题干

试管婴儿、试管苗和克隆羊三者均属于生物工程技术的杰出成果,下面叙述正确的是(    )

A:都属于无性生殖,能保持母本性状

B:都利用了细胞或组织培养技术

C:都充分体现了体细胞的全能性

D:都不会发生基因重组和变异

上一题 下一题 0.0难度 选择题 更新时间:2018-11-01 09:38:05

答案(点此获取答案解析)

B

同类题2

阅读自然,探索科学奥秘。
神秘的运动常数
①鸟儿在空中飞翔时,翅膀每分钟拍几次?每次幅度有多大?鱼儿在水里游动时,尾巴每分钟摆几回?每回幅度又是多少?这些问题看上去似乎没什么意义,难道空中的飞鸟和水里的游鱼不是完全自由的吗?它们的运动不是完全无拘无束的吗?
②答案真是出乎我们的意料。看上去自由自在、无拘无束的飞行动物和游水动物,它们的运动似乎恰恰遵循着严格的规律。科学家们通过大量的观察和实验,发现飞行动物和游水动物的运动过程中存在着一个神秘的常数,那就是它们的翅膀和尾巴的振动频率、振幅以及运动速度三者之间的关系———振动频率乘以振幅再除以运动速度,计算得到的数值总是落在0.2-0.4之间。
③比如,世界上最小的鸟类———蜂鸟,其双翅展开仅有10厘米左右,飞翔时振幅大约是20厘米。蜂鸟的飞行速度非常快,大约能达到50米/秒。而科学家们以往观察到的蜂鸟飞行时双翅震动频率在50—70赫兹之间,根据上述公式计算出来的蜂鸟的运动常数就落在0.2-0.4之间。
④与蜂鸟形体相差悬殊的海豚也是科学家们观察的一个对象。成年海豚的体长一般在3米左右,在大海里游动时,海豚尾巴摆动的振幅大约是1米,频率每分钟30次左右,游泳速度是每小时100公里。根据公式计算出来的海豚的运动常数是0.3,也落在0.2-0.4之间。
⑤这不仅仅是巧合,研究者们做了大量的试验,发现几乎所有的飞行动物和游水动物的运动机制中都存在这个神秘的常数。事实上,只有当这一运动常数的值处于0.2-0.4之间,动物们才能达到最佳的运动状态。
⑥虽然目前科学家们还不清楚为什么会存在这个运动常数,它的值又为什么恰恰落在0.2-0.4之间,但这一神奇常数似乎像物理学中著名的光速不变原理那样放之四海而皆准,也像光速一样神秘。它能够帮助生物学家们根据动物化石的身体构造,判断出那些早已灭绝了的动物曾经具有怎样的运动速度。甚至,不能排除,这一常数很可能对外星生物也同样适用。很多科学家都表示:“如果在其它星球上真的存在游水生物或飞行生物的话,我们相信,它们的运动也遵循着同样的规律。”
⑦此外,这一常数还能够帮助军方研制出各种高性能的飞行器。比如说,一个翼长15厘米、翼振振幅为10厘米的机械间谍如果要做有效飞行的话,其最佳翼振频率应该是30次/秒左右。目前美国军方已经着手研制类似于飞鸟的有翼飞行器,在配备上微型摄像机之后,这样的机械鸟将可能渗入敌方的任何机密要塞进行刺探。当然,它们最感兴趣的还是能够研制出体积更小,无孔不入的机械昆虫,相信这一天的到来也为时不晚。(据《科技之谜》罗声/编译)

同类题4

阅读短文,根据短文内容,回答问题。

    When it comes to the letter grade on your test or homework, you might notice that there is no letter E. Have you ever thought about why that is so?

    In the A, B, C, D and F grading system, the first four letters are typically considered passing grades. An F in this system simply stands for“fail”.The word“fail”happens to start with the letter F, which seems to leave out the letter E. The fact”.Some schools have U grade for“unsatisfactory”,or I grade for “incomplete”.

    Even with all this said, we should also point out that E grade actually has been used pretty commonly throughout the history of letter grades in the US.

    The first college in the US to use a letter grading system like the ones we use today is Mount Holyoke College. In 1897, they began to use the following grading scale:

A: 95-100%(excellent)       B: 85-94%(good)      C: 76-84%(fair)

D: 75%(barely passed)       E: below 75%(failed)

    We can see the inclusion of E instead of F.

    Gradually,the letter grading system became more popular throughout the US. However, many schools decided to drop the E grade and go straight to F.There is no evidence(证据) to really support this, but one possible explanation is that teachers were worried that some students and parents might mistake E for “excellent”.

    Why do we have letter grades? Well, part of the reason is that they made grading simpler during a time of great change for schools. As the 20th century began,growing cities and an increase in immigration led to larger school classrooms. Most teachers at that time thought this new letter grading system was an easy, fair and clear way to grade students.

    Today, more and more people argue that letter grades don't fully reflect(反映) student learning. However,as teachers try to improve grading methods, many parents continue to favor the letter grades they got as kids, they are familiar(熟悉) and easy for parents to understand. So while they might not be perfect, the letter grades probably aren't going away any time soon.