2.6 Structure of DNA and RNA

Question 1

What are nucleic acids made of?

Answer 1

Lots of nucleotides linked together.

Question 2

What are nucleotides made of?

Answer 2

• A pentose sugar (5 carbons)
• A nitrogenous base
• A phosphate group - acidic, negatively charged part

The base and the phosphate group are covalently linked to the sugar.

Question 3

How are nucleotides linked together?

Answer 3

Covalent bonds form between the phosphate of one nucleotide and the pentose sugar of the next molecule This creates a strong backbone for the molecule.

Question 4

How do nucleic acids store genetic information?

Answer 4

There are 4 different nitrogenous bases and therefore 4 different nucleotides, the order of these nucleotides determines the order of amino acids.

Question 5

What are the differences between RNA and DNA?

Answer 5

• The sugar in DNA is deoxyribose and the sugar in RNA is ribose. DNA has one fewer oxygen.
• DNA is double stranded and RNA is single stranded
• The four bases in DNA are: cytosine, guanine, adenine, and thymine. Whereas RNA has uracil instead of thiamine.

Question 6

What are the four bases?

Answer 6

• Guanine
• Adenine
• Cytosin
• Thymine (RNA = Uracil)

Question 7

How do we draw DNA molecules?

Answer 7

• Circles for phosphates
• Pentagons for sugars
• Squares for bases

Question 8

What is the structure of DNA?

Answer 8

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

• Each strand consists of nucleotides linked by covalent bonds
• The two strands are parallel but run in opposite directions.
• The strands are held together by hydrogen bonds between the complementary pairs of nitrogenous bases.

Question 9

What were the flaws in Crick and Watson’s models of DNA structure?

Answer 9

Their first model consisted of a triple helix, with bases on the outside of the molecule and magnesium holding the two strands together with ionic bonds to the phosphate on each nucleotide.
It was difficult to get all parts of this model to fit together satisfactorily and it was rejected when Franklin pointed out there would not be enough magnesium available to form the cross links between the strands. It also did not fit the fact that there were equal amount of adenine and thymine.

They then worked out that the base pairs were equal in length and therefore would fit between two phosphate backbones. Then to make the bases fit they had to be antiparallel.