A-level生物知识点学习笔记，带你巩固DNA部分重要考点
在物理实验中，误差的产生往往是我们不能避免的，因此我们往往需要需要定量地分析实验误差(quantitative analysis of experimental errors)，而这部分ALevel物理考点往往在AS和A2物理考试中都有着非常深入的考察，并且这个技能将会对我们将来的专业发展有着非常大帮助。下面未来小编就带大家一起来分析一下定量分析实验误差中的一些重点内容，一起来了解一下吧！
一、random error与systematic error
在科学实验中，每一次测量都无法做到完美的准确，换句话说，每一次测量都是有误差的。误差的来源有很多种，可能是设备不够精密（low precision instrument），可能是测量过程中的随机因素，也可能是测量过程的不规范。
实验误差总体上被分为两类：random error and systematic error.
Random error causes data to fluctuate around the true value.随机误差使测量数据在真实值上下浮动。
常见的随机误差有：测量长度时尺子的最小刻度带来的误差，测量时间时人的反应时间带来的误差。
减小随机误差的方法有：repeat measurement and take average,use an instrument with higher precision.
Systematic error causes all the data to be either smaller or greater than the true value.系统误差使测量数据要么都比真实值大，要么都比真实值小。
常见的系统误差有：zero error when using a voltmeter or a balance,parallax error when using a measuring cylinder,effect of air resistance等等。
减小系统误差的方法可以是：check zero error before measuring,make sure eye-sight is parallel to the lowest point on the surface of liquid.
举个栗子：（CIE 9702/12/M/J/19）
怎么做呢？
题目告诉我们p与q是directly proportional，代表p-q图应该是穿过原点的一条直线。
A的特征是第三个点明显与其他数据不符。
The third data point is clearly inconsistent with the others.We say it is an anomalous point.This is not due to random error.故A不对。
B的特征是best-fit line不是直线。这很可能是由实验设计产生的systematic error，导致图像不成正比关系了，和random error无关。故B不对。
C的特征是data points scatter uniformly around the best-fit line，完全就是random error的特点。故C正确。
D的特征是best-fit line没经过原点，很可能是all the data are higher than the true values，也就是是由systematic error导致的。故D不对。
二、precision and accuracy
We say“data is precise”or“data has high precision”if distance between data points are close.
We say“data is accurate”or“data has high accuracy”if the mean value of data is close to the true value.
Precise与accurate的具体含义我们直接从例题来看：（CIE 9702/12/O/N/18）
怎么做呢？
我们来判断一下X,Y,Z是否满足precise与accurate的定义：
X:分布看起来比较宽比较胖，代表数据彼此之间差别比较大。分布是关于中心轴对称的，代表平均值和V非常接近。所以X是accurate but not precise。
Y:分布很窄很瘦，代表数据彼此差别较小。分布不是对称的，能够看出平均值应当实在最高值偏左一点，明显和V不同。所以Y是precise but not accurate。
Z:分布明显不对称，平均值在最高值靠右的地方，明显大于V。分布的最高值看似很窄，但右边的尾巴代表数据与平均值存在着较大差别。所以Z是neither accurate nor precise。所以答案选A。
三、absolute uncertainty and percentage uncertainty
想象一个实验：我想要测量一个长方型的周长(C)和面积(A)。实验室中无法直接测量面积和体积，那么我采取的方法是用直尺测量长(L)宽(W)，然后使用C=2L+2W,A=L*W来找到答案。
我们已经讨论过，每当我用直尺测量时，都会产生一个random error，通常这个误差的大小就是尺子上的最小刻度。那么问题是：计算值C和A有没有误差？如果有，是多大？
这是在科学实验中无可避免的一步：我们知道测量值和测量误差(measured value and its uncertainty)，那么如何找到计算值的误差(uncertainty in calculated value)?
这在物理中被称作propagation of error。
以上就是小编关于ALevel物理考点中定量分析实验误差的解析，同学们是不是已经完全掌握了呢？如果在备考中还有哪些内容了解不是很清楚的话，欢迎随时咨询我们的在线老师，让专业的ALevel课程老师一对一为你进行课程的指导吧！
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A-level生物知识点学习笔记，带你巩固DNA部分重要考点
脱氧核糖核酸（英文DeoxyriboNucleic Acid，缩写为DNA）是生物细胞内含有的四种生物大分子之一核酸的一种。DNA携带有合成RNA和蛋白质所必需的遗传信息，是生物体发育和正常运作必不可少的生物大分子。在A-level生物知识点学习中，Nucleic Acids和Protein synthesis是我们学习和考察的重点章节，今天未来小编就带大家解析一下这部分中的一些重点内容，希望对大家有所帮助。
Part 1：Nucleic Acids
DNA is the genetic material,which carries the genetic information.
Chromosome is a DNA molecule associate with the histone proteins.
Gene is the small section of DNA,which can be used to make the proteins.
■DNA
-Deoxyribonucleic acids
-Double helix structure with antiparallel strands。
-DNA is the polymer of DNA nucleotides(Polynucleotides)
■RNA
-Ribonucleic Acids
-Made up by RNA nucleotides.
-Messenger RNA:Copy the genetic information from DNA in the nucleus and bring the information to the ribosome in cytoplasm.
-Transfer RNA:Carry the specific amino acid to the ribosome.
-Ribosomal RNA:Form the subunits of the ribosome.
l Nucleotide contains a pentose sugar,a phosphate group and a nitrogen-containing base:
■Each nucleotide is linked to the next one by phosphodiester bonds between the phosphates of a nucleotide and pentose of another nucleotide to form the polynucleotide.
■Commentary base pairing:purine bases pair with pyrimidine bases(A-T,G-C)
-Adenine always pairs with thymine(uracil in RNA)via 2 hydrogen bonds.
-Guanine always pairs with cytosine via 3 hydrogen bonds.
■Two polynucleotide strands of the DNA is held by the hydrogen bonds between the complementary base pairing.
Part 2.DNA replication
■Occurs during interphase before the cell division to ensure each daughter cell get correct amount of genetic material.
■DNA replicates by semi-conservative replication:Each daughter DNA molecule contains one strand from parent DNA and one newly formed strand.
■Process:
1.The DNA double helix structure is unwound and unzipped by helicase.
2.Both separated DNA strands are used as the template strands.
3.Free nucleotides joined with templet strand and held by the hydrogen bonds between the complementary base pairs.
4.DNA polymerase catalyses the formation of the phosphodiester bonds between DNA nucleotides.
5.Due to the new DNA strands have to formed from 5’end to 3’end,and the DNA is antiparallel,one daughter DNA strand is formed discontinuously.The DNA segments are connected by the DNA ligase.
6.Two identical daughter DNA molecules are formed.
Part 3：Protein synthesis
■Two strands of DNA:sense strand(coding strand)+antisense strand(template strand)
■The genes are on the antisense strand
■Transcription(occurs in the nucleus):
1.Double strands of DNA are unwound and unzipped and the antisense strand is used as the template.
2.RNA nucleotides pair up with their complementary bases on the template strand via hydrogen bonds.
3.RNA polymerase catalyse the formation of the phosphodiester bonds between the RNA nucleotides.
4.The mRNA molecules is formed,and then the hydrogen bonds between the mRNA and DNA template strand are broken.
5.The DNA rewind into the double helix structure and mRNA molecule leaves the nucleus via nuclear pore.
■Translation:(occurs on the ribosome)：
1.The mRNA moves into the cytoplasm and attached by ribosomes to provide the binding site for tRNA.
2.tRNA will brings the specific amino acids and bind with the mRNA and ribosome via the complementary base pairing between the codon and anticodon.
3.Then,second tRNA bring another amino acids and bind with the mRNA.
4.The peptide bond form between the adjacent amino acids.
5.The first tRNA detached from the its amino acids and he ribosome moves along the mRNA molecule,and a new tRNA with the amino acids will bind with the next codon.
6.The polypeptide is formed gradually until the ribosome reach the stop codon,then the formation of the polypeptide is completely.It will fold in the secondary and tertiary structure immediately.
7.The mRNA can be read by several ribosomes at the same time to produce many polypeptide.
以上就是未来小编关于A-level生物知识点中有关DNA和RNA部分的一些重点内容的总结和分析，希望能够帮助同学们更好的做好知识点的复习和巩固。更多A-level学习问题，欢迎随时咨询我们。
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