Environment & Gene Expression: Examples (HL) (HL IB Biology)

Effect of air pollution on gene expression

• Gene expression in cells may be affected by a range of external conditions e.g. diet, temperature, chemicals or air pollution
• Exposure to the chemicals in polluted air is of particular concern in urbanised or industrialised areas
  • Some of these substances may cause damage to lung tissue which results in the development of conditions such as asthma or chronic obstructive pulmonary disease
  • Air pollution may also have a negative impact on cardiovascular health
• Exposure to air pollution may change methyl tags on DNA or histone proteins, which will affect patterns of gene expression in cells
  • One effect of this may be increased levels of inflammation in the body and a higher risk of developing cardiovascular disease or certain pulmonary conditions
  • Inflammation in the respiratory system may result in scarring and the eventual thickening of tissue which may decrease the rate of diffusion of oxygen into the blood
• Suggested treatments to reduce the impact of air pollution include exercise and a diet high in B vitamins

• During egg and sperm development in mammals, most epigenetic tags are removed 
  • This is to remove the changes made to methylation patterns caused by environmental influences and prevent them from being passed on to offspring
• Some of the epigenetic tags may be retained and passed on to the next generation by a process called imprinting
  • During imprinting, epigenetic tags may also be added to the DNA in sperm and egg cells
  • This results in only one copy of a gene being expressed (or 'switched on') while the other copy is suppressed (or 'switched off')
• Organisms will typically inherit two functional copies of a gene (one from each parent)
  • However, only one functional copy of an imprinted gene will be inherited by offspring since the other copy will be silenced by the presence of epigenetic tags
  • In sperm development, maternal genes are epigenetically silenced while paternal genes will be silenced during egg development

Epigenetic origins of phenotypic differences in tigons and ligers

• Tigons and ligers are both the result of crossbreeding lions and tigers
  • Male tiger x female lion = tigon
    • Tigons are about the same size or even smaller than their parents
  • Male lion x female tiger = liger
    • Ligers are typically larger than both lions and tigers
• The phenotypic differences between tigons and ligers are due to genetic imprinting
• Lions and tigers have different reproductive habits which may affect the expression of the gene responsible for growth
  • Male lions will pass genes on that will encourage growth, while female lions have imprinted genes which will discourage growth
  • Neither male nor female tigers will pass down genes to discourage growth in their offspring
• When a male lion carrying genes that will encourage growth is crossed with a female tiger which does not have imprinted genes to discourage growth the resulting liger offspring will be able to grow much larger than its parents
• This is an example of how phenotypic differences between tigons and ligers have an epigenetic origin

Epigenetic Inheritance Diagram

The phenotypic differences between tigons and ligers are due to different patterns of epigenetic inheritance

• Variation between members of the same family may be due to several factors such as:
  • Genetic factors - differences in the genome lead to differences in the phenotype
  • Environmental factors - examples would include a change in skin colour due to sun exposure 
  • A combination of both genetic and environmental factors - such as height and weight
• Monozygotic twin studies can be used to determine the contribution of genetics and the environment to phenotypic variation
  • Monozygotic twins are also called identical twins as they originate from the same zygote that splits into two during development
    • This leads to two individuals that are genetically identical
  • Dizygotic (or non-identical) twins are the result of two different egg cells that were fertilised at the same time
    • These twins are genetically different from one another
• Specific phenotypic traits can be measured between monozygotic twins and the results could indicate whether genetics or the environment are responsible for any measurable variation
  • E.g. if separated twins show traits that are similar, then there is a good probability that genes are responsible while significant differences in traits may indicate that environmental factors had an influence
• Results for monozygotic and dizygotic twin studies can be compared and used to determine the extent to which genetics or the environment are responsible for differences
  • E.g. if both monozygotic and dizygotic twins have similar traits, it is possible that it could be due to exposure to the same environmental factors
  • If monozygotic twins share more similarities with each other than with dizygotic twins, then it could be due to genetic factors that monozygotic twins have in common with each other
• Despite having the same genetic information, there are still some variation between monozygotic twins that could be attributed to epigenetic changes between them
  • These changes would include methylation of DNA or acetylation of histone tails

Monozygotic and Dizygotic Twins Diagram

Monozygotic twins are genetically identical because they develop from the same zygote whereas dizygotic twins are genetically distinct as they develop from two different zygotes

The effect of a hormone on gene expression

• Steroid hormones, such as oestrogen (also known as oestradiol), progesterone and testosterone, act as ligands that affect gene expression
  • This is covered in more detail under the subtopic of Intracellular Receptors

The effect of lactose on gene expression in bacteria

• Regulatory genes control structural genes and their levels of protein production
  • Regulatory genes sometimes have control over several structural genes at once
  • Structural genes in prokaryotes can form an operon: a group or a cluster of genes that are controlled by the same promoter
    • The lac operon found in some bacteria is one of the most well-known of these
    • The lac operon controls the production of the enzyme lactase (also called β-galactosidase) and two other structural proteins
    • Lactase breaks down the substrate lactose so that it can be used as an energy source in the bacterial cell
    • It is an inducible enzyme that is only synthesized when lactose is present
    • This helps prevent the bacteria from wasting energy and materials

Structure of the lac operon

• The components of the lac operon are found in the following order:
  • Promoter for structural genes
  • Operator
  • Structural gene lacZ that codes for lactase
  • Structural gene lacY that codes for permease (allows lactose into the cell)
  • Structural gene lacA that codes for transacetylase
• Located to the left (upstream) of the lac operon on the bacterium's DNA there is also the:
  • Promoter for regulatory gene
  • Regulatory gene lacI that codes for the lac repressor protein
• The lac repressor protein has two binding sites that allow it to bind to the operator in the lac operon and also to lactose (the effector molecule)
  • When it binds to the operator it prevents the transcription of the structural genes as RNA polymerase cannot attach to the promoter
  • When it binds to lactose the shape of the repressor protein distorts and it can no longer bind to the operator

Lac Operon Diagram

The components of the Lac operon along with the upstream regulatory gene and its associated promoter

When lactose is absent

• The following processes take place when lactose is absent in the medium that the bacterium is growing in:
  • The regulatory gene is transcribed and translated to produce lac repressor protein
  • The lac repressor protein binds to the operator region upstream of lacZ
  • Due to the presence of the repressor protein, RNA polymerase is unable to bind to the promoter region
  • Transcription of the structural genes does not take place
  • No lactase enzyme is synthesized

The repressor protein binding to the operator region of the Lac operon and preventing transcription of the structural gene

When lactose is present

• The following processes take place when lactose is present in the medium that the bacterium is growing in:
  • There is an uptake of lactose by the bacterium
  • The lactose binds to the second binding site on the repressor protein, distorting its shape so that it cannot bind to the operator site
  • RNA polymerase is then able to bind to the promoter region and transcription takes place
  • The mRNA from all three structural genes is translated
  • Enzyme lactase is produced and lactose can be broken down and used for energy by the bacterium

Lactose binding to the repressor protein which frees up the operator region of the Lac operon so RNA polymerase can bind and begin transcription of the structural genes