Chapter 18 Flashcards
What does Alternative Splicing do?
It deals with the removal of introns
Why is Alternative Splicing carried out?
Introns must be removed from (RNA transcript) in order for coding sequences to be expressed properly.
How is Alternative Splicing carried out? Give an example.
-Introns can be removed separately or in combination, depending on how the splicing machinery interacts with the RNA
Ex. If two successive introns are removed together, the exon between them will also be removed.
What is the end product of Alternative Splicing?
- The splicing machinery has the opportunity to modify the coding sequence of an RNA by deleting some of its exons
- Instead of duplicating genes, the alternate splicing of transcripts makes it possible for a single gene to encode different polypeptides.
What is an example of Alternative Splicing?
Troponin T:
-Transcripts of this gene are spliced in different ways to create a large array of mRNAs.
-When translated, many different troponin T polypeptides are produced.
What are 2 examples of inducible gene expressions in eukaryotes?
- Temperature (Heat Shock)
2.Signal Molecules (Hormones)
(Steroid and Peptide)
Explain how Heat Shock can induce transcription.
When organisms are subjected to high temperatures, they respond by synthesising a (heat shock) protein to help stabilise the internal cellular environment.
-The heat stress induces the transcription of genes encoding for the proteins
Explain how signal molecules can induce transcription.
Hormone-induced gene expression is mediated by specific sequences in the DNA called hormone response elements (HREs)
HREs are located near the genes they regulate and bind specific proteins, which act as TF.
Hormones circulate through the body, make contact with their target cells, and then initiate a response to regulate the expression of certain genes.
What are the 2 classes of hormones?
Steroid and Peptide
What is a steroid hormone?
They are small, lipid-soluble molecules derived from cholesterol which have little to no trouble passing through cell membranes.
How do steroid hormones work?
- The steroid hormone enters its target cell and combines with a receptor protein.
- The hormone/receptor complex binds to a hormone response element in the DNA.
- The bound complex stimulates transcription.
- The transcript is processed and transported to the cytoplasm.
- The mRNA is translated into proteins.
What are Peptide Hormones?
They are linear chains of amino acids which are too large to pass through cell membranes.
Therefore, they send signals to the inner part of the cell by using a membrane bound receptor protein (outside of cell).
Like all other polypeptides, these molecules are encoded by genes
How do Peptide Hormones work?
- The hormone binds to a receptor protein in the membrane of its target cell. (A conformational change in the receptor eventually leads to changes in other proteins inside the cell)
2.The hormone/receptor complex activates a cytoplasmic protein.
- The activated cytoplasmic protein transduces a signal to the nucleus.
- The signal induces a transcription factor
to bind to DNA. - The bound transcription factor stimulates transcription.
- The transcript is processed and transported to the cytoplasm.
- The mRNA is translated into proteins.
What 3 things do enhancers exibit?
- They act over large distances
2.Their influence on gene expression is independent of orientation—they function equally well in either the normal or inverted orientation within the DNA
3.Their effects are independent of position—they can be located upstream, downstream, or within an intron of a gene and still have profound effects on the gene’s expression
What 2 chemical domains do transcriptional inducing proteins have?
- DNA-binding domain
- Transcriptional activation domain
How many domains does a steroid hormone receptors (proteins) have?
Three
Steroid hormone receptors have a third domain that specifically binds the steroid hormone.
What causes transcriptional activation with proteins involved in control of transcription?
It involves physical interactions between proteins.
A TF that has bound to an enhancer may make contact with one or more proteins at other enhancers, or directly with proteins that have bound in the promoter region.
Through these contacts, the transcriptional activation domain of the factor may then induce conformational changes in the assembled proteins, causing RNA polymerase to bind and initiate transcription.
Explain the Zinc Finger TF motif.
It is a short peptide loop that forms when two cysteines and two histidines bind a zinc ion
The peptide segment between the two pairs of amino acids then juts out a finger
These fingers play important roles in DNA binding
Explain the Helix-turn-Helix motif.
It contains 3 short helices of amino acids separated from each other by turns
The helical segment closest to the carboxy terminus is required for DNA binding
The other helices are involved in the formation of protein dimers.
Explain the Leucine Zipper motif.
It is a stretch of amino acids with a leucine at every seventh position.
Polypeptides with this feature can form dimers by interactions between the leucines in each of their zipper regions.
The zipper sequence is adjacent to a positively charged stretch of amino acids.
When two zippers interact, these charged regions splay out in opposite directions, forming a surface that can bind to negatively charged DNA.
Explain the Helix-loop-Helix motif.
It is a stretch of two helical regions of amino acids separated by a nonhelical loop.
The helical regions permit dimerization between two polypeptides.
If the motif is adjacent to a stretch of basic (positively charged) amino acids, dimerization occurs.
These amino acids can bind to negatively charged DNA.
What 2 RNAs are included in the RNAi Pathways?
siRNA and miRNA
What is RNAi? (RNA Interference)
Posttranscriptional mechanisms play an important role in regulating the expression of genes.
These small, noncoding RNAs include siRNA and miRNA.
By base pairing with target sequences in messenger RNA molecules, these small RNAs interfere with gene expression.
How are SiRNA and miRNA created?
These molecules start off as larger double stranded RNA which is diced by proteins that are double stranded RNA-specific endonucleases. (Dicers)
The siRNA and miRNA produced are base-paired throughout their lengths except at their 3′ ends, where two nucleotides are unpaired.
These ends allow the molecule to be inserted into the gene and cause interference.
