Define nucleotide.

Answer 1

A single unit of nucleic acid

Question 2

Define nucleic acid (give examples).

Answer 2

• Very large molecules that are constructed by linking together nucleotides to form a polymer.
• DNA and RNA

Question 3

Outline components of nucleotides.

Answer 3

• Phosphate group:
• Acidic
• Negatively charged
• Nitrogenous base:
• Contains nitrogen
• Has one or two rings in its structure
• Pentose sugar:
• Five carbon atoms
• If the sugar is deoxyribose the polymer is Deoxyribose Nucleic Acid
• If the sugar is ribosome the polymer is Ribose Nucleic Acid

Question 4

What are the nitrogenous bases in DNA and RNA?

Answer 4

• Adenine (A)
• Guanine (G)
• Thymine (T)
• Cytosine (C)

RNA Shares the same bases except that Uracil (U) replaces Thymine

Question 5

How is the genetic code made up?

Answer 5

• Nucleotides a linked into a single strand via a condensation reaction
• Bonds are formed between the phosphate of one nucleotide and the pentose sugar of the next
• The phosphate group (attached to the 5’-C of the sugar) joins with the hydroxyl (OH) group attached to the 3’-C of the sugar
• This results in a phosphodiester bond between the two nucleotides and the formation of a water molecule
• Successive condensation reactions between nucleotides results in the formation of a long single strand

Question 6

Describe the number of strands in DNA and RNA.

Answer 6

DNA: Two anti-parallel, complementary strands form a double helix
RNA: Single stranded, and often, but not always, linear in shape

Question 7

Explain DNA’s anti-parallel double helix structure.

Answer 7

• Each polynucleotide chain (strand) consists of a chain of nucleotides bonded covalently
• Two polynucleotide chains of DNA are held together by hydrogen bonds between complementary base pairs:
• Adenine pairs with thymine (A=T) via two hydrogen bonds
• Guanine pairs with cytosine (G=C) via three hydrogen bonds
• In order for bases to be facing each other and thus able to pair, the two strands must run in opposite directions (i.e. they are anti-parallel)
• As the polynucleotide chain lengthens, the atoms that make up the molecule will arrange themselves in an optimal energy configuration. This position of least resistance results in the double-stranded DNA twisting to form a double helix with approximately 10 - 15 bases per twist

Question 8

Outline Watson and Crick’s experiment.

Answer 8

• Used stick-and-ball models to test their ideas on the possible structure of DNA.
• Allowed them to visualize the molecule and to quickly see how well it fitted the available evidence
• Their first model, a triple helix, was rejected for several reasons:
• Ratio of Adenine to Thymine was not 1:1 (as discovered by Chargaff)
• Required too much magnesium (identified by Franklin)
• From their setbacks they realized:
• DNA must be a double helix
• The relationship between the bases and base pairing
• The strands must be anti-parallel to allow base pairing to happen
• Because of the visual nature of their work the second and the correct model quickly suggested:
• Possible mechanisms for replication
• Information was encoded in triplets of bases