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CH. 11 : Transcriptional Control of Gene Expression Jennifer Brown.

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2 CH. 11 : Transcriptional Control of Gene Expression Jennifer Brown

3 INTRODUCTION  Gene expression: the entire process whereby the information in a particular gene is decoded into a particular protein.  RNA polymerase is required to initiate transcription  mRNA is then synthesized at a certain site, transported from the nucleus to cytoplasm, then translated into protein  Ribosomes, tRNA, and translation factors all aid in this process

4 Control of Transcription Initiation  This is the first step and most important mechanism for determining whether genes are expressed and how much of encoded mRNAs (proteins) are produced  Gene control allows for the correct expression of the correct genes during development  Regulation of transcription initiation is most widespread form of gene control

5 Transcription Promoters  The promoter is the DNA sequence that specifies where transcription begins  Transcription factors may bind upstream or downstream from the promoter  This allows for complex control of gene expression

6 RNA Polymerases  Eukaryotic cells contain three different RNA polymerases which contain 2 large and 3 small core subunits  RNA polymerase I is located in the nucleolus and transcribes genes encoding pre-rRNA which is processed into 28S, 5.8S, and 18S rRNAs  RNA polymerase III transcribes genes encoding tRNA, 5S rRNA, and other small stable RNAs

7 RNA Polymerase II  Transcribes ALL protein-coding genes and function in production of mRNAs  Contains a carboxyl-terminal domain which the other two do not have  Carboxyl end of largest subunit contains a stretch of 7 amino acids  This sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser is repeated multiple times  Also initiates transcription of genes at the DNA sequence encoding the capped 5’ end of the mRNA

8 Transcription-Control Regions  Protein-binding DNA sequences that regulate protein-coding genes  Promoter: control elements plus TATA- box or initiator  Enhancers: long distance transcriptional- control elements which can occur upstream or downstream from a promoter and are cell-type specific

9 Types of Promoter Sequences in Eukaryotic DNA  3 Types  TATA box: this is the most common type found upstream from start site and rapidly transcribes genes  Initiators: these have cytosine at -1 position and adenine at start site (+1)  CpG Islands: located upstream from start site and have low rate of transcribed genes


11 Transcription Factors  Activate or repress expression of protein- coding genes  Example is GAL4 which is composed of a N- terminus DNA-binding domain and a C- terminus activation domain  Transcription repressors are functionally converse of activators

12 Classes of DNA Binding Proteins  Homeodomain Proteins: contain conserved 60- residue DNA-binding motif  Zinc-Finger Proteins: regions fold around a central Zinc ion and produce compact domain from short polypeptide chain  Leucine-Zipper Proteins: contain the amino acid Leucine at every 7 th position and bind to DNA as dimers  Basic Helix-Loop-Helix Proteins: similar in structure to basic-zipper motif


14 Heterodimeric Transcription Factors  Allow activation domains of each monomer to be placed together in different combinations  Each monomer with different DNA- binding specificity increases number of DNA sequences the family of transcription factors can bind  Allows for combinatorial complexity

15 Transcription Initiation by RNA Polymerase II  General Transcription Factors: initiation factors that place polymerase molecules at transcription start sites and help template strand enter active site  Example in Polymerase II: TFIIA, TFIIB, etc.  Required for synthesis of RNA from most genes


17 Regulatory Proteins  Act in concert with other proteins to modulate chromatin structure  This influences ability of transcription factors to bind to promoters  Also interact with large multiprotein complex = mediator  This binds with Pol II and directly regulates assembly of transcription preinitiation complexes

18 Regulation of Transcription- Factor Activity  Expression of transcription factor by a cell is regulated  Activities of those factors expressed are also controlled indirectly  This is done by interaction between proteins on surface of cell and by external hormones and growth factors

19 Nuclear Receptors  All have unique N-terminal region  Response elements bind several nuclear receptors and can be:  Inverted  Direct repeats  Heterodimeric:located exclusively in nucleus  Homodimeric: found in cytoplasm in absence of ligans

20 Mechanisms for Terminating Transcription  Differ for each of 3 RNA Polymerases  RNA Polymerase I: termination requires polymerase-specific termination factor  RNA Polymerase II: terminates after polymerizing a series of U residues  RNA Polymerase III: doesn’t terminate until after a sequence is transcribed that directs cleavage and polyadenylation of RNA


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