1.5.1 DNA + RNA

• Many scientists initially doubted that DNA carried the genetic code due to the relative simplicity of DNA as a chemically simple molecule with few components
• Structure of DNA:

  • Polymer of nucleotides (polynucleotide)
  • Each nucleotide formed from deoxyribose, a phosphate group, and a nitrogen-containing organic base
  • Phosphodiester bonds join adjacent nucleotides
  • 2 polynucleotide chains held together by hydrogen bonds
  • Double helix
• Describe the basic functions of DNA and RNA in all living cells? and draw them;

  DNA Holds genetic information which codes for polypeptides (proteins) RNA Transfers genetic information from DNA to ribosomes There are different types of RNA.

  
• Name the two types of molecule from which a ribosome is made?

  • RNA and proteins

  
• Describe the differences between a DNA nucleotide and an RNA nucleotide

  • Pentose sugar is deoxyribose - DNA
  • Pentose sugar is ribose - RNA
  • Base can be thymine- DNA
  • Base can be uracil - RNA

  
• Describe how nucleotides join together to form polynucleotides;

  ● Condensation reactions, removing water molecules ● Between phosphate group of one nucleotide and deoxyribose/ribose of another ● Forming phosphodiester bonds

  
• Why did many scientists initially doubt that DNA carried the genetic code?

  The relative simplicity of DNA - chemically simple molecule with few components.

  
• Describe the structure of DNA?

  ● Polymer of nucleotides (polynucleotide) ● Each nucleotide formed from deoxyribose, a phosphate group and a nitrogen-containing organic base ● Phosphodiester bonds join adjacent nucleotides ● 2 polynucleotide chains held together by hydrogen bonds ● Between specific complementary base pairs - adenine / thymine and cytosine / guanine ● Double helix

  
• Describe the structure of (messenger) RNA ;

  ● Polymer of nucleotides (polynucleotide) ● Each nucleotide formed from ribose, a phosphate group and a nitrogen-containing organic base ● Bases - uracil, adenine, cytosine, guanine ● Phosphodiester bonds join adjacent nucleotides ● Single helix

  
• Compare and contrast the structure of DNA and (messenger) RNA?

  • Pentose sugar is deoxyribose -DNA
  • Pentose sugar is ribose -RNA
  • Has the base thymine -DNA
  • Has the base uracil -RNA
  • Double stranded / double helix -DNA
  • Single stranded / single helix -RNA
  • Long (many nucleotides) -DNA
  • Shorter (fewer nucleotides) -RNA
  • Has hydrogen bonds / base pairing -DNA
  • Does not-RNA

  
• Suggest how the structure of DNA relates to its functions?

  ● Two strands → both can act as templates for semi-conservative replication

  ● Hydrogen bonds between bases are weak → strands can be separated for replication

  ● Complementary base pairing → accurate replication

  ● Many hydrogen bonds between bases → stable / strong molecule

  ● Double helix with sugar phosphate backbone → protects bases / hydrogen bonds

  ● Long molecule → store lots of genetic information (that codes for polypeptides)

  ● Double helix (coiled) → compact

  
• Suggest how you can use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases

  1. % of adenine in strand 1 = % of thymine in strand 2 (and vice versa)
  2. % of guanine in strand 1 = % of cytosine in strand 2 (and vice versa) Because of specific complementary base pairing between 2 strands

  
• Exam insight: common mistakes ❌-(1/3)

  ❌*Using letters instead of naming bases.*

  ✅Bases should be written in full, as they appear in the specification.

  
• Exam insight: common mistakes ❌-(2/3)

  ❌*Not naming the pentose sugar when describing DNA or RNA nucleotides.*

  ✅DNA nucleotides have the pentose sugar deoxyribose, while RNA nucleotides have the pentose sugar ribose.

  
• Exam insight: common mistakes ❌-(3/3)

  ❌“DNA is strong because of hydrogen bonds.”

  ✅Hydrogen bonds are weak individually, but strong in high numbers. You need to say that there are many hydrogen bonds.