Describe cloning (reproductive and therapeutic)
WHAT is cloning?
Because there is no meiosis and no fertilisation between two gametes, the offspring's DNA comes from a single donor. This is why the copy is genetically the same (barring mutations and mitochondrial DNA, discussed below).
WHY does it work at all? Every somatic cell of your body has the full genome (this is genomic equivalence). A skin cell only uses skin genes, but it still contains the genes for a whole animal. The egg cytoplasm contains factors that "reset" the nucleus, switching genes back on so the full body plan can be built again.
HOW the two types differ
Both start the same way (SCNT → early embryo). They diverge at the blastocyst stage:
| Feature | Reproductive cloning | Therapeutic cloning |
|---|---|---|
| Goal | Produce a whole living organism | Produce embryonic stem cells / tissue |
| Embryo fate | Implanted into a surrogate uterus | Grown in vitro; inner cell mass harvested |
| End product | A new individual (e.g. Dolly) | Stem cells → replacement tissue for a patient |
| Continues to birth? | Yes | No (embryo not implanted) |
Reproductive cloning — deriving the steps from scratch
Let's build the protocol by reasoning, not memorising.
- We want a genetic copy of donor animal D. So the DNA must come only from D. → Take a somatic cell from D (its nucleus has D's full genome).
- A nucleus alone can't develop. It needs egg cytoplasm to reprogram + supply machinery. → Take an egg from another female of the same species.
- We must remove the egg's own genes, else the offspring would be a mix. → Enucleate the egg.
- Combine them. Insert D's nucleus into the enucleated egg (micro-injection or fusion by electric pulse). Now the egg has D's diploid genome.
- Trigger division. An electric/chemical shock mimics fertilisation → egg begins mitosis → forms an embryo (blastocyst).
- Grow the whole animal. Implant the embryo into a surrogate mother's uterus → pregnancy → birth of a clone of D.
Therapeutic cloning — deriving the steps
Same first 5 steps as above (SCNT → blastocyst). Then:
- Do NOT implant. Instead grow the blastocyst in vitro.
- Harvest the inner cell mass → these are embryonic stem (ES) cells, which are pluripotent (can become any body cell type).
- Differentiate the ES cells (using specific growth factors) into the tissue the patient needs — e.g. insulin-producing pancreatic cells, neurons, cardiac muscle.
- Transplant into the patient.
Why a clone is not a perfect copy
Recall Feynman: explain to a 12-year-old
Imagine every cell in a sheep has the full "recipe book" to build a whole sheep, but skin cells only ever cook the "skin chapter." To clone, we take that recipe book (the nucleus) out of a skin cell and drop it into an empty egg — an egg we've hollowed out. The egg is magic: it says "start from page one!" and reboots the whole recipe. If we then put this rebooted egg into a mummy sheep's tummy, it grows into a new sheep that looks just like the recipe's owner — that's reproductive cloning (like Dolly). But if instead we keep the tiny ball of cells in a dish and use it to grow just a patch of new tissue (like fresh insulin cells for a sick person), that's therapeutic cloning — no baby, just a repair part that the patient's body won't fight, because it's made from their own recipe.
Active-recall flashcards
What is a clone?
What does SCNT stand for and mean?
Why must the egg be enucleated before nuclear transfer?
What property of somatic cells makes cloning possible?
At which stage do reproductive and therapeutic cloning diverge?
Fate of the embryo in reproductive cloning?
Fate of the embryo in therapeutic cloning?
Which sheep was Dolly a clone of, and what was the proof?
What makes therapeutic-cloned tissue not rejected by the patient?
Why is a clone NOT 100% identical?
How does SCNT differ from IVF?
What are ES cells and why useful?
Connections
- Stem Cells and Pluripotency — source of tissues in therapeutic cloning
- Totipotency and Genomic Equivalence — the principle that lets a somatic nucleus rebuild an organism
- Mitochondrial DNA Inheritance — why clones aren't perfect copies
- Immune Rejection and MHC/HLA — why patient-derived tissue avoids rejection
- IVF and Assisted Reproduction — contrast: sexual vs asexual
- Recombinant DNA and Molecular Cloning — "cloning" of genes vs whole organisms
- Ethical Issues in Biotechnology — debate around embryo use
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, cloning ka matlab hai ek aisi copy banana jo genetically bilkul identical ho — matlab do parents ka DNA mix karne wala normal tareeka hum skip kar dete hain. Iska core technique hai SCNT (Somatic Cell Nuclear Transfer): donor ke body cell (somatic cell) ka nucleus nikaalo, aur ek aise egg mein daalo jiska apna nucleus pehle hi hata diya gaya ho (enucleated egg). Egg ka cytoplasm us nucleus ko "reset" kar deta hai, taki wo dobara ek pura jeev bana sake. Yeh possible isliye hai kyunki har body cell mein poora genome hota hai — isko genomic equivalence kehte hain.
Ab reproductive aur therapeutic cloning shuru mein same hote hain (dono SCNT se blastocyst banate hain), par blastocyst stage par alag ho jaate hain. Reproductive cloning mein embryo ko ek surrogate mother ke uterus mein daal dete hain, aur usse ek poora naya animal paida hota hai — jaise famous Dolly sheep. Dolly ka face Finn-Dorset breed jaisa tha (nucleus donor jaisa), isse proof mila ki identity nucleus se aati hai, egg ya surrogate se nahi.
Therapeutic cloning mein hum embryo ko uterus mein daalte hi nahi — dish mein hi rakhte hain aur usse embryonic stem cells nikaal lete hain. Yeh pluripotent hote hain, matlab kisi bhi tarah ki cell ban sakte hain — jaise insulin banane wale beta cells diabetic patient ke liye. Agar nucleus patient ka apna ho, to tissue patient ka hi hoga, isliye body use reject nahi karti — yehi iska sabse bada faayda hai.
Ek important baat yaad rakho: clone 100% identical nahi hota. Kyunki mitochondrial DNA egg se aata hai (nucleus se nahi), aur environment/epigenetics bhi look aur behaviour change kar dete hain. Exam mein steel-man mistakes zaroor cover karna: therapeutic cloning se baby nahi banta, aur SCNT IVF se alag hai (IVF sexual hai, SCNT asexual).