2.1.3 - Nucleic acids and nucleotides

• DNA is made of the pentose sugar deoxyribose

  rna is made of the pentose sugar ribose
• Polynucleotide strands are formed from condensation reactions between nucleotides to form strong phosphodiester bonds and make the sugar phosphate backbone.
• DNA is a double helix of 2 deoxyribose polynucleotide strands (2 sugar phosphate backbones)

  hydrogen bonds form between complementary base pairs on strnds that run antiparallel.
• The two purine bases are adenine and guanine which are two ringed molecules.
• The three pyrimidine bases are thymine, cytosine and uracil and they are one ring molecules.
• DNA replication is semi conservative because strands from original DNA molecules act as templates.

  New DNA molecule contains 1 old strand and 1 new strand. Specific base pairing enables genetic material to be conserved accurately.
• New strands are formed during semi-conservative replication.

  1. free nucleotides attach to exposed bases by complementary base pairing
  2. dna polymerase joins adjacent nucleotides on new strand in 5’ to 3’ direction through condensation reactions to form phosphodiester bonds
  3. hydrogen bonds re form
• Genetic code is degenerate - more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)
• Genetic code is universal - same bases and sequences used by all species
• A gene determines the sequence of amino acids as it consists of base triplets that code for specific amino acids
• DNA can be purified by precipitation by adding ethanol and a salt to an aqueous solution. Nucleic acids precipitate out of solution. Centrifuge to obtain pellet of nucleic acid. Wash pellet with ethanol and centrifuge again.
• Transcription produces mrna and it occurs in the nucleus
• The process of transcription

  1. RNA polymerase binds to start region on a gene
  2. section of DNA uncoils into 2 strands with exposed bases. Antisense strand acts as a template
  3. free nucleotides are attracted to their complementary bases
  4. rna polymerase joins adjacent nucleotides by phosphodiester bonds
• Translation produces an amino acid chain and it occurs in cytoplasm or on ribosomes
• Translation :

  1. ribosome moves along mrna until ’start’ codon
  2. trna anticodon attaches to complementary bases on mrna
  3. condensation reactions between amino acids on trna form peptide bonds
  4. process continues to form polypeptide chain until ’stop’ codon is reached
• Adenosine triphosphate has three inorganic phosphate groups attached to a ribose sugar which is attached to an adenine base
• A mutation Is an alteration to the DNA base sequence. Mutations often arise spontaneously during DNA replication.
• Genetic code is non overlapping - each triplet is only read once
• In DNA, two antiparallel DNA strands twist round eachother to form a double helix
• DNA forms a right handed helix, with about 10. Nucleotide pairs for each turn
• In the condensation reaction between nucleotides H is lost from either C3 or C5 on sugar and the OH is lost from the phosphate group
• ATP is used for energy as small amounts of energy are used to break off the end phosphaste but it releases lots of energy
• Complementary bases occur in equal proportions

  so if 33% of the dna bases is A; then 33% of the bases are T
• DNA replicates during interphase - the S phase
• In the meselson and stahl experiment:

  it showed DNA replication as semi conservative because the percentage of DNA that had N15 base decreased and the percentage of dna that had the N14 base increased (semi) but there were always 2 strands of N15 DNA (showing its ‘conservative’ nature)
• A gene is a section of DNA that codes for the sequence of amino acids in a polypeptide
• Alleles are different versions of the same gene
• Shape and behaviour of proteins depends on the sequence of amino acids.
• Genetic code is always read from the template strand - antisense strand
• Practical investigation to purify dna:

  1. crush piece of fruit using pestle and mortar
  2. add detergent (washing up liquid)
  3. add protease enzyme - found in pineapple
  4. filter to remove solids
  5. add salt to help dna clump together
  6. pour cold alcohol into mixture - dna is insoluble in alcohol so will float to top
• Crushing fruit in DNA practical helps to break down the cellulose cell wall
• Adding detergent in DNA practical helps dissolve the cell surface / plasma membrane
• Adding protease enzyme in DNA investigation breaks down histone proteins associated with DNA.
• In transcription:

  the gene coding for a protein is exposed by DNA helicase catalysing the splitting of hydrogen bonds holding bases together.

  Free rna nucleotides form a complementary strand by lining up base pairs of antisense strand

  rna polymerase helps form phosphodiester bonds in between RNA nucleotides
• After mRNA leaves the nucleus through a nuclear pore and the DNA reforms, translation occurs
• Adenosine diphosphate has two inorganic phosphate groups attached to a ribose sugar which is attached to an adenine base