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45 Cards in this Set

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  • Back
How do we know if something is a mutagen?
Amen test
what is ames test
mimics what happens when animals are exposed to chemicals

- bacteria are exposed to chemicals with mamilliam liver enzyme

ames test procedure
salmonella typhimurium

- these bacteria have various mutations that they cannot synthetize histidine

- plated on medium lacking histine and then they count number of mutations after exposure to chemicals

number of revertants from His- to HIS +
a compound is mutagenic if INCREASE in reversion rate
Direct repair of DNA damage
1. identify and reverse damage

- proofreading of DNA pol

repair of UV-induced photoproducts
pyrimidine dimers can be repaired by:

photoreactive repair

photoreactive repair
enzyme: photolyase uses visible light to break bonds between pyrimidine dimers

which gene encodes photolyase?
gene phr (photoreactive repair) gene
repair damage done by alkylating agents
enzymes remove added groups restoring base to original structure

- ex: o6-methyguanine --> guanine by enzyme

--> methyltransferase

--> enzyme is deactivated after one process

nucleotides can be removed and replaced if they cannot be repaired
double strand acts as template
UV repair
a short segment surrounding DNA photoproduct can be removed

which genes encode the enzymes to reapir UV repair
uvr-A, uvr-B, uvr-C, uvr-
process of UV repair
2 uvr-A + 1 uvr-B bind to strand opposite photoproduct

* uvr-A leaves

uvr-C joins uvr-B to form a cleaving complex that cleaves phosphodiester bonds on either side of photoproduct

uvr-D(helicase) + dna pol 1 bind and fill in missing nucleotides

nucleotide exicision repair
dna glycolysase remove modified purine base leaving an apurinic site

- AP nuclease removes the remainder of the nucleotide

- dna pol + dna ligase fill in gap

dna damage signalling system
active throughout cell cycle - looking for damage
key molecules in damage signalling system
ATM (kinase) phosphorylates p53 repair pathway
p53 controls
1. pause of cell cycle by G1-S

2. apoptosis

what are levels of p53 in healthy cell
they are low
what happens to p53 when a damage is notified
the levels increases and two possible pathways:
two pathways of p53
1. pause in cell cycle to repair damage

--> when p5 increases due to increase in ATM then initiates cell pause in G1 becayse p21 increases and inhibits the formation of cyclin-CDK

?? what does this do?
2. apoptosis and p53
p53 activates transcription of BAX gene which encodes slow acting inhibitor BCL2

- BCL2 represses apoptosis

but when damage cell:

-- the formation of BAX causes BCl2 to bind to BAX and therefore allows the cell to go to apoptosis

inhibits apoptosis

when in presence of BAX gene it binds to it and therefore cell goes to apoptosis

cyclin-CDK inhibitor
mutation in DNA damage repair genes
Li-fraumeni syndrome - mutation in p53 repair pathway
homologs and sister chromatids do not separate properly
normal number of chromosome pairs
abnormal number of chromosome pairs
nondisjuction in germ-line cells

anueploid gametes:

one daughter cell (n+1)

one daughter cell (n-1)

fusion of these with normal gametes (anueploid zygotes)

(2n+1) trisomic

(2n-1) monosomic

nondisjunction in meiosis II

failure of sister chromatids

of 4 gametes produced:

2 are normal

1 (n+1) --> + n = zygote (2n+1) trisomic

1 (n-1) --> + n = zygote (2n-1) monosomic

gene balance
change in gene dosage leads to imbalance in gene products

- most animals are highly sensitive

- plants are not affected

anueploidy in humans
autosomal chromosomes

only trisomics 13,18,21 are observed

monosmies: not observied

multiple sex-chromosome anueploids are observed

both trisomies and monosmies survive on sex-chromosome

trisomies of autosomal chromosomes

trisomies and monosomies of sex chromosomes
47 xxy47 xyy47 xxx45 xo
autosomal trisomy 13
autsomal trisomy 18

autosomal trisomy 21
down syndrome
trisomy 21 - down syndrome
a small number of genes on chromosome 21 are linked to abnormalities



down syndrome critical region

where gene: DYRK known to produce similiar symptoms in mice

sex trisomies and monosomies
47 xxy (kla?)

47 xyy (jacob syndrome)

47 xxx (triple x?)

45 xo (turner syndrome)

turner syndrome
45 XO

SHOX gene is insufficient to direct normal development

haploinsufficiency of this gene plays a central role in producing the symptoms of this syndrome

in trisomies
meiosis results in two chromosomes going to one pole

1 chromosome going to other pole

-- half gametes will be normal half will be aneuploidy

-- those that are aneuoploidy are unlikely to survive --> semiinfertility of aneuiploidy

result of mitotic nondisjunction early in embryo

- turner syndrome causes occur in females that are mosaic

some 45XO, some 46 XX

some have 47 XXX

how does mosaicism occur
46 xx zygote - mitosis : 46 + 46

46a = normal

46b = mitotic nondisjunction --? 45 + 47