DNA

• what are rna and dna?


  - important information carrying molecules

  - DNA holds genetic information

  - RNA transfers genetic information from DNA to ribosomes
• What are the components of a nucleotide?


  - phosphate group

  - pentose sugar

  - nitrogenous/nitrogen containing organic base
• what are the organic bases in DNA?


  Adenine

  Thymine

  Guanine

  Cytosine
• what are the organic bases in rna?


  Adenine

  Uracil

  Cytosine

  Guanine
• polynucleotide formation


  - nucleotides join together by condensation which forms a phosphodiester bond between the phosphate group of one one nucleotide and the pentose sugar of another

  - this is a covalent bond and makes the sugar phosphate backbone of the nucleic acid stable and strong

  - a DNA molecule is a double helix structure which consists of 2 polynucleotide chains held together by hydrogen bonding between specific complementary base pairings

  - adenine always pairs with thymine

  - cytosine always pairs with guanine
• what is the basis of the genetic code?

  the sequence of the bases along the nucleotide chain is variable
• what is the function of DNA?

  codes for amino acids (3 bases = 1 amino acid) so long strands of DNA code for polypeptide chains and proteins
• what is RNA?


  - a relatively short polynucleotide chain which is normally a single stranded helix

  - used to transfer genetic information from DNA to ribosomes
• sugar-phosphate backbone and double helix - function


  - provides strength and stability

  - protects bases and hydrogen bonding between bases
• long molecule - function


  can store a lot of information
• helix structure - function


  compact
• base sequence - function


  codes for amino acids and therefore proteins
• double stranded - function


  allows semi conservative replication, as each strand can act as a template
• complementary base pairing - function


  allows accurate replication
• hydrogen bonds between bases are weak - function


  hydrogen bonds can be broken so allows strands to separate for replication
• many hydrogen bonds in the whole molecule - function


  strong and stable molecule
• process of dna replication

  stage 1: dna helicase breaks hydrogen bonds between complementary bases in the polynucleotide strands which cause the DNA helix to unwind

  
  

  stage 2: free new dna nucleotides are attracted to the exposed bases on the template strands. both strands act as templates. hydrogen bonds form between complementary bases

  
  

  stage 3: dna polymerase joins the adjacent nucleotides together via a condensation reaction to form phosphodiester bonds
• why is dna replication described as semi conservative?


  - Each strand in the original molecule acts as a template

  - Each new dna molecule contains one strand from the original molecule and one new strand
• why does dna polymerase run in opposite directions?

  - dna strands are antiparallel

  - in dna replication, the active site of dna polymerase is only complementary to the 3' end due to the different shape of nucleotides

  - this means that dna polymerase runs in opposite directions along the two template strands