3.B.1 Gene Regulation. Gene regulation results in differential gene expression, leading to cell specialization.

Transcription

1 3.B.1 Gene Regulation Gene regulation results in differential gene expression, leading to cell specialization.

2 We will focus on gene regulation in prokaryotes first.

3 Gene regulation accounts for some of the phenotypic differences between organisms with similar genes.

4 Gene expression can involve: Regulatory Sequences Within Genes Regulatory Genes Small Regulatory RNAs (srna)

5 Operons are a type of regulatory sequence consisting of clusters of genes under the control of a single regulatory region. Prokaryotes only!

6 Promoters are nucleotide sequences that allow the genes of an operon to be transcribed. RNA polymerase binds to the promoter region to begin transcription.

7 Terminators are nucleotide sequences that mark the end of a gene or operon.

8 Both positive and negative control mechanisms regulate gene expression in bacteria and viruses. Positive Control Stimulate Transcription Negative Control Inactivate Transcription

9 Operators are segments of DNA that a regulator molecule can bind to.

10 Repressors are small regulatory proteins that halt transcription. They bind to the operator region of an operon and prevent RNA polymerase from binding.

11 The expression of specific genes can be inhibited by the presence of a repressor. A repressor binds to the operator site of an operon, preventing RNA polymerase from binding and therefore preventing transcription of the operon (negative control).

12 Inducers are small proteins that stimulate transcription. They bind to repressors, inactivating them so that RNA polymerase can bind to the operator and begin transcription.

13 The expression of specific genes can be turned on by the presence of an inducer. An inducer disables repression of the operon (positive control).

14 Positive Control Stimulate Transcription Inducer Negative Control Inactivate Transcription Repressor

15 A regulatory gene is a sequence of DNA encoding a regulatory protein (such as a repressor) or RNA.

16 Certain genes are continuously expressed; that is, they are always turned on, regardless of environmental conditions.

17 Example of Prokaryotic Gene Regulation: The Trp Operon

18 The trp operon consists of a group of genes that code for the enzymes necessary to synthesize tryptophan, an amino acid.

19 The trp operon is an example of negative feedback. When there is too much tryptophan, tryptophan itself acts as a corepressor, which activates the repressor that shuts down this operon.

20 The trp operon is an example of a repressible operon; it is usually on but can be turned off when there is too much tryptophan.

21 Example of Prokaryotic Gene Regulation: The Lac Operon

22 The lac operon consists of a group of genes in e. coli that allow the bacteria to metabolize lactose when lactose is present in the gut of its host.

23 When there is no lactose present, e. coli does not need to produce the enzymes to break down lactose, instead using glucose as its primary nutrient.

24 In the absence of lactose, the lac repressor protein is made, which binds to the operator and halts the binding of RNA polymerase.

25 In the presence of lactose, an inducer binds to the repressor, altering its shape so that it is no longer able to bind to the operator. RNA polymerase can now bind and begin transcribing the operon.

26 The lac operon is an example of an inducible operon; it is usually off but can be turned on in the presence of lactose.

27 In eukaryotes, gene expression is complex and control involves regulatory genes, regulatory elements and transcription factors that act in concert.

28 Transcription factors are proteins that bind to specific DNA sequences and/or other regulatory proteins. They work alone or in complex.

29 Some of these transcription factors are activators (increase expression), while others are repressors (decrease expression).

30 Enhancers are short regions of DNA that can be bound with proteins to enhance transcription.

31 Regulatory sequences are stretches of DNA that interact with regulatory proteins to control transcription.

32 The combination of transcription factors binding to the regulatory regions at any one time determines how much, if any, of the gene product will be produced.

33 Learning Objectives: LO 3.18 The student is able to describe the connection between the regulation of gene expression and observed differences between different kinds of organisms. [See SP 7.1] LO 3.19 The student is able to describe the connection between the regulation of gene expression and observed differences between individuals in a population. [See SP 7.1] LO 3.20 The student is able to explain how the regulation of gene expression is essential for the processes and structures that support efficient cell function. [See SP 6.2] LO 3.21 The student can use representations to describe how gene regulation influences cell products and function. [See SP 1.4]