After these modifications, the RNA molecule leaves the nucleus, enters the cytoplasm, and undergoes translation process of synthesizing proteins.
In contrast, bacterial transcription occurs in the cytoplasm and does not involve any of the post-transcriptional modifications.
As a result, the transcribed RNA can immediately be used to synthesize proteins; therefore, translation immediately follows transcription in bacteria. It is found in some viruses such as HIV. During which of the following phase s of the cell cycle does transcription occur?
This is a normal cellular process that is required for cells to grow and function properly because these RNA molecules are eventually converted to proteins, the building blocks of cells. Growth of cells occurs in G1 and G2 phases; therefore, transcription occurs during both of these phases. Note that DNA replication occurs during S phase; therefore, no DNA molecules will be available for transcription during S phase and transcription will be halted.
A bacteria is known to have a defect in a protein that codes for the sigma factor. What will you most likely observe in this bacteria? Increased post-transcriptional modifications. Complete halt of transcription because there is an increased degradation of RNA polymerase.
Complete halt of transcription because RNA polymerase stays as a holoenzyme. Sigma factor is a special molecule in bacteria that is used to initiate transcription. In a bacterial cell, RNA polymerase is typically kept in its inactive form, called holoenzyme. When it is needed for transcription, RNA polymerase is converted to its active form by sigma factor.
Which of the following could be part of a promoter region in bacteria? A Pribnow box is a type of promoter region in bacteria that contains a sequence similar to the eukaryotic TATA box.
Transposable elements are those that can move around within the genome, which increases genetic diversity. Also, due to alternative splicing of introns, multiple distinct proteins can be synthesized from the same exact mRNA transcript. One example of this is antibody production. This also increases genetic diversity, if this occurs in the germ-line cells.
For this question, we need to consider how histone acetyltransferases affect histones. Then, we need to determine how these modified histones affects the expression of genes. First, it's important to note that histones are proteins that mostly contain positive charges.
As a result of this, histones are able to associate with DNA very well, since DNA contains a negatively charged backbone. In this tightly bound conformation, the collection of DNA and proteins are referred to as hererochromatin. What's more is that when the DNA is tightly bound like this, the transcription machinery in the cell is physically blocked from associating with genes. Thus, gene expression is lowered.
Histone acetyltransferases are enzymes that attach acetyl groups to the positively charged lysine residues that are part of histones. Remember, the positive charge of these lysine residues is what allows the histones to associate with the DNA.
When acetyl groups are added, the positive charge on these histones becomes neutralized. As a result, the histones are no longer able to associate with the DNA. What this means is that the transcription machinery in the cell is now able to physically access the genes, allowing gene expression to increase.
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The polymerase comprised of all five subunits is called the holoenzyme. A promoter is a DNA sequence onto which the transcription machinery, including RNA polymerase, binds and initiates transcription. In most cases, promoters exist upstream of the genes they regulate. The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all the time, some of the time, or infrequently. Although promoters vary among prokaryotic genomes, a few elements are evolutionarily conserved in many species.
At the and regions upstream of the initiation site, there are two promoter consensus sequences , or regions that are similar across all promoters and across various bacterial species Figure. Once this interaction is made, the subunits of the core enzyme bind to the site.
The A—T-rich region facilitates unwinding of the DNA template, and several phosphodiester bonds are made. The transcription initiation phase ends with the production of abortive transcripts, which are polymers of approximately 10 nucleotides that are made and released. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it.
Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals. One is protein-based and the other is RNA-based. Rho-dependent termination is controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the end of the gene, the polymerase encounters a run of G nucleotides on the DNA template and it stalls.
As a result, the rho protein collides with the polymerase. The interaction with rho releases the mRNA from the transcription bubble. Rho-independent termination is controlled by specific sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C—G nucleotides. The result is a stable hairpin that causes the polymerase to stall as soon as it begins to transcribe a region rich in A—T nucleotides. This, coupled with the stalled polymerase, induces enough instability for the core enzyme to break away and liberate the new mRNA transcript.
Activation of the RNA polymerase complex enables transcription initiation, and this is followed by elongation of the transcript. In turn, transcript elongation leads to clearing of the promoter, and the transcription process can begin yet again. Transcription can thus be regulated at two levels: the promoter level cis regulation and the polymerase level trans regulation. These elements differ among bacteria and eukaryotes.
In bacteria, all transcription is performed by a single type of RNA polymerase. This polymerase contains four catalytic subunits and a single regulatory subunit known as sigma s.
Interestingly, several distinct sigma factors have been identified, and each of these oversees transcription of a unique set of genes. Sigma factors are thus discriminatory, as each binds a distinct set of promoter sequences. A striking example of the specialization of sigma factors for different gene promoters is provided by bacterial sporulation in the species Bacillus subtilis. This bacterium exists in two states: vegetative growing and sporulating.
Genes involved in spore formation are not normally expressed during vegetative growth. Remarkably, expression of a gene encoding a novel sigma factor turns on the first genes for sporulation.
Each of these sigma factors recognizes the promoters of the genes in its group, not those "seen" by other sigma factors. This simple example illustrates how transcription can be regulated in both cis and trans to cause changes in cell function. Therefore, while bacteria accomplish transcription of all genes using a single kind of RNA polymerase, the use of different sigma factor subunits provides an extra level of control.
Interestingly, RNA pol II is uniquely sensitive to amatoxins, such as a-amanitin of the extremely toxic Amanita genus of mushrooms Weiland, , a fact that researchers have been able to exploit for the purposes of polymerase studies - although recreational mushroom hunters should beware! Thus, while eukaryotic transcription is far more complex than bacterial transcription, the main difference between the two types of transcription lies in RNA polymerase.
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