2细胞基本知识概要

第二章 细胞基本知识概要 本章内容提要: 第一节 细胞的基本概念 第二节 非细胞形态的生命体 -------病毒及其与细胞的关系 第三节 原核细胞与古核细胞 第四节 真核细胞基本知识概要 第一节 细胞的基本概念 What is a cell? Cells are structural units that make up plants and animals, also there many single cell organisms. What cells all have in common is they are small 'sacks' composed mostly of water. The 'sacks' are made from a phospholipid bilayer. The membrane is semi-permeable (allowing some things to pass in or out of the cell and blocking others), there are also other methods of transport that we will get into later. So what is in a cell? The cell as we mentioned is a fluid like membrane that surrounds the contents of the cell. Each component will be discussed in more detail later. 一、细胞是生命活动的基本单位 1、一切有机体都由细胞构成，细胞是构成有机体的基本单位； 2、细胞具有独立的、有序的自控代谢体系，细胞是代谢与功能的基本单位 3、细胞是有机体生长与发育的基础 4、细胞是遗传的基本单位，细胞具有遗传的全能性 5、没有细胞就没有完整的生命 二、细胞的基本共性 1.所有的细胞表面均有由磷脂双分子层与镶嵌蛋白质构成的生物膜，即细胞膜。 2.所有的细胞都含有两种核酸:即DNA与RNA作为遗传信息复制与转录的载体。 3.作为蛋白质合成的机器─核糖体，毫无例外地存在于一切细胞内。 4.所有细胞的增殖都以一分为二的方式进行分裂。 第二节 非细胞形态的生命体 —病毒及其与细胞的关系 一、Basic characteristics of viruses Simply stated, viruses are merely genetic information surrounded by a protein coat. They may contain external structures and a membrane. Viruses are obligate（专性的） intracellular parasites--meaning that they require host cells to reproduce. In the viral life cycle, a virus infects a cell, allowing the viral genetic information to direct the synthesis of new virus particles by the cell. There are many kinds of viruses. Those infecting humans include polio（脊髓灰质炎）, influenza, herpes（疱疹）, and human immunodeficiency virus (HIV) causing AIDS. 二、viral reproduction(i.e. the course of viral infection, e.g. HIV infection) 1、Attachment(getting in) On the surface membrane of all living cells are complex protein structures called "receptors". A receptor is often compared to a lock into which a specific key or "ligand" will fit. There are at least two receptors on T-lymphocytes to which the human immunodeficiency virus (HIV) sticks. The primary receptor, called "CD4", is shown on the right in the diagram. But a second receptor that loops through the cell membrane 7 times is critical for infection to occur. HIV infection of a lymphocyte requires attachment of the virus to the cell membrane through both of these "ligand-receptor" links. In cells whose "7-transmembrane receptor" is different, the HIV "key" no longer matches the lymphocyte "lock" and attachment is incomplete. Those cells may avoid infection by HIV. Tight attachment of the viral particle to receptors on the lymphocyte membrane enables fusion with the cell membrane. The viral contents, including viral RNA (shown in yellow) then empty into the cell's cytoplasm. Like other viruses that infect human cells, HIV commandeers the host's machinery to make multiple copies of itself. 2、Reverse Transcription: Converting viral RNA into DNA An enzyme (protein) that's part of the human immunodeficiency virus reads the sequence of viral RNA nucleic acids that have entered the host cell and transcribes the sequence into a complementary DNA sequence. That enzyme is called "reverse transcriptase" . Without reverse transcriptase, the viral genome couldn't become incorporated into the host cell, and couldn't reproduce. Reverse transcriptase sometimes makes mistakes reading the RNA sequence. The result is that not all viruses produced in a single infected cell are alike. 3、Integration of Viral DNA Once the viral RNA has been reverse-transcribed into a strand of DNA, the DNA can then be integrated (inserted) into the DNA of the lymphocyte. The virus has its own enzyme called "integrase" that facilitates incorporation of the viral DNA into the host cells DNA. The integrated DNA is called a provirus. 4、Transcription: Back to RNA As long as the lymphocyte is not activated or "turned-on", nothing happens to the viral DNA. But if the lymphocyte is activated, transcription of the viral DNA begins, resulting in the production of multiple copies of viral RNA. This RNA codes for the production of the viral proteins and enzymes (translation) and will also be packaged later as new viruses. 5、Translation: RNA -> Proteins There are only 9 genes in the HIV RNA. Those genes have the code necessary to produce structural proteins such as the viral envelope and core plus enzymes like reverse transcriptase, integrase, and a crucial enzyme called a protease. 6、Viral Protease When viral RNA is translated into a polypeptide sequence, that sequence is assembled in a long chain that includes several individual proteins (reverse transcriptase, protease, integrase). Before these enzymes become functional, they must be cut from the longer polypeptide chain. Viral protease cuts the long chain into its individual enzyme components which then facilitate the production of new viruses. 7、Assembly and Budding （release) Finally, viral RNA and associated proteins are packaged and released from the lymphocyte surface, taking with them a swatch(样品, 样本 )of lymphocyte membrane containing viral surface proteins. These proteins will then bind to the receptors on other immune cells, facilitating continued infection. Budding viruses are often exactly like the original particle that initially infected the host. In the case of HIV, however, the resulting viruses exhibit a range of variations which makes treatment difficult. 三、病毒与细胞在起源与进化中的关系 病毒是非细胞形态的生命体，它的主要生命活动必须要在细胞内实现。病毒与细胞在起源上的关系，目前存在3种主要观点: 1.生物大分子→病毒→细胞 病毒 2.生物大分子 细胞 3.生物大分子→细胞→病毒 现在来说，第二种观点和第三种观点比较容易接受，而且第三种观点越来越有说服力。 认为病毒是细胞演化的产物的观点主要依据如下: 彻底的寄生性； 病毒核酸与哺乳动物细胞DNA某些片断的相似性； 病毒可以看成是核酸与蛋白质形成的复合大分子。 第三节 原核细胞与古核细胞 一、Cell types Living cells are divided into two types - prokaryotic and eukaryotic (sometimes spelled procaryotic and eucaryotic). This division is based on internal complexity. Eukaryotic cell Cell Prokaryotic cell 二、Basic characteristics of Prokaryotic cell 1. 遗传的信息量小，遗传信息载体仅由一个环状DNA或RNA构成； 2. 细胞内没有分化为以膜为基础的具有专门结构与功能的细胞器和细胞核膜。 三、原核细胞的主要代表 1、支原体 为什么说支原体是一个细胞 （1）能在培养基上生长，具有典型的细胞膜； （2）具有环状的双螺旋DNA作为遗传信息量的载体； （3）mRNA与核糖体结合形成多聚核糖体，指导蛋白质的合成； （4）以一分为二的方式分裂繁殖。 支原体是最小、最简单的细胞。 2、细菌 1）、细菌的三种形态:球状、杆状和螺旋状 2)、细菌细胞的核区与基因组:细菌的核区实际主要由一个环状的DNA分子组成；现在也可以把细菌的环状DNA理解为细菌基因组。 3）、细菌细胞的表面结构: A. 细胞膜:主要功能是选择性的交换物质----吸收营养物质，排出代谢废物，并且有分泌与运输蛋白的作用。 B. 细胞壁: 所有细菌的细胞壁的共同成分是肽聚糖，由乙酰氨基葡萄糖、乙酰胞壁酸与四五个氨基酸短肽聚合而成的多层网状大分子结构。 C. 细胞壁特化结构:a. 中膜体-----细胞膜内陷而形成的；b. 荚膜-----是一层松散的粘液物质，有一定程度的保护作用；c. 鞭毛-----细菌的运动器官，与真核生物的鞭毛不同，它是由一种称为鞭毛蛋白的弹性蛋白所构成。 4）、细菌细胞的核糖体——部分附着在细胞膜内侧，大部分游离于细胞质中，与蛋白质的合成密切相关。 5）、细菌细胞核外DNA------质粒，是裸露环状DNA，在遗传工程研究中很重要。 6）、细菌细胞的内生孢子，即芽孢，是细菌对不良环境或营养耗尽时的反应。 3. 蓝藻细胞:是最简单的自养植物类型之一。 基本特征:1）中心质------相当于细菌的核区，是遗传物质DNA所在部位。 2）光合片层-----位于细胞质部分，是同心环状的膜片层结构，上边附着有藻胆蛋白体（包括藻蓝蛋白，一藻蓝蛋白和藻红蛋白），能够把光能传递给叶绿素a，进行原始光和作用。 3）细胞质内含物 4）细胞表面结构 5）细胞分裂 四、原核细胞与真核细胞的比较 1、原核细胞与真核细胞最根本的区别 : （1）、细胞膜系统的分化和演变。 细胞内部结构和职能的分工是真核细胞区别于原核细胞的重要标志。 （2）、遗传信息量与遗传装置的扩增与复杂化。 遗传信息重复序列与染色体多倍性的出现是真核细胞区别于原核细胞的另一重要标志。 （3）、真核细胞内，遗传信息的转录与翻译有严格的阶段性和区域性，而在原核细胞内则是转录与翻译可以同时发生 五、原核细胞与真核细胞基本特征的比较（p36) 六、原核细胞与真核细胞的遗传结构装置和基因表达的比较(p37) 七、古细菌 古细菌（archaebacteria）与真核细胞曾在进化上有过共同历程 主要证据 （1）细胞壁的成分与真核细胞一样，而非由含壁酸的肽聚糖构成，因此抑制壁酸合成的链霉素， 抑制肽聚糖前体合成的环丝氨酸，抑制肽聚糖合成的青霉素与万古霉素等对真细菌类有强的抑制生长作用，而对古细菌与真核细胞却无作用。 （2）DNA与基因结构:古细菌DNA中有重复序列的存在。此外，多数古核细胞的基因组中存在内含子。 （3）有类核小体结构:古细菌具有组蛋白，而且能与DNA构建成类似核小体结构。 （4）有类似真核细胞的核糖体:多数古细菌类的核糖体较真细菌有增大趋势，含有60种以上蛋白，介于真核细胞（70～84）与真细菌（55）之间。抗生素同样不能抑制古核细胞类的核糖体的蛋白质合成。 （5）5S rRNA:根据对5S rRNA的分子进化分析，认为古细菌与真核生物同属一类，而真细菌却与之差距甚远。5S rRNA二级结构的研究也说明很多古细菌与真核生物相似。 第四节 真核细胞基本知识概要 一、真核细胞的基本结构体系 1.生物膜系统:以脂质及蛋白质成分为基础的生物膜结构系统； 2.遗传信息表达结构系统:以核酸（DNA或RNA）与蛋白质为主要成分的遗传信息表达系统 3.细胞骨架系统:由特异蛋白分子装配构成的细胞骨架系统。 二、细胞的大小及其分析 各类细胞直径的比较 三、植物细胞与动物细胞的比较 植物细胞特有的结构: 1. 细胞壁 2. 液泡 3. 叶绿体