Genetic Material in Cells and Light Microscopy

Genetic instructions control cell activities

Cells are the basic units of living organisms. Their genetic material is DNA, which carries instructions controlling how they develop and function. These instructions allow cells to make proteins, including enzymes that control chemical reactions. This connects genetic material to everyday cell activities such as growth and using nutrients.

Plant and animal cells are eukaryotic cells. Their main genetic material is enclosed in a nucleus, where DNA is packaged into several long, linear chromosomes. The nucleus controls cell activities because it contains these instructions—not because it physically carries out every process itself.

Other structures carry out particular jobs. Many chemical reactions happen in the cytoplasm, the cell membrane controls movement of substances into and out of the cell, and ribosomes make proteins. The genetic instructions and these working structures therefore act together.

Bacteria have DNA without a nucleus

Bacteria are prokaryotic organisms. Their cells are generally much smaller than plant and animal cells and have no nucleus. Instead, their main DNA forms a long circular chromosome in the cytoplasm. This DNA still controls cell activities and provides instructions needed when the cell reproduces.

Bacteria may also contain plasmids: small circles of extra DNA separate from the main chromosome. Plasmids carry additional genetic instructions. For example, a plasmid may carry instructions that give a bacterium resistance to an antibiotic, allowing it to survive exposure to that antibiotic. Plasmids can be transferred between bacteria, so useful instructions can spread.

The important distinction is where the DNA is kept and how it is organised. Both eukaryotic and prokaryotic cells have genetic material; only eukaryotic cells enclose their main genetic material in a nucleus.

How a light microscope reveals cells

A light microscope passes visible light through a specimen and uses lenses to produce an enlarged image. The objective lens sits above the specimen; the eyepiece lens is the lens you look through. Both magnify the image.

The slide rests on the stage, with the specimen above a hole through which light passes. Stage clips hold it in place. The coarse focus makes large adjustments to bring cells roughly into focus, while the fine focus makes small adjustments to sharpen their outlines. The light source and diaphragm let you adjust illumination and contrast.

A specimen must be thin enough for light to pass through it. Overlapping layers make structures difficult to distinguish. Many cells are almost transparent, so adding a stain makes structures such as the nucleus stand out against their surroundings.

Preparing plant and animal cell slides

For this investigation, use a light microscope, clean slides and cover slips, forceps, a mounted needle, dropping pipettes, iodine solution, methylene blue, filter paper and a sterile cotton bud. Onion epidermis provides a plant specimen, while cheek cells provide an animal specimen.

Onion epidermis: use forceps to peel a small piece of the thin skin from the inner surface of a fleshy onion layer. Lay it flat in the centre of a slide, removing folds so that cells do not overlap. Add a drop of iodine solution to improve contrast.

Cheek cells: gently rub the inside of your own cheek with a sterile cotton bud. Smear the collected cells thinly across a separate slide, then put the used bud straight into disinfectant. Add a drop of methylene blue to make the nuclei easier to see.

For either specimen, rest one edge of a cover slip beside the drop, holding it at roughly $45^\circ$ with a mounted needle. Lower it slowly so that the liquid spreads beneath it without trapping air. The cover slip flattens and protects the specimen and keeps liquid away from the objective lens. Use filter paper at its edge to absorb surplus liquid.

Wear eye protection and avoid getting stains on skin or clothing. Do not ingest stains or share cotton buds. Handle glass and the mounted needle carefully. If cutting onion tissue, cut on a tile away from your fingers. Carry the microscope with one hand supporting its base and the other holding its arm.

Finding cells and bringing them into focus

1. Select the lowest-power objective lens and clip the slide onto the stage, with the specimen centred over the hole. Low power gives a wide field of view, making cells easier to find.
2. Switch on or adjust the light. Watching from the side, bring the objective close to the slide without touching it.
3. Look through the eyepiece and use coarse focus to move the objective and slide farther apart until the cells appear. Use fine focus to sharpen the image.
4. Adjust illumination and the diaphragm until cell outlines and stained structures are clear.
5. Centre the cells you want to examine before selecting a higher-power objective. At higher power, use fine focus only.

The view is ready to record when individual cells have sharp outlines and their stained nuclei can be distinguished. If the image remains blurred, return to low power and check the focus, specimen thickness and slide preparation.

Total magnification is found by multiplying the two lens magnifications:

$$\text{total magnification}=\text{eyepiece magnification}\times\text{objective magnification}$$

For example, a ×10 eyepiece and a ×40 objective produce ×400 magnification. The image is enlarged 400 times; this does not mean every tiny structure becomes visible.

Interpreting and recording what you see

Onion epidermis cells usually look like closely packed, brick-shaped compartments. Their clear boundaries are cell walls, and iodine helps reveal the nuclei within them. This onion bulb tissue does not contain chloroplasts, so a plant cell need not show every structure in a general textbook diagram.

Cheek cells are flatter, irregularly rounded cells that may lie separately or overlap. Methylene blue reveals a darker nucleus within the cytoplasm. They have a cell membrane but no cell wall.

Seeing nuclei in both specimens supports their classification as eukaryotic cells. However, you cannot make out individual DNA molecules or plasmids with a school light microscope. A diagram of genetic material explains its organisation; it is not a picture of everything this practical reveals.

Record a representative group of cells using clear, single pencil lines without shading. Add a descriptive title, label visible structures with straight ruled lines and record the lens magnifications. The drawing should show your observations rather than an idealised cell.

Genetic material

• Plant and animal cells: eukaryotic; main DNA is packaged into linear chromosomes inside a nucleus.
• Bacteria: prokaryotic; no nucleus; main DNA is a circular chromosome in the cytoplasm.
• Plasmids: extra small DNA circles that may carry useful instructions, such as antibiotic resistance, and can pass between bacteria.