Name: dna copying: leading and lagging strands
Uploaded: 2020-09-24T15:06:53.000Z
Duration: 2 min 40 s
Channel: JoVE

DNA 复制是通过利用现有 DNA 链作为模板 合成新 DNA 链来实现的。这两个新双螺旋分别包含一个原始模板链 和一个新合成的子链，这就是为什么这个过程 称为半保留复制的原因。为了开始复制，一种叫做解旋酶的酶解开了 DNA 螺旋，打破了两条链之间的氢键。然后，它将单个链分离，形成一个 Y 形结构，叫做复制叉，模板链可以被其他酶访问。另一种酶叫做引物酶，它会在每条模板链上 添加称为引物的短 RNA 片段。这些引物对 DNA 的合成至关重要，因为 DNA 聚合酶只能向现有链中 添加核苷酸。DNA 聚合酶在两个模板 DNA 链上 添加正在生长的子链。根据 DNA 配对规则，核苷酸的添加由原始 DNA 链的序列 引导。其中一条子链，也就是前导链，在复制叉运动的方向上合成。另一条链，叫后随链，沿相反方向合成。这导致前导链合成为连续聚合物，而后随链被合成为 短片段。这是因为 DNA 只能在 5'到 3'方向合成。在加入到正在生长的聚合物中之前，核苷酸以脱氧核糖核苷三磷酸的形式存在，三个磷酸附着在糖的 第五个碳上。这种游离三磷酸核苷酸与 3'羟基 发生反应。这是在生长链末端连接到糖的 第三个碳上的羟基。这个反应引起焦磷酸盐的释放，并在两个核苷酸之间形成 磷酸二酯键。新链合成后，RNase H 或 DNA 聚合酶 的其他变体去除引物，并在其位置合成 DNA。然后，片段之间的缝隙被 DNA 连接酶密封，形成一个连续链。核苷酸的添加一直持续到 两个复制叉相遇，从而完成复制。

<ol>
	<li>Pray， L. （2008） DNA 复制的主要分子事件。自然教育 1（1）:99 </li>
</ol>

脱氧核糖核酸（dna）复制

DNA Replication

DNA 复制包括分离双螺旋线的两个线束，每个线束用作复制新互补线束的模板。  复制后，每个双链 DNA 包括一条亲链或“旧”链和一条“新”链。 这被称为半系统复制。 由此产生的 DNA 分子具有相同的序列，并均匀地分为两个子细胞。 
 原核生物中的复制 
 DNA 复制使用大量蛋白质和酶。 酶氦酶分离了 DNA 的两股。 随着螺旋酶沿着 DNA 移动，它将两条线分开，形成一个称为复制叉的 Y 形结构。 此后，酶原酶添加了称为底漆的 RNA 短段，以通过 DNA 聚合酶 (即 DNA 合成的酶) 对 DNA 进行合成。 在细菌中，已知有三种主要类型的 DNA 聚合酶： DNA pol I ， DNA pol II 和 DNA pol III DNA pol III 是 DNA 合成所需的酶； DNA pol I 和 DNA pol II 主要需要进行修复。 DNA pol III 增加了脱氧核糖核酸，每一种都与模板股上的核苷酸互补，一对一生长中的 DNA 链中的 3 ’ -OH 组。 DNA 聚合酶 III 只能沿 5 ’至 3 ’方向延伸。 添加这些核苷酸需要能量。 该能量存在于每个核苷酸附带的三个磷酸盐组的键中，类似于能量如何存储在三磷酸腺素 (ATP) 的磷酸盐键中。 当磷酸盐之间的键为破损且释放二磷酸盐时，释放的能量允许通过脱水合成形成共价磷酸二酯键。 
 DNA 双螺旋是反平行的；即一条线束以 5 ’到 3 ’的方向为导向，另一条线束以 3 ’到 5 ’的方向为导向。 在复制过程中，一条对 3 ’到 5 ’父母 DNA 链的补充，由于聚合酶可以在这一方向添加核苷酸，因此会向复制叉持续合成一条链。 这股持续合成的股称为股前股。 另一条线是 5 ’到 3 ’父母 DNA 的补充，它与复制叉成了一条距离，因此聚合酶必须移回复制叉，以开始将碱添加到新底漆中，这也是朝着远离复制叉的方向。 它会这样做，直到它撞击到之前合成的股，然后它再次返回。 这些步骤可产生称为 Okazaki 片段的小 DNA 序列片段，每个片段均由 RNA 底物分离。 含有冈崎碎片的股称为滞后股，据说其合成是不连续的。 
 DNA 聚合酶 III 合成后，引物通过 DNA 聚合酶 I 的核酸外切酶活性被去除，并填充缝隙。 新合成 DNA (取代 RNA 底漆) 之间的缺口 以前合成的 DNA 由酶 DNA 连体封堵，该连体催化一个 DNA 片段的 3'-OH 端与另一个片段的 5 ’磷酸盐端之间的共价磷酸二酯连杆的形成，从而稳定了 DNA 分子的糖磷酸骨干。 
 真核生物中的复制 
 真核基因组基因组比原子核基因组多了很多化合物，而且通常由多条线性染色体组成。 例如，人类基因组中每一个小类染色体组有 30 亿碱对，复制过程中插入了 60 亿碱对。 每一个真核染色体都有多种复制来源；人类基因组有 30,000 至 50,000 个复制来源。 复制的率值约为每秒 100 个核苷酸—比原子核复制慢 10 倍。 
 真核生物中复制的基本步骤与原核生物中的步骤相同。 在开始复制之前，必须将 DNA 作为模板提供。 真核 DNA 是高度超卷和包装的，这由许多蛋白质 (包括组性蛋白) 促成。 在启动复制后，在类似于原核生物的过程中，通过真核 DNA 聚合酶促进伸长。 股前 δ 由真核多糖酶 pol ε 连续合成，而滞后股由 pol 合成。 酶二核苷酶 H (RNase H) 取代了细菌中的 DNA 聚合酶，去除了 RNA 底漆，随后将其替换为 DNA 核苷酸。 残留的缝隙由 DNA 韧带密封。 
 与原核生物一样，真核 DNA 聚合酶只能沿 5 ’到 3 ’方向添加核苷酸。 在前束中，合成将持续到染色体末端或另一个沿相反方向发展的复制叉。 在滞后的链上， DNA 被短时间合成，每一种都由单独的引物启动。 当复制叉到达线性染色体末端时，没有地方为 DNA 片段在染色体末端复制制作底漆。 因此，这些目标仍然没有配对，随着时间的推移，随着细胞继续分裂，它们可能会逐渐缩短。 
 本文改编自 <a href="https://openstax.org/books/microbiology/pages/11-2-dna-replication"> Openstax, 微生物学，第 11.2 章： DNA 复制</a>。

脱氧核糖核酸（DNA）复制

DNA复制涉及双螺旋结构中两条链的分离，每条链作为模板用于合成新的互补链……

Education

教育

JoVE Core

Chemistry

化学

Biochemistry

生物化学

KEY TERMS AND CONCEPTS

关键术语与概念

dna copying: leading and lagging strands

DNA replication copies genetic material before a cell divides. Each original DNA strand serves as a template for a new complementary strand. The result is semiconservative replication, which means each new DNA molecule contains one old strand and one new strand. The two daughter cells then receive DNA with the same sequence.

Replication starts when helicase separates the two strands of the double helix. This creates a Y-shaped replication fork. Primase then adds short RNA primers that give DNA polymerase a place to begin DNA synthesis. In bacteria, the main DNA polymerases are DNA pol I, DNA pol II, and DNA pol III. DNA pol III carries out DNA synthesis, while DNA pol I and DNA pol II mainly help with repair.

DNA pol III adds deoxyribonucleotides one at a time to the 3'-OH end of the growing strand. It can extend DNA only in the 5' to 3' direction. The energy for this process comes from the three phosphate groups on each nucleotide. When a phosphate bond is broken and diphosphate is released, that energy helps form the phosphodiester bond in the DNA backbone.

Because the DNA double helix is antiparallel, the two template strands face opposite directions. The strand copied toward the replication fork is made continuously and is called the leading strand. The other strand is copied away from the fork. It must start over with new primers as the fork opens, so it is made in short stretches called Okazaki fragments. This strand is the lagging strand, and its synthesis is discontinuous.

After DNA pol III finishes, DNA pol I removes the RNA primers in bacteria, and the gaps are filled in. DNA ligase then seals the remaining nicks by forming covalent phosphodiester bonds between adjacent DNA fragments. This step strengthens the sugar-phosphate backbone of the new DNA molecule.

Eukaryotic DNA replication follows the same basic steps, but the genome is larger and more complex. Eukaryotic chromosomes are linear and have many origins of replication. In humans, the genome has 3 billion base pairs per haploid set, and about 6 billion base pairs are copied during replication. Human chromosomes also have about 30,000 to 50,000 origins of replication, and replication occurs at about 100 nucleotides per second, which is much slower than in prokaryotes.

Before replication begins, eukaryotic DNA must be made available as a template. It is tightly packed and supercoiled, and histones help organize this packaging. After initiation, eukaryotic DNA polymerases carry out elongation. Polymerase delta (pol δ) synthesizes the leading strand, and polymerase epsilon (pol ε) synthesizes the lagging strand. RNase H removes the RNA primers, DNA nucleotides replace them, and DNA ligase seals the final gaps. Because eukaryotic DNA polymerase also works only in the 5' to 3' direction, the lagging strand is still made in short pieces. Near the ends of linear chromosomes, there is no place to add a final primer, so these ends remain unpaired and can become shorter over time as cells keep dividing.

DNA Copying: Leading and Lagging Strands

functional groups

Functional Groups in Organic Chemistry

Functional groups in organic chemistry are groups of atoms that give molecules specific properties. When these groups attach to a carbon skeleton, they can change how the molecule behaves. Some functional groups make a molecule hydrophilic, or water-attracting. Others make it hydrophobic, or water-repelling. Functional groups are a key part of organic chemistry and are also found in major biological molecules. They appear in carbohydrates, proteins, lipids, and nucleic acids. Each functional...

structure of lipids

Lipids, Fats, and Cell Membranes

Lipids are nonpolar molecules that do not mix well with water. They are hydrophobic, which means “water fearing,” because they contain mostly nonpolar carbon-carbon and carbon-hydrogen bonds. In cells, lipids store long-term energy as fats, help insulate plants and animals, and form protective water-repellent layers on feathers and fur. They are also part of cell membranes and serve as building blocks for many hormones. Common lipids include fats, oils, waxes, phospholipids, and steroids. A...

chemistry of the cell

Cell Chemistry: Water and Macromolecules

Cell chemistry includes water, organic molecules, and inorganic ions. These materials shape how cells store energy, build structures, and carry out life processes. Most cellular chemistry and metabolism happen in the watery cytoplasm. Water is a key part of the cell because it has special properties tied to its polarity. In a water molecule, hydrogen and oxygen form polar covalent bonds, so the molecule has no net charge but does have a slightly positive end and a slightly negative end. Oxygen...

dna base pairing

Chargaff’s Rules and DNA Pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T). Watson and Crick later showed why these matches matter. A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. These pairings keep the DNA double helix a...

chemistry of carbohydrates

Carbohydrates as Energy and Storage Molecules

Carbohydrates are important energy sources in human and animal diets. Grains, fruits, and vegetables are natural sources of carbohydrates. They help provide energy to the body, especially through glucose, a simple sugar found in starch and many staple foods. Carbohydrates are identified by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule. This means the ratio of carbon to hydrogen to oxygen is usually 1:2:1. The name carbohydrate comes from this pattern,...

protein and protein structure

How Protein Shape Controls Function

Proteins are a major class of organic molecules in living systems, and their shape helps determine what they do. They can act as structural, regulatory, contractile, or protective molecules. They also take part in transport, storage, membranes, toxins, and enzymes. All proteins are polymers of amino acids arranged in a linear sequence. A protein’s function depends on its three-dimensional form. For example, an enzyme works when its active site can bind a specific substrate. If local changes or...

amino acids

Amino Acids and Protein Building

Amino acids are the building blocks of proteins. Each amino acid has the same basic structure. A central carbon atom, called the alpha carbon, is bonded to an amino group, a carboxyl group, and a hydrogen atom. Each amino acid also has an R group, or side chain, attached to the central carbon. The R group is different for every amino acid. It helps determine whether an amino acid is acidic, basic, polar, or nonpolar. Scientists call these molecules amino acids because they contain both amino...

peptide bonds

Peptide Bond Formation and Shape

Peptide bonds join amino acids into peptides and polypeptides. They form when the carboxyl group of one amino acid reacts with the amino group of another. This dehydration reaction releases a water molecule and creates a covalent link. As more amino acids are added, the chain becomes a polypeptide. Each polypeptide has a free amino group at one end. This end is the N-terminal, or amino-terminal. The other end has a free carboxyl group. This end is the C-terminal, or carboxyl-terminal. The...

nucleic acids

DNA and RNA: Genetic Instructions

DNA and RNA are the nucleic acids that store genetic information and help direct cell activity. These macromolecules carry the cell’s genetic blueprint and the instructions it uses to function. DNA is the genetic material in all living organisms, from single-celled bacteria to multicellular mammals. In eukaryotic cells, DNA is found in the nucleus and also in organelles such as chloroplasts and mitochondria. In prokaryotes, DNA is not enclosed in a membrane. The full genetic content of a cell...

dna replication

DNA replication copies genetic material before a cell divides. Each original DNA strand serves as a template for a new complementary strand. The result is semiconservative replication, which means each new DNA molecule contains one old strand and one new strand. The two daughter cells then receive DNA with the same sequence. Replication starts when helicase separates the two strands of the double helix. This creates a Y-shaped replication fork. Primase then adds short RNA primers that give DNA...

from dna to protein

The Genetic Code and Codons

The genetic code and codons explain how cells turn DNA information into proteins. Genes first direct the sequence of mRNA, and mRNA then directs the sequence of amino acids that make up proteins. Specific proteins and RNAs carry out this decoding. Cells copy DNA into mRNA so the original DNA stays protected while the cell uses the message for protein synthesis. Copying DNA to RNA is relatively simple. One nucleotide is added to the mRNA strand for every nucleotide read on the DNA strand.

polymers

Polymer Chains and How They Form

Polymers are long chains of molecules made from repeating units called monomers. The word polymer comes from the Greek words "poly," meaning many, and "mer," meaning parts. Polymers can be found in nature, such as DNA and proteins, or made synthetically, such as plastics. A polymer’s shape helps define its properties. Some polymers form linear chains, while others are branched or arranged in complex networks. Functional groups on a polymer also affect how it behaves. The repeating units in a...

key terms and concepts

Explore how DNA replication uses leading and lagging strands, RNA primers, DNA polymerase, ligase, and semiconservative copying in bacteria and eukaryotes.

DNA Copying: Leading and Lagging Strands | JoVE K12

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

DNA Copying: Leading and Lagging Strands

Description

Transcript

Tags