? 發問時間： 科學生物學 · 1 0 年前

# 請問DNA bend的問題

Assume that a poly(A) tract five base pairs long produces a 20° bend in a DNA strand. Calculated the total (net) bend produced in a DNA if the center base pairs (the third of five) of two successive (dA)5 tracts are located (a) 10 base pairs apart ; (b) 15 base pairs apart. Assume 10 base pairs per turn in the DNA double helix.

Answer: (a) 40° (b) 0°

1.我想請問一定要是(aaAaa)都在一起才會有bend的能力嗎?

2.(a)的意思是這樣aaAaa000000aaAaa嗎？還是不是這樣？

(aaAaa00000)(000aaAaa00)或是(0aaAaa0000)(0000aaAaa0)等等，變得會有很多

in phase跟out of phase是怎麼辦定的

2 個已更新項目:

3.(b)你說是out of phase是怎麼看的？怎麼樣算是out of phase？

(aaAaa00000)(aaAaa00000)是這樣才算in phase嗎？

(00aaAaa000)(000aaAaa00)這種就不算in phase嗎？

### 1 個解答

• 菲菲
Lv 4
1 0 年前
最佳解答

Answer :

When bending elements are repeated in phase with the helix turn

(i.e., every 10 base pairs) as in (a) ,

the total bend is additive ;

when bending elements are repeated out of phase by one half-turn as in (b),

they cancel each other out.

Thus, the net bend is ((a) 40° (b) 0°

2010-08-02 22:37:16 補充：

基本上, A-tracts 不過是個名詞,它表示了在DNA上有著連續之A(Adenine)之motif 構造,

而通常這樣子之A baes pairs 連續約4~6個bp 時,在DNA上會出現轉彎的情形,也就是俗稱

之A-tracts .

2010-08-02 22:37:41 補充：

在一些paper內容中,您可參考見其 :

( Adenine-rich sequences (A-tracts) occur strategically in regions of curvature and draw continued interest because the origin and nature of bending remain unresolved. We simulated two A-tracts whose structures had also been determined : CGCGA6CG and CGCA6GCG.)

2010-08-02 22:38:07 補充：

而最近的發現指出,這樣子連續A所構成的轉彎motif似乎跟複製及調節有著一些相關聯

(當然這並非本題之重點)

這題其實有點語焉不詳,它的意思是: 連續5個A的A-tracts是以第3個A為轉彎的中心

(也就是最中間的那個A),而轉彎的程度為約20度(當然實際情形更複雜),因此當連續兩個

A-tracts時,自然的角度加成為40度,但若是連續3個A-tracts 時會因為DNA自我構型的維持性

而轉回0度(一圈約10個base,超過10個base就轉回來,畢竟還是B-form DNA),至於為什麼?

沒見到有書寫,實際上我也沒看到有人很明白的寫在paper裡!!

2010-08-02 22:38:53 補充：

唯一我有看到的一個解釋是在期刊中寫道 :

Long A-tract avoidance is restricted exclusively to nucleosome-occupied exon regions.

由此推知可能是因為組蛋白的纏繞而使它不得以繞回0度

(出自於: Nucleic Acids Research Advance Access published online on August 21, 2009)

2010-08-02 22:39:19 補充：

提供你一個NCSA的網址,它將整個A-tracts的故事寫的還算完整 :

而提供了3段A-tracts會抵銷其轉彎的敘述在:

http://nar.oxfordjournals.org/cgi/content/full/26/...

2010-08-02 22:39:50 補充：

我猜你沒時間讀全部,因此我擷取了有提到的部份:

In each case, one of these motifs is separated from an identical repeat by 15–16 bp (1.5 helical turns), which effectively cancels its bending contribution , while the other two sequence motifs remain in phase, separated by either one or two DNA helical turns.

2010-08-02 22:42:38 補充：

另有參考資料:

1. DNA bending by asymmetrically tethered cations:infuence of tether £exibility

2. Intrinsic Curvature in the VP1 Gene of SV40: Comparison of Solution and Gel Results

<最後, in phase指的是在一個B-form DNA 的turn ;10個base 而 out phase則顯而易見的指的是超過一個turn的部份 !!>

參考資料： 與你用同本課本的我,若想交流交流可寄信予我, papersssssssss