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Western University
Biochemistry 2280A

Topic 18:  Purine = 2 ring structure, A and G, attach to c1 via N9 position  Pyrimidine = 1 ring structure, C and T, attach via the N1 position  CG has 3 hydrogen bonds, AT has only 2  The anomeric carbon of ribose is in the B-configuration when attached to the nitrogenous base, and are joined in an N-glycosidic bond  Phosphates attach to the C5 by a phosphoester linkage, make the DNA negative  5’ 3’  The double helix is stabilized by base stacking and base pairing  DNA is 2nm wide and can be very long. The distance between bases is 0.34nm Topic 19:  Chromatin is a complex of DNA and its organizing proteins  Histones have fewer than 200 amino acids, and have many lysine and arginine (which are positively charged), and so interacts well with the negative DNA  DNA wraps around a cluster of 8 histones to make the nucleosome core particle. The DNA makes 1.7 circuits around the histone core (147 base pairs) o Nucleosomes compact the DNA by a factor of 3  The nucleosomes can pack into a 30nm-chromatin fibre, and is stabilized in a ‘zig-zag’ by histone H1. The DNA has been compacted by a factor of 100  The 30nm chromatin fibres form large loops of 30000-200000 base pairs Topic 22-24:  DNA RNA  protein, gene expression  To express the genetic information stored in DNA, the nucleotide sequence of a gene is transcribed into RNA o Transcription is catalyzed by RNA polymerase  RNA differs from DNA, as it contains ribose, and uracil, single-stranded, and can fold into 3D shapes  Cells can make mRNA, rRNA (ribosomes), and tRNA  Transcription begins at DNA sites called promoters. To initiate, RNA polymerases require transcription factors at the promoter in eucaryotes, only needs a sigma factor in bacteria  Most genes are composed of exons with Introns, and during transcription, both introns and exons are transcribed  Introns are removed via RNA splicing, catalyzed by snRNPs, and exons are joined  mRNAs go through processing before they leave the nucleus, capping and addition of a poly-A- tail, this occurs as splicing and transcription are happening, and then complete mRNAs are transported to the cytoplasm  translation takes place in the cytoplasm on a ribosome  the nucleotide sequence in mRNA is read out in sets of codons, each codon representing one amino acid  64 possible codons: 4x4x4, for 20 amino acids, so many amino acids are specified by more than one codon (‘wobble’)  Aminoacyl tRNA synthetases link amino acids to their appropriate tRNAs. Each tRNA has an anticodon, which will match a codon in the mRNA  Protein synthesis begins when a ribosome assembles at the start codon AUG. Synthesis stops when it reaches a stop codon (UAA, UAG, or UGA)  The linking of amino acids into the
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