LESSON 2: FACTORS AFFECTING THE EXPRESSION OF GENES

Cells and organisms use literally thousands and thousands of different types of proteins to survive, grow and reproduce. All of these proteins in turn are coded for in the DNA that exists in cells. Imagine, though, if all of the proteins coded for in an organism’s DNA were produced all of the time. The results would be disastrous, not to mention wasteful. It is crucial for organisms to control what proteins are produced and active at different times and under different conditions. A number of different mechanisms occur in cells to insure that the proper proteins are produced at the right times. These mechanisms act at all different stages of protein synthesis, from before transcription to after translation.

Regardless of how carefully protein synthesis is controlled, mutations, or changes in the DNA, can change the proteins that are produced. Mutations may have little or no effect or can have significant effects on an organism. The effect of mutations depends on exactly what change occurs in the DNA, and in what type of cell it occurs. The genetic changes that mutations create are also the raw material for evolution.

Learning Objectives

• Understand the importance of controlling protein synthesis.

• Be able to describe several mechanisms cells use to control protein synthesis.

• Be familiar with different types of mutations and the effect they have on proteins.

• Understand the link between mutations and evolution.

Topics covered in this Lesson

• Control of Gene Expression
• Errors in the Code

Controls of Gene Expression

The biochemical processes that occur in cells and organisms are generally highly controlled – protein synthesis is no exception. It is important to control protein synthesis at all times during the life of an organism so that the organism can grow, develop, reproduce and respond to its environment. There are many mechanisms that cells use to control the production of proteins so that the correct proteins are active at the right time and place. These mechanisms can act before, during and after transcription and translation. There are some similarities and differences in the control of gene expression between eukaryotic and prokaryotic cells.

Learning Objectives

• Comprehend the importance of controlling gene expression.

• Be able to define inducers and repressors, and understand the differences and similarities between them.

• Understand the structure of the bacterial operon, including the promoter, operator, and structural genes.

• Be able to describe the significance of the sigma factors of RNA polymerase in gene expression.

• Be able to define positive regulation, and use the lac operon as an example.

• Be able to define negative regulation, and use the trp operon as an example.

• Understand the roles of the G-cap and the poly-A tail of mRNA in control of gene expression.

• Describe how complementary base-pairing in mRNA can effect gene expression.

Errors in the Code

It is important for cells to maintain their DNA so that they are able to produce proteins that function correctly. However, it is unavoidable that some changes will occur over time in an organism’s DNA. Think your own DNA - you are composed of billions of cells, each with several billion base pairs of DNA. Protecting that much DNA is a nearly incomprehensible job, so it should be no surprise that changes in DNA, called mutations,
occur on a regular basis.

Mutations can happen in several ways, and they can range from changes in a single nucleotide to the duplication of an entire chromosome or even an entire genome. Accordingly, the effect that mutations have on an organism may be little or none to significant.

Learning Objectives

• Be able to define mutations
• Know the difference between:
  • somatic and germ-line mutations
  • point and chromosomal mutations
  • spontaneous and induced mutations
• Be able to define and describe the consequences of the following types of mutations:
  • silent (synonymous) mutations
  • missense mutations
  • nonsense mutations
  • frame-shift mutations
• Know the four main types of chromosomal mutations.
• Be able to describe the link between mutations and evolution.