2.3 Nucleic Acids

Question 1

Describe the structure of a nucleotide

Answer 1

A nucleotide consists of:

• phosphate
• base (adenine, thymine, cytosine, guanine)
• deoxyribose sugar

Question 2

How do nucleotides join together to make DNA?

Answer 2

• condensation reaction joins nucleotides together

* phosphodiester bond (joined by the phosphate)

Question 3

What is the difference between RNA and DNA?

Answer 3

•RNA has nucleotides that contain a ribose sugar
-DNA has a pentose sugar called deoxyribose

Both have a phosphate group and a base attached

•RNA uracil (a pyrimidine) replaces thymine

•RNA is made up of a single polynucleotide chain
-DNA is made up of 2 polynucleotide chains (each made up of lots of nucleotides joined together)

Question 4

What is the difference between purines and pyrimidines?

Answer 4

Purine base:
Contains 2 carbon-nitrogen rings joined together

Pyrimidine base:
Contains 1 carbon-nitrogen ring
Is smaller than a purine base

Question 5

Describe the synthesis of polynucleotides

Answer 5

• nucleotides join up to form polynucleotides (forming a phosphodiester bond
• chain of sugars and phosphates is known as the sugar-phosphate backbone

Question 6

Describe the breakdown of polynucleotides

Answer 6

•they can be broken down into nucleotides again by breaking the phosphodiester bond

Question 7

Describe the structure of ADP

Answer 7

•ADP contains the base adenine, the sugar ribose and 2 phosphate groups

Question 8

Describe the structure of ATP

Answer 8

• ATP contains the base adenine, the sugar ribose and 3 phosphate groups
• Provides energy for chemical reactions in the cell
• Synthesised from ADP + inorganic phosphate
• Energy is stored in the phosphate bond
• Can be broken back down into ADP + inorganic phosphate when energy is released

Question 9

Describe semi-conservative replication

Answer 9

• Parent/original DNA unwinds - catalysed by DNA helicase, which breaks hydrogen bonds and DNA gyrase, which unwinds the DNA and prevents supercoiling
• Strands separate and bases become exposed, both strands act as a template
• Free nucleotides with complementary bases are attracted to exposed bases. This is catalysed by DNA polymerase
• Hydrogen bonds form between bases. DNA polymerase and DNA ligase join nucleotides to form new strands.
• Phosphodiester bonds form between nucleotides. Hydrogen bonds reform. Sugar-phosphate backbone reforms
• Results in 2 new daughter strands, each containing one parent/original strand