Cloning | 克隆

📚 Cloning | 克隆

Cloning is a cornerstone topic in OCR A‑Level Biology, encompassing the production of genetically identical organisms, tissues, or cells. This article covers natural and artificial cloning in plants and animals, the step‑by‑step processes of tissue culture, embryo splitting, and somatic cell nuclear transfer (SCNT), as well as the applications, ethical considerations, and common examination pitfalls. A thorough understanding of these concepts will equip you for both structured questions and synoptic essays.

克隆是OCR A‑Level生物学中的一个核心主题，涉及基因相同的生物体、组织或细胞的产生。本文涵盖动植物中自然和人工克隆、组织培养、胚胎分割及体细胞核移植（SCNT）的逐步过程，以及应用、伦理考量和常见考试误区。全面理解这些概念将帮助你应对结构化问题和综合性论述题。

---

1. What Is Cloning? | 什么是克隆？

Cloning is the process of generating a genetically identical copy of an organism, cell, or DNA sequence. In the context of whole organisms, a clone arises from a single ancestor through asexual reproduction or artificial intervention, meaning all members of a clone share an identical nuclear genome.

克隆是指生成生物体、细胞或DNA序列的遗传相同拷贝的过程。就整个生物体而言，克隆起源于通过无性繁殖或人工干预的一个祖先，这意味着一个克隆的所有成员拥有相同的细胞核基因组。

It is essential to distinguish between gene cloning (copying DNA fragments, often using plasmids) and reproductive cloning (creating whole animals). OCR focuses chiefly on reproductive cloning in plants and animals, though gene cloning features in genetic engineering.

必须区分基因克隆（通常利用质粒复制DNA片段）和生殖性克隆（创造整个动物）。OCR主要关注动植物中的生殖性克隆，但基因克隆出现在基因工程部分。

---

2. Natural Cloning in Plants | 植物的自然克隆

Many plants reproduce asexually through vegetative propagation. New individuals grow from a fragment of the parent plant, such as runners in strawberry, bulbs in onion, or tubers in potato. These offspring are natural clones, carrying exactly the same alleles as the parent.

许多植物通过营养繁殖进行无性生殖。新个体从母体植物的一部分长出，例如草莓的匍匐茎、洋葱的鳞茎或马铃薯的块茎。这些后代是自然克隆，携带与母体完全相同的等位基因。

Vegetative propagation relies on mitosis to produce genetically identical cells, followed by differentiation. Since there is no fertilisation or meiosis, genetic variation only arises through mutation. This uniformity is advantageous in stable environments but risky if conditions change.

营养繁殖依赖有丝分裂产生遗传相同的细胞，随后进行分化。由于没有受精或减数分裂，遗传变异仅通过突变产生。这种均一性在稳定环境中有利，但若环境改变则存在风险。

---

3. Artificial Cloning in Plants: Tissue Culture | 植物人工克隆：组织培养

Tissue culture, also called micropropagation, is used to produce thousands of genetically identical plants from a small piece of tissue, known as the explant. The process begins by excising meristematic tissue, which is free from viral infection, from a desirable parent plant.

组织培养，也称微繁殖，用于从一小块组织（称为外植体）中产生数以千计的遗传相同植物。该过程从切取无病毒感染的分生组织开始，取自理想的母体植物。

The explant is sterilised with ethanol or bleach and placed on a nutrient agar medium containing sucrose, minerals, vitamins, and a balanced ratio of auxin and cytokinin plant hormones. Under aseptic conditions, cells divide and form a callus – a mass of undifferentiated cells.

外植体用乙醇或漂白剂消毒后，置于含有蔗糖、矿物质、维生素以及平衡的生长素和细胞分裂素比例的琼脂营养培养基上。在无菌条件下，细胞分裂并形成愈伤组织——一团未分化的细胞。

By adjusting hormone levels, the callus is encouraged to differentiate into roots and shoots, eventually producing small plantlets. These plantlets are transferred to soil for hardening off. The entire procedure ensures that all plantlets are clones of the original parent.

通过调整激素水平，愈伤组织被诱导分化出根和芽，最终产生小植株。这些小植株被转移到土壤中进行炼苗。整个过程确保所有小植株都是原始亲本的克隆。

---

4. Advantages of Micropropagation | 微繁殖的优势

Micropropagation allows rapid multiplication of plants that are difficult to grow from seed or that reproduce slowly. Because it uses meristem tissue, the resulting plants are often virus‑free, a critical factor in commercial horticulture and agriculture.

微繁殖能快速增殖难以从种子生长或繁殖缓慢的植物。由于使用分生组织，所得植株常常不带病毒，这是商业园艺和农业中的一个关键因素。

The method can be used to preserve rare or endangered species by generating large numbers of identical individuals without damaging wild populations. Furthermore, genetically uniform crops simplify harvesting and increase yield predictability.

该方法可用于保存珍稀或濒危物种，通过大量繁殖相同个体而不破坏野生种群。此外，遗传一致的作物简化了收获并提高了产量可预测性。

However, tissue culture is labour‑intensive and requires sterile facilities, making it expensive. Uniformity also means that a single disease could wipe out the entire population, highlighting the lack of genetic diversity.

然而，组织培养劳动密集且需要无菌设施，成本较高。均一性还意味着一种病害就可能消灭整个种群，凸显了遗传多样性的缺乏。

---

5. Animal Cloning: Embryo Splitting | 动物克隆：胚胎分割

Embryo splitting mimics the natural process that gives rise to identical twins. In a laboratory, a fertilised egg (zygote) is allowed to divide until it forms a ball of totipotent cells, usually at the 8‑ or 16‑cell stage. Each cell is still capable of developing into a complete embryo.

胚胎分割模拟了产生同卵双胞胎的自然过程。在实验室中，受精卵被允许分裂成具有全能性细胞的球体，通常在8或16细胞期。每个细胞仍能发育为完整的胚胎。

The zona pellucida is removed, and the embryo is carefully split into individual cells using a fine knife or enzyme treatment. Each separated cell is then placed into an empty surrogate zona, cultured briefly, and implanted into the uterus of a surrogate mother.

去除透明带，然后用精细刀片或酶处理将胚胎小心分割成单个细胞。每个分离的细胞随后放入一个空白的代用透明带，短暂培养后植入代孕母体的子宫。

Because all cells come from the same fertilised egg, the resulting offspring are genetically identical clones of each other. However, they are not clones of an existing adult; they are genetically new individuals, albeit identical to one another.

因为所有细胞都来自同一个受精卵，产生的后代是彼此遗传相同的克隆。但它们并非成年体的克隆；它们是基因上全新的个体，尽管彼此相同。

---

6. Somatic Cell Nuclear Transfer (SCNT) | 体细胞核移植

Somatic cell nuclear transfer is the technique used to produce a clone of an adult animal. In SCNT, the nucleus from a somatic (body) cell of the donor animal is transferred into an enucleated egg cell, i.e., an egg whose own nucleus has been removed.

体细胞核移植是用于产生成年动物克隆的技术。在SCNT中，从供体动物的体细胞中取出细胞核，移入一个去核的卵细胞，即自身细胞核已被移除的卵细胞。

The somatic cell nucleus contains the full diploid genome of the donor. By placing it into the egg cytoplasm, the nucleus is reprogrammed to an embryonic state, effectively resetting gene expression so that it can direct development from scratch.

体细胞核包含供体的完整二倍体基因组。通过将其置于卵细胞质中，细胞核被重编程为胚胎状态，有效地重置基因表达，使其能够从头指导发育。

The reconstructed egg is stimulated with an electric pulse or chemical signal, which triggers cell division and embryo formation. After a few days of culture, the resulting embryo is transferred into the uterus of a surrogate mother, where it develops to term.

通过电脉冲或化学信号刺激重构卵，触发细胞分裂和胚胎形成。培养数天后，所得胚胎被移入代孕母体的子宫，在那里发育至足月。

---

7. The Dolly the Sheep Process | 多莉羊的过程

Dolly the sheep, born in 1996, was the first mammal cloned from an adult somatic cell. Scientists at the Roslin Institute used SCNT: they took an udder cell from a Finn Dorset ewe and cultured it under low‑nutrient conditions, forcing the cell into a quiescent G0 phase to make its nucleus more receptive to reprogramming.

多莉羊诞生于1996年，是第一只由成年体细胞克隆的哺乳动物。罗斯林研究所的科学家使用SCNT：他们从一只芬兰多塞特母羊身上获取乳腺细胞，在低营养条件下培养，迫使细胞进入静止的G0期，使其细胞核更易接受重编程。

An egg cell was harvested from a Scottish Blackface ewe and enucleated. Using a micropipette, the donor nucleus was injected into the enucleated egg. A brief electric pulse fused the two cells and activated development, mimicking the signal of fertilisation.

从一只苏格兰黑脸母羊采集卵细胞并去核。用微量移液器将供体细胞核注入去核卵。短暂的电脉冲融合了两者并激活发育，模拟受精信号。

The fused cell began dividing in culture to form an embryo, which was then implanted into a Scottish Blackface surrogate. Dolly was born as a clone of the original Finn Dorset ewe, demonstrating that the nucleus from a differentiated adult cell retains all genetic information needed to build a complete organism.

融合细胞在培养中开始分裂形成胚胎，然后植入苏格兰黑脸代孕母体。多莉作为原始芬兰多塞特母羊的克隆出生，证明来自已分化成年细胞的细胞核保留了构建完整生物体所需的全部遗传信息。

---

8. Applications of Animal Cloning | 动物克隆的应用

Animal cloning holds great promise in agriculture, where elite livestock with high milk yield or disease resistance can be duplicated to create entire herds without the uncertainty of selective breeding. It also enables the production of transgenic animals for pharmaceutical purposes, known as pharming.

动物克隆在农业上大有前景，可复制高泌乳量或抗病性的优良牲畜，以创建整群动物，避免了选择育种的不确定性。它还能生产用于制药的转基因动物，即“药牧”。

In conservation, SCNT has been proposed to save endangered species by cloning individuals from preserved somatic cells. Although inter‑species surrogacy adds complexity, promising strides have been made with species such as the gaur and the black‑footed ferret.

在保护生物学中，SCNT被提议通过保存的体细胞克隆个体来拯救濒危物种。虽然跨物种代孕增加了复杂性，但在白肢野牛和黑足雪貂等物种上已取得了有希望的进展。

Medical applications include the potential generation of patient‑specific stem cells for regenerative medicine. Therapeutic cloning, where embryos are created for the purpose of harvesting embryonic stem cells, aims to treat conditions like Parkinson’s disease without immune rejection.

医学应用包括为再生医学产生患者特异性干细胞的潜力。治疗性克隆旨在制造胚胎以获取胚胎干细胞，从而治疗帕金森病等疾病并避免免疫排斥。

---

9. Problems and Limitations of Cloning | 克隆的问题与局限

Cloning efficiency is extremely low; many reconstructed embryos fail to implant or develop normally. Dolly was the only successful birth out of 277 attempts. Cloned animals often suffer from large offspring syndrome, where organs are unusually enlarged, leading to breathing and circulatory difficulties.

克隆效率极低；许多重构胚胎无法着床或正常发育。多莉是277次尝试中唯一成功出生的个体。克隆动物常患有巨大后代综合征，器官异常增大，导致呼吸和循环困难。

Many clones exhibit premature ageing and health problems, possibly because the donor nucleus retains epigenetic marks of its age, so telomeres may be shorter. Dolly developed arthritis early and was euthanised at six, roughly half the normal lifespan of her breed.

许多克隆动物表现出早衰和健康问题，可能因为供体核保留了其年龄的表观遗传标记，因此端粒可能较短。多莉早年患有关节炎，并在六岁时被安乐死，约为其品种正常寿命的一半。

There is also a risk of reduced genetic diversity in livestock populations if cloning replaces traditional breeding, making animals more susceptible to epidemics. Additionally, the technique is technically demanding and raises significant ethical questions.

如果克隆取代传统育种，家畜种群还有遗传多样性降低的风险，使动物更易受到流行病侵袭。此外，该技术操作要求高，并引发了重大的伦理问题。

---

10. Ethical Considerations | 伦理考量

Reproductive cloning of humans is almost universally opposed and is illegal in many countries. Beyond human cloning, the welfare of surrogate mothers and cloned animals is a major concern; many pregnancies result in miscarriage, and surviving clones may experience suffering.

人类的生殖性克隆几乎遭到普遍反对，并且在许多国家是非法的。除了人类克隆，代孕母体和克隆动物的福利是主要关注点；许多妊娠导致流产，存活的克隆动物可能经受痛苦。

The creation and destruction of embryos for therapeutic cloning raise questions about the moral status of the embryo. Supporters argue that the potential medical benefits justify the use of early‑stage embryos that would otherwise be discarded, while opponents maintain that embryos should be afforded respect from conception.

为治疗性克隆而创造和破坏胚胎引发了对胚胎道德地位的疑问。支持者认为，潜在的医学益处使使用早期胚胎成为正当，而这些胚胎原本也会被丢弃；反对者则坚持，自受孕起胚胎就应受到尊重。

There are also concerns about ‘cloning for spare parts’, commodification of life, and the psychological impact on clones. Open ethical debate continues, and students should be prepared to discuss these nuances in synoptic essays.

还有对“为备件而克隆”、生命商品化以及克隆体心理影响的担忧。公开的伦理辩论仍在继续，学生应准备好讨论这些细微之处，以应对综合性论述题。

---

11. Exam Tips and Common Misconceptions | 考试技巧与常见误解

Examiners frequently ask students to describe the steps of tissue culture or SCNT sequentially. Use precise terms such as ‘explant’, ‘callus’, ‘enucleated egg’, and ‘electric pulse’. Always link each step to its purpose, e.g., agar provides anchorage and nutrients, while hormones direct differentiation.

考官常要求学生按顺序描述组织培养或SCNT的步骤。要使用精确术语，如“外植体”“愈伤组织”“去核卵”和“电脉冲”。始终将每个步骤与其目的联系起来，例如琼脂提供固着和营养，而激素指导分化。

Do not confuse embryo splitting with SCNT. Embryo splitting produces clones of one another, not of an adult, and relies on totipotent cells. In contrast, SCNT generates a clone of an adult by reprogramming a differentiated nucleus.

不要混淆胚胎分割和SCNT。胚胎分割产生彼此克隆，而非成年体的克隆，并依赖全能性细胞。相反，SCNT通过对一个已分化核进行重编程来产生成年体的克隆。

Another pitfall is assuming clones are phenotypically identical in every way. Environmental factors and epigenetic differences cause variation even among clones. When evaluating cloning, always balance benefits with limitations, and include ethical perspectives.

另一个误区是假设克隆体在表型上完全相同。环境因素和表观遗传差异甚至会导致克隆之间的变异。在评价克隆时，始终要平衡益处与局限，并纳入伦理视角。

---

12. Summary of Key Points | 考点总结

Cloning can be natural (e.g., vegetative propagation, identical twins) or artificial (tissue culture, embryo splitting, SCNT). Plant tissue culture uses meristem explants grown on sterile nutrient agar with hormones to produce whole plants, while animal cloning relies on transferring a diploid nucleus into an enucleated egg.

克隆可以是自然的（如营养繁殖、同卵双胞胎）或人工的（组织培养、胚胎分割、SCNT）。植物组织培养使用在无菌营养琼脂上生长的分生组织外植体和激素以产生完整植株，而动物克隆依赖将一个二倍体核移入去核卵。

The key stages of SCNT include nuclear removal, fusion, activation, embryo culture, and transfer to a surrogate. Cloning offers uniform crops, elite livestock, and potential for conservation and medicine, but is constrained by low efficiency, health problems, reduced diversity, and ethical dilemmas.

SCNT的关键阶段包括去核、融合、激活、胚胎培养和移植到代孕母体。克隆可提供均一的作物、优良牲畜以及保护和医学潜力，但受限于低效率、健康问题、多样性降低和伦理困境。

Students should be able to compare different cloning techniques, evaluate their advantages and disadvantages, and articulate well‑reasoned arguments on the ethics of cloning. Mastering these details will ensure top marks in the OCR A‑Level examination.

学生应能够比较不同的克隆技术，评价其优缺点，并就克隆的伦理提出有说服力的论证。掌握这些细节将确保在OCR A‑Level考试中获得高分。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）