Lecture 26. Gene Expression in Prokaryotes and Eukaryotes Flashcards
What produces distinct cell types ?
Differential gene expression patterns
Why in eukaryotes is the cell under complex control ?
They contain many different cell types and what makes one cell type different from another is the specific cohort of genes that each cell expresses
What are some cellular activities ?
- During cell division
- In response to stimulation with glucose
- In response to stress or starvation
What is gene expression fundamental to ?
What an organism can do and whether it can adapt to changing circumstances
Where do different cell types arrive ?
Embryonic development
What initiates different patterns of gene expression ?
Developmental signals
When can cell differentiation occur ?
Occurs during multiple stages of development including in the adult
What are the cells in the early embryo called ?
Blastomere
What types of cells do cells in the early embryo begin as ?
Totipotent
What are totipotent cells ?
Can differentiate into any cell type
In response to signals, what do cells that are totipotent differentiate into ?
Pluripotent, multipotent and finally fully differentiated cells
What are housekeeping genes ?
Genes that are expressed in practically all cells within a given organism
What do housekeeping genes include ?
RNA and proteins that are required to build common cellular structures, organelles and carry out basic functions of the cell
What are some types of housekeeping genes ?
- Structural proteins
- Enzymes for cellular metabolism
- Proteins and other enzymes for organelle synthesis and maintenance
What are tissue-specific genes ?
Genes which are relatively unique to each cell type
Where are tissue specific genes expressed ?
Specific cell types
What are the four mechanisms of different patterns of gene expression ?
- Transcriptional
- Post-transcriptional
- Translational
- Post translational
What does the transcriptional mechanism involve ?
Factors that affect RNA transcription
What does the post-transcriptional mechanism involve ?
Factors that affect stability or composition of RNAs
What does the translational mechanism involve ?
Factors that affect translation of mRNAs into protein
What does the post-transcriptional mechanism involve ?
Factors that affect protein function of activity state
What does RNA polymerase regulate RNA synthesis in tandem with ?
Transcription activators and repressors
At its most basic level, how is gene expression regulated ?
Through binding of an RNA polymerase to the regulatory elements upstream of a gene
What is found close to the transcription start sites of most genes ?
Specific DNA sequences
In prokaryotes, how many RNA polymerases are utilised ?
1
In eukaryotes, how many RNA polymerases are utilised ?
3
What is e. coli RNA polymerase comprise of ?
2 alpha subunits, 1 beta, 1 beta prime, 1 omega and 1 sigma
What is the function of the sigma subunit in e.coli RNA polymerase ?
Recognizes the specific sequences that are common in bacteria promoters and help to position the RNA polymerase at the right place upstream of the transcriptional start site of the gene
In bacterial promoters, where do the common sequence elements begin ?
The TATA box and the TTGACAT
Where is the TATA box ?
- 10 base pairs
Where is the TTGACAT ?
- 35 base pairs
What is another name of the TATA box ?
Pribnow box
What are the TATA box and the TTGACAT recognised by ?
RNA polymerase specifically the sigma subunit
What is the function of the sigma subunit in bacterial promoters (generally) ?
Positions the polymerase at the correct position to initiate transcription and also separates the strands of double helix apart to enable transcription to begin
Upon initiation of transcription, what happens to the sigma subunit ?
It dissociates from the promoter
What happens in chain termination ?
RNA polymerase recognises specific sequences within DNA that act to terminate RNA synthesis
What do the sequences which act to terminate RNA synthesis form ?
Hairpins which form in the growing RNA molecules
What is intrinsic termination sequences ?
Sequences which terminate RNA synthesis cause hairpins to form in a growing RNA molecule that causes it to fall off the DNA template
What can a protein called Rho do ?
Can recognize certain sequence elements that are also termination signals and upon binding to these elements can the bind to RNA polymerase causing it to fall off the DNA and release the RNA
What is Rho-dependent termination ?
Requires Rho to bind to RNA polymerase causing it to fall off the DNA and release the RNA
What is the function of transcriptional repressors ?
Work by sitting on or close to the promoter regions of the DNA, preventing RNA polymerase or initiator/activator proteins from starting transcription initiation therefore turning off the genes
In prokaryotes are promoters usually on or off ?
Constitutively on
In a prokaryote, what stops the gene promotors from being constitutively on ?
The binding or activity of RNA polymerase being blocked or impeded by binding of a transcriptional repressor to the promoter
What is the function of the lac repressor ?
Represses the genes that are involved in lactose utilisation in bacteria
What are bacterial genes often co-transcribed within ?
Operons
What is an operon ?
Groups of related genes that are co-transcribed under the control of the same repressor and promoter
What type of genes are often in operons ?
Bacterial genes
Why are bacterial genes organised into operons ?
They live in harsh and ever changing environments where food sources vary rapidly, requiring the bacterium to be able to rapidly express new sets of genes that are required to deal with varying food sources
What is the lac operon controlled by ?
Lac repressor protein
What three enzymes does the growth on lactose depend on ?
- B-galactoside
- Permease
- Thiogalactoside transacetylase
What are some characteristics of the genes that encode the enzymes B-galactosidase, permease and thiogalactoside transacetylase
- The three genes were tightly linked on the chromosome
- All were induced coordinately
- The ratio of Lac Z:A:Y proteins remain constant
How does the lac operon work ?
- lacI encodes a repressor LacI that switches off the whole operon in the absence of lactose (by binding to the operator for the Lac Operon)
- When lactose is present, this binds to the Lacl repressor protein and displaces it from the promoter, permitting the Lac Operon to switch on and express the genes required for lactose utilisation
What are the genes required for lactose utilisation ?
- LacZ
- LacA
- LacY
What proteins are considered as stable ?
Keratin, secreting proteins
What proteins are considered as short lived or unstable ?
Transcription factors
