请教讨论下OG那篇Embryo那篇最后几句话意思？

梦不落

Passage 14

       Nearly a century ago, biologists found that if they

separated an invertebrate animal embryo into two parts

at an early stage of its life, it would survive and develop

as two normal embryos. This led them to believe that the

 (5)  cells in the early embryo are undetermined in the sense

that each cell has the potential to develop in a variety of

different ways. Later biologists found that the situation

was not so simple. It matters in which plane the embryo

is cut. If it is cut in a plane different from the one used

(10)        by the early investigators, it will not form two whole

embryos.

    A debate arose over what exactly was happening.

Which embryo cells are determined, just when do they-

become irreversibly committed to their fates, and what

(15) are the “morphogenetic determinants” that tell a cell

what to become? But the debate could not be resolved

because no one was able to ask the crucial questions

in a form in which they could be pursued productively.

Recent discoveries in molecular biology, however, have

(20)  opened up prospects for a resolution of the debate.

Now investigators think they know at least some of the

molecules that act as morphogenetic determinants in

early development. They have been able o show that,

   in a sense, cell determination begins even before an egg

(25) is fertilized. 

   Studying sea urchins, biologist Paul Gross found

that an unfertilized egg contains substances that func-

tion as morphogenetic determinants. They are located

in the cytoplasm of the egg cell; i.e., in that part of the

(30) cell’s protoplasm that lies outside of the nucleus. In the

unfertilized egg, the substances are inactive and are not

distributed homogeneously. When the egg is fertilized,

the substances become active and, presumably, govern

the behavior of the genes they interact with. Since the

(35) substances are unevenly distributed in the egg, when the

fertilized egg divides, the resulting cells are different

from the start and so can be qualitatively different in

their own gene activity.

       The substances that Gross studied are maternal

(40) messenger RNA’s --products of certain of the maternal

genes. He and other biologists studying a wide variety

of organisms have found that these particular RNA’s

direct, in large part, the synthesis of histones, a class

of proteins that bind to DNA. Once synthesized, the

(45) histones move into the cell nucleus, where section of

DNA wrap around them to form a structure that resem-

bles beads, or knots, on a string. The beads are DNA

segments wrapped around the histones; the string is the

intervening DNA. And it is the structure of these beaded

(50)
                    DNA strings that guides the fate of the cells in which

   they are located.

 

请问绿色部分的是什么意思呢？我的理解是synthesized DNA有三种不同的structure. but what does intervening DNA mean?

而且，这三种structure的DNA只有一种，the beaded DNA controls the fate of the cells. 是这样的吗？

那如果是这样的话，那也就是DNA决定了细胞的命运。可是文中第二段里面的 morphogenetic determinants （红色部分）应该指的是RNA啊，那也就是说RNA决定了cell啊，那这不是自相矛盾吗？

高中学的生物也都忘光了，大家讨论下啊：）

谢谢！

jimmyskean

高中学的生物也都忘光了

gonghao

是这样的以上的内容牵涉到分子生物学的基本内容

比较的复杂，试着解释一下：

DNA在空间的排布上的结构分为很多的层次

第一层：双螺旋结构（唯物主义的地球人一般都知道），A－T、C－G的排列，螺旋成一个双分子链，但不是均匀的分布，因为ATCG在空间结构上有大小的差异，所以我们看到的双分子螺旋结构中有大沟小沟之分。，

第二层：双分子螺旋结构成链后，可自己再次螺旋，就像大家吃的麻花一样，一根麻花看成一个双份子链，并且正因为由大沟小沟之分，使得有些分子能嵌入两个互相螺旋的DNA之间，比如嵌入大沟和大沟之间，（在文中嵌入的就是组蛋白）

第三层：再第二层的基础上再螺旋一次，并且麻花由多个多种的组蛋白包裹连接

第四层：空间的极其复杂的结构

这里说的是第二结构，翻译一下最后一段

The substances that Gross studied are maternal messenger RNA’s --products of certain of the maternal genes.  Gross研究的物质是一种【RNA的信使】（生化语言），这种信使是母体基因的产物

He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA.他和其他的生物学家研究了大量的不同的有机体后发现，这些RNA信使直接引导组蛋白的合成，这种组蛋白是一种嵌入DNA空间结构的一种蛋白。

Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. 这些组蛋白一旦合成后，就会进入细胞核（细胞核里由DNA当然是针对高等动物），在那里组蛋白被DNA包裹，这样的一个包裹结构形成了一种特殊的结构，类似于一种珠子或者是节（类似于绳子打结后的突起），并且排成一列（因为有N多得组蛋白，有N多得包裹结构，通过DNA连接，因为DNA是ATCG连接后得双份子链）

The beads are DNA segments wrapped around the histones（这里解释了上面说得话，“珠子”指前面得包裹结构是NA片断包裹着得组蛋白）; the string is the intervening DNA（排成一列是因为DNA得相互连接，所以看起来是一列，像一条绳子）.

And it is the structure of these beaded DNA strings that guides the fate of the cells in which they are located.（这样得一个通过DNA连接的包裹有组蛋白的空间结构指导了细胞最终的fate）

这里的包裹结构的形成的原因就是：

第二层：双分子螺旋结构成链后，可自己再次螺旋，就像大家吃的麻花一样，一根没有绕起来的麻花看成一个双份子链，并且正因为由大沟小沟之分，使得有些分子能嵌入两个互相螺旋的DNA之间，比如嵌入大沟和大沟之间，（在文中嵌入的就是组蛋白）

最后，高等生物有DNA做为遗传物质，低等的生物如低等的细菌会以RNA作为遗传物质

且DNA单链，RNA的单链可以进行复制的，从而实现遗传物质的遗传

对于高等生物来说，受精的过程分为两步：父亲一方解旋一条单链，母亲一方释放一条单链，着两条单链进行ATCG的排列，尽可能的形成新的DNA双份子链，由于两条链不可能完全重合，有些部分可能无法实现ATCG的连接则可能造成空间上的DNA分子链的局部变构，空间跨越连接，或是进行空间重排，从而使得我们在不可能和父母完全相同，但是大部分相同，无论是宏观的长相还是微观的DNA

但愿我说清楚了，分子生物学那叫一个吐啊

[此贴子已经被作者于2006-5-31 14:03:59编辑过]