Biological Molecules: Nucleic Acids - Structure of DNA & RNA

Question 1

Role of DNA & RNA

Answer 1

• Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules

• DNA is used to store genetic information
• RNA transfers genetic information from DNA to the ribosomes
• Ribosomes are formed from RNA and proteins

Question 2

Nucleotide Structure

Answer 2

• Both DNA and RNA are polymers of nucleotides
• Each nucleotide is formed from:
• a pentose (sugar with 5 carbon atoms)
• a nitrogen-containing organic base
• a phosphate group

Question 3

Formation & Structure of Polynucleotides

Answer 3

• Many nucleotides join together to form polynucleotide strands

• Nucleotides join together via a condensation reaction between the phosphate group of a nucleotide and the sugar group of another nucleotide
- This forms a phosphodiester bond (consisting of the phosphate group and two ester bonds)

• The chain of sugar and phosphate is called the sugar-phosphate backbone

Question 4

DNA Structure: Double Helix

Answer 4

• A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs
- DNA molecules are really long and coiled up very tightly, so a lot of genetic information can fit into a small space in the cell nucleus

Question 5

DNA Structure: Nucleotide

Answer 5

• The components of a DNA nucleotide are:

• deoxyribose
• a phosphate group
• one of the organic bases adenine, cytosine, guanine or thymine

Question 6

DNA Structure: Complementary Base Pairing

Answer 6

• Two DNA polynucleotide strands join together by hydrogen bonds between the bases
• Each base can only join with one particular partner - this is called complementary base pairing
• Adenine (A) pairs with thymine (T)
• Held together by 2 hydrogen bonds
• Guanine (G) pairs with cytosine (C)
• Held together by 3 hydrogen bonds

• The size of the bases - a larger purine must always pair with a smaller pyrimidine

• Purines have two carbon rings: adenine and guanine
• Pyrimidines have one carbon ring: thymine and cytosine

Question 7

DNA Structure: Antiparallel Chains

Answer 7

• The two polynucleotide strands in DNA run in opposite directions to each other - they are antiparallel

• In DNA, the strands run in opposite directions, creating one 5’ to 3’ strand and one 3’ to 5’ strand
- The directions are named according to the position number of the carbon atoms in the pentose sugar of each nucleotide

• In a mononucleotide, carbon five is bonded to the phosphate group and carbon three has a hydroxyl group
• However, in a polynucleotide, carbon three is instead bonded to the phosphate group of the next nucleotide, creating a 5’ to 3’ strand

Question 8

How is the structure of DNA suited to its function?

Answer 8

• Made of polynucleotides
- Large molecule to store a lot of genetic information

• Polynucleotides held together by weak hydrogen bonds between the bases

• Each hydrogen bond is weak so the polynucleotide chains can be easily separated for replication and transcription
• High number of hydrogen bonds, collectively they stabilise the helix

• Double helix

• Genetic code of nitrogenous bases protected in the centre
• Each strand can be used as a template for replication

• Sugar phosphate backbone joined by phosphodiester bonds

• Strong bonds, don’t break which reduces likelihood of mutations
• Adds stability to the double helix

• Purine base always base pairs with pyrimidine base
- Means 2 strands are parallel but run in opposite directions (antiparallel)

Question 9

RNA Structure: (Poly)nucleotide

Answer 9

• An RNA molecule is a relatively short polynucleotide chain

• RNA is composed of only one polynucleotide chain
• It is single-stranded

• The components of an RNA nucleotide are:

• ribose
• a phosphate group
• one of the organic bases adenine, cytosine, guanine or uracil
• adenine pairs with uracil

Question 10

Comparing DNA & RNA

Answer 10

• Shape

• DNA: Double stranded
• RNA: Single stranded

• Pentose sugar

• DNA: Deoxyribose
• RNA: Ribose

• Bases

• DNA: Adenine, guanine, cytosine, thymine
• RNA: Adenine, guanine, cytosine, uracil

• Size

• DNA: Long
• RNA: Short

Question 11

Watson & Crick

Answer 11

• In the 1800s, scientists doubted that DNA could carry genetic code because it has a relatively simple chemical composition
• By 1953, experiments showed that DNA carried genetic code
• The double-helical structure of DNA was first proposed by Watson and Crick