Nucleic Acids 和 Protein synthesis 一直是A Level的重点章节，也是经常被同学们提问的地方。下面崩大家整理了一份内部复习资料，在即将考试期间提供给同学们，希望能帮助到大家！
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.
■ 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.
■ 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.
Alevel生物中最有效的实验题答法——PEE答题法，专治各种疑难实验题。我们就用这道AQA考试局的生物AS真题来举例子。请看下图：
这道题的背景不是非常复杂，涉及到的几个关键词，例如：inhibitor（抑制剂），mitosis（有丝分裂），trial（实验组）。
在前一道小题中也解释了kenisin 是一种控制细胞分裂涉及到的酶。kenisin的功能一旦被抑制剂抑制，就会形成那种单面的spindle fibre。这种畸形的spindle fibre越多，就越会降低细胞分裂的速率。
好了，我们再看一下问题是什么：
Q：一个学生根据实验数据得出结论：在接下来的实验治疗过程中，用浓度为100nmol dm-3 的抑制剂来抑制肿瘤生长是最合适的。实验的数据支持这个结论吗？为什么？
这个问题我们要通过两方面回答，先说是，为什么。再说不是，为什么。当然，你的观点是不算分的，给出支持你观点的数据和理由才是给分点。
P: point，观点，对/错，支持/不支持。
E: evidence, 证据，你提出的观点依据究竟是什么。
E: explain, 解释，原因究竟是什么。
具体解题思路如下：
Point: 该生的观点正确。
Evidence:在浓度到达100nmol dm-3之后，形成的畸形spindle fibre的数量增长并不明显。
Explain: 浓度过高的抑制剂可能对身体造成伤害。
Point: Yes
Evidence: When the concentration is above 100, the increase in % of cells not undergoing mitosis is very little.
Explain: High concentration of inhibitors may be harmful to human bodies.
Evidence: 在浓度到达100nmol dm-3 之后，还有残余的少量癌变细胞。
Explain: 残余的少量细胞还会继续繁殖，形成肿瘤。
Point: No.
Evidence: When the concentration is at 100, there are still 7% cancer cells undergoing mitosis.
Explain: The cancer cells that survived may continue to form tumors.
注意：写的时候把PEE的题头去掉。
这是一个交互式网站，上面有大量的图片和动画，
对AS生物中的细胞学、显微镜学、微生物学、免疫部分的学习都非常有帮助
美国亚利桑那大学生物项目，上面有非常好的动画、教程和在线测试
提供DNA、基因、基因组成等方面的在线教程
这个是CIE AS与A Level化学网站，
但是里面的部分化学内容对生物分子的Section 2学习非常有用
英国皇家化学学会网站，对生物分子Section B的学习非常有帮助
布里斯托生物伦理教育项目，上面有很多 A Level课程讨论的伦理话题
归属于WHO，在上面可以找到关于传染性疾病和非传染性疾病的最新信息
美国疾病控制中心网站，同样能在这个网站上找到关于疾病的最新信息
密苏里植物园官方网站，非常适合了解植物
同样包含非常多有用的植物图片，填补上一个网站的空缺部分
包含非常多电子显微镜下的动物细胞和组织的照片
这个网站上有很多人体解剖学和组织学的图片
美国加利福尼亚大学古生物博物馆网站提供的很多生物进化的学习资源
诺贝尔奖官方网站，上面有非常多的教育资源，而且还有诺奖得主的介绍
www.wellcometreeofl ife.org/
同样属于 Wellcome Trust基金会，可以帮助你A Level课程生物多样性的学习
