DNA and Gene Expression – how they make us who we are

By Megan Widdows

Although it is so small that it can­not be seen by the naked eye, your DNA con­tains all the genet­ic inform­a­tion needed to make up tens of thou­sands of genes in your gen­ome. These genes are respons­ible for determ­in­ing many of your phys­ic­al char­ac­ter­ist­ics such as hair and eye col­our. Your genes also con­tain the inform­a­tion needed to pro­duce pro­teins and oth­er poly­mers such as RNA that carry out vari­ous essen­tial func­tions in your body. These are referred to as gene products, and it is by pro­du­cing these products that our DNA is able to be expressed in the body. 

How­ever, not all of these genes or their products are use­ful all the time, so it would be an inef­fi­cient use of our cells’ energy to express them all, all the time. This means cells need ways of switch­ing on and off the expres­sion of genes to enable them to carry out dif­fer­ent func­tions, or to respond to changes in our bod­ies or environment. 

The pro­cess of switch­ing genes on or off is known as epi­gen­et­ic modi­fic­a­tion, where changes in an organ­ism are achieved by alter­ing the expres­sion of cer­tain genes rather than chan­ging the genet­ic code dir­ectly. This pro­cess occurs in all organ­isms, from humans right down to bac­teria and viruses. 

The most strik­ing examples of indi­vidu­als dif­fer­ing dra­mat­ic­ally from one anoth­er, des­pite hav­ing the same under­ly­ing gen­ome come from spe­cies with caste colon­ies. In bio­logy, a caste refers to the divi­sion of a colony of indi­vidu­als into sub-groups that are each spe­cially adap­ted to carry out a par­tic­u­lar func­tion. This is com­mon in insects such as ants, bees, wasps and ter­mites who all have Queens that are used for repro­duc­tion. These castes also include work­ers and/or sol­diers who are often sterile, which means they can­not repro­duce. Instead, these work­ers carry out spe­cial­ised tasks, defend the colony and even care for those mem­bers who can reproduce.

Bumble­bee colon­ies, one example of a caste colony, are foun­ded by a single queen who will first lay female eggs that were fer­til­ised the year before. These will devel­op and become dip­loid female work­er bees. Dip­loid means they have two sets of chro­mo­somes, one from each par­ent. The queen then lays more eggs res­ult­ing in new dip­loid queens and hap­loid male drones, which have just one set of chro­mo­somes from their moth­er only. The males and new queens leave the colony and don’t return. Once out of the colony, there are many more males than females so sev­er­al males have to com­pete with one anoth­er to fer­til­ise a queen. Many are unsuc­cess­ful so end up being unable to pass on their genes to the next generation. 

This raises an inter­est­ing evol­u­tion­ary ques­tion. Since most male drones and female work­er bees don’t have chance to repro­duce, how is their genet­ic mater­i­al, and the inform­a­tion needed to main­tain the caste sys­tem passed on to the next generation?

This is pos­sible to be due to a type of epi­gen­et­ic modi­fic­a­tion known as DNA methyl­a­tion. This is the addi­tion of a small hydro­car­bon group known as a methyl group (CH₃) to a cytosine base that is paired to a guan­ine base in an organism’s DNA. Cytosine and Guan­ine (C and G) are two of the four bases that make up DNA. 

Areas of DNA that con­tain lots of paired cytosine and guan­ine are known as “CpG islands. These islands are often loc­ated close to spe­cial “pro­moter” regions which are sequences of DNA to which spe­cif­ic pro­teins can bind, ini­ti­at­ing the expres­sion of that gene. How­ever, when a CpG island is methyl­ated the pro­teins can­not bind so expres­sion of that gene does not occur. 

There are large dif­fer­ences in the levels of DNA methyl­a­tion between castes, spe­cific­ally on the genes that are respons­ible for many key bio­lo­gic­al func­tions includ­ing repro­duc­tion. The methyl­a­tion of these key genes means that they can be switched on or off in dif­fer­ent castes. For example, repro­duct­ive genes are not required in the sterile work­er bees so these will be methyl­ated and there­fore not expressed. 

This means that the dif­fer­ent roles and asso­ci­ated char­ac­ter­ist­ics with­in a bumble­bee colony are not linked to genet­ic dif­fer­ences but are thought to be down to epi­gen­et­ic changes. This one example high­lights the import­ance of con­trolling gene expres­sion, allow­ing dif­fer­ent bees to take on dif­fer­ent func­tions and sup­port the sur­viv­al of the colony. There are count­less oth­er examples in every organ­ism, includ­ing humans. 

The study men­tioned in this art­icle can be found here. A gloss­ary of terms can be found below.

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