What Happens in Cells


DNA is the abbreviation of the chemicals full name which is ‘Deoxyribose Nucleic Acid’. This chemical is a long chain of repeating units, such chemicals are known as polymers and the repeating units as monomers. The sequence of these monomers produces a code that carries all the information needed to grow, repair, reproduce, and function as an organism.

The structure of DNA was discovered by James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin in 1952, using a technique called X-Ray Crystallography in which images of molecules can be formed from the use of X-rays.

DNA is made of two strands of deoxyribose sugar molecules held together by nucleotide base molecules, of which there are four; thymine, adenine, guanine, and cytosine. Thymine (T) is always found opposite adenine (A) and guanine (G) opposite cytosine (C). These are known as the base pairs. The sequence of these pairs forms the genetic code that controls the cell. These form the code that makes up the genes in all living organisms.

What Happens in Cells, figure 1

DNA double helix discovered by Watson and Crick with information supplied by Rosalind Franklin.

Chromosomes found in the nucleus of eukaryotic cells, the strands of nucleoids and the plasmids in prokaryotic cells are all made of DNA with the same double helix structure.


Enzymes are biological catalyst that speed up and control the reactions in organisms that make up their metabolism. These include the enzymes for respiration in the mitochondria, chlorophyll in the chloroplasts and amylase secreted in saliva that starts the digestion of starch into sugars.

Like all catalysts, they increase the speed of chemical reactions, but they are not changed by the reaction, meaning they can be used more than once within a cell to perform functions.

Enzymes are reaction specific, they only work on one reaction. This is because of the way they function. Enzymes are a protein molecule which have one place on their surface that is specially shaped to allow a specific substrate molecule to bind with it at this site. This site is called the active site. When a substrate molecule binds with the active site the reaction can occur. Once this reaction is complete the products are released from the active site. As the active site is not changed by the reaction the enzyme can continue to catalysis more reactions.

The active site works because of its shape, it mirrors the shape of the substrate molecule. So only a molecule of the correct shape can fit into the active site.

This idea is known as the lock and key hypothesis.

What Happens in Cells, figure 1

The lock and key hypothesis.

As long as the active site is undamaged and remains the same shape the reaction can occur.

Enzymes are sensitive to temperature, pH and toxins. Any of these can result in a change in the shape of the active site and prevent the reaction from occurring. A change in shape of the active site is known as denaturing.

What Happens in Cells, figure 2

Denaturing can be caused by changes in temperature, pH or the presence of toxins.

Denatured enzymes cannot catalyse reactions.

What are the four bases in DNA and what is the correct pairing these along the DNA double helix?
thymine is found opposite adenine and guanine opposite cytosine
When someone is ill the medical professionals always observe the patient's body temperature. Explain why a high temperature can be damaging to the patient.
high temparature can lead to denaturing