
English: 
 
Do you recognize this molecule?
This is DNA, or
deoxyribonucleic acid.
By the end of this
video, you will
be able to identify the key
structural features of DNA,
as well as describe
the importance
of those features for function.
During this video, we will look
at different representations
of the DNA molecule to
better view certain details,
but all views represent
this same structure.
 
Inside the cell, you
will most commonly
find double- stranded DNA, in
which two strands intertwine
to form a double helix.
The most common form of
the DNA double helix,
which is what we
will discuss here,
is also called B-form DNA.
Now, let's move to a more
simplified representation
of DNA to discuss the details.
We can unwind the
double helix like this
so that we can see the
chemical structure inside.

Chinese: 
你认识这个分子吗？
这是DNA 或叫脱氧核糖核酸
看完这个视频
你就能识别DNA的主要结构特征
和描述这些特征对实现功能的重要性了
在这个视频里 我们以不同的形式表现DNA
以更好地观察到某些细节
但所有形式描述的都是同一结构
在细胞内部 最常会看到双链DNA
两条链缠绕在一起形成双螺旋
我们要讲的 就是DNA双螺旋最常见的形式
也被称为B型DNA
接下来 让我们用一个简化的DNA模型来讲解细节
像这样把双螺旋解开
我们就能看到内部的化学结构了

English: 
Each strand is a
polynucleotide, meaning
the strand is made up of
many individual units called
nucleotides.
A nucleotide has three
components: the five-carbon sugar,
a phosphate
group, and one
of four possible
nitrogenous bases--
adenine, guanine,
thymine, and cytosine.
The nitrogenous base is
always attached at the
1' carbon of the sugar.
If we count from
there, we can see
that there is a phosphate
between the 5' carbon
of one sugar and the 3' carbon of the neighboring sugar.
 
The sugar is called
deoxyribose because it
is missing a hydroxyl group
at the 2' carbon which
is present in ribose.
Because of this, nucleotides
in DNA, deoxyribonucleic acid,
are called deoxynucleotides.
 

Chinese: 
每一条链都是多聚核苷酸链
意味着DNA链由许多核苷酸单元组成
一个核苷酸有三部分
一个戊糖 一个磷酸基团
还有四种可能的含氮碱基之一——
腺嘌呤 (adenine)、鸟嘌呤 (guanine)、胸腺嘧啶 (thymine) 和胞嘧啶 (cytosine)
含氮碱基总是连接在戊糖的1'碳上
如果从1'数起
就会发现在5'碳和相邻戊糖的3'碳之间有一个磷酸基团
这个戊糖被称作脱氧核糖
因为它在2'碳位脱去了一个核糖中本该存在的羟基
因此 DNA里的核苷酸 即脱氧核糖核酸
被叫做脱氧核糖核苷酸

English: 
Nucleotides attach to each
other in the DNA strand
by phosphodiester bonds.
The phosphate group
of one nucleotide
binds to the 3' oxygen
of the neighboring nucleotide.
Thus, we can see that the sugars
and phosphate groups make up
the DNA backbone.
The carbon numbering
is key to describing
the directionality of the DNA
strand, 5' to 3'.
 
Looking within the sugars, there
is an intrinsic orientation
difference between
the two strands.
On the top strand, you can
see that the 5' carbon
of each sugar is on the left
and the 3' carbon is
on the right.
The opposite is true
for the bottom strand.
Reading left to
right, that makes
the top strand orientation
5' to 3'
and the bottom strand
orientation 3' to 5'.
These strands are also sometimes
called Watson and Crick.
Keep in mind that this
double-stranded DNA is still

Chinese: 
DNA中的核苷酸通过磷酸二酯键相互连接
核苷酸上的磷酸基团
连接上邻位核苷酸的3'氧
因此可以发现 脱氧核糖和磷酸基团组成了DNA的骨架
碳原子的编号对于描述DNA  5'到3'的方向性至关重要
观察脱氧核糖  两条链内部存在方向的差别
上面的那条 每个脱氧核糖上的5'碳都在左侧
而3'碳都在右侧
底部的链则相反
从左到右
顶部链的方向是5'到3'
底部链则是3'到5'
双螺旋有时也被称为沃森和克里克链
记住 双链DNA依然是双螺旋形式的

Chinese: 
我们只是通过解开和压平了螺旋来简化描述
从而能更好地看清原子结构
虽然核苷酸通过共价键聚合在一起构成骨架
两条DNA链通过碱基间的非共价键氢键相互作用
每个碱基都会和另一条链上的互补碱基生成多个氢键
通过氢键相互连接
这一单位叫做碱基对
氢键使得碱基对存在一定的特异性
胸腺嘧啶倾向于与腺嘌呤配对
形成两个氢键 而胞嘧啶倾向于与鸟嘌呤配对
生成三个氢键
胸腺嘧啶和胞嘧啶是嘧啶
以单环结构为特征 腺嘌呤和鸟嘌呤是嘌呤
它们都有双环

English: 
a double helix and we have
simplified the representation
by flattening and unwinding
the helix here to better see
the atomic structure.
Although the nucleotides
come together
through covalent
bonds in the backbone,
the two DNA strands interact
through non-covalent hydrogen
bonds between the bases.
Each base forms
multiple hydrogen bonds
with its complementary base
on the opposite strand.
Bound together by
hydrogen bonds,
each unit is called a base pair.
The hydrogen bonding
contributes to the specificity
of base pairing.
Thymine preferentially
pairs with adenine
through two hydrogen bonds and
cytosine preferentially pairs
with guanine through
three hydrogen bonds.
Thymine and cytosine are called
pyrimidines, characterized
by their single ring structure,
and adenine and guanine
are called purines,
which have double rings.
 

Chinese: 
AT或者TA、GC或者CG碱基对的几何形态都是相同的
使得螺旋具有对称性 碱基也能堆积起来
这和骨架之间的距离和碱基连接到骨架上的角度有关
其他的碱基配对 比如GT
没有这类的几何构型 不能形成作用较强的氢键
并且会阻碍螺旋的形成
DNA的双螺旋结构具有很强的规律性
每圈螺旋大概是10碱基对长
除了碱基之间的氢键
碱基堆积也加固了双螺旋结构
碱基堆积也加固了双螺旋结构
当碱基里 相邻芳香环堆积在一起
共享电子 就形成了π-π键的相互作用
规律性的螺旋结构
形成两部分重复交替的空间

English: 
The geometry of the AT or
TA and GC or CG base pairs
is the same, allowing for
symmetry and base stacking
in the helix.
This mostly has to do with the
distance between the backbones
and the angles to which the
bases attach to the backbone.
Other base pairs,
like GT, for example,
do not have the same geometry,
cannot form strong hydrogen
bonds, and disturb the helix.
 
The double helix structure
of DNA is highly regular.
Each turn of the helix measures
approximately ten base pairs.
 
In addition to the hydrogen
bonding between the bases,
the stacking of the bases also
stabilizes the double helix
structure.
These pi-pi
interactions form when
the aromatic rings
of the bases stack
next to each other and share
electron probabilities.
 
The regularity of
the helical structure
forms two repeating
and alternating

Chinese: 
叫大沟(major grooves)和小沟(minor grooves)
这些沟的作用是识别碱基对
以及作为蛋白质的结合位点
大沟含有携带特殊信息的碱基对
而小沟则主要是非特异性碱基对
因为蛋白质能作用的氢键受体和供体位于沟内
这样 DNA既能被看作是特异性序列
也能被看作为非特异性序列
使得蛋白质能够结合到正确的基因上
完成指定任务
这就是DNA双螺旋
你们已经知道了对DNA功能造成影响的结构特征
我希望你们能和我们一起享受对这奇妙分子的探索
教育无边界-TLF联合出品 |翻译:等着被打颜二喜 | 校对:口蘑白的菇 | 审阅：Mycroft

English: 
spaces, called the
major and minor grooves.
These grooves act as
base pair recognition
and binding sites for proteins.
The major groove contains
base pair specific information
while the minor groove is
largely base pair nonspecific.
This is because of the patterns
of hydrogen bond acceptors
and donors that proteins can
interact with in the grooves.
In this way, the DNA
can be acted upon
in either a sequence specific
or non-sequence specific manner,
allowing proteins to
position themselves correctly
in the genome to carry out
their designated tasks.
 
This is the DNA double
helix, and you've now
learned the structural features
that influence its function.
We hope you've enjoyed exploring
this amazing molecule with us.
 
