DNA, Enzymes and Cell Division
DNA — The Code of Life
Inside every cell is DNA, the molecule that carries genetic instructions for building and running an organism. DNA is a polymer — a long molecule made of repeating sub-units called nucleotides. It consists of two strands twisted around each other to form a double helix. The strands are held together by pairs of complementary bases: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). This is known as complementary base pairing.
The sequence of bases along the DNA acts as a code. Each group of three bases codes for a particular amino acid. This triplet code determines the order in which amino acids are assembled during protein synthesis at the ribosomes. Different sequences of amino acids produce different proteins, each with a unique shape and function.
DNA is a polymer made of two strands forming a double helix. The sequence of bases provides a code that determines the order of amino acids assembled into proteins at the ribosomes. Base pairs: A–T and C–G.
From DNA to Protein
The journey from gene to protein involves two key stages. First, in the nucleus, the relevant section of DNA is used as a template to produce a molecule of mRNA (messenger RNA). This mRNA carries the copied code out of the nucleus and into the cytoplasm. Second, at the ribosomes, the code on the mRNA is read and used to assemble amino acids in the correct order. The resulting chain of amino acids folds into a specific 3D shape, forming a functional protein.
Proteins are essential for life — they include enzymes (biological catalysts), structural proteins (such as collagen), hormones (such as insulin), and antibodies (part of the immune system). Because the DNA base sequence determines which proteins are made, DNA ultimately controls the cell's characteristics and functions.
Enzymes — Biological Catalysts
Enzymes are proteins that act as biological catalysts, speeding up the chemical reactions that occur inside cells without being used up themselves. They play a central role in metabolism. Without enzymes, metabolic reactions would be too slow to sustain life.
Each enzyme has a uniquely shaped region called the active site. Only a substrate with the correct complementary shape can fit into the active site, like a key fitting into a lock. This is the lock and key hypothesis. When the substrate binds, the enzyme-substrate complex forms, the enzyme catalyses the reaction, and the products are released. The enzyme is then free to accept another substrate molecule.
Temperature — as temperature increases, molecules move faster and collide more frequently with the enzyme, increasing the rate of reaction. However, above the optimum temperature, the enzyme begins to denature — the bonds holding the active site in shape break, the active site changes shape permanently, and the substrate can no longer fit. The rate drops sharply. Different enzymes have different optimum temperatures — not all enzymes work best at 37°C.
pH — each enzyme has an optimum pH. For most human enzymes this is around pH 7, but pepsin in the stomach works best at around pH 2. Moving away from the optimum changes the active site shape, reducing activity. Extreme pH values cause permanent denaturation.
Substrate concentration — increasing substrate concentration raises the rate because more substrate molecules collide with enzyme active sites per unit time. Eventually, all active sites are occupied simultaneously (the enzyme is saturated) and the rate plateaus.
Enzyme concentration — more enzyme molecules mean more active sites available, so the rate increases (assuming excess substrate is present).
A common practical involves testing the effect of amylase on starch. Amylase is a digestive enzyme that breaks down starch into simpler sugars. Monitor the reaction by taking samples at regular intervals and testing with iodine solution — iodine turns blue-black when starch is present, but stays brown-orange once the starch has been fully digested.
To investigate the effect of temperature, set up water baths at different temperatures. Place test tubes containing amylase and starch in each water bath, allow them to equilibrate, then mix and start timing. Sample at intervals and test with iodine. Record the time taken for starch to be fully digested at each temperature.
To calculate the rate of reaction: \(\text{rate} = \frac{1}{\text{time}}\). A shorter time means a faster rate. Plot rate against temperature to find the optimum. Control variables include the volume and concentration of amylase and starch, and the pH of the buffer solution.
Enzymes have a specific active site that fits only one substrate (lock and key hypothesis). Above the optimum temperature or at extreme pH, the active site changes shape permanently — the enzyme is denatured and can no longer catalyse the reaction.
Mitosis and the Cell Cycle
Cells divide to produce new cells for growth, repair and replacement of damaged tissue. The cell cycle has several stages: first the cell grows and increases in size, then the DNA replicates so there are two copies of every chromosome, the cell grows a little more, and finally it divides by mitosis. Mitosis produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.
During DNA replication, the two strands of the double helix unzip. Each strand then acts as a template, with new complementary nucleotides added. The result is two identical copies of the original DNA molecule — one for each daughter cell. This ensures that genetic information is passed on faithfully during cell division.
Differentiation is the process by which a cell becomes specialised to perform a particular function. Specialised cells have specific structures suited to their role. For example, red blood cells are biconcave with no nucleus to maximise space for haemoglobin; nerve cells have long axons to transmit impulses over distance; root hair cells have extensions to increase surface area for absorption.
Stem cells are undifferentiated cells that can divide and produce different cell types. Embryonic stem cells can differentiate into any cell type in the body — they are described as pluripotent. Adult stem cells (e.g. in bone marrow) can produce a limited range of cell types, such as blood cells. Meristems in plants contain stem cells that allow plants to continue growing and differentiating throughout their lives — unlike most animal cells, which lose the ability to differentiate early in development.
The use of embryonic stem cells raises ethical questions because the embryo is destroyed in the process. Some people argue the potential medical benefits — such as treating conditions like Parkinson's disease or diabetes — outweigh this concern. Others oppose it on moral or religious grounds. Adult stem cells avoid this controversy but have a more limited range of differentiation.
Mitosis produces two genetically identical daughter cells for growth and repair. Stem cells are undifferentiated cells that can divide to produce different cell types. Embryonic stem cells can form any cell type; adult stem cells have a more limited range.