Cell & Nuclear Division

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शिक्षकों के लिए: Cell & Nuclear Division (Biology, HL) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके शिक्षार्थी लाइव गेम की तरह खेलें।

लेसन नोट्स

Cell Division and Cytokinesis

According to cell theory, new cells are produced from pre-existing ones; a parent cell divides to produce two daughter cells.
Nuclear division occurs first, followed by cytokinesis, the division of the cytoplasm, which produces two genetically identical daughter cells in mitosis.
In animal cells, cytokinesis involves a cleavage furrow formed by a contractile ring of actin and myosin proteins that pinches the cell in two.
In plant cells, a cell plate forms at the equator and develops into a new cell wall, separating the daughter cells.
Cytokinesis is usually equal, but can be unequal, as in oogenesis (forming an ovum and polar bodies) and budding in yeast.
Each daughter cell must receive at least one mitochondrion (and a chloroplast in plant cells) because these organelles can only be made by dividing pre-existing structures.

Nuclear Division: Mitosis and Meiosis

Mitosis produces two genetically identical daughter nuclei that are also identical to the parent nucleus, maintaining the chromosome number and genome.
Mitosis is used for growth, repair of damaged tissues, replacement of cells, and asexual reproduction.
Meiosis produces four genetically different daughter cells, each with half the number of chromosomes of the parent cell (haploid, n).
Meiosis is used to produce gametes (sex cells) and is important for generating genetic diversity.
Before nuclear division, DNA replication occurs during interphase, creating two identical sister chromatids joined at a centromere.
During anaphase, sister chromatids separate and are pulled to opposite poles; after separation, each chromatid is considered an individual chromosome.

Meiosis

Chromosome Structure and Condensation

DNA is a very long molecule that must fit inside a much smaller nucleus; it is packaged with histone proteins.
DNA wraps around eight histone proteins to form a nucleosome; the complex of DNA and histones is called chromatin.
During prophase, chromatin condenses by supercoiling to form visible chromosomes.
Microtubules are tubulin fibres that form part of the cytoskeleton and are responsible for chromosome movement during cell division.
Microtubules are made of α-tubulin and β-tubulin dimers that can be added or removed to change length.
Motor proteins carry chromosomes along microtubules to the equator of the cell.

From cell to gene

Phases of Mitosis

Prophase: chromosomes condense and become visible; centrosomes move to opposite poles; spindle fibres emerge; the nuclear envelope breaks down and the nucleolus disappears.
Metaphase: chromosomes line up at the equator (metaphase plate); spindle fibres attach to centromeres via kinetochores; each sister chromatid is attached to a spindle fibre from opposite poles.
Anaphase: sister chromatids separate at the centromere; spindle fibres shorten and pull the separated chromosomes to opposite poles.
Telophase: chromosomes arrive at poles and decondense; nuclear envelopes reform; spindle fibres break down; new nucleoli form.
Cytokinesis follows nuclear division, dividing the cytoplasm to form two daughter cells.
Remember the order of mitosis stages with PMAT: Prophase, Metaphase, Anaphase, Telophase.

Mitosis

Identifying Mitosis Stages in Micrographs

Interphase: most cells are in this stage; chromatin is visible but chromosomes are not, so nuclei appear dark.
Prophase: chromosomes are visible and the nuclear envelope is breaking down; only one group of chromosomes is present.
Metaphase: chromosomes are lined up along the middle of the cell.
Anaphase: chromosomes are moving away from the middle towards opposite poles, often appearing V-shaped.
Telophase: chromosomes have arrived at opposite poles, begin to uncoil, and the nuclear envelope is reforming; two groups of chromosomes are present.
Cytokinesis: in animal cells a cleavage furrow forms; in plant cells a cell plate forms at the metaphase plate and expands towards the cell wall.

Meiosis and Genetic Variation

Meiosis involves two divisions: Meiosis I (reduction division) separates homologous chromosomes, and Meiosis II separates sister chromatids.
During prophase I, homologous chromosomes pair up to form a bivalent; this pairing is called synapsis.
Crossing over occurs between non-sister chromatids at points called chiasmata, exchanging genetic material and producing new allele combinations.
Random orientation of bivalents at metaphase I means each homologous chromosome is randomly attached to one pole, and bivalents assort independently.
The number of possible chromosome combinations from random assortment is 2ⁿ, where n is the number of homologous pairs.
Non-disjunction occurs when chromosomes fail to separate correctly during anaphase I or II, leading to gametes with an abnormal chromosome number.
Down syndrome (Trisomy 21) is caused by non-disjunction of chromosome 21, resulting in three copies and a total of 47 chromosomes.

The Cell Cycle and Its Control

The cell cycle is the regulated sequence of events between one cell division and the next, consisting of interphase, nuclear division (mitosis), and cell division (cytokinesis).
Interphase includes G1, S, and G2 phases; it is the longest and most active phase, where the cell grows and carries out normal functions.
G1 phase: cell grows, synthesises RNA, enzymes, and proteins; may receive signal to divide.
S phase: DNA replication occurs, resulting in chromosomes with two identical sister chromatids.
G2 phase: cell continues to grow, DNA is checked and repaired, and proteins for division (e.g., tubulin) are produced.
The cell cycle is controlled by cyclins (proteins) and cyclin-dependent kinases (enzymes); cyclins bind to kinases to trigger specific events.
There are three checkpoints: at G1, G2, and M; these must be overridden before the next stage begins.

Uncontrolled Cell Division and Tumour Formation

Mutations in genes controlling the cell cycle can lead to cancer.
Proto-oncogenes code for proteins that stimulate normal cell division; mutation converts them to oncogenes, leading to increased or permanently activated proteins and uncontrolled division.
Tumour-suppressor genes code for proteins that inhibit cell division or promote apoptosis; mutations may result in reduced or deactivated proteins, also leading to uncontrolled division.
Malignant tumours are cancerous: they grow rapidly, invade and destroy surrounding tissues, and can metastasise (spread via blood or lymph).
Benign tumours are not cancerous: they grow slowly, do not invade other tissues, and do not metastasise.
Carcinogens such as UV radiation, tobacco, asbestos, and processed meat can initiate malignant tumour formation.

Mitotic Index

The mitotic index is the proportion of cells in a sample that are undergoing mitosis.
It is calculated using the formula: mitotic index = number of cells with visible chromosomes / total number of cells.
The answer can be multiplied by 100 to give a percentage.
A high mitotic index indicates a high rate of cell division, which may suggest the presence of a tumour.
To study mitosis, root tip meristems can be used; cells are stained and squashed to spread them out for observation.
The mitotic index formula must be remembered as it is not provided in exams.

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साइन अप करें — फ्री लॉग इन

प्रैक्टिस सवाल

फ्री प्रीव्यू — 64 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.Which of the following is the correct definition of the mitotic index?
Easy
- AThe proportion of cells in a sample that are undergoing mitosis
- BThe number of cells in a sample that are in interphase
- CThe total number of cells in a sample
- DThe number of cells that are undergoing meiosis
2.Which of the following is the correct formula for calculating the mitotic index?
Easy
- Anumber of cells in interphase / total number of cells
- Bnumber of cells undergoing mitosis / total number of cells
- Ctotal number of cells / number of cells undergoing mitosis
- Dnumber of cells undergoing mitosis / number of cells in interphase
3.Mitosis produces two genetically identical daughter nuclei.
Easy
True or false?
4.Meiosis produces four haploid nuclei from one diploid nucleus.
Easy
True or false?
5.Which of the following correctly describes the term 'chromatid'?
Easy
- AA single strand of DNA that is replicated during interphase
- BOne of the two identical strands of DNA that make up a chromosome after DNA replication
- CThe region where two chromatids are joined together
- DA pair of homologous chromosomes
6.What is the name of the narrow region that joins two sister chromatids together?
Easy
- ACentromere
- BCentrosome
- CChiasma
- DKinetochore
7.Which of the following statements about homologous chromosomes is correct?
Easy
- AThey carry the same genes in the same order but may have different alleles
- BThey are identical copies of each other produced by DNA replication
- CThey are only found in haploid cells
- DThey are separated during anaphase of mitosis
8.Crossing over occurs between non-sister chromatids of homologous chromosomes.
Easy
True or false?

और सवाल

9.Non-disjunction results in gametes with the correct haploid number of chromosomes.
Easy
True or false?
10.During which phase of the cell cycle does DNA replication occur?
Medium
- AG1 phase
- BS phase
- CG2 phase
- DM phase
11.Which of the following is a role of mitosis in living organisms?
Medium
- AProduction of gametes
- BGrowth and repair of tissues
- CGeneration of genetic variation
- DReduction of chromosome number
12.In a sample of onion root tip cells, 20 cells were undergoing mitosis out of a total of 75 cells. Calculate the mitotic index.
Medium
- A0.27
- B0.73
- C3.75
- D0.20
13.Which of the following statements about the cell cycle is correct?
Medium
- AInterphase is the shortest phase of the cell cycle
- BDNA replication occurs during the G2 phase
- CCyclins are proteins that control the cell cycle
- DMitosis is part of interphase
14.During anaphase of mitosis, sister chromatids are pulled to opposite poles by spindle fibres.
Easy
True or false?
15.A cell has 24 chromosomes. How many chromosomes will each daughter cell have after meiosis?
Medium
- A12
- B24
- C48
- D6
16.Which of the following contributes to genetic variation during meiosis?
Medium
- ACrossing over and random orientation of bivalents
- BDNA replication and cytokinesis
- CMitosis and binary fission
- DCondensation of chromosomes and formation of spindle fibres
17.Match each stage of mitosis with its correct description.
Medium
- Prophase
- Metaphase
- Anaphase
- Telophase
- Chromosomes line up at the equator
- Chromosomes condense and nuclear envelope breaks down
- Chromosomes arrive at opposite poles and decondense
- Sister chromatids separate and are pulled to opposite poles
18.Place the stages of mitosis in the correct sequence.
Medium
- Prophase
- Metaphase
- Anaphase
- Telophase
19.Which of the following is the correct definition of an oncogene?
Easy
- AA gene that stimulates normal cell division
- BA gene that inhibits cell division
- CA mutated gene that can lead to uncontrolled cell division
- DA gene that promotes apoptosis
20.Tumour-suppressor genes code for proteins that inhibit cell division.
Easy
True or false?
21.Which of the following statements about the cell cycle are correct? (select all that apply)
Medium
- AInterphase is the longest phase of the cell cycle
- BDNA replication occurs during the S phase of interphase
- CCyclins are proteins that control the cell cycle
- DMitosis is the stage where the cytoplasm divides
- EThe G2 phase occurs before the S phase
22.Match each stage of the cell cycle with its correct description.
Medium
- G1 phase
- S phase
- G2 phase
- M phase
- Cell growth and protein synthesis
- DNA replication
- Further growth and preparation for mitosis
- Nuclear division and cytokinesis
23.Place the stages of the cell cycle in the correct sequence.
Medium
- G1 phase
- S phase
- G2 phase
- Mitosis
- Cytokinesis
24.Which of the following is the correct definition of a bivalent?
Medium
- AA pair of homologous chromosomes
- BA pair of sister chromatids
- CA single chromosome with two chromatids
- DA pair of non-homologous chromosomes
25.During meiosis I, homologous chromosomes are separated, while during meiosis II, sister chromatids are separated.
Easy
True or false?
26.Which of the following processes contribute to genetic variation during meiosis? (select all that apply)
Medium
- ACrossing over
- BRandom orientation of bivalents
- CDNA replication
- DNon-disjunction
- EIndependent assortment
27.Which of the following statements about non-disjunction is correct?
Medium
- AIt results in gametes with the correct haploid number of chromosomes
- BIt occurs when homologous chromosomes fail to separate during meiosis I
- CIt always leads to Down syndrome
- DIt occurs during mitosis
28.A doctor examines a tissue sample from a tumour and finds a very high mitotic index compared with normal tissue from the same organ. What does this most likely indicate?
Medium
- AThe tumour cells are dividing rapidly and the tumour may be malignant
- BThe tumour cells have stopped dividing and are benign
- CThe tumour cells are undergoing meiosis to form gametes
- DThe tissue sample contains mostly cells in G0 phase
29.A student counts 250 cells in a root tip squash. 60 cells are in prophase, 15 in metaphase, 10 in anaphase and 15 in telophase. What is the mitotic index, expressed as a percentage?
Medium
- A40%
- B10%
- C60%
- D4%
30.Which change in the cell cycle would most directly explain the uncontrolled division seen in cancer cells?
Medium
- AA mutation causing a cyclin-dependent kinase complex to remain permanently active
- BA mutation preventing the S phase from occurring
- CA mutation increasing the rate of cytokinesis only
- DA mutation causing the nuclear envelope to reform earlier in telophase
31.A cell biologist treats cultured cells with a drug that prevents the breakdown of the nuclear envelope. Which stage of mitosis would be directly blocked?
Medium
- AProphase
- BMetaphase
- CAnaphase
- DTelophase
32.A student studies a photomicrograph of a root tip and sees a cell in which the chromosomes are arranged in a single line across the middle of the cell. Which statements about this cell are correct? (select all that apply)
Medium
- AThe cell is in metaphase
- BSpindle fibres are attached to the centromeres of the chromosomes
- CThe sister chromatids have already separated
- DThe chromosomes are equidistant from the two poles
- EThe nuclear envelope has fully reformed
33.A photomicrograph of a root tip shows a cell containing two groups of chromosomes, one at each end of the cell, with a cleavage furrow forming between them. Which stage is this cell in?
Hard
- ATelophase
- BProphase
- CMetaphase
- DAnaphase
34.In a root tip squash, the majority of cells are usually seen in interphase because interphase is the longest phase of the cell cycle.
Easy
True or false?
35.A high mitotic index in a tissue sample indicates that a large proportion of the cells are undergoing meiosis.
Medium
True or false?
36.Match each observation from a photomicrograph of a root tip with the stage of mitosis it indicates.
Medium
- Chromosomes visible as two groups at opposite poles
- Chromosomes lined up along the equator
- Chromosomes moving away from the equator with a V shape
- Chromosomes visible with the nuclear envelope breaking down
- Telophase
- Metaphase
- Anaphase
- Prophase
37.A student observes five cells in a root tip squash and identifies the stage of each. Place the stages in the order in which they occur during mitosis.
Medium
- Prophase
- Metaphase
- Anaphase
- Telophase
38.A cell from a plant has 24 chromosomes. How many different chromosome combinations can be produced in its gametes by random orientation of bivalents during meiosis I? Use the formula 2ⁿ.
Hard
- A4096
- B24
- C12
- D48
39.A human diploid cell (2n = 46) undergoes meiosis. A student states that the number of possible chromosome combinations in the gametes is 2⁴⁶. Why is this incorrect?
Hard
- AThe formula uses n = 23, the number of homologous pairs, giving 2²³
- BThe formula uses n = 92, the number of chromatids, giving 2⁹²
- CThe formula should use 2n = 46 as the exponent
- DThe number of combinations is fixed at 46
40.A karyogram of a foetus shows three copies of chromosome 21, giving a total of 47 chromosomes. Which event during meiosis most directly explains this result?
Challenge
- ANon-disjunction of the 21st pair of homologous chromosomes during anaphase I
- BCrossing over between non-sister chromatids of chromosome 21
- CRandom orientation of bivalents at metaphase I
- DFailure of DNA replication during interphase
41.Name the protein complex that packages DNA into nucleosomes.
Medium
42.Complete the sentence: The division of the cytoplasm is called ___.
Easy
The division of the cytoplasm is called ____.
43.A student counted 42 cells in a sample, 32 of which had visible chromosomes. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
44.A cell has 24 chromosomes. Calculate the number of possible chromosome combinations that can be generated in gametes during meiosis. Use the formula 2ⁿ, where n is the number of homologous chromosome pairs.
Medium
45.A sample of garlic root tip tissue contained 36 cells in interphase, 14 in prophase, 5 in metaphase, 3 in anaphase and 6 in telophase. Calculate the mitotic index for this tissue. Give your answer to 2 decimal places.
Medium
46.A student examined a root tip squash and recorded 55 cells in interphase and 20 cells with visible chromosomes. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
47.Complete the sentence about cytokinesis in plant cells.
Medium
In plant cells, a ____ forms at the equator of the cell and expands towards the cell wall of the parent cell, separating the daughter cells.
48.Which protein complex packages DNA into nucleosomes?
Easy
49.The cells of domestic cats contain 38 chromosomes. Calculate the number of possible chromosome combinations that can be generated in domestic cat gametes during meiosis. Use the formula 2ⁿ, where n = the number of chromosome pairs.
Hard
50.Complete the sentence about identifying mitotic stages from photomicrographs.
Medium
In prophase there is only one group of chromosomes in the cell, while in telophase there are ____ groups, one at each pole.
51.A cell biologist counts 400 cells in a tumour sample and finds that 120 of them are undergoing mitosis. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
52.Calculate the mitotic index for this tissue sample. Give your answer to 2 decimal places.
Medium
53.Outline the events of crossing over during meiosis.
Hard
54.Explain how the process of crossing over during meiosis I increases genetic variation in the gametes produced.
Hard
55.Explain why the halving of the chromosome number during meiosis is essential in organisms with a sexual life cycle.
Hard
56.Outline the events in the cell cycle that result in the development of a new yeast cell during budding.
Hard
57.Explain why the majority of the cells counted in this sample were in interphase.
Medium
58.A student counted 42 cells in a sample, 32 of which had visible chromosomes. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
- A2.76
- B0.76
- C1.76
- D-0.24
59.A cell has 24 chromosomes. Calculate the number of possible chromosome combinations that can be generated in gametes during meiosis. Use the formula 2ⁿ, where n is the number of homologous chromosome pairs.
Medium
- A4916
- B4096
- C3686
- D4506
60.A sample of garlic root tip tissue contained 36 cells in interphase, 14 in prophase, 5 in metaphase, 3 in anaphase and 6 in telophase. Calculate the mitotic index for this tissue. Give your answer to 2 decimal places.
Medium
- A-0.56
- B0.44
- C1.44
- D2.44
61.A student examined a root tip squash and recorded 55 cells in interphase and 20 cells with visible chromosomes. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
- A-0.73
- B1.27
- C2.27
- D0.27
62.The cells of domestic cats contain 38 chromosomes. Calculate the number of possible chromosome combinations that can be generated in domestic cat gametes during meiosis. Use the formula 2ⁿ, where n = the number of chromosome pairs.
Hard
- A576717
- B471859
- C524288
- D629146
63.A cell biologist counts 400 cells in a tumour sample and finds that 120 of them are undergoing mitosis. Calculate the mitotic index. Give your answer to 2 decimal places.
Medium
- A2.3
- B1.3
- C-0.7
- D0.3
64.Calculate the mitotic index for this tissue sample. Give your answer to 2 decimal places.
Medium
- A1.44
- B-0.56
- C0.44
- D2.44

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