Tut 10 Flashcards
(15 cards)
List the levels of gene regulation in eukaryotes.
DNA organisation
Transcription
mRNA splicing + modification
mRNA transport
mRNA stability (degradation)
Translation mRNA to protein
Protein modification (post-translational)
How does modification of DNA by methylation influence eukaryotic gene expression?
DNA methylation has DNA methylase which effectively acts to add methyl group (cytosine residue) into the promoter region of DNA. When the methyl group (C residue) is in the 5’ prime translated region, Methylation will turn off the gene and inhibit transcription.
The TATA box to be activated, all the promoter has to sit there and bind, and then RNA polymerase can bind to it. Methylation eventually cut this off, so RNA polymerase cannot stick to the DNA and cannot initiate transcription.
- DNA polymerase is not affected by methylation
What is responsible for uncoiling DNA during full DNA replication?
DNA HELICASE
Because helicase separates and unwinding the double-stranded DNA into single-stranded, and is breaking down (uncoupled) the H-bonding between A-T or G-C, but these will close up again and rejoining when helicase moves apart.
> single stranded binding proteins hold apart the hydrogen bonds as DNA helicase uncoils
Topoisomerase involved in un-supercoiling DNA, and prevents the DNA double helix ahead of replication fork from getting too tightly wound as the DNA opens up. It cuts the backbone of DNA in a control fashion, so it allows the DNA to flip to uncoil and removes the torsional energy then reseal.
> Supercoiling builds potential energy, and you get torsional stress(Strain on hydrogen bonds between base pairs). Strain could potentially cause a random break in the DNA.
What parts of DNA strands cannot be replicated by DNA polymerases? What is the significance of this, and how is it overcome?
The end of 3’ prime and end of 5’ prime of DNA, as DNA polymerase cannot fill the RNA primer site at ends. So DNA polymerase will miss the ends as it translates. Thus, it will get shorter DNA over repeated replication.
Overcomed how? Telomerase maintains overhanging, which fill the sapce in the ends, meaning you can regenerate the cut away part of DNA
(Telomerase present in stem cell is activated)
(Telomerase present in a somatic cell is inactivated)
How is DNA replication controlled? Explain how checkpoints work.
There are four specific steps at which this monitoring occurs have been termed checkpoint control.
- Cyclin D is linked with CDK4 + CDK6 and is progress past G1/S boundary –> checks all required components for replication are available (amp, gmp, enzymes, dna poly, sugars, phosphates, etc.)
- Cyclin E and A is linked with CDK2 (initiation of DNA replication in S phase)
- Cyclin B is linked with CDK1 (G2 to M phase transition)
Cell cycle checkpoints: all the cyclin linked to particular kinase that effectively ticks the box and the cell is not allowing to move to the next phase until the checking system is done.
What is the basic structure of a eukaryotic transcription factor? Generally, how does the function of eukaryotic transcription factors differ from DNA binding proteins in prokaryotes?
DNA-binding domain and Trans-activation domain
(helix-turn-helix, zinc-finger, leucine zipper)
- Eukaryotes – regulated by activators, which means transcription factors mostly turned on–cells are more likely to be at rest until needed.
- Prokaryotes – regulated by repressor, which means transcription factors mostly switched off – ‘default setting’ for the genes is ON
How can changes (via regulation) to messenger RNA processing be used to change protein gene expression in eukaryotes? (i.e. make different proteins from the same DNA code). Provide an example.
mRNA splicing + modification
mRNA transport
mRNA stability (degradation)
Translation mRNA to protein
(Alternative splicing can result in different proteins from the same gene–more than one mRNA can be made from the same gene)
Pre-mRNA has to go through some processing steps to become an mRNA that can be translated into a protein. Capping- a 7 methylguanosine is added to the 5’ end.
polyadenylation - the 3’ end is cleaved and a poly-A tail is added. Then mRNA is spliced.
The introns are removed, and the exons remained, becoming the mRNA.
In alternative splicing, one pre-mRNA may be spliced into different ways depends on which exons are kept and which introns are cut.
E.g calcitonin peptide spliced into calcitonin gene-related peptide
What are the main components of RNA silencing mechanism in eukaryotes? How does the mechanism target specific mRNAs for degradation or blocking translation?
- siRNA - Small interfering RNA
- miRNA - Micro-RNA
- RISC - RNA induced silencing complex
- RITS - RNA induced transcription silencing complex
Long double-stranded RNAs processed by Dicer enzyme into siRNA or miRNA. (The double-stranded precursor of miRNA and siRNA binds to dicer which cuts the mRNA into short segments and hands it over to argonaute). Depending on the way argonaute attaches to mRNA, you can block or cut.
siRNA and miRNA usually have approximately 21 base pairs, single-stranded RNA sequence within the cell and is not protected will not last for a long time and eventually will be degraded.
Cutting- siRNA cut in the middle, 7’ methylguansine is not existed and poly A tail presence only and therefore, permanently degraded.
Blocking- miRNA is roles for blocking, because the sequence of mRNA binding is not fully binds (partial binding), so the inhibition of mRNA is not permanent. (Not sure the answer..)
Under what circumstances might a lethal dominant allele persist in the population?
For dominant lethal alleles to exist, the affected individual must reproduce before dying.
Tightly curled hair is caused by a dominant gene in humans. If a heterozygous curly haired person marries a person with straight hair, what answer below represents the correct fraction of hair type in their offspring?
1/2 straight
A recessive allele in tigers causes the white tiger. If two normally pigmented tigers are mated and produce a white offspring, what % of their remaining offspring would be expected to have normal pigmentation?
75%
Which one of the following statements is CORRECT?
A conditional mutant is one whose expression is influenced by some environmental condition
Assume that a cross is made between AaBb and aabb plants and that the offspring fall into approximately equal numbers of the following groups: AaBb, Aabb, aaBb, aabb. These results are consistent with the following circumstance:
Independent assortment
Which one of the following statements regarding population genetics is CORRECT?
Selection is the differential reproduction of genotypes, resulting from their variable fitness.
