Gene Expression Flashcards
Discuss why every cell does not express every gene.
Every somatic cell has the same genetic content, but gene regulation controls which genes are expressed. This is what dictates whether a cell becomes an eye cell or a lung cell.
It is the different gene expression patterns that give rise to complex organisms. In other words, the key to multicellularity is the specialization of cells due to gene regulation.
Describe the components of a prokaryotic operon and how each contributes to gene regulation.
-promoter- RNA polymerase binds to this
-operator- Regulator binds to this, can block RNA polymerase
-structural genes- Genes coding for something but not involved in the regulation process
-regulator gene: This is separate and has its own promoter. It encodes for the regulator protein than affects gene regulation.
Compare and contract positive and negative regulators.
Positive regulator protein is an activator and triggers a higher level of expression of an operon through transcription.
Negative regulator protein is a repressor and prevents the expression of operon through transcription.
Compare and contrast inducible and repressible gene regulation.
Inducible System:
Transcription goes from low to high.
Repressible System:
Transcription goes from high to low.
Describe the processing of eukaryotic pre-mRNA.
1: Addition of 5’ methyl cap
-Protects mRNA from degradation
-Involved in the initiation of translation
2: Addition of 3’ poly-A tail
-Created by poly-A polymerase
-Protects mRNA from degradation
-Part of the sequence gets recognized and leads to cleavage site that enzymes can recognize
-Polyadenylation takes place at 3’ end
-This poly-A tail (consisting of adenine containing nucleotides) does not encode for anything so it doesn’t matter if it degrades
3: Removal on noncoding sequences (introns)
-Pre-mRNA splicing done by spliceosomes.
-RNA splicing removes introns and joins exons.
Recognize the effects of alternative splicing.
-A single primary transcript can be spliced into different mRNAs by different combinations of exons. This is why there are more mRNA sequences than gene sequences.
-15% of human genetic disorders are due to altered splicing. BUT 35-59% of human genes exhibit some form of altered splicing.
Relate post-translational modification to gene regulation.
Proteins are modified in several ways after they are released from the ribosome.
-cleavage or trimming of the protein
-attachment of carbohydrates (such as glycoproteins)
-phosphorylation of serines or tyrosines
-complexing with metals
Exons
The parts of the code that actually get translated into protein sequences are called exons. They are eventually expressed.
RNA Splicing
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence. Thus, the mature mRNA contains less material than the DNA; It contains exons and some untranslated 3’ and 5’ regions, and the stop codons, but no introns. Therefore, mRNA is an expression of the DNA information that is relevant to making protein.
Spliceosomes
In some cases, RNA splicing is carried out by spliceosomes.
Spliceosomes consist of a variety of proteins including a protein called small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites.
Operon
A group of prokaryotic genes with a single promoter (transcribed as a single mRNA). The genes in an operon encode proteins that all function in a given process.
-promoter
-operator
-structural genes
Ex: the lac operon
What could cause a dominant gene to not be expressed?
If the c gene is epistatic to a, the c gene can prevent the dominant a gene from being expressed
Function of regulator gene and regulator protein
Regulator gene encodes for the regulator protein.
Regulator protein binds DNA at the operator.
In a negative repressible operon, the
regulator protein is synthesized as an
inactive repressor
Co-activator
Positive inducible (0 to 1):
Molecule (substrate) that interacts with the activator to make it functional (allowing transcription)
Co-repressor
Negative repressible (1 to 0)
Molecule (product) interacts with the activator to make it functional (preventing transcription)
In the presence of allolactose (the inducer),
the regulator protein known as the lac
repressor
cannot bind to the operator
Given that the the trp operon is a negative
repressible operon, what happens to the trp
repressor in the absence of the tryptophan
co-repressor?
It cannot bind to the operator and transcription takes place.
What is the central dogma of biology and what are the different levels of gene regulation during this process?
The central dogma of biology is that genetic information flows from DNA to RNA to protein through transcription and translation.
-Epigenetics (alteration of structure that can prevent transcription from occurring)
-Transcriptional Regulation (in prokaryotes)
-mRNA Processing (in eukaryotes)
-Post Translational Modification
Why can eukaryotes regulate gene expression more closely than prokaryotes?
Eukaryotes can regulate gene expression more closely than prokaryotes because they can regulate transcription and translation separately since transcription occurs in the nucleus and translation in the cytoplasm.
Describe the process of epigenetics
Alteration of DNA structure that can prevent transcription from occurring.
Nucleosomes consist of DNA coiled tightly around histone proteins. The positive histone tails react with the negatively charged DNA phosphate group to prevent transcription.
Acetylation of the tails can weaken their interaction and help some transcription factors to occur.
Histone components of a nucleosome are dynamic (changing) but can be inherited.
Heterochromatin
Tightly packaged chromatin that does not allow access to the DNA sequence.
Euchromatin
Loosely packaged chromatin that allows spaces for transcriptional machinery.
Positive Repressible
-Activator regulator protein can trigger higher expression of operon.
-Since it’s repressible, it starts at some level of transcription and turns off.
-Feedback Repression: Activator will be turned off by product and become nonfunctional.
