case 6 Flashcards
What is methylation (histone addition)?
An enzyme that catalyses the addition or removal of specific methyl groups. Addition of methyl groups to histones can lead to the condensation of chromatin and reduced transcription. Often the addition of a particular chemical group may create a new binding site for enzymes that further modify chromatin structure.
What is acetylation?
The addition of an acetyl (-COCH3 ) to an amino acid in a histone tail – appears to promote transcription by opening the chromatin structure.
What is methylation (DNA addition)?
long stretches of inactive DNA, (such as that of inactivated mammalian X chromosomes.) are generally more methylated than regions of actively transcribed DNA. On a smaller scale, the DNA of individual genes is usually more heavily methylated in cells in which those genes are not expressed. Removal of the extra methyl groups can turn on some of these genes. One methylated, genes usually stay that wat through successive cell division in a given individual. (at DNA sites where one strand is already methylated, enzymes methylate the correct daughter strand after round of DNA replication. Methylation patterns are thus passed on to the daughter cells)
What is an operator?
Genes are switched on or off by changes in the metabolic status of the cell. One basic mechanism for this control of gene expression is described as the operon model . Together, the operator, the promoter, and the genes they control—the entire stretch of DNA required for the enzyme in question.
Explain an operon.
The on-off switch is a segment of DNA called an operator. the operator is the operon’s switch for controlling transcription. the operon is the unit of genetic function. consisting of an promoter, an operator and a coordinately regulated cluster of genes whose products
function in a common pathway
This is how it works; ? the operon in question is turned on; that is, RNA polymerase can bind to the promoter and transcribe the genes of the operon. The operon can be switched off by a protein that is called the … repressor. There are two types of negative gene regulation and one of positive.
Explain repressor.
A repressor binds to the operator and blocks attachment of RNA polymerase to the promoter.
- A repressor is protein specific for the operator.
- A repressor protein is encoded by a regulatory gene
- the binding of repressors to operators is reversible. An operator alternates between two states: one with the repressor bound and one without the repressor bound. The relative duration of the repressor-bound state is higher when more active repressor molecules are present
- repressor can bind to operon active shape turns gen of
- repressor can not bind to operon inactive shape turns gen on.
Explain corepressor.
A small molecule that cooperates with repressor protein to switch an operon off.
Example: The trp repressor is synthesized in the inactive form, which has little affinity for the trp operator. Only when a tryptophan molecule binds to the trp repressor at an allosteric site does the repressor protein change to the active form that can attach to the operator, turning the operon off. Tryptophan functions in this system as a corepressor, a small molecule that cooperates with a repressor protein to switch an operon off. As tryptophan accumulates, more tryptophan molecules associate with trp repressor molecules, which can then bind to the trp operator and shut down production of the tryptophan pathway enzymes. If the cell’s tryptophan level drops, many fewer trp repressor proteins would have tryptophan bound, rendering them inactive; they would dissociate from the operator, allowing transcription of the operon’s genes to resume.
later
What are two types of small ncRNAs and explain?
Explanation;
- microRNAs (miRNAs) small, single stranded RNA molecules capable of binding to complementary sequences in mRNA molecules. (
o A longer RNA precursor is processed by cellular enzymes into an miRNA, a single-stranded RNA of about 22 nucleotides that form a complex with one or more proteins. The miRNA allows the complex to bind to any mRNA molecule with at least seven or eight nucleotides of complementary sequence. The miRNA-protein complex than degrades the target mRNA or less, often simply blocks the translation.
o Mostly encoded in intronic regions (will never be achieved)
o Can be in exons of other genes.
o Always regulated with the host gene. (if the whole things is transcribed introns and exons.)
o Has an extra processing step
If origin = intron splice some
Else DROSHA removes 5çap +poly (A) taile
- Small interfering RNAs (siRNA) similar in function and size as the miRNA.
20-24 base pairs it is double stranded. Two hanging nucleotides on both ends. Comes outside and from inside the body.
What are the similarities and differences between siRNA and miRNA.
Similarities;
- Both can associate with the same proteins producing similar results.
- Use the same protein complex
Differences
- Subtle differences in structure of their precursors, (which in both cases are RNA molecules that are mostly double-stranded)
- mRNA is not truly double stranded.
Explain the dicer.
Is an protein that can detect and then binds to dsRNA (double stranded RNA). Than cleaves into shorter dsRNA. (still double stranded but chopped op in pieces) (does it for both siRNA and miRNA)
Explain the RISC complex.
RNA-induced silencing complex. It need RNA to function. It silences RNA expression/translation. Most important gene is the Argonaute.
- The in chopped pieces from dicer will come to Argonaute.
- Argonaute (Cuts down the nucleotide pairs. So 11 wil be 1 1. This is now called single stranded RNA(ssRNA))
- ssRNA + risc finds complementary mRNA
- result No translation. (ribosome can’t process) lower gene expression.
What is an inducer?
A specific small molecule that binds to a bacterial repressor protein and changes the repressors shape so that it cannot bind to an operator, thus switching an operon on.
What is an activator?
A protein that binds to DNA and stimulates gene transcription. In prokaryotes, activators bind in or near the promoter: in eukaryotes, activators generally bind to control elements in enhancers
What does eRNA do?
Help chromatin looping and Transcription Factor binding
