10.04听写 自己的材料 颜色不知道为什么又弄不上去了
As we learn more about the DNA in human cells and how it controls the growth and development of cellsThen maybe we can explain a very important observationThat when we try to grow most human cells in libratory, they seem programmed to divide only a certain number of times before they dieNow this differs with the type of cell, some cells like Nerve cells only divide 7 to 9 times in their total lifeOthers like skin cells will divide many, many more timesBut finally, the cells stop renewing themselves and they dieAnd in the cells of the human body itself, in the cells of every organ of almost every type of tissues in the body, the same thing will happen eventuallyOK, you know that all of persons genetic information is contained on very long pieces of DNA, called Chromosomes46 of them are in the human cells that's 23 pairs of these chromosomes are of very lengths and sizesNow if you look at this rough drawing of one of them, one chromosomes is about to divide into 2You see that it sort of looks like, well actually it’s much more complex than this but it reminds us a couple of springs linked together to coil up pieces of DNA And If you stretch them out, you will find they contain certain genes certain sequences of DNA that help to determine how the cells of the body will developWhen researchers look really carefully at the DNA in Chromosomes though, they were amazed we all were to find that only a fraction of it, maybe 20-30 percents converts into meaningful genetic informationIt's incredible at least it was to me, but if you took away all the DNA that codes for genesYou still have maybe 70% of the DNA left over that's the so called junk DNA, though the word junk is used sort of townies and cheekThe assumption is that even these DNA doesn't make up any of the genes, it must serve some other purposeAnyway, if we examine these ends of these coils of DNA, we will find a sequence of DNA at each end of every human Chromosomes, called a telomereNow a telomere is a highly repetitious and genetically meaningless sequence of DNA, what we were calling junk DNA.But it does have any important purpose, it's sort of like the plastic tip on each end of shoelaceIt means not help you tie your shoe but that little plastic tip keeps the rest of the shoelace, the shoe string from unraveling into weak and useless threadsWell telomere at the end of Chromosomes seems to do about the same thing------ protect the genes the genetically functional parts of the Chromosomes from being damagedEvery time the Chromosome divides, every time one cell divides into 2Pieces of the ends of the Chromosome, the telomeres, get broken offSo after each division, the telomere gets shorterAnd one of the things that may happen after a while is that pieces of the genes themselves get broken off the ChromosomesSo the Chromosome is now losing important genetically information and is no longer functionalBut as long as the telomeres are at certain length, they keep this from happeningSo it seems that when the, by looking at the length of the telomeres on specific Chromosomes, we can actually predict pretty much how long certain cells can successfully go on dividingOther some other cells just seems to keep on dividing regardless, which mean not be always a good thing if it gets out of controlBut when we analyze the cells chemically, we find something very interesting, a chemical in them, and an enzyme called telomeraseAs bits of the telomere break off from the end of Chromosome, this chemical, this telomerase can rebuild it, can help resemble the protected DNA, the telomere that Chromosome is lostSomeday we may be able to take any cell and keep it alive functioning and reproducing itself essentially forever through the use of telomeraseAnd in the future, we may have virtually immortal nerve cells and immortal skin cells of whatever because of these chemical, telomerase can keep the telomere on the ends of Chromosomes from getting any shorter作者: maxx0108 时间: 2011-10-5 00:20:38
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As we learn more about the DNA in humancells and how it controls the growth and development of cells
Then maybe we can explain a very importantobservation
That when we try to grow most human cells in libratory,they seem programmed to divideonly a certain number of times before they die
Now this differs with the type of cell, some cells like Nerve cells only divide7 to 9 times in their total life
Others like skin cells will divide many, many more times
But finally, thecells stop renewing themselves and they die
And in the cells of the human body itself, in the cells of every organ of almostevery type of tissuesin the body, the same thing will happen eventually
OK, you know that all of persons geneticinformation is contained on very long pieces ofDNA, called Chromosomes
46 of them arein the human cellsthat's 23 pairs of these chromosomes are of very lengths andsizes
Now if you look atthis rough drawing of one of them, one chromosomesis about to divide into 2
You see that it sortof looks like, well actually it’s much more complex thanthis but it reminds us a couple ofsprings linked together to coil up pieces of DNA
And If you stretch them out, you will find they contain certain genescertain sequences of DNA that help to determine how the cells of the body will develop
When researchers look really carefully at the DNA in Chromosomes though, they were amazed we all wereto find that only a fraction of it, maybe 20-30 percents converts intomeaningful genetic information
It's incredible at least it was to me, butif you took away all the DNA that codes for genes
You still have maybe 70% of the DNA left over that'sthe so called junk DNA, though the word junkis used sort of towniesand cheek
The assumption is that even these DNAdoesn't make up any of the genes, it mustserve some other purpose
Anyway, if we examine these ends of these coils of DNA, we will find a sequence of DNAat each end of every human Chromosomes, called a telomere
Now a telomereis a highly repetitious and genetically meaningless sequence of DNA, what we were calling junk DNA.
But it does have anyimportant purpose, it's sort of like theplastic tip on each end of shoelace
It means nothelp you tie your shoe but that little plastic tip keepsthe rest of the shoelace, the shoestring from unraveling into weak and uselessthreads
Well telomere at the end of Chromosomes seems to do about the same thing------protect the genes the genetically functional partsof the Chromosomes from being damaged
Every time the Chromosomedivides, every time one cell divides into 2
Pieces of theends of the Chromosome, the telomeres, get broken off
So after each division, the telomere getsshorter
And one of the thingsthat may happen after a while is that pieces of thegenes themselves get broken off the Chromosomes
So the Chromosomeis now losing important genetically informationand is no longer functional
But as long as thetelomeres are at certain length, they keep this from happening
So it seems that when the, by looking at thelength of the telomeres on specific Chromosomes,
wecan actually predict pretty much how long certaincells
can successfully go ondividing
Other some other cells just seems to keep on dividing regardless, whichmean not be always a good thing if it getsout of control
But when we analyzethe cells chemically, we find something veryinteresting, a chemical in them, and an enzyme calledtelomerase
As bits of the telomere break off fromthe end of Chromosome, this chemical,this telomerase can rebuildit, can help resemble the protected DNA, the telomere that Chromosomeis lost
Someday we may be able to take any cell andkeep it alive functioning and reproducing itself essentially forever through the use of telomerase
And in the future,we may have virtually immortal nerve cells and immortal skin cells ofwhatever because of these chemical, telomerase can keep thetelomere on the ends of Chromosomesfrom getting any shorter