DNA replication & repair

DNA structure: nucleotides are the

building blocks of nucleic acids

DNA structure: each nucleotide consists of

a 5-carbon sugar (deoxyribose in DNA), a phosphate group, and a nitrogen-containing base

structure of DNA: DNA (&RNA) are formed from

nucleotides that are linked together through a phosphodiesterase backbone in a linear direction

DNA structure: nitrogen bases

Adenine (A) & guanine (G) are purines; thymine (T) & cytosine (C) are pyrimidines

DNA structure: the amount of adenine always equals the

amount of thymine; the amount of guanine always equals the amount of cytosine

DNA structure: base pairing

two kinds ofphase pairings formalong the length of the double-stranded DNA molecule: A-T & G-C; each species has its own unique sequence of base pairs in its DNA

DNA structure: the structure proposed by Watson & crick represents the B form of DNA which is the most common conformation, the B form has 10 base pairs per turn

if relative humidity surrounding the DNA is reduced to 75%, the molecule assumes the A form which has 11 base pairs per turn

DNA structure: both the A & B forms are right-handed →

the helix turns clockwise

DNA structure: living cells also contain a small proportion of Z-DNA, which forms a

left-handed helix, the exact function of Z-DNA is still unknown

DNA replication is

bidirectional, each strand forms a daughter strand → asymmetrical replication fork

one new DNA strand is synthesized continuously using primase

called the leading strand

the other new strand is synthesized discontinuously using primase and okazaki fragments

called the lagging strand

the leading and lagging strands are due to only

being able to add to the 3’ end

DNA damage: ultraviolet (UV) radiation cross-links adjacent pyrimidines,

this creates dimers and interrupts base pairing

DNA damage: gamma rays and x-rays ionize the molecules surrounding DNA,

this creates free radicals that can alter bases or break strands

light repair: most organisms have a photoreactivation pathways, catalyzed by the enzyme DNA photolyase

this enzyme is used to reverse the effects of UV exposure; placental mammals, including humans, do not have a photoreactivation pathway

excision repair: nucleotide excision repair (NER) is used to reduce the severity of

helix distorting DNA damage, including pyrimidine dimer formation

excision repair: (NER): the damaged fragment is cleaved by

excinucleases; DNA polymerase fills in the gap; DNA ligase seals the DNA

excision repair: base excision repair (BER) involves the

recognition & removal by DNA glycosylase; DNA polymerase fills in the gap

double-strand break repair: DNA ends attract a protein called Ku (a two-polypeptide dimer), which

protects the endsfrom degradation until end-joining is complete

double-strand break repair

Ku recruits DNA protein kinase (DNA-PK) to align the broken ends; phosphorylation of DNA-PK activates DNA helicase

hershey & chase experiment: labeled bacteriophage DNA & protein with 32P & 35S, respectively in 1952

the pages attached to bacteria & injected their genes; host cells were found to contain 32P-labeled DNA