BIOC 2300 Lecture Notes - Lecture 4: The Double Helix, Oswald Avery, Gc-Content
3 May 2018
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4 – Nucleic Acids: the Central Dogma
January 11th, 2016
• First NS that demonstrated that DNA is the genetic material in cells: Oswald Avery
Complementary nature of DNA – the basis of life
• The “central dogma”
• DNA replication/synthesisDNARNA synthesis/transcriptionRNAprotein
synthesisamino acids
DNA: a quick guide:
• DNA consists of two strands of nucleotide polymers
• each nucleotide consists of a base, sugar (deoxyribose) and phosphodiester linkage
• four types of bases: A and G are purines, C and T are pyrimidines
• DNA strands are anti-parallel: running in opposite directions from the 5’ to 3’ end (by
convention)
• A pairs with T (or U in RNA), while G pairs with C, forming 2 and 3 hydrogen bonds,
respectively
• melting (denaturation) temperature of DNA increases with GC content
Nucleotides: a closer look…
• the monomeric repeating unit of nucleic acids is a nucleotide, made up from:
o a nitrogenous aromatic base, either a purine (adenine, guanine) or pyrimidine
(cytosine, thymine or uracil)
o a pentose sugar: ribose in RNA and 2’-deoxyribose in DNA
o phosphates attached to the 5’ and/or 3’ hydroxyl groups of the sugar
• to maintain the helix width, a purine (2 ring) always base pairs with a pyrimidine (1 ring)
• base pair specificity arises from the number of H bonds
The DNA double helix
• most common form consists of 2 strands of the nucleotide polymers with base pairs in the
middle, stacked perpendicular to the helix axis
• note the polar exterior and non-polar interior, plus major and minor grooves
• the pitch of the helix is 3.4nm (or 10 base pairs) one complete turn
• The two strands are antiparallel and conventionally read in the 5’ to 3’ direction
DNA Denaturation and Renaturation :
• The double helix is stabilized by H bonds, Mg2++ binding to PO4 backbone and by the
hydrophobic effect
• The denaturation (or melting T) temperature of DNA increases with the GC content – due
to the number of H bonds
• This can be reversible renaturation
• *If cooled to fast mis-matched
If cooled slowly correct refolding
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
January 11th, 2016: first ns that demonstrated that dna is the genetic material in cells: oswald avery. Complementary nature of dna the basis of life: the central dogma , dna replication/synthesis dna rna synthesis/transcription rna protein synthesis amino acids. If cooled slowly correct refolding: conditions can often be established to allow hybridization of perfectly complementary olgionucleotides (short, single stranded nucleotide polymer) while those with a single mis-matched base pair remain single stranded rna/rna probes. *if dna is laid out it would be 2m in length. Genome structure: the nucleosome is the basic structural unit of chromatin, condensation is due to basic histone proteins interacting with an acidic dna backbone, histone modifications help regulate packing and gene expressions (epigenetics) Short region of dna beads on a string (chromatin) 30nm chromatin fibre of packed nucleosomes section of chromosome condensed section of chromosome mitotic chromosome.
