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Assessment 1.3

Regulation of Gene Expression

Transcription Regulation Nomenclatre cis acting is based solely on the sequence of the DNA
trans acting is based on something additional to the DNA itself , something outside of DNA usually a protein
Structure of an Operon is a cluster of genes with related functions acting as a coordinated unit and controlled by a regulatory sequence, present only in prokaryotes
Components of the Operon structural genes code for the protein, what u want out of a gene; eukaryotic cells code for only one protein (monocistronic) and prokaryotic genes code for multiple proteins (polycistronic)
promoter-influences the efficiency of gene transcription
repressor-protein which inhibits transcription of a specific gene when bound to DNA
activator- protein which enhances transcription of a specific gene when bound to DNA
operator- is a sequence of DNA which can bind a repressor and/or an activator
Transcriptional Regulation in Prokaryotes: promoter recognition Promoter(determines how efficient binds) recognition by the sigma subunit of RNA polymerase is a crucial step in prokaryotic transcription
Consensus Sequence-DNA sequence which will be similar in all genes , the sequence is about 35 bases prior to transcription and is called Pribnow Box (TTGACA), after this there is about 17 bases long called a spacer,
then about 10 bases prior to transcription start is the TATA Box (TATAAT).
No natural promoter resembles the consensus sequence, but in general, the more closely a promoter resembles the promoter sequence, the more efficiently that particular promoter initiates transcription
Prokaryotic Operon Catabolic Pathways- breakdown (enzymes) Genes are kept turned off until the compound that is catabolized by that specific gene product is present
Anabolic (synthesize) Genes are kept turned on until the product of the anabolic pathway is present
In this way genes are only expressed when needed and the cell isn't wasting energy on unneeded proteins
lac Operon-look at page 11 of notes consist of 3 structual genes that breakdown lactose plus adjacent regulatory genes
transcriptional control of these genes residues in two regions: repressor and operator. mRNA transcribed from the repressor gene codes for a repressor protein, transcription of this gene is completely uncontrolled.
The repressor protein binds to the operator region in such a way as to prevent RNA polymerase from binding thus blocking transcription of the 3 structual genes.
lactose and allolactose are inducers of the lac operon. when lactose or its metabolite are present in the cell, the cell is able to producte the enzyme required for lactose metabolism
trp Operon biosynthesis consumes energy, therefore it is to a cell's advantage not to undergo unnecessary biosynthesis.
In this system, binding of a small molecule ligand to a repressor activates the repressor .
consists of a regulatory region and 5 structural genes that code for proteins in tryptophan biosynthesis.
trp repressor protein binds to tryptophan and the protein-tryptophan ligand complex binds to the operator to block transcription, so in this case of the trp operon, the tryptophan repressor comples acually turn off transcription.
Transcriptional Control in Eukaryotes RNA polymerase II always requires several accessory protein factors to initiate transcription, these proteins bind to certain promoter sequences which include TATA Box, CAAT Box or GC rich sequence
Hormonal Regulation of Transcription intracellular receptor, hormone bind to a protein receptor in the cell can affect transcription.
The hormone receptor complex binds to DNA and enhances transcription directly by facilitating RNA Polymerase II attachment.
Alternatively, the hormone receptor complex could interact with a repressor protein, either causing augmentation or inhibition of the repressor
Cell Surface Receptor Regulation of Gene Expression cell surface receptors can influence gene expression by altering cellular chemistry, resulting in an increase or decrease in intracellular cAMP or calcium (GPCR). this will activate cAMP response element binding proteins (CREB proteins) to bind to cAMP
response elements (CRE) on the DNA. This binding leads to activation of gene expression
Transcriptional Control in Eukaryotes Enhancers-activate transcription and may lie 100's to 1000's of base pairs distant from the gene
Silences- inhibition of transcription
Translational Control in Eukaryotes transcriptional control is the most common form of gene regulation
translational control is more common for eukaryotes than prokaryotes
RNAi RNA interference-method of silencing the translation of certain genes by controlling the RNA for those genes; thought to have developed an intracellular "immune system", control expression of endogenous genes (virus genes),
Targets intracellular double stranded RNA (dsRNA)
RNAi Mechanism 1. dsRNA is recognized within the cell by a protein called dicer, the dicer cuts the dsRNAinto short double stranded RNA segments called small interfering RNA (siRNA)
2. siRNA are recognized by the RNA-induced silencing complex (RISC)
3. RISC will then select one strand of the siRNA and hunt for a complementary mRNA strand
4.Once RISC finds a complementary strand of mRNA it degrades it (chops it up); this controls replication of viruses)
Created by: lisagoette