cell bio chp. 8

What is gene expression?

a gene makes a product that is useful to the cell or organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity

What is it called when cells become specialized in structure, composition, and function?

cell differentiation

Why do different cells express different sets/combinations of genese?

because of differential gene expression

All cells have the same ___ and ___.

DNA; genes

Negative control (gene expression):

turns off or decreases gene activity; via repressor proteins/molecules

Positive control (gene expression):

turns on or increases gene activity; via activator proteins/molecules

Pre-transcriptional and transcriptional control (1):

controlling when and how often a given gene is transcribed

Post-transcriptional control (2,3, and 4):

controlling how an RNA transcript is spliced or processed; selecting which RNAs are exported from the nucleus

Pre-translational and translational control (5):

regulating how quickly certain RNAs are degraded; selecting which mRNAs are translated into protein; how often a given mRNA is translated

Post-translational control (6 and 7):

regulating how quickly certain proteins are degraded; controlling how a polypeptide is processed/activated

Pre-transcriptional control occurs..

at the DNA level of regulation via changes in chromosome and DNA structure by regulatory proteins and chemical modifications (prevents or allows transcription)

What impacts regulation?

the packaging of DNA into chromosomes and the structure

When histone acetyltransferases add acetyl groups tp the tails of histones the results are…

the loosening of the chromatin structure, freeing up activation/promoter sites

Histone-modifying enzymes can also prevent transcription by…

removing the activating groups and making the chromatin more compacted

Histone deacetylases:

removes acetyl groups from the histones, causing histones to become more compacted

Histone methyltransferases:

add (methylation) or remove (demethylation) methyl groups to the histone-tails; increases or decreases transcription of genes depending on the a.a.

What do methyl groups act as?

a blockage, preventing transcription factors and RNA polymerase the ability to bind to DNA nucleotides of the promoter (gene silencing)

In prokaryotic cells, which stage is the most regulated level?

the initiation stage

Silencers:

specific DNA sites that are binding sites for transcriptional repressors

Enhancers:

specific DNA sites that are binding sites for transcriptional activators

If there aren’t any regulatory proteins, what will some gene promoters allow?

RNA polymerase to weakly bind and initiate transcription (constitutive/basal level)

To activate transcription, what must be done?

a transcriptional activator binds to the DNA and helps/enhances RNA polymerase binding to the promoter

When transcriptional proteins bind to DNA and RNA polymerase, what happens to the RNA polymerase?

it gets closer to the promoter; called recruitment and results from cooperative binding

What is the Lac operon?

a set of 3 genes found in E.coli, involved in the transport of lactose into the cell and breakdown into glucose; consists of a promoter, a terminator, and an operator

Lac promoter:

directs transcription of all three genes as a single mRNA molecule

LacZ:

produces the enzyme B-galactosidase; breaks down lactose into galactose and glucose

LacY:

produces the protein lactose permease; transports lactose into the cell

LacA:

produces the enzyme thiogalactoside transacetylase; breaks down thiogalactoside that is transported into the cell by lactose permease

When are the Lac genes expressed?

when lactose is present and glucose isn’t

Lac repressor protein regulation occurs by?

allolactose: an absence of lactose in the cell,, the Lac repressor proteins is able to bind to the Lac promoter and prevent transcription (Lac inducer)

What happens when glucose is present in Lac operon?

the Lac repressor binds to the operator region and prevents RNA polymerase from binding; when it’s absent CAP binds to the CAP site and RNA polymerse, bring it to the promoter

Which cells have more complex regulator binding sites and multiple sites for each gene?

eukaryotic cells

Combinatorial control:

process by which groups of transcription regulators work together to determine the expression of a single gene

What are the functions of capping and polydenylation?

stabilizes 5’ and 3’ ends of the RNA by preventing exonuclease degradation, facilitates nuclear export, marks it as a specific type of RNA, stimulates translation, and additional transcript processing

What else does splicing aid in?

nuclear export, initiation of translation, and effecting RNA lifespan/degradation

What is the start codon?

AUG

Translation repressor proteins:

prevent the attachment of ribosomes by binding to specific sequences in the 5’ untranslated region

What is post-translational control/regulation dependent on?

polyribosomes and characteristics of the polypeptide/protein

What enzyme degrades proteins by a process known as proteolysis?

proteases (involves hydrolizing the peptide bonds)

Proteosomes are…

large protein machines/complexes found in the cytosol and nucleus that have a protease component

Which protein marks a protein for destruction?

ubiquitin

After the protein is marked for destruction, what happens next?

the ubiquitylated protein is recognized by the proteosome and degraded