2.6 Structure of DNA and RNA

Question 1

What are the two types of nucleic acids present in cells?

Answer 1

1. DNA: deoxyribonucleic acid
   - stores the genetic blueprint for cells
   - double stranded (forms a double helix
   - bases involved are adenine, thymine, guanine and cytosine
   - deoxyribose pentose sugar
2. RNA: ribonucleic acid
   - single stranded
   - bases involved are adenine, uracil, guanine and cytosine
   (thymine has been replaced by uracil)
   - ribose pentose sugar

Question 2

Nucleic acids (DNA and RNA) and polymers of…

Answer 2

Nucleotides

Question 3

Nucleotides are composed of…

Answer 3

• 5-carbon pentose sugar - forms a stable backbone
• phosphate group
• nitrogenous base - carries the code

Question 4

Bonds in a polymer/nucleotide chain

Answer 4

To create a polymer/polynucleotide chain, the nucleotide units link together through a phosphodiester bond (a covalent bond) between the sugar of a nucleotide (lower left corner) and the phosphate of another nucleotide by a condensation reaction

Question 5

What are the complementary bases in DNA

Answer 5

• Adenine and Thymine → 2 hydrogen bonds
• Guanine and Cytosine → 3 hydrogen bonds

Question 6

Strands and bonds to form DNA

Answer 6

DNA is a double helix molecule made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

Question 7

Watson and Crick DNA models

Answer 7

Watson and Crick assembled a DNA model using the following key experimental discoveries:
- DNA is composed of nucleotides made up of a sugar, phosphate and base – Phoebus Levene, 1919
- DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) – Erwin Chargaff, 1950
- DNA is organized into a helical structure (X-ray) – Rosalind Franklin, 1953 (data shared without permission)

Watson and Crick decided to combine what was known about chemical content of DNA with information from X-ray diffraction studies. They used model making which allowed them to make several arrangements of scale model until found best one that fitted all the data