Cell Biology and Cell Division

Cell Biology and Cell Division is the heaviest unit in the Botany paper — 5 of the 40 questions — and the most predictable. One question usually comes from the cell theory or the prokaryote/eukaryote comparison, two or three from organelle structure and function, and one or two from the cell cycle and the differences between mitosis and meiosis. The numbers are what examiners love: 70S and 80S, 9+2, 46 and 23, 2C and 4C. Learn the tables and count chromosomes carefully; this unit is where careless students lose marks they knew.

7.1 Prokaryotic and eukaryotic cells; the cell theory

How the cell was discovered, and the cell theory

The cell theory in its modern form says four things: the cell is the structural and functional unit of all living organisms; all organisms are made of one or more cells; new cells arise only by division of existing cells; and all cells are chemically similar, containing DNA, RNA, protein and a plasma membrane.

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Exceptions. The theory has three standard exceptions:

1. Viruses, viroids and prions are acellular — they have no cell, no cytoplasm and no ribosomes, and cannot reproduce outside a host.
2. Coenocytic (aseptate) structures — the hyphae of Mucor and Rhizopus are a single continuous mass of cytoplasm with many nuclei and no cross walls; the latex vessels of plants and striated muscle fibres are also multinucleate. One "cell" holds many nuclei.
3. Prokaryotes have no membrane-bound nucleus or organelles, so the classical statement "every cell has a nucleus" does not apply to them — though they are still cells.

Prokaryotic and eukaryotic cells compared

7.2 The cell wall, the membrane and the organelles

The cell wall

Only plants, fungi and bacteria have a wall; animal cells do not. A plant cell wall has three parts, laid down in this order:

Plasmodesmata are fine cytoplasmic channels that pass through the wall and connect the cytoplasm of adjacent cells. The wall's functions are support, protection, a limit to water uptake (which is why a plant cell does not burst in pure water), and transport through the plasmodesmata. Bacterial walls are peptidoglycan, fungal walls chitin — a fact the paper tests every year.

The plasma membrane

The membrane is a phospholipid bilayer about 7.5 nm thick. The phosphate heads face outwards into water; the fatty-acid tails face inwards, away from it. Proteins float in or through this bilayer, and cholesterol sits between the phospholipid tails. Because the proteins move about in a fluid lipid sea, Singer and Nicolson (1972) called it the fluid-mosaic model.

The membrane is selectively permeable: small non-polar molecules (O₂, CO₂) cross by simple diffusion, water by osmosis through channels called aquaporins, and ions and sugars through pumps and carriers, often against the gradient using ATP.

Mitochondria

A mitochondrion is 0.5–1 μm wide and 2–8 μm long, bounded by two membranes. The outer membrane is smooth; the inner one is folded into finger-like cristae that project into the jelly-like matrix. On the inner membrane sit the F₁ particles (oxysomes), which carry the enzymes of the electron transport chain and ATP synthase. The matrix holds the enzymes of the Krebs cycle, circular DNA, 70S ribosomes and phosphate granules.

Because a mitochondrion has its own DNA and ribosomes it is called semi-autonomous: it makes some of its own proteins but depends on nuclear genes for the rest, and it divides on its own. Mitochondrial DNA is inherited from the mother in humans. The mitochondrion is the powerhouse — the site of aerobic respiration and oxidative phosphorylation. Cells that do heavy work (heart muscle, flight muscle, the tapetum of an anther) are packed with them.

Chloroplasts

A chloroplast is a lens-shaped organelle 5–10 μm long, also bounded by two membranes. Inside is the stroma, a protein-rich fluid holding the enzymes of the Calvin cycle, starch grains, circular DNA and 70S ribosomes. From the stroma, flat membranous sacs called thylakoids arise; where they are stacked like coins they form a granum, and the unstacked thylakoids that join one granum to the next are the intergranal (stroma) lamellae. The chlorophyll and the photosystems sit in the thylakoid membrane, not in the stroma — a favourite trap. Algal chloroplasts carry a protein body, the pyrenoid, around which starch is laid down.

Endoplasmic reticulum

The ER is a network of membrane-bound tubules and flattened sacs (cisternae) continuous with the outer nuclear membrane.

• Rough ER carries ribosomes on its surface and is the site of protein synthesis and transport — abundant in cells that secrete enzymes (pancreas, salivary gland).
• Smooth ER has no ribosomes. It makes lipids and steroids, detoxifies drugs and poisons (liver), stores Ca²⁺ (sarcoplasmic reticulum of muscle) and carries out glycogen breakdown.

Golgi body

The Golgi body (Golgi apparatus, dictyosome in plants) is a stack of flattened, curved cisternae with swollen margins. It has two faces: the cis (forming) face receives vesicles from the ER, and the trans (maturing) face releases vesicles towards the membrane. Its jobs are to modify, sort, package and secrete — adding sugar chains to proteins (glycosylation), forming lysosomes, producing cell-wall material, and in plant cells laying down the cell plate at cytokinesis.

Lysosomes

A lysosome is a single-membrane vesicle, formed from the Golgi, filled with about 40 hydrolytic (acid hydrolase) enzymes — protease, lipase, nuclease, phosphatase — that work best at pH ≈ 5. They digest material taken in by the cell (heterophagy) and worn-out organelles of the cell itself (autophagy). If the lysosome membrane ruptures, its enzymes digest the cell's own contents; Christian de Duve, who discovered them, therefore called lysosomes the "suicidal bags" of the cell. They are common in animal cells and in fungi; in mature plant cells the large central vacuole does much of the same work.

Ribosomes

A ribosome is a tiny granule of rRNA and protein, with no membrane, first seen by George Palade. It is the site of protein synthesis. The prokaryotic ribosome is 70S, made of a 50S and a 30S subunit; the eukaryotic cytoplasmic ribosome is 80S, made of 60S and 40S subunits. Mitochondria and chloroplasts contain 70S ribosomes — evidence for their bacterial origin. Several ribosomes translating one mRNA form a polysome.

Nucleus

The nucleus is bounded by a double membrane, the nuclear envelope, perforated by nuclear pores that let RNA and proteins pass between nucleus and cytoplasm. Inside are the nucleoplasm, the chromatin (DNA with histones) and one or more nucleoli. The nucleolus is not an organelle with a membrane; it is the site where rRNA is made and ribosomal subunits are assembled. The nucleus holds the genetic material and controls the cell's activities.

Chromosomes

During division the chromatin condenses into chromosomes. A chromosome that has replicated has two identical chromatids joined at the centromere, a narrow region (the primary constriction) that carries the kinetochore to which spindle fibres attach. The ends of each chromatid are telomeres, which protect the DNA from being degraded or fused. Some chromosomes have a satellite beyond a secondary constriction.

The DNA itself is wrapped twice round an octamer of histones — two molecules each of H2A, H2B, H3 and H4 — with about 147 base pairs per turn, forming a nucleosome; the string of nucleosomes is coiled further to make the visible chromosome.

Cilia and flagella

Eukaryotic cilia and flagella have the same internal structure: a core, the axoneme, of nine pairs of microtubules arranged in a ring around a central pair — the 9+2 arrangement. The pairs are linked by dynein arms, which slide the microtubules past one another using ATP, so the structure bends. At the base is a basal body with a 9+0 arrangement, identical to a centriole. Cilia are short and numerous; flagella are long and few (the sperm tail, Chlamydomonas).

A bacterial flagellum is completely different: a single fibril of the protein flagellin, with no 9+2 and no membrane, that rotates like a propeller driven by a proton gradient. Do not mix the two.

Cell inclusions

Cell inclusions are non-living materials in the cytoplasm — they are not organelles and have no membrane of their own:

Plant cells and animal cells compared

7.3 The cell cycle; amitosis, mitosis and meiosis

The cell cycle

The cell cycle is the sequence of events by which a cell duplicates its contents and divides into two. It has two main phases:

A cell that leaves the cycle and stops dividing — a mature neuron, a muscle fibre — enters G₀, a resting state from which it may never return. The checkpoints are controlled by cyclins and cyclin-dependent kinases (CDKs); if DNA is damaged at the G₁/S checkpoint the cell arrests and repairs it, or kills itself (apoptosis). Loss of checkpoint control is a step towards cancer.

Amitosis

Amitosis (direct division) is division of the nucleus without spindle formation and without condensation of chromosomes: the nucleus simply constricts and divides into two. It occurs in bacteria (binary fission), in some yeasts and protozoa, and in damaged or ageing cells. It produces no variation and is not a substitute for mitosis in the cells of higher organisms.

Mitosis

Mitosis is the division of a nucleus into two genetically identical daughter nuclei with the same chromosome number. It is one continuous process, described in four stages:

Cytokinesis follows. In an animal cell the membrane invaginates as a cleavage furrow (a contractile ring of actin and myosin) and pinches the cell in two. In a plant cell the rigid wall prevents pinching, so vesicles from the Golgi line up at the equator and fuse to form a cell plate, which becomes the middle lamella and then the new walls.

Significance of mitosis: it keeps the chromosome number constant (2n → 2n); it produces genetically identical cells, so it is the basis of growth, repair and replacement of tissues (skin, gut lining, blood cells), asexual reproduction (binary fission, budding, cuttings) and the vegetative propagation of plants.

Meiosis

Meiosis is a reduction division: one diploid cell gives four haploid cells after two divisions with only one round of DNA replication.

Significance of meiosis: it halves the chromosome number, so that fertilisation restores the diploid number and the species number stays constant across generations; it produces variation through crossing over and independent assortment, which is the raw material of evolution; and it shuffles alleles so that no two gametes are alike.

The two sources of variation. Crossing over (pachytene of prophase I) exchanges segments between non-sister chromatids of homologous chromosomes, producing recombinant chromatids. Independent assortment (metaphase I) is the random orientation of each bivalent: with n pairs, a gamete can receive 2^n different combinations of maternal and paternal chromosomes — 2^{23}, over eight million, in a human.

Mitosis and meiosis compared

Formula and fact sheet

Practice questions

Set A — Recall (30 questions)

1. The cell theory was proposed byaHooke and LeeuwenhoekbSchleiden and SchwanncVirchow and PasteurdBrown and Purkinje
2. "Omnis cellula e cellula" — every cell arises from a pre-existing cell — was stated byaSchwannbSchleidencHookedVirchow
3. The cell was first discovered byaRobert Hooke in corkbLeeuwenhoek in pond watercRobert Brown in orchiddSchwann in cartilage
4. Which of the following is absent from a prokaryotic cell?aribosomesbplasma membranecmembrane-bound nucleusdcell wall
5. The ribosomes of a prokaryotic cell are of the typea80Sb60Sc40Sd70S
6. The genetic material of a bacterium lies in theanucleoidbnucleuscmitochondriondchloroplast
7. The middle lamella of a plant cell wall is made mainly ofacellulosebligninccalcium pectatedchitin
8. The fluid-mosaic model of the plasma membrane was proposed byaDanielli and DavsonbSinger and NicolsoncRobertsondPalade
9. The inner membrane of a mitochondrion is folded intoacristaebgranacthylakoidsdcisternae
10. A stack of thylakoids in a chloroplast is called aacristabstromaccisternadgranum
11. Rough endoplasmic reticulum differs from smooth ER in bearingaDNAbribosomescchlorophylldhydrolytic enzymes
12. The organelle that modifies, packages and secretes cell products is thealysosomebribosomecGolgi bodydperoxisome
13. Lysosomes are called "suicidal bags" because theyacontain DNAbmake ATPcdigest fooddhold enzymes that can digest the cell's own contents
14. The 80S ribosome of a eukaryote is made of the subunitsa60S and 40Sb50S and 30Sc40S and 30Sd60S and 50S
15. The nucleolus is the site ofaDNA replicationbrRNA synthesis and ribosome assemblycATP synthesisdprotein digestion
16. In a nucleosome, DNA is wound aroundaa single histonebnon-histone protein onlycan octamer of histonesdRNA
17. The region at which the two chromatids of a chromosome are joined is theacentromerebtelomerecchiasmadsatellite
18. The 9+2 arrangement of microtubules is found inabacterial flagellabprokaryotic ciliaccentriolesdeukaryotic cilia and flagella
19. Which of these is a non-living cell inclusion?amitochondrionbribosomeccalcium oxalate crystaldGolgi body
20. The longest phase of the cell cycle isaM phasebinterphasecanaphasedtelophase
21. DNA replicates duringaG₁ phasebG₂ phasecM phasedS phase
22. The stage of mitosis in which chromosomes line up at the equator isametaphasebanaphasecprophasedtelophase
23. In plant cells, cytokinesis takes place by formation of aacleavage furrowbcontractile ringccell platedchitin septum
24. Synapsis and crossing over occur inamitosisbprophase I of meiosiscprophase II of meiosisdanaphase I
25. The chromosome number is halved duringameiosis Ibmeiosis IIcmitosisdS phase
26. Meiosis II is called equational because itahalves the chromosome numberbdoubles the DNAcpairs the homologuesdseparates sister chromatids without changing the number
27. Direct division of the nucleus without spindle formation isamitosisbamitosiscmeiosisdendomitosis
28. Chiasmata are first clearly visible inaleptotenebzygotenecdiploteneddiakinesis
29. Colchicine arrests a dividing cell ataprophasebanaphasectelophasedmetaphase
30. A human somatic cell has 46 chromosomes. The number in a gamete isa23b46c92d12

Set B — Understanding (18 questions)

1. Viruses are an exception to the cell theory because theyahave a nucleusbare prokaryotescare acellular and have no celldlack nucleic acid
2. A coenocytic hypha of Mucor is an exception because itais one mass of cytoplasm with many nuclei and no cross wallsbhas no nucleuscis a virusdhas a cellulose wall
3. A plant cell placed in pure water does not burst becauseait has no membranebit has a contractile vacuolecit lacks a cell walldthe rigid wall resists the turgor pressure
4. Mitochondria and chloroplasts are called semi-autonomous because theyalack DNAbhave their own DNA and 70S ribosomes but depend on nuclear genescdivide only with the nucleusdhave 80S ribosomes
5. The lysosome membrane is important because itakeeps the hydrolytic enzymes away from the cytoplasmbmakes ATPcsynthesises proteindcontains DNA
6. A cell that secretes large amounts of digestive enzyme would have manyachloroplastsblysosomescrough ER and Golgi bodiesdcilia
7. The nuclear pore allowsafree passage of ribosomesbexchange of RNA and proteins between nucleus and cytoplasmcATP synthesisdDNA replication
8. During anaphase of mitosis, a human cell containsa23 chromosomesb46 chromosomesc46 pairs of chromosomesd92 chromosomes
9. Meiosis is important for evolution mainly because itaproduces new gene combinations by crossing over and independent assortmentbdoubles the chromosome numbercmakes identical cellsdrepairs tissues
10. Mitosis in a haploid cell producesahaploid gametes onlybdiploid cellsctwo haploid cells identical to the parentdfour haploid cells
11. Spindle fibres attach to a chromosome at theatelomerebsatellitecchiasmadkinetochore of the centromere
12. In a cell with 2n = 8, the number of bivalents at metaphase I isa8b4c16d2
13. A cell in G₀ahas left the cycle and is not dividingbis in S phasechas no DNAdis undergoing meiosis
14. Colchicine-treated cells show a doubled chromosome number because colchicineabreaks the DNAbprevents DNA replicationcprevents spindle formation, so the nucleus re-forms undividedddissolves the cell wall
15. The significance of crossing over is that itahalves the chromosome numberbexchanges segments between non-sister chromatidscseparates sister chromatidsddoubles the DNA
16. A plant cell differs from an animal cell at cytokinesis because the plant cellahas no spindlebdivides by binary fissioncforms a cleavage furrowdforms a cell plate from Golgi vesicles
17. The enzyme used to separate cells by digesting the middle lamella isapectinasebcellulasecligasedamylase
18. If the DNA content of a cell in G₁ is 2C, the content at metaphase of mitosis isa2CbCc4Cd8C

Set C — Application (12 questions)

1. A human cell with 46 chromosomes completes meiosis. The number of cells and the chromosome number in each area2 and 46b4 and 23c4 and 46d2 and 23
2. A cell has 2n = 24. At anaphase of mitosis, the number of chromosomes in the cell isa12b24c36d48
3. An organism has 2n = 6. Ignoring crossing over, how many different combinations of maternal and paternal chromosomes can a gamete receive?a3b6c8d64
4. A cell in G₁ has 10 chromosomes and 10 DNA molecules. At metaphase I of meiosis it hasa10 chromosomes and 20 DNA moleculesb20 chromosomes and 20 DNA moleculesc10 chromosomes and 10 DNA moleculesd5 chromosomes and 10 DNA molecules
5. After meiosis I, each daughter of a 2n = 10 cell hasa10 chromosomes and 10 DNA moleculesb5 chromosomes and 5 DNA moleculesc10 chromosomes and 20 DNA moleculesd5 chromosomes and 10 DNA molecules
6. A drug blocks formation of the cell plate. Which cells are affected?aanimal cells onlybplant cells onlycbacterial cells onlydboth plant and animal cells
7. An onion root-tip cell has 2n = 16. At anaphase of mitosis it containsa8 chromosomesb16 chromosomesc32 chromosomesd64 chromosomes
8. Two genes lie on different chromosomes. The gametes of a heterozygote AaBb are produced in the ratioa1 : 1 : 1 : 1b3 : 1c9 : 3 : 3 : 1d1 : 2 : 1
9. A cell has 2n = 46. After meiosis II, the DNA content of each cell compared with G₁ isathe samebhalfcdoubledfour times
10. A moss gametophyte has 12 chromosomes in each cell. The sporophyte cells havea6b12c48d24
11. Crossing over occurs between two linked genes in 20 % of meioses. The recombination frequency isa10 %b20 %c40 %d5 %
12. A cell with 2n = 8 undergoes meiosis. The number of chromatids per cell at metaphase II isa4b16c8d2