Topic 2: Cells, Viruses and Reproduction of Living Things — Edexcel A-Level Biology B
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Topic 2: Cells, Viruses and Reproduction of Living Things explained
Topic 2: Cells, Viruses and Reproduction of Living Things covers 8 specification points, including Nucleoid, plasmids, 70S ribosomes and cell… and Viruses i Understand that the classification….
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2.1 Eukaryotic and prokaryotic cell structure and function i Understand that cell theory is a unifying concept that states that cells are a fundamental unit of structure, function and organisation in all living organisms. ii Understand that in complex organisms, cells are organised into tissues, organs, and organ systems. iii Know the ultrastructure of prokaryotic cells and the structure of organelles, including: nucleoid, plasmids, 70S ribosomes and cell wall. iv Be able to distinguish between Gram positive and Gram negative bacterial cell walls and understand why each type reacts differently to some antibiotics. v Know the ultrastructure of eukaryotic cells and the functions of organelles, including: nucleus, nucleolus, 80S ribosomes, rough and smooth endoplasmic reticulum, mitochondria, centrioles, lysosomes, Golgi apparatus, cell wall, chloroplasts, vacuole and tonoplast. vi Know how magnification and resolution can be achieved using light and electron microscopy. vii Understand the importance of staining specimens in microscopy.
Cell theory defines cells as life's fundamental unit, forming tissues, organs and systems. Prokaryotes contain a nucleoid, plasmids, 70S ribosomes and a peptidoglycan wall. Gram-positive walls are thick peptidoglycan; Gram-negative have a thin peptidoglycan layer and an outer lipid membrane blocking antibiotics like penicillin. Eukaryotes feature a nucleus (contains DNA), nucleolus (makes ribosomes), 80S ribosomes (protein synthesis), RER (proteins), SER (lipids), mitochondria (ATP), centrioles (spindles), lysosomes (digestion), Golgi (modifies proteins), cell wall (support), chloroplasts (photosynthesis) and vacuole with tonoplast (turgor). Light microscopes use lenses to magnify; electron microscopes use electron beams for higher resolution. Staining adds contrast.
2.2 Viruses i Understand that the classification of viruses is based on structure and nucleic acid types as illustrated by λ (lambda) phage (DNA), tobacco mosaic virus and Ebola (RNA) and human immunodeficiency virus (RNA retrovirus). ii Know the lytic cycle of a virus and latency. iii Know that viruses are not living cells and so antivirals must work by inhibiting virus replication. iv Know that as viruses can be difficult to treat once infection has occurred, the focus of disease control should be on preventing the spread, as exemplified by the 2014 Ebola outbreak in West Africa. v Be able to evaluate the ethical implications of using untested drugs during epidemics.
Viruses are classified by structure and nucleic acid type. λ phage has DNA; tobacco mosaic virus and Ebola have RNA; HIV is an RNA retrovirus. The lytic cycle involves attachment, entry, replication, assembly and release; latency is when viral DNA integrates into the host genome and remains dormant. Viruses are not living cells, so antivirals must inhibit replication. Because treating established infection is difficult, disease control focuses on preventing spread, as in the 2014 Ebola outbreak in West Africa. Evaluating untested drugs during epidemics raises ethical issues of consent, safety and equity.
2.3 Eukaryotic cell cycle and division i Know that the cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis. ii Understand what happens to genetic material during the cell cycle, including the stages of mitosis. iii Understand how mitosis contributes to growth, repair and asexual reproduction.
The eukaryotic cell cycle is a regulated process producing two genetically identical daughter cells. It consists of interphase (G1, S, G2), mitosis and cytokinesis. During interphase, DNA replicates and the cell grows. Mitosis ensures genetic continuity and involves four stages. In prophase, chromosomes condense and become visible as two sister chromatids joined by a centromere. In metaphase, chromosomes align at the cell equator. In anaphase, centromeres divide and sister chromatids are pulled to opposite poles by spindle fibres. In telophase, chromosomes decondense and nuclear envelopes reform around each set of genetic material. Finally, cytokinesis divides the cytoplasm. Mitosis is essential for the growth of multicellular organisms, the repair of damaged tissues by replacing cells, and asexual reproduction in some species.
2.4 Sexual reproduction in mammals i Understand the processes of oogenesis and spermatogenesis. ii Understand the events of fertilisation from the first contact between the gametes to the fusion of nuclei. iii Understand the early development of the embryo to blastocyst stage.
Oogenesis begins before birth, pauses in prophase I, and resumes each cycle: one primary oocyte completes meiosis I to give a secondary oocyte and a polar body, and meiosis II finishes only after fertilisation, producing one ovum and further polar bodies. Spermatogenesis is continuous from puberty: spermatogonia divide by mitosis, grow into primary spermatocytes, then meiosis I and II yield four haploid spermatids that mature into sperm. At fertilisation, sperm first contact and bind the zona pellucida, the acrosome releases enzymes, membranes fuse, and the sperm nucleus enters so the two haploid nuclei fuse. The zygote then divides by mitosis to a morula and forms a blastocyst with an inner cell mass and trophoblast.
2.5 Sexual reproduction in plants i Understand how a pollen grain forms in the anther and the embryo sac forms in the ovule. ii Understand how the male nuclei formed by division of the generative nucleus in the pollen grain reach the embryo sac, including the roles of the tube nucleus, pollen tube and enzymes.
In the anther, diploid microspore mother cells undergo meiosis to form four haploid microspores; each microspore undergoes mitosis to produce a pollen grain with a generative nucleus and a tube nucleus inside a protective wall. In the ovule, a diploid megaspore mother cell undergoes meiosis to yield four megaspores; one surviving haploid megaspore divides mitotically three times to form the embryo sac containing eight nuclei, including the egg cell and two polar nuclei. During pollination and germination, the tube nucleus controls pollen tube growth through the style, secreting enzymes that digest style tissue. The generative nucleus divides by mitosis into two haploid male gametes, which travel down the pollen tube to enter the embryo sac for double fertilisation.
CORE PRACTICAL 2: Use of the light microscope, including simple stage and eyepiece micrometers and drawing small numbers of cells from a specialised tissue.
Core Practical 2 develops skill in using a light microscope to observe and draw cells from a specialised tissue. You set up the microscope, select an appropriate objective lens, and focus the specimen. A stage micrometer is a slide with a known scale; an eyepiece graticule is a scale in the eyepiece. Calibrate the graticule against the stage micrometer at each magnification, then use the calibrated graticule to measure cell dimensions. Draw a small number of cells accurately, with clear continuous lines, correct proportions, labels and a stated magnification. Record raw measurements and show your calibration calculation so the scale is traceable.
CORE PRACTICAL 3: Make a temporary squash preparation of a root tip to show stages of mitosis in the meristem under the light microscope. iv Understand how meiosis results in haploid gametes, including the stages of meiosis. v Understand that meiosis results in genetic variation through recombination of alleles, including independent assortment and crossing over. vi Understand what chromosome mutations are, as illustrated by translocations. vii Understand how non-disjunction can lead to polysomy, including Down’s syndrome, and monosomy, including Turner’s syndrome.
Core Practical 3: cut a root tip, place in acetic orcein, warm, and squash under a cover slip to spread meristem cells into a single layer. Under a light microscope, identify mitosis stages (prophase, metaphase, anaphase, telophase). Meiosis produces haploid gametes via two divisions: meiosis I (prophase I, metaphase I, anaphase I, telophase I) separates homologous chromosomes; meiosis II (prophase II, metaphase II, anaphase II, telophase II) separates chromatids. Genetic variation arises by independent assortment at metaphase I and crossing over in prophase I. Chromosome mutations alter structure, e.g. translocations. Non-disjunction causes polysomy (Down’s syndrome) or monosomy (Turner’s syndrome).
CORE PRACTICAL 4: Investigate the effect of sucrose concentrations on pollen tube growth or germination. iii Understand the process of double fertilisation inside the embryo sac to form a triploid endosperm and a zygote.
Core Practical 4 tests how sucrose concentration affects pollen germination or pollen tube growth. Pollen is placed on a suitable growth medium containing a range of sucrose concentrations, incubated, then examined under a microscope; the percentage of grains that germinate or the length of pollen tubes is recorded and compared. In double fertilisation, one male nucleus fuses with the egg cell to form a diploid zygote, while the other male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus. The endosperm nourishes the developing embryo, and the zygote develops into the embryo of the seed.
Your focus
- Describe cell theory and the levels of organisation from cells to organ systems.
- Identify prokaryotic structures (nucleoid, plasmids, 70S ribosomes, cell wall) and state their structural features.
- Compare Gram-positive and Gram-negative cell walls, including the thin peptidoglycan layer, and explain how the outer membrane affects antibiotic response.
Show all 27 objectives
- Identify eukaryotic organelles and state the specific function of each named structure, including the nucleus and 80S ribosomes.
- Explain how magnification and resolution are achieved in light and electron microscopy and why staining is important.
- Describe how viruses are classified based on structure and nucleic acid type, using named examples.
- Explain the lytic cycle and latency in viruses.
- Justify why antivirals must inhibit virus replication and why disease control focuses on preventing spread, using the 2014 Ebola outbreak as an example.
- Evaluate the ethical implications of using untested drugs during epidemics.
- Describe the three main stages of the eukaryotic cell cycle and their outcomes.
- Explain the behaviour of genetic material during interphase and each specific stage of mitosis.
- Relate the process of mitosis to growth, repair and asexual reproduction in organisms.
- Compare oogenesis and spermatogenesis in terms of timing, products and chromosome number.
- Describe the sequence of fertilisation from gamete contact to fusion of nuclei.
- Outline early embryo development from zygote through morula to blastocyst.
- Describe how meiosis in the anther produces haploid microspores that develop into pollen grains.
- Explain how meiosis and mitosis in the ovule produce the embryo sac containing the egg cell and polar nuclei.
- Explain how the generative nucleus divides into two male nuclei and how the tube nucleus and enzymes enable the pollen tube to reach the embryo sac.
- Use a light microscope safely and correctly to observe a specialised tissue.
- Calibrate an eyepiece graticule against a stage micrometer and use it to measure cells.
- Produce accurate biological drawings of a small number of cells with labels and stated magnification.
- Describe how to make a temporary squash preparation of a root tip and identify mitotic stages under a light microscope.
- Explain how meiosis I and II produce haploid gametes, naming the stages from prophase I to telophase II.
- Distinguish chromosome mutations such as translocation from non-disjunction, and relate non-disjunction to polysomy and monosomy with named examples.
- Describe a method to investigate the effect of sucrose concentration on pollen germination or pollen tube growth.
- Explain how double fertilisation produces a diploid zygote and a triploid endosperm nucleus.
- Relate the products of double fertilisation to the development of the embryo and the nutrition of the seed.
Topic 2: Cells, Viruses and Reproduction of Living Things exam tips
Marking Points
- States cell theory is a unifying concept where cells are the fundamental unit of structure, function and organisation, forming tissues, organs and systems.
- Describes prokaryotic structures: nucleoid (unbound circular DNA), plasmids (small DNA rings), 70S ribosomes (protein synthesis) and cell wall (peptidoglycan).
- Distinguishes Gram-positive (thick peptidoglycan) from Gram-negative (thin peptidoglycan layer between the plasma membrane and an outer lipid membrane) walls, explaining that the outer membrane blocks some antibiotics like penicillin.
- Identifies eukaryotic organelles and functions: nucleus (contains genetic material), nucleolus (synthesises ribosomes), 80S ribosomes (protein synthesis), RER, SER, mitochondria, centrioles, lysosomes, Golgi apparatus, cell wall, chloroplasts, vacuole and tonoplast.
- Explains magnification and resolution: light microscopes use glass lenses to magnify and light waves (lower resolution), while electron microscopes use electromagnets and shorter-wavelength electron beams for higher resolution.
- Explains that staining specimens provides contrast to distinguish specific organelles and structures.
- Classification of viruses is based on structure and nucleic acid type, illustrated by λ phage (DNA), tobacco mosaic virus (RNA), Ebola (RNA) and HIV (RNA retrovirus).
- The lytic cycle includes attachment, entry of viral nucleic acid, replication using host machinery, assembly of new virions and release by lysis.
- Latency is a dormant state in which viral DNA is integrated into the host genome and may reactivate later.
- Viruses are not living cells, so antivirals must target virus replication rather than cell metabolism.
- Because established infection is difficult to treat, disease control focuses on preventing spread, as in the 2014 Ebola outbreak in West Africa.
- Evaluating untested drugs during epidemics involves weighing potential benefits against risks, consent and equity.
- The cell cycle is regulated and results in two identical daughter cells, consisting of interphase, mitosis and cytokinesis.
- During interphase, DNA replication occurs so each chromosome consists of two sister chromatids.
- In prophase, chromosomes condense; in metaphase, they align at the equator; in anaphase, centromeres divide and chromatids separate to opposite poles; in telophase, nuclear envelopes reform.
- Mitosis ensures genetic material is equally distributed, maintaining genetic continuity.
- Mitosis contributes to growth by increasing cell number, repair by replacing damaged cells, and asexual reproduction.
- Oogenesis produces one ovum and polar bodies per primary oocyte, with meiosis I completed before ovulation and meiosis II completed after fertilisation.
- Spermatogenesis produces four haploid spermatids from each primary spermatocyte, and spermatogenesis is continuous from puberty.
- Fertilisation begins with sperm contact and binding at the zona pellucida, involves the acrosome reaction, membrane fusion, and ends with fusion of the haploid nuclei.
- The zygote undergoes cleavage by mitosis to form a morula, which develops into a blastocyst with an inner cell mass and an outer trophoblast.
- In the anther, diploid microspore mother cells divide by meiosis to form haploid microspores; each microspore divides by mitosis to form a generative nucleus and a tube nucleus enclosed in a pollen grain.
- In the ovule, a diploid megaspore mother cell divides by meiosis, and one surviving haploid megaspore undergoes mitosis to form the embryo sac containing eight nuclei (including the egg cell and two polar nuclei).
- The generative nucleus divides by mitosis to produce two haploid male nuclei (gametes).
- The tube nucleus controls pollen tube growth, directing secretion of hydrolytic enzymes that digest style tissue to reach the embryo sac.
- The pollen tube delivers the male gametes directly to the micropyle of the ovule.
- Sets up and uses a light microscope correctly, selecting an appropriate objective lens and focusing the specimen.
- Calibrates an eyepiece graticule against a stage micrometer at a stated magnification.
- Uses the calibrated graticule to measure cell dimensions and records the measurements with units.
- Draws a small number of cells from a specialised tissue with accurate proportions, clear continuous lines and correct labels.
- States the magnification of each drawing and shows the calibration calculation.
- Core Practical 3 requires a temporary squash of a root tip meristem, stained and pressed to give a single layer of dividing cells for light microscopy.
- Mitosis stages visible in the meristem are prophase, metaphase, anaphase and telophase; cells can be counted to estimate the proportion in each stage.
- Meiosis I (prophase I, metaphase I, anaphase I, telophase I) separates homologous chromosomes, halving the chromosome number.
- Meiosis II (prophase II, metaphase II, anaphase II, telophase II) separates sister chromatids to produce haploid gametes.
- Genetic variation in meiosis arises from independent assortment of homologous chromosomes and crossing over between non-sister chromatids, recombining alleles.
- Chromosome mutations are structural changes to chromosomes, illustrated by translocation, in which part of one chromosome becomes attached to another.
- Non-disjunction produces gametes with an extra or missing chromosome; polysomy includes Down’s syndrome and monosomy includes Turner’s syndrome.
- Core Practical 4 uses a controlled range of sucrose concentrations with pollen on a growth medium, and measures either the percentage of pollen grains germinating or the length of pollen tubes.
- A control or comparison is needed, such as a set sucrose concentration series, with other variables kept constant to make the comparison valid.
- In double fertilisation, one male nucleus fuses with the egg cell to form a diploid zygote.
- The second male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus, which develops into the nutritive endosperm.
- The zygote develops into the embryo and the endosperm provides nutrients for the developing seed.
Examiner Tips
- 💡Learn organelle functions as cause-and-effect statements, for example the nucleolus makes ribosomal RNA, rather than as a bare list.
- 💡Use a table to compare Gram-positive and Gram-negative walls, explicitly mentioning the thin peptidoglycan layer and the outer membrane's role in antibiotic resistance for Gram-negative bacteria.
- 💡When a question asks about microscopy, link the wavelength of radiation (light vs electron beam) to resolution and the lens arrangement (glass vs electromagnets) to magnification.
- 💡Learn the specific examples of viruses and their nucleic acid types: λ phage (DNA), tobacco mosaic virus (RNA), Ebola (RNA) and HIV (RNA retrovirus).
- 💡When describing the lytic cycle, use the correct sequence: attachment, entry, replication, assembly, release.
- 💡For ethical evaluation, consider arguments for and against using untested drugs, such as potential benefits versus unknown side effects and the need for informed consent.
- 💡Learn the stages of mitosis in order (prophase, metaphase, anaphase, telophase) and describe the specific behaviour of chromosomes in each.
- 💡Remember that interphase includes G1, S and G2 phases, with DNA replication occurring specifically in the S phase.
- 💡Be able to explain how mitosis is involved in growth, repair and asexual reproduction, giving concrete examples like tissue regeneration.
- 💡Sequence oogenesis and spermatogenesis as timelines, noting where meiosis pauses and resumes.
- 💡Use the correct order for fertilisation: contact, binding, acrosome reaction, membrane fusion, entry of sperm nucleus, fusion of nuclei.
- 💡Name embryo stages in order: zygote, cleavage divisions, morula, blastocyst.
- 💡Link each structure to its role, for example zona pellucida in sperm binding and trophoblast in implantation.
- 💡Learn the sequence anther to pollen grain and ovule to embryo sac as two parallel flow diagrams, labelling ploidy at each stage.
- 💡Practise explaining the division of the generative nucleus and the role of the tube nucleus in one sentence each, so the two roles are never swapped.
- 💡When describing pollen tube growth, always name the enzyme action on the style as the mechanism, not just growth towards the ovule.
- 💡Always record the stage micrometer reading and the graticule reading together so the calibration can be checked.
- 💡Show the calibration calculation clearly, including the magnification at which it was made.
- 💡When drawing, use a sharp pencil, keep lines continuous, and label structures with straight lines that do not cross.
- 💡Learn the practical sequence in order: root tip, stain, warm, squash, single layer, light microscope, identify stages.
- 💡Name the specific stages of meiosis I and II when describing how haploid gametes are formed.
- 💡Link each named condition to its cause: Down’s syndrome to an extra chromosome 21, Turner’s syndrome to a single X chromosome.
- 💡State clearly whether you are measuring germination percentage or pollen tube length, and describe how the measurement is taken.
- 💡Use a sucrose concentration series and identify the independent variable, dependent variable and at least two controlled variables.
- 💡For double fertilisation, write the ploidy of each product: zygote diploid, endosperm triploid, and link each to its role in the seed.
Common Mistakes
- Confusing magnification with resolution: magnification enlarges an image, whereas resolution is the ability to distinguish two close points as separate.
- Assuming all bacterial cell walls react identically to antibiotics: Gram-negative bacteria have an outer lipid membrane that prevents certain antibiotics (like penicillin) from reaching the thin peptidoglycan layer.
- Mixing up 70S and 80S ribosomes: prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes.
- Treating the nucleoid as a true membrane-bound nucleus: prokaryotes have a nucleoid region of unbound circular DNA without a nuclear envelope.
- Misunderstanding: all viruses contain DNA. Correction: some viruses have RNA genomes, such as tobacco mosaic virus, Ebola and HIV.
- Misunderstanding: viruses are living cells. Correction: viruses are not living cells; they are obligate intracellular parasites and antivirals must inhibit their replication.
- Misunderstanding: latency is the same as the lytic cycle. Correction: latency is a dormant state where viral DNA is integrated into the host genome, whereas the lytic cycle leads to immediate replication and cell lysis.
- Misunderstanding: antibiotics can treat viral infections. Correction: antibiotics target bacteria; antivirals are needed to inhibit virus replication.
- Misunderstanding that mitosis is the entire cell cycle. Correction: mitosis is one stage; the cell cycle also includes interphase and cytokinesis.
- Stating that DNA replication occurs during mitosis. Correction: DNA replication occurs during the S phase of interphase, before mitosis begins.
- Confusing the events of anaphase and telophase. Correction: in anaphase, sister chromatids are pulled apart; in telophase, they reach the poles and nuclear envelopes reform.
- Misunderstanding that daughter cells are genetically different. Correction: mitosis produces genetically identical daughter cells.
- Thinking oogenesis yields four equal gametes: it yields one ovum plus polar bodies, whereas spermatogenesis yields four spermatids.
- Believing meiosis II in the oocyte completes before ovulation: it completes only after fertilisation.
- Describing fertilisation as fusion of gametes without the acrosome reaction and membrane fusion steps: these precede nuclear fusion.
- Confusing morula and blastocyst: the morula is a solid ball of cells, while the blastocyst has a fluid-filled cavity, inner cell mass and trophoblast.
- Thinking the pollen grain is diploid: it is haploid, because it arises from a haploid microspore produced by meiosis in the anther.
- Confusing the roles of the two pollen nuclei: the tube nucleus controls pollen tube growth, whereas the generative nucleus divides to give the two male nuclei.
- Believing the pollen tube pushes through the style by mechanical force alone: enzymes digest the style tissue to allow the tube to grow towards the ovule.
- Assuming the embryo sac contains only the egg cell: it contains eight nuclei, including the egg cell and two polar nuclei.
- Using an uncalibrated eyepiece graticule: the graticule units mean nothing until calibrated against a stage micrometer at that magnification.
- Forgetting to recalibrate when changing objective lens: the graticule value changes with magnification, so calibration must be repeated.
- Drawing cells with sketchy or double lines and no magnification: biological drawings need clear continuous lines, correct proportions and a stated magnification.
- Measuring cells without units or without recording the calibration: measurements must be traceable to the stage micrometer scale.
- Thinking the squash shows meiosis: the root tip meristem shows mitosis, so the stages identified are prophase, metaphase, anaphase and telophase, not meiosis I and II.
- Confusing independent assortment with crossing over: independent assortment is the random orientation of homologous pairs at metaphase I, whereas crossing over is exchange between non-sister chromatids in prophase I.
- Believing non-disjunction always causes monosomy: it can give an extra chromosome (polysomy, as in Down’s syndrome) or a missing chromosome (monosomy, as in Turner’s syndrome).
- Treating translocation as a change in chromosome number: translocation is a structural chromosome mutation, not aneuploidy.
- Thinking only one fertilisation occurs: double fertilisation involves two fusion events, producing a zygote and a triploid endosperm nucleus.
- Describing the endosperm as diploid: it is triploid because one male nucleus fuses with two polar nuclei.
- Treating sucrose concentration as the only variable without controlling temperature, pH or incubation time, which weakens the comparison.
- Recording pollen tube growth without a clear measurement method, such as counting germinated grains or measuring tube length under a microscope.