One thing I have never been able to understand about recombination... if a segment of DNA crosses over to another homolog why wouldn't there be a bunch of mismatched DNA bases, such as A thymine across from a thymine or a guanine across from a guanine etc.. I don't understand why the crossover regions aren't completely ruined segments of DNA... can anyone please answer this?.. does DNA polymerase I edit crossover regions to match up the bases or something?
@CommonSense90 This is due to the segment of DNA that crosses over being homogolous pieces of DNA. Sure there are mismatches, but its not two random pieces of DNA doing this. The majority is the same.
@CommanderQ commanderQ your answer is wrong when there'd be mismatches then dna helix wouldn't remain stable and if there's no mismatch then there's no variation.
@CommanderQ yeah but that mismatched base would then easily be broken and if majority of bases (eg 20 in 1 ) remains same then no variation would occur. there must be any further molecular mechanism behind this fact i m gonna ask this question to my teacher tomorrow.
@tarascotti There is a reason evolution takes such a long time. If you had a high number of variations, then it is entirely possible you'll end up repressing a functional protein, or simply stopping it from working. The mismatch would be resolved when DNA polymerase copies the two strands of DNA. The two template strands would create two different copied strands that differ where the mismatches were before.
@tarascotti The mismatched base would not be broken as you put it, as it is covalently linked to its neighbours by the phosphate backbone. The bonding between base pairs is simply hydrogen bonding. It takes about 12 kCal to disassociate a hydrogen bond, and about 400 kCal to disassociate a covalent one.
@CommanderQ it will remain attached to sugar and phosphate backbone but the bonding between nitrogenous bases will be easily broken and this doesn't happen with normal chromosomes if it were to happen in every crossing over then all chromosomes would remain broken at certain places. i hope you got it.
@tarascotti Despite there being specific DNA repair mechanisms for mismatched nucleotides, the very nature of DNA not matching exactly and then being replicated to produce two different copies is what makes evolution possible.
I suggest you take a look at Wikipedia's DNA mismatch repair page.
@CommonSense90 you are right i also dont understand it and also when two non sister chromatids exchange their dna segments they get paired with new strands it means it is correct base pairing as prior to crossover that results in no genetic variation. but still how do alleles shuffle ?? we need to find out this answer.
It's only the Holliday model, in which DNA forms a Holliday junction (that implies a heteroduplex) and to solve, to cut the Holliday junction you can cut in a vertical or horizontal way, that's the last image. However, nowadays Holliday's model is no longer accepted, the accepted model now is double-strand break model (DSB model or also called Szostak model).
Nice video, clearly explained, but explains old model for recombination. The new model is basically the same, but starts with a double stranded break, then 5' degradation. THen strand invasion and DNA synthesis.
Liked the animation of the holiday junction resolution- makes the whole thing easier to visualise.
they're both right. It depends on the second nick whether you geht patch recombination (lower) oder recombination with exchange DNA-rests (upper). If the nick is done in the exchanged strands there will be only a small piece of exchanged DNA in both doublestrands (patch) and if it is nicked in the other parental strands you come to the upper result.
@Onadragonfly in two homologous chromosomes the genes A and B (with their respective alleles or mutations a and b) got mixed up or moved from different chromosomes but in the other two chromosomes these genes remained in their original place
hmm, this looks left handed to me. If I picture looking down the axis of the DNA strand and move forward in either direction, the strands rotate leftwise, or counter clockwise. Good catch by MDGonDemand... We can just say this is Z-DNA =)
Take your left hand, point your thumb in the direction that you're scanning along the DNA, and notice how your other fingers wrap in the direction that the backbone is twisting (counterclockwise as you're scanning away from yourself). That should convince you that it is left handed.
It's a pretty weird process, if the base pairs don't match, there is a repair system that kicks in called the "mismatch repair mechanism". It will replace the bases that don't fit. And yep, one is from mom, the other from dad.
some great inforamtion here thanks
prchecker 1 month ago
this is 1/3 models of recombination meselson something model I think...
FixerNK 2 months ago
exonuclease, not endo
vick8989 8 months ago
One thing I have never been able to understand about recombination... if a segment of DNA crosses over to another homolog why wouldn't there be a bunch of mismatched DNA bases, such as A thymine across from a thymine or a guanine across from a guanine etc.. I don't understand why the crossover regions aren't completely ruined segments of DNA... can anyone please answer this?.. does DNA polymerase I edit crossover regions to match up the bases or something?
CommonSense90 9 months ago
@CommonSense90 This is due to the segment of DNA that crosses over being homogolous pieces of DNA. Sure there are mismatches, but its not two random pieces of DNA doing this. The majority is the same.
CommanderQ 8 months ago
@CommanderQ commanderQ your answer is wrong when there'd be mismatches then dna helix wouldn't remain stable and if there's no mismatch then there's no variation.
tarascotti 4 months ago
@tarascotti If you have one in 20 base pairs different, that is not going to make the helix become unstable. The majority is still similar.
CommanderQ 4 months ago
@CommanderQ yeah but that mismatched base would then easily be broken and if majority of bases (eg 20 in 1 ) remains same then no variation would occur. there must be any further molecular mechanism behind this fact i m gonna ask this question to my teacher tomorrow.
tarascotti 4 months ago
@tarascotti There is a reason evolution takes such a long time. If you had a high number of variations, then it is entirely possible you'll end up repressing a functional protein, or simply stopping it from working. The mismatch would be resolved when DNA polymerase copies the two strands of DNA. The two template strands would create two different copied strands that differ where the mismatches were before.
CommanderQ 4 months ago
@tarascotti The mismatched base would not be broken as you put it, as it is covalently linked to its neighbours by the phosphate backbone. The bonding between base pairs is simply hydrogen bonding. It takes about 12 kCal to disassociate a hydrogen bond, and about 400 kCal to disassociate a covalent one.
CommanderQ 4 months ago
@CommanderQ it will remain attached to sugar and phosphate backbone but the bonding between nitrogenous bases will be easily broken and this doesn't happen with normal chromosomes if it were to happen in every crossing over then all chromosomes would remain broken at certain places. i hope you got it.
tarascotti 4 months ago
@tarascotti Despite there being specific DNA repair mechanisms for mismatched nucleotides, the very nature of DNA not matching exactly and then being replicated to produce two different copies is what makes evolution possible.
I suggest you take a look at Wikipedia's DNA mismatch repair page.
CommanderQ 4 months ago
@CommonSense90 you are right i also dont understand it and also when two non sister chromatids exchange their dna segments they get paired with new strands it means it is correct base pairing as prior to crossover that results in no genetic variation. but still how do alleles shuffle ?? we need to find out this answer.
tarascotti 4 months ago
This has been flagged as spam show
It's only the Holliday model, in which DNA forms a Holliday junction (that implies a heteroduplex) and to solve, to cut the Holliday junction you can cut in a vertical or horizontal way, that's the last image. However, nowadays Holliday's model is no longer accepted, the accepted model now is double-strand break model (DSB model or also called Szostak model).
Sorry if my English is as good as it should be.
mjamutemia 1 year ago
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mjamutemia 1 year ago
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mjamutemia 1 year ago
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mjamutemia 1 year ago
Nice video, clearly explained, but explains old model for recombination. The new model is basically the same, but starts with a double stranded break, then 5' degradation. THen strand invasion and DNA synthesis.
Liked the animation of the holiday junction resolution- makes the whole thing easier to visualise.
LordYon13 1 year ago
@LordYon13 , can you explain me that? are there 2 explanations for the same thing? which is right and wich is wrong...or are the both right?
i've heard about that one in wich the double strand break and there's 5' degradation...but im little bit confused right now...
thanks
PhyloidealCreations 1 year ago
@PhyloidealCreations
they're both right. It depends on the second nick whether you geht patch recombination (lower) oder recombination with exchange DNA-rests (upper). If the nick is done in the exchanged strands there will be only a small piece of exchanged DNA in both doublestrands (patch) and if it is nicked in the other parental strands you come to the upper result.
I hope I could help you.
greez from Germany
DrAMoKK 11 months ago
i mean, there are 2 didatic or academic explanations to this...or there are 2 scientific model theories racing to explain it??
PhyloidealCreations 1 year ago
neato
gtyxgh 1 year ago
very helplful video
myrymics 1 year ago
What's the difference between the 2 recombinant DNA in the end?
Onadragonfly 2 years ago
@Onadragonfly in two homologous chromosomes the genes A and B (with their respective alleles or mutations a and b) got mixed up or moved from different chromosomes but in the other two chromosomes these genes remained in their original place
Yamanekoman 1 year ago
Great video!
7dmatto 2 years ago
exelent
talkaudaykiran 2 years ago
No, it's right handed. It's correct.
SuperLeandroBros 2 years ago
hmm, this looks left handed to me. If I picture looking down the axis of the DNA strand and move forward in either direction, the strands rotate leftwise, or counter clockwise. Good catch by MDGonDemand... We can just say this is Z-DNA =)
mattchuuu1 2 years ago
Lol, i still seeing it right handed.
SuperLeandroBros 2 years ago
Take your left hand, point your thumb in the direction that you're scanning along the DNA, and notice how your other fingers wrap in the direction that the backbone is twisting (counterclockwise as you're scanning away from yourself). That should convince you that it is left handed.
Sc0t0ma 2 years ago
Excellent video, except... the DNA is all left handed and it should be right handed lol
MDGonDemand 2 years ago
wow. i thought this was a music video. goddamn title was the same as my song. if i understand wat was happening i would be stoked. yay science.
Kdawgbling666 2 years ago
is it normal that that much base peirs mach?
if not how can they combine like that?
is one of them from father and one of them from mother?
kamyarghofrani 3 years ago
It's a pretty weird process, if the base pairs don't match, there is a repair system that kicks in called the "mismatch repair mechanism". It will replace the bases that don't fit. And yep, one is from mom, the other from dad.
MDGonDemand 2 years ago
thank you very much.
kamyarghofrani 2 years ago
if its like this, shoudnt both DNAs be the same thing?
kamyarghofrani 3 years ago
GREAT!!! THANK YOU.. I was exactly looking for this. Pretty clear and illustrative.
lhtd 3 years ago
you don't have to watch it, if it's so boring! this is made for students in biology or medecine so don't rate something you don't care about.
erev4n 3 years ago 2
This comment has received too many negative votes show
So boring....
DocCallihan 4 years ago