How do almost all cells of a multicellular organism contain the same DNA, yet specialized cells have very specific and unique functions?
Differential gene expression- different cell types express different subsets of genes
Typical human cell only expresses ~20% of its protein-coding genes
How is eukaryotic gene expression regulated?
Chromatin remodeling
Regulation of transcription initiation
RNA processing
mRNA degradation
Translation regulation
Protein processing and degradation
What must happen in order for a gene to be expressed (transcribed)?
The chromatin must be remodeled
DNA is packaged with proteins (histones) and needs to be unwound because the tighter the chromosome is wound up, the harder it is for DNA to get in
Errors in remodeling can lead to many different diseases including cancer
How can histone tails be modified?
By the addition or removal of specific chemical groups
-Acetylation (-COCH3)
-Methylation (CH3)
-Phosphorylation (PO4)
Acetylation (-COCH3)
Loosens DNA coiling (increase gene expression)
HATs (histone acetyltransferases) and HDACs (histone deacetylases)
Methylation (CH3)
Tightens DNA coiling (decreasing gene expression)
Phosphorylation (PO4)
Loosens DNA coiling
How does DNA methylation cause coiling to tighten?
Adds methyl groups to cytosine residues which triggers chromatin to condense (inactivates the gene)
Once a gene is methylated it will usually remain that way even through successive rounds of divisioun
-Can occur throughout life due to environmental factors (plays a role in cancer development)
What type of modification is DNA methylation?
Nucleotide modification
Can be inherited from mother to progeny
Epigenetics
Study of changes in gene expression not due to change in DNA sequence.
Chromatin modifications don't change the DNA sequence but they may be passed onto future generations
May explain the differences seen in identical twins (i.e. why one develops a disease and the other doesn't)
A researcher found a method she could use to manupulate and quantify phosphorylation and methylation in embryonic cells in culture. One of her colleagues suggested she try increased methylation of C nucleotides in a mammalian system. Which of the following results would she most likely see?
C) Increased chromatin condensation
Regulatory Sequences
Most eukaryotic genes have associated segments of noncoding DNA that serve as binding sites for proteins to start transcription
Have promoter-proximal elements such as enhancers and silencers
Enhancers
Far away from promoter
Can be upstream, downstream or even in an intron
This is gene expression, gene not as expressed is a mutation in this area
Silencers
Inhibit transcription
Repressors bind to silencers
Difference between regulatory sequence and proteins
Enhancers and silencers are regulatory sequence
Activator and repressors are regulatory proteins
Which of the following mechanisms is (are) used to coordinate the expression of multiple, related genes in eukaryotic cells?
E) A specific combination of control elements in each gene's enhancer coordinates the simultaneous activation of the genes
General transcription factors
Are essential for the transcription of all proteins coding genes
Interact with TATA box, other proteins, and RNA polymerase to form transcription initiation complex
Low rate of transcription
Specific transcription factiors
Bind to regulatory sequences
Strongly increase or decrease transcription rate (activators, repressors)
Can also recruit proteins for chromatin remodeling
In eukaryotes, general transcription factors...
B) bind to other proteins or to a sequence element within the promoter called the TATA box
What does regulation depend on?
The combination of control elements
Different cell types will have different activators and repressors
Genes with related functions that need to be coordinately expressed share a specific combo of control element (when appropriate activators are present they bind to the control elements regardless of where the gene is in the genome)
Rats that are the offspring of parents that were raised with poor nutrition have a greater chance of developing disorders similiar to type 2 diabetes, regardless of the alleles they inherit. This shows that...
D) patterns of gene expression may be inherited
Post-transcriptional regulation
Expression does not depend on transcription alone. The protein still needs to be made and folded so that it is functional
How can cells regulate gene expression in several ways after transcription
RNA processing
Initiation of translation
Protein processing and degradation
Alternative splicing
Different mRNAs can be produced from the same primary transcript
Regulatory proteins control intron-exon choices in a cell specific manner
MicroRNA (miRNAs)
Small single-stranded RNAs that can bind to complementary sequences in mRNAs
Forms a complex with proteins (RISC= RNA-inducing silencing complex)
At least 1100 human miRNAs regulate expression of most genes
Regulate when a protein is being expressed
RNA interference process
Starts in a hairpin structure and then a dicer dices it up and gives you one strand of miRNA.
If the miRNA is an exact match with the mRNA then it degrades it.
If the miRNA is an incomplete match then the translation is blocked
Post-translational control
Regulates polypeptides and is any regulation after DNA has been polymerized
Ubiquitin molecules
Proteins are marked for destruction by the addition of ubiquitin molecules
Ub-tagged proteins are recognized by a proteasome and degraded
Which of the following is most likely to have a small protein called ubiquitin attached to it?
A) A cyclin that usually acts in G1, now that the cell is in G2
In eukaryotic gene expression, the step prokaryotes do not use is...
