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USMLE biochemistry


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Chromatin struture
substance that forms chromosomes. DNA(-) looped around histones."beads of strings"
Heterochromatin Vs. Euchromatin
Hetero-condensed, inactive
Eu- less condensed, active
"truly transcribed"
Nucleotide: Purine
A&G (G has KEYtone) with 2 rings,
"PURe As Gold"
Nucleotide: Pyrimidine
C,U,T(THYmine has a meTHYL)
"CUT the PY"
Transition vs. Transversion
TransItion substitute Identical type, transVerion conVersion b/w types
Mutations in DNA
silent-same aa
missense- different aa
nonsense-early stop codon
Prokaryotic DNA replication
single origin of replication,
continuous syn. on leading strand, discont. syn. on laging strand (Okazaki fragments).
DNA polymerase III
elongates chain by adding deoxynulceotides 5' to 3'
DNA polymerase I
degrades RNA primer
Eukaryotic DNA polymerase
have seperate polymerases for specific functions.
Eukaryotic genome
has several origins of replication, AT base pair sequences
DNA Repair: Single stranded
glycosylase recognizes and removes damaged base. Endonuclease breaks bases at 5' side. Exonuclease removes nucleotides. DNA polymerase fills gap. DNA ligase seals
DNA repair defects
Xeroderma pigmentosum=UVlight
Bloom's syndrome= radiation
Fanconi's anemia=cross linking agents
Xeroderma pigmentosum
defective excision repair, inability to repair thymidine dimers. Sx: dry skin, melanoma
DNA/RNA/Protein synthesis direction
5' to 3'
largest, made by RNA polymerase II
most abundant, syn. by RNA polymerase I
smallest, syn. by RNA polymerase III
RNA polymerase II
opens DNA at promotor site. inhibited by alpha amanitin in death cap mushrooms

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