2.6 - Structure of DNA and RNA

Question 1

The nucleic acids DNA and RNA are polymers of nucleotides

—————-Nucleotides consist of three parts

Answer 1

1. Sugar –> five carbon atoms = pentose sugar
2. Phosphate group –> acidic + negatively charged part of nucleic acid
3. Base –> nitrogen + either one or two rings of atoms in its structure

Question 2

how are nucleotides linked together into a chain

Answer 2

• To link nucleotides together into a chain or polymer, covalent bonds are formed between the phosphate of one nucleotide and the pentose sugar of the next nucleotide
• This creates a strong backbone for the molecule of alternating sugar and phosphate groups, with a base linked to each sugar
• There are four different bases in both DNA and RNA = 4 different nucleotides
• The nucleotides can be linked together in any sequence because the phosphate and sugar used to link them are the same in every nucleotide

Question 3

There are three important differences between the two types of nucleic acid

Answer 3

1. The sugar within DNA is deoxyribose and the sugar within RNA is ribose
2. There are usually two polymers of nucleotides in DNA but only one in RNA
3. There are four bases in DNA = adenine, cytosine, guanine, thymine –> In RNA the four bases = adenine, cytosine, guanine, uracil

Question 4

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

Answer 4

• Each strand consists of a chain of nucleotides linked by covalent bonds
• The two strands are parallel but run in opposite directions so they are said to be antiparallel. One strand is oriented in the direction 5’ to 3’ and the other is oriented in the direction 3’ to 5’
• The two strands are wound together to form a double helix
• The strands are held together by hydrogen bonds between the bases –> A pairs with T and G pairs with C –> this is known as complementary base pairing –> AT and GC complement each other by forming base pairs

Question 5

Application: Crick and Watsons elucidation of the structure of DNA using model-making

Answer 5

• Their success was based on using the evidence to develop possible structures of DNA and testing them by model making
• Their first model was a triple helix  bases on the outside of the molecule and magnesium holding the two strands together in ionic bonds to the phosphate group on each strand
• The helical structure and spacing between subunits in the helix fitted the X-ray diffraction pattern obtained by Rosalind Franklin
• The pieces didn’t fit together and it was pointed out that there was not enough magnesium to form cross links between the strands
• The first model also did not take into account evidence that amount of adenine equals amount of thymine and amount of cytosine equals the amount of guanine
• By creating a physical model for DNA Benson and Crick tried to create the structure with accurate pieces
• The finished structure was accurate and immediately suggested a mechanism for copying DNA