[主题活动] 【CASK EFFECT】0910F阅读全方位锻炼--越障【SCI】 1-2 [复制链接]

草木也知愁
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发表于 2009-7-11 13:36:18 |显示全部楼层

【CASK EFFECT】0910G阅读能力基础自测（速度、难度、深度、越障、真题、RAM）
https://bbs.gter.net/forum.php?mod=viewthread&tid=910464&highlight

【CASK EFFECT】0910G阅读全方位锻炼--难度【LSAT】汇总贴
https://bbs.gter.net/thread-982016-1-1.html

【CASK EFFECT】0910G阅读全方位锻炼--速度【CET】汇总贴
https://bbs.gter.net/thread-982018-1-1.html

【CASK EFFECT】0910F阅读全方位锻炼--越障【SCI】汇总贴
https://bbs.gter.net/thread-982020-1-1.html

【CASK EFFECT】0910G阅读全方位锻炼--真题【GRE】（后期推出）

【CASK EFFECT】0910G阅读全方位锻炼--深度【FICTION】（后期推出）

【CASK EFFECT】0910F阅读全方位锻炼--RAM 汇总贴（后期推出）

本贴为“越障”训练的汇总贴

规则很简单：

我每天贴出1000字左右的一篇文字（从我平时看的书或者paper里摘的）

没有别的要求，只要大家坚持读完就可以

如果你能坚持一个月，你会发现自己的阅读进化了~

[注]
1、直接在电脑屏幕面前做，虽然GRE阅读是在纸上考，但是这个过程会遏制你做笔记，同时给你的阅读造成视觉障碍，也就是把难度训练和抗干扰训练同步结合，增加效率（初期会很累，但是既然大家想要成为高手，那么就别对自己太温柔）
2、不用苛求速度，看完即可

The algorithm USCHANGE in chapter 2 is an example of a greedy strategy: at
each step, the cashier would only consider the largest denomination smaller
than (or equal to) M. Since the goal was to minimize the number of coins returned
to the customer, this seemed like a sensible strategy: youwould never
use five nickels in place of one quarter. A generalization of USCHANGE, BETTERCHANGE
also usedwhat seemed like the best option and did not consider
any others, which is whatmakes an algorithm “greedy.” Unfortunately, BETTERCHANGE
actually returned incorrect results in some cases because of its
short-sighted notion of “good.” This is a common characteristic of greedy
algorithms: they often return suboptimal results, but take very little time to
do so. However, there are a lucky few greedy algorithms that find optimal
rather than suboptimal solutions.
5.1 Genome Rearrangements
Waardenburg’s syndrome is a genetic disorder resulting in hearing loss and
pigmentary abnormalities, such as two differently colored eyes. The disease
was named after the Dutch ophthalmologist who first noticed that people
with two differently colored eyes frequently had hearing problems as well.
In the early 1990s, biologists narrowed the search for the gene implicated in
Waardenburg’s syndrome to human chromosome 2, but its exact location remained
unknown for some time. There was another clue that shed light on
the gene associated with Waardenburg’s syndrome, that drew attention to
chromosome 2: for a long time, breeders scrutinized mice for mutants, and
one of these, designated splotch, had pigmentary abnormalities like patches
of white spots, similar to those in humans with Waardenburg’s syndrome.
Through breeding, the splotch gene was mapped to one of the mouse chromosomes.
As gene mapping proceeded it became clear that there are groups
of genes in mice that appear in the same order as they do in humans: these
genes are likely to be present in the same order in a common ancestor of
humans and mice—the ancient mammalian genome. In some ways, the
human genome is just the mouse genome cut into about 300 large genomic
fragments, called synteny blocks, that have been pasted together in a different
order. Both sequences are just two different shufflings of the ancient mammalian
genome. For example, chromosome 2 in humans is built from fragments
that are similar to mouse DNA residing on chromosomes 1, 2, 3, 5, 6,
7, 10, 11, 12, 14, and 17. It is no surprise, then, that finding a gene in mice
often leads to clues about the location of the related gene in humans.
Every genome rearrangement results in a change of gene ordering, and a
series of these rearrangements can alter the genomic architecture of a species.
Analyzing the rearrangement history of mammalian genomes is a challenging
problem, even though a recent analysis of human and mouse genomes
implies that fewer than 250 genomic rearrangements have occurred since the
divergence of humans and mice approximately 80 million years ago. Every
study of genome rearrangements involves solving the combinatorial puzzle of finding a series of rearrangements that transform one genome into another.
Figure 5.1 presents a rearrangement scenario in which the mouse X chromosome
is transformed into the human X chromosome.1 The elementary
rearrangement event in this scenario is the flipping of a genomic segment,
called a reversal, or an inversion. One can consider other types of evolutionary
events but in this book we only consider reversals, the most common
evolutionary events.
Biologists are interested in the most parsimonious evolutionary scenario,
that is, the scenario involving the smallest number of reversals. While there is
no guarantee that this scenario represents an actual evolutionary sequence, it
gives us a lower bound on the number of rearrangements that have occurred
and indicates the similarity between two species.2
Even for the small number of synteny blocks shown, it is not so easy to verify
that the three evolutionary events in figure 5.1 represent a shortest series
of reversals transforming the mouse gene order into the human gene order
on the X chromosome. The exhaustive search technique that we presented
in the previous chapter would hardly work for rearrangement studies since
the number of variants that need to be explored becomes enormous for more
than ten synteny blocks. Below, we explore two greedy approaches thatwork
to differing degrees of success.