Molecular mechanisms of crossing over

Mol
ecul
arMechani
sm ofCr
ossi
ngOv
er:

Therear
etwoimpor
tantt
heor
ies:
1.Copychoi
cet
heor
yand

2.Breakageandr
euni
ont
heor
ytoexpl
aint
hemechani
sm of
crossi
ngov er
.

i
.CopyChoi ceTheory:
ThistheorywasproposedbyBell
ing.Thi stheorystatesthatt
he
enti
rerecombinantsect
ionorpartarisesfrom thenewl y
synthesi
sedsecti
on.Thenon-si
sterchr omat i
dswhencomei ncl
ose
contacttheycopysomesecti
onofeachot herresult
ingin
recombinati
on.Accordi
ngtothi
st heory,physicalexchangeof
preformedchromati
dsdoesnottakepl ace.

Thenon-sisterchromatidswhencomet ogetherduri
ngpair i
ng,
copy
partofeachot her.Thus, r
ecombi
nantchr omosomeorchr omati
ds
havesomeal l
elesofonechr omatidsandsomeofot her
.The
i
nformationmaybecopi edbyonestrandorbot hthestr
ands.When
onl
yonest randcopi es,non-r
eci
procalrecombinantisproduced.




Ifcopyprocessinvol
vesbothstrandsofchromosomes, r
ecipr
ocal
recombinantsareproduced.Assume,therear
etwochr omosomes,
viz.
,ABandab.Whent hei
rchromatidscomeinclosecontactthey
copyeachot herandresulti
nAbandaBr e-
combinat
ionsbesides
parent
alcombi nat
ions.