Cellular control Flashcards
Silent mutations
A point mutation involving a change to the base triplet, where the triplet still codes for the same amino acid. The secondary and tertiary structure of the protein remains unchanged.
Missence mutations
A change to the base triplet sequence that leads to a change in the amino acid sequence in a protein. Altering primary structure could change the tertiary structure which could alter its shape preventing it from carrying out its usual function.
Nonsense mutations
Alters the base triplet so that it becomes a termination (stop) triplet. This will produce a very abnormal protein that won’t function. It will most likely be degraded within the cell.
Insertion and deletion mutations
If nucloetide base pairs, not in pairs of 3, are inserted or deleted from the gene, all subsequent base triplets are altered. This is a frameshift. The primary and therefore tertiary structure of the protein is severely altered so protein won’t function properly and will be rapidly degraded within the cell.
When is the lac operon used?
In E. coli, if glucose is absent and the disaccharide lactose is present, lactose induces the production of the enzymes lactose permease, which allows lactose to enter the cell, and beta-galactisidase, which hydrolyses lactose to glucose and galactose.
What does the lac operon consist of?
A length of DNA, around 6000 base pairs long contains:
- An operator region (lac0)
- structural genes, lacZ codes for beta-galactosidase and lacY codes for lactose permease.
- The promoter region, P, is next to lacO
- A small distance away from the lac operon is the regulatory gene, I
I____P O lacZ lacY
How does the lac operon work?
When the regulatory gene is expressed, the repressor protein (coded for at I) binds to the operator region. It also overlaps slightly onto the promoter region preventing RNA polymerase from binding to the promoter region. This means that the repressor protein prevents the structural genes lacZ and lacY from being expressed so the enzymes for lactose metabolism are not made.
When lactose is added it will bind to the repressor protein altering its shape so it can no longer bind to the operator region. RNA polymerase can bind to the promoter region and begin transcribing the structural genes into mRNA that will be translated into the two enzymes.
What are transcription factors?
proteins or short non-coding pieces of RNA that act within the nucleus to control which genes are turned on or off.
How do transcription factors work?
They slide along a part of a DNA molecule looking to bind to their specific promoter regions. They then aid or inhibit the attachment of DNA polymerase to the DNA and activate or suppress the transcription of the gene.
Why are transcription factors important?
They regulate gene expression in eukaryotes making sure different genes in different types of cells are activated or suppressed. Some are involved in the cell cycle. Tumour suppressor genes help regulate cell division via transcription factors. Mutations of these genes can lead to cancer.
What are introns?
Introns are the non-coding regions of DNA
What are exons?
Exons are the coding, or expressed, region of DNA
How is DNA divided into introns and exons?
All the DNA of a gene is transcribed, this is results in primary mRNA. The primary mRNA is edited and the RNA introns are removed. The remaining RNA exons are joined together. Endonuclease enzymes may be involved in the editing and splicing processes.
How does cAMP activate enzymes and stimulate transcription?
- A signalling molecule (e.g protein hormone) binds to a receptor on the plasma membrane of cell
- This activate a transmembrane protein which activates a g protein
- Activated G protein activates adenyl cyclase enzymes
- Activated adenyl cyclase catalyses formation of cAMP from ATP
- cAMP activates protein kinase A (PKA)
- Activated PKA catalyses phosphorylation of proteins hydrolysing ATP in the process. Phosphorylation activates enzymes in cytoplasm.
- PKA may phosphorylate another protein e.g CREB
- This enters nucleus and acts as a transcription factor to regulate transcription.
What is a homeobox sequence? And homeodomain sequence?
A sequence of 180 base pairs (excluding introns) found within genes that are involved in regulating patterns of anatomical development. the 60 amino acid sequence it forms is called a homeodomain sequence which can fold into a particular shape and bind to DNA, regulating the transcription of adjacent genes. The shape of the protein containing the homeodomain sequences has two alpha helices connected by one turn.
