Nucleic acids

DNA

• carries genetic code in all living organisms

• certain viruses have RNA as their genetic material, but they are not considered living

nucleotide components

• DNA and RNA are made of repeat units called nucleotides

• nucleotide contains

  • pentose sugar

  • nitrogenous base

    • adenine, thymine (uracil in RNA), guanine, cytosine

    • purines: adenine and guanine

    • pyrimidines: cytosine, thymine, uracil

  • phosphate group

• phosphate group and nitrogenous base are covalently bonded pentose sugar

• atoms in nucleotide numbered from right in clockwise direction

linking nucleotides

• phosphate group of one nucleotide forms covalent bond to pentose sugar of next one, forming polynucleotide

  • this happens by condensation reaction, one water molecule released for each bond formed

• this forms sugar-phosphate backbone

RNA structure

• single stranded polynucleotide with ribose as pentose sugar

• OH on 2C

• adjacent RNA nucleotides linked by condensation reaction

  • forms phosphodiester bond between pentose sugar of one nucleotide and phosphate group of another

DNA structure

• double helix of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs

• H on 2C

• each DNA polynucleotide has a 3’ end and a 5’ end

  • because they are anti parallel, one is the 5’ to 3’ strand and the other is the 3’ to 5’ strand

• nitrogenous base of each nucleotide project out from backbone towards interior of double stranded DNA

hydrogen bonding

• hydrogen bonding occurs between same pairs of bases:

  • A & T = 2 hydrogen bonds between them

  • G & C = 3 hydrogen bonds between them

• this is complementary base pairing

genetic code

• DNA molecules carry genetic code as sequence of nitrogenous bases in nucleotides

  • they can combine in very diverse sequences of different lengths

  • enormous capacity for storing genetic data with great economy

    • number of genes contained in even most simplistic forms of life is massive

• one strand of DNA molecule carries the base sequence that will be read by enzymes, this is the coding strand

• sequence of bases forming genes on coding strand determine order of amino acids in proteins that are synthesised

conservation of genetic code

• genetic code is universal

• the same triplet codes code for the same amino acids in all living things

• this provides evidence for universal common ancestor

• many coding and non-coding sequences remain unchanged over time despite mutations, these are conserved sequences

  • similarity in these sequences indicate shared universal ancestry

directionality of DNA/RNA

• 5'‘ to 3’ directionality

• important to ensure that genetic code is copied, transcribed, and translated correctly

purine to pyrimidine bonding

• base pairings A-T and C-G are equal in length, so DNA helix has same 3D structure regardless of base sequence

• stability of double helix is increase by H-bonds between complementary base pairs

nucleosomes

• eukaryotic DNA is associated with histone proteins

• histone packages DNA into nucleosomes

  • nucleosomes consist of a strand of DNA coiled around 8 histone proteins

  • DNA takes 2 turns around histone core, held in place by additional histone protein which is attached to linker DNA

  • nucleosomes help to protect DNA

Hershey & chase experiment

• proved that DNA is the heritable material, not protein

• how it happened:

  • DNA contains phosphorus, not sulfur

  • amino acids (proteins) contain sulfur, not phosphorus

  • Bacteria grown in separate media containing radioactive sulfur or radioactive phosphorus were infected with viruses

  • offspring viruses contained either S labelled proteins or P labelled DNA

  • unlabelled bacteria were then infected separately with either type of virus

    • bacteria should then contain the heritable material after infection

  • blender was used to remove attached viruses from bacteria

    • viruses remained in the supernatant, bacteria formed a pellet

  • only bacteria infected by the P-labelled viruses were shown to be radioactive

  • DNA was transferred to the bacteria

NOS: technological developments open up new possibilities for experiments

• when radioisotopes were made available to scientists as research tools, the Hershey–Chase experiment became possible

chargaff’s data NOS

• problem of induction

  • general conclusions being drawn from specific observations

  • it’s impossible to prove a hypothesis generated by inductive reasoning as absolutely true, we cannot be sure that general observations made in the past will be true in the future

• falsification of existing hypotheses

  • scientist discovered pentose sugars and suggested that structure of nucleic acid was a tetranucleotide

  • Chargaff falsified this

Chargaffs discovery

• purine bases = pyrimidine bases

• adenine bases = thymine bases

• guanine bases = cytosine bases