Q.Given that the average duplication time of E.coli is 20 minutes. How much time will two E.coli cells take to become 32 cells ?
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Modes of Asexual Reproduction
Asexual reproduction takes several distinct forms depending on the organism. Many single-celled organisms reproduce by binary fission, splitting into two roughly equal daughter cells (as in Amoeba and Paramecium), or by budding, an unequal division that produces a smaller attached bud which later separates and matures (as in yeast). Fungi and simple algae commonly produce specialised asexual structures — motile zoospores, or spore-bearing conidia — while simple animals reproduce through budding (Hydra) or gemmule formation (sponges). Under harsh conditions, Amoeba can also encyst and later un …
2 cells need 4 doublings to become 32 cells; at 20 min each, total = 80 minutes.
E. coli reproduces by binary fission, so each generation the population doubles. The duplication (generation) time is given as 20 minutes.
Starting from 2 cells and doubling each 20-minute cycle:
| Divisions | Time (min) | Number of cells |
|---|---|---|
| Start | 0 | 2 |
| 1 | 20 | 4 |
| 2 | 40 | 8 |
| 3 | 60 | 16 |
| 4 | 80 | 32 |
So we need 4 doublings to go from 2 cells to 32 cells.
Number of cells =2×2n, where n = number of divisions. Setting 2×2n=32 gives 2n=16=24, so n=4.
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- CBSE 2026Set 57/2/11 markMCQQ.For Question numbers 13 to 16, two statements are given – one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from Code (A), (B), (C) and (D) as given below : (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not correct explanation for Assertion (A). (C) Assertion (A) is true, Reason (R) is false. (D) Assertion (A) is false, Reason (R) is true. Assertion (A) : Apomixis leads to production of genetically identical off-springs. Reason (R) : Apomixis involves fertilization between two genetically similar plants.
›Reveal solutionSolution
Apomixis produces genetically identical offspring because it bypasses meiosis and fertilization entirely, not because it involves fertilization between similar plants.
Let’s first understand what apomixis actually is. In flowering plants, the normal route to seed formation involves two key events: meiosis (which halves the chromosome number and creates genetic variation) and fertilization (fusion of male and female gametes). Apomixis is a remarkable exception — it is a form of asexual reproduction that mimics sexual reproduction. The plant produces seeds without the fusion of gametes. The embryo develops directly from the maternal tissues of the ovule, such as the nucellus or the integuments, or from an unreduced egg cell that never underwent meiosis. Because there is no meiosis and no fertilization, the offspring are exact genetic copies of the mother plant — clones.
Now look at the Assertion: “Apomixis leads to production of genetically identical offspring.” This is perfectly true. Since the embryo is formed from maternal cells without any genetic recombination, every seedling is a clone of the parent. This is why apomixis is so valuable in agriculture — it allows farmers to propagate hybrid varieties indefinitely without the seeds losing their desirable traits. …
- CBSE 2025Set F1 markMCQQ.Which of the following types of reproduction takes place in Amoeba under favourable condition?(a) Fragmentation(b) Multiple fission(c) Budding(d) Binary fission
›Reveal solutionSolution
In favourable conditions Amoeba divides by binary fission; multiple fission is its unfavourable-condition (cyst) response.
Amoeba reproduces asexually. Under favourable conditions it undergoes binary fission: the nucleus divides mitotically and the cytoplasm splits, giving two equal daughter Amoebae.
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- CBSE 2024Set 57/3/11 markMCQQ.In which one of the following floral plants are many embryos formed in the seeds without fertilisation of the egg cell ? (A) Black pepper (B) Mustard (C) Groundnut (D) Citrus
›Reveal solutionSolution
The question asks for a plant where many embryos form in seeds without egg fertilisation — this is apomixis (specifically adventive embryony). Among the options, Citrus is the classic example where nucellar cells develop into embryos without fertilisation, so the correct answer is (D) Citrus.
The key concept here is apomixis — a form of asexual reproduction in plants where seeds are formed without the fusion of gametes. The specific type described in the question is adventive embryony (also called nucellar embryony), where embryos develop directly from diploid cells of the nucellus or integuments, not from the fertilised egg. This means multiple embryos can arise in a single seed, a phenomenon called polyembryony.
Why does this happen? In normal sexual reproduction, the egg cell is fertilised by a sperm to form the zygote, which develops into the embryo. In adventive embryony, the egg may or may not be fertilised, but additional embryos arise from surrounding maternal tissue — so they are genetically identical to the mother plant. This is a survival strategy that preserves a successful genotype.
Now, let's examine each option:
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Black pepper (Piper nigrum) — It reproduces sexually through normal fertilisation. No known mechanism of adventive embryony or apomixis is reported in this plant. Seeds typically contain a single embryo from the fertilised egg.
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Mustard (Brassica spp.) — Mustard is a strictly sexual plant. It undergoes normal double fertilisation (syngamy and triple fusion) to form the embryo and endosperm. Polyembryony or apomixis does not occur naturally in mustard.
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Groundnut (Arachis hypogaea) — Also called peanut. It reproduces sexually. After fertilisation, the flower stalk elongates and pushes the ovary into the soil (geocarpy), but the embryo develops from the fertilised egg only. No multiple embryos from nucellar tissue. …
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- CBSE 2023Set ANNUAL1 markMCQQ.Asexual reproductive structures in Hydra are -(a) Bud(b) Conidia(c) Zoospores(d) Gemmules
›Reveal solutionSolution
Hydra's characteristic asexual method is budding, not conidia formation, zoospore formation or gemmule formation.
In Hydra, asexual reproduction occurs mainly through budding. A localised outgrowth called a bud arises on the body wall by repeated mitotic divisions. The bud grows, develops tentacles and a mouth of its own, and eventually detaches from the parent body to lead an independent life as a new Hydra.
- Conidia are asexual spores produced by certain fungi (e.g., Penicillium), not by Hydra. …
- CBSE 2023Set ANNUAL1 markMCQQ.What type of binary fission occurs in Paramecium?(a) simple binary fission(b) transverse binary fission(c) longitudinal binary fission(d) oblique binary fission
›Reveal solutionSolution
Paramecium undergoes transverse binary fission.
In binary fission a single parent divides into two daughter cells. The plane of division differs between organisms. In Paramecium the division plane runs across the body, perpendicular to the long axis, so it is transverse binary fission. (In …
- CBSE 2022Set ANNUAL1 markMCQQ.Penicillium produces(a) mitospore(b) zoospore(c) meiospore(d) both (A) and (C)
›Reveal solutionSolution
Penicillium's asexual conidia are mitospores, so the answer is (A).
Penicillium is an ascomycete fungus. Its common mode of reproduction is asexual, by the production of chains of non-motile spores called conidia borne on brush-like conidiophores. Because these conidia are produced by ordinary mitotic divisions, they are mitospores. Fungi do not produce motil …
- CBSE 2021Set D1 markMCQQ.Binary fission occurs in(a) Amoeba(b) Paramecium(c) Both (A) and (B)(d) None of these
›Reveal solutionSolution
Binary fission (parent splits into two) is seen in both Amoeba and Paramecium, so the answer is (C) Both (A) and (B).
Binary fission is a form of asexual reproduction in which the parent cell divides into two nearly equal daughter cells, each becoming an independent organism. It is common in unicellular organisms.
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- CBSE 2019Set ANNUAL1 markMCQQ.Yeast reproduces by means of-(a) Budding(b) Fragmentation(c) Pollination(d) All of these
›Reveal solutionSolution
Yeast reproduces asexually by budding.
In budding, a small outgrowth (bud) develops on the parent body, receives a nucleus and cytoplasm, grows, and then separates to become a new individual. Yeast (and Hydra) reproduce this way. Fragmentation oc …
- CBSE 2017Set ANNUAL1 markMCQQ.In which of the following organism, asexual reproduction occurs by buds?(a) Chlamydomonas(b) Hydra(c) Penicillium(d) Sponge
›Reveal solutionSolution
Hydra reproduces asexually by budding; the other options use different asexual methods.
In budding, a small outgrowth (bud) develops on the parent body, grows by repeated mitotic division, develops its own tentacles and mouth, and eventually detaches (or stays attached to form a colony) as a new individual genetically identical to the parent.
- (a) Chlamydomonas — reproduces asexually by forming zoospores, not buds.
- (b) Hydra — reproduces asexually by budding; a bud forms on the side of the body wall, grows, and pinches off as a new hydra. Correct. …
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