Prokaryotic Gene Expression - Online Tutor, Practice Problems & Exam Prep

Prokaryotic gene expression involves simultaneous transcription and translation within the cytoplasm, specifically in the nucleoid region, unlike eukaryotes where these processes occur separately in the nucleus and cytoplasm. Sigma factors play a crucial role in initiating transcription by binding to promoter sequences, enabling RNA polymerase recruitment. Prokaryotic mRNAs are typically polycistronic, carrying multiple genes, while eukaryotic mRNAs are monocistronic and require processing. Understanding these differences is essential for grasping the efficiency and regulation of gene expression in prokaryotes.

Location of Transcription & Translation

In this video, we're going to begin our lesson on prokaryotic gene expression by first talking about the location of transcription and translation, which is different between eukaryotes and prokaryotes. And so in eukaryotes or in eukaryotic organisms that have a nucleus, transcription first occurs in the nucleus of the eukaryotic organism. And then later, translation will occur in the cytoplasm or in the rough endoplasmic reticulum or rough ER of the eukaryotic cell. Now in prokaryotes, on the other hand, which lack a membrane-bound nucleus, transcription and translation both occur simultaneously in the cell's cytoplasm, specifically in the region of the cell known as the nucleoid.

Notice in our image down below that we're showing you the locations of transcription as a star and the location of translation as this green shape that you see here. Notice on the left we're showing you prokaryotes, and on the right we're showing you eukaryotes. Notice that prokaryotes have this nucleoid region represented by this dashed circle here, and that nucleoid region is not membrane-bound. So it's not to be confused with the nucleus that is found in eukaryotic cells, which is membrane-bound. Notice that with the eukaryotes they have a nucleus, a membrane-bound nucleus. And this is the location of transcription. Transcription, represented by the star here, is going to occur in the nucleus of eukaryotes. And translation, notice, does not occur in the nucleus. Translation will occur outside of the nucleus either in the rough endoplasmic reticulum, or the rough ER, or just somewhere in the cytoplasm.

Now with the prokaryotes over here on the left, again, they have this nucleoid region, not to be confused with the nucleus. And the nucleoid is not membrane-bound, and that's why you can see it's represented by this dashed line. And what you'll notice is that transcription, which is again represented by the star, and translation, this shape right here, both occur simultaneously in the cell's cytoplasm, specifically in this nucleoid region. And so, this box that you see right here is labeling the cytoplasm of the cells. So, what you'll notice is that in prokaryotes and in eukaryotes the locations of transcription and translation are going to be different. This concludes our brief introduction to the location of transcription and translation. We'll be able to get some practice and learn more about prokaryotic gene expression as we move forward in our course. So I'll see you in our next video.

Which of the following statements is true?

• A. In eukaryotes, transcription occurs in the nucleoid, while translation occurs in the rough ER.
• B. In prokaryotes, transcription and translation both occur in the cytoplasm.
• C. There is not difference between where transcription and translation occur in prokaryotes and eukaryotes.

Simultaneous Transcription & Translation in Prokaryotes

In this video, we're going to further emphasize the idea of simultaneous transcription and translation that occurs only in prokaryotes and not in eukaryotes. Unlike eukaryotes or eukaryotic organisms that have a nucleus and have transcription and translation occur in different locations within the cell, prokaryotic organisms or prokaryotes, and their prokaryotic gene expression can actually start translation of a messenger RNA before the messenger RNA or the mRNA is even fully synthesized. Free ribosomes that are in the cytoplasm of the prokaryotic cell are able to bind to the messenger RNA and initiate translation while the messenger RNA is still being transcribed before the messenger RNA is even fully synthesized. In fact, multiple ribosomes are capable of translating the mRNA at once, making prokaryotic gene expression very efficient because transcription and translation can occur simultaneously. And that's very efficient. This simultaneous transcription and translation only occurs in prokaryotes. It does not occur in eukaryotes because in eukaryotes, transcription and translation occur in different locations within the cell because the eukaryotes have a membrane-bound nucleus. Whereas in prokaryotes, they do not have a membrane-bound nucleus, and this allows for transcription and translation to both occur in the same place in the cytoplasm of the prokaryotes.

If we take a look at this image down below, it focuses on the simultaneous transcription and translation that occurs only in prokaryotes. Here we have the RNA polymerase, which is in the process of elongating the messenger RNA, meaning transcription is in the middle of its formation. The messenger RNA is still being formed here. But notice that ribosomes, these green structures you see here, labeled as ribosomes, are capable of binding to the messenger RNA at this ribosome binding site over here and starting the translation process before transcription is even over. Transcription and translation are occurring simultaneously. You'll notice that each of these ribosomes is creating a protein, binding amino acids together. There can be multiple ribosomes bound to the messenger RNA at once so that multiple proteins are being built simultaneously as the RNA polymerase is continuing transcription. Once again, this simultaneous transcription and translation only occurs in prokaryotes, like bacteria, for example, that do not have a nucleus. But simultaneous transcription translation does not occur in eukaryotes. This concludes our lesson on simultaneous transcription and translation in prokaryotes, and we'll be able to get some practice moving forward with these concepts as well as learn more about prokaryotic gene expression. So I'll see you all in our next video.

PRACTICE:In prokaryotes, as soon as RNA polymerase synthesizes the 5' end of mRNA, ribosomes come in and initiate translation before transcription is completed. This is called ____.

• A. Simultaneous transcription & translation.
• B. Post-transcription.
• C. Co-transcription.
• D. Coupled transcription and translation.
• E. Co-translation

Prokaryotic Sigma (σ) Factors

This video, we're going to further emphasize the importance of prokaryotic sigma factors. Recall in our previous lesson videos when we talked about the initiation of transcription, we mentioned that sigma factors are important for prokaryotes to initiate transcription. Here we're recalling from our previous lesson videos that prokaryotic transcription initiation requires a sigma factor molecule. The sigma factor molecule is going to bind to the promoter sequence and help to recruit the RNA polymerase so that the RNA polymerase can bind and initiate transcription. Now, cells can actually have many different sigma factors, and these different sigma factors can recognize different promoters on the DNA. Different sigma factors are needed to express different genes within prokaryotes. Standard sigma factors are used for expressing genes during routine growth. Standard sigma factors are almost always going to be expressed because they're always needed for routine growth. However, alternative sigma factors are going to recognize different promoters and control the expression of alternative gene groups. These alternative gene groups are not routinely expressed and are only going to be needed in very specific, certain scenarios. These alternative sigma factors will need to be present in order for these alternative gene groups to be expressed. For example, alternative sigma factors control the expression of specific gene groups during heat shock in bacterial cells. That's what we're showing you over here on the left-hand side as an example of these alternative sigma factors.

Again, these sigma factors are only present in prokaryotes, and they are not used by eukaryotes. Over here on the top, what we have is a prokaryotic cell, and notice it's saying it isn't that hot, I'll be fine. This prokaryotic cell is just in its normal temperature conditions. However, if we heat shock this bacterial cell here, by applying lots of heat in a short period of time, then, this bacterial cell will need to rely on heat shock proteins, and those heat shock proteins will need to be expressed. The only way those heat shock proteins can be expressed is if the appropriate sigma factors are available. If the sigma factor is present, like what we see over here on the left-hand side, then transcription of the heat shock genes will occur, and those heat shock proteins will protect the bacterial cell from high temperatures. Notice on the left-hand side over here, we're showing you the bacterial cell surviving the heat shock, and it's saying the heat never bothered me anyway. Now on the right-hand side, what we're showing you is what happens if the sigma factor is not present. If the sigma factor is absent and not available then no transcription of the heat shock genes will occur. If there are no heat shock proteins made then that is ultimately going to lead to the death of the bacterial cell. These prokaryotic sigma factors must be present in order for these specific genes to be expressed at the right time when they are needed.

Over here on the right-hand side, we're just showing you some examples of gene groups that are regulated by alternative sigma factors. Not that you need to memorize this list here, but just showing you a few different examples. For example, heat shock, like what we just talked about, relies on the presence of very specific alternative sigma factors. The stationary phase of survival, during a normal growth curve for bacteria, nitrogen assimilation, flagellar synthesis, misfolded protein response, and ion transport and uptake are all examples of some processes that require and rely on alternative sigma factors. The biggest takeaway from this video is that prokaryotic organisms rely on sigma factor molecules to initiate transcription during very specific situations for specific genes to be expressed. This here concludes our brief lesson on prokaryotic sigma factors, and we'll be able to apply these concepts as we move forward and learn more about prokaryotic gene expression as well. So I'll see you all in our next video.

PRACTICE:_____ allows RNA polymerase to recognize a specific promoter sequence of a gene.

• A. Rho factor.
• B. Omega factor.
• C. Beta subunit of RNA polymerase.
• D. Alpha subunit of RNA polymerase.
• E. Sigma factor.

PRACTICE:During which stage of bacterial transcription are sigma (σ) factors involved?

• A. Initiation.
• B. Elongation.
• C. Termination.
• D. Splicing.

Prokaryotic vs. Eukaryotic mRNA

In this video, we're going to briefly point out some of the important differences between prokaryotic and eukaryotic mRNAs. Recall from our previous lesson videos that eukaryotic mRNAs must be processed after transcription. For example, the addition of a 5' cap and the addition of a poly(A) tail, as well as the removal of introns, all must occur for eukaryotic mRNAs to be expressed. However, prokaryotic mRNAs actually do not have any introns, and so these prokaryotic mRNAs do not require processing after transcription like what eukaryotic mRNAs require.

On the left-hand side over here, what we're showing you is a prokaryotic mRNA, which you'll notice consists only of exons and there are no introns in the prokaryotic mRNA. However, with the eukaryotic mRNA, again, recall from our previous lesson videos, that the mRNA is initially transcribed as a premature mRNA, a pre-mRNA that contains both exons and introns. The introns are these blue regions and we know from our previous lesson videos that these introns must be removed during the RNA processing and splicing, as well as the addition of a 5' cap and a poly(A) tail. This is what a fully mature eukaryotic mRNA might look like.

You can compare the differences between the two. This here concludes our brief lesson on prokaryotic versus eukaryotic mRNA. So, I'll see you on our next video.

Monocistronic mRNA vs. Polycistronic mRNA

In this video, we're going to briefly differentiate between monocistronic mRNA and polycistronic mRNA. Eukaryotes tend to only make monocistronic mRNA. Whereas prokaryotes, on the other hand, can either make monocistronic mRNA or they could make polycistronic mRNA. Now, monocistronic, as its name implies with the root 'mono', is referring to mRNA molecules that can only carry one single gene. Whereas polycistronic, on the other hand, as its name implies with the root 'poly', which means many or multiple, is referring to mRNA molecules carrying multiple genes.

These polycistronic mRNA molecules tend to contain what are known as spacers. These spacers are random noncoding sequences that are found between different genes. If we take a look at our image down below, you'll notice on the left-hand side, we're focusing on monocistronic mRNA molecules. The root 'mono' means that it's only going to contain one single gene. Notice that this monocistronic mRNA molecule only has one gene which is gene A, and this gene A is being flanked by a start codon and a stop codon. The gene A encodes for protein A, as you see here. Because there's only one single gene, gene A, this is a monocistronic mRNA molecule. Within gene A, if it is a eukaryotic monocistronic mRNA, there will be introns and exons. Introns get removed whereas exons are going to remain and get spliced together. Eukaryotic organisms tend to make monocistronic mRNA like what you see here containing only a single gene such as gene A.

Now on the right-hand side, what we're showing you is polycistronic mRNA, and the root 'poly' means many or multiple, so it's going to contain multiple genes. Notice that in the image, it not only contains gene A, which codes for protein A, but it also contains gene B, which codes for protein B. Because there are two genes within this mRNA molecule right here, gene A and gene B, this is an example of a polycistronic mRNA molecule. Notice that in between gene A and gene B, is this region here in pink. This region here in pink is what we call the spacer. Once again, the spacer region is going to be a random noncoding sequence that's found between different genes. Spacers are not to be confused with introns because introns are not found between different genes; introns are found within a single gene, and those introns eventually get removed. However, the spacers are not found inside a gene; the spacers are found between different genes.

Eukaryotes tend to only make monocistronic mRNA, like what you see on the left side. However, prokaryotes can make either monocistronic or polycistronic mRNA. Prokaryotes can do either of these. We'll be able to see some examples of polycistronic mRNA molecules later in our course when we start to talk about the lac operon and operons in prokaryotes. But for now, this here concludes our brief lesson on the difference between monocistronic mRNA and polycistronic mRNA, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

PRACTICE:'Cistron' is another word for 'gene'. Which of these answers are characteristics of polycistronic mRNA?
a) Only found in prokaryotes.  
b) Single mRNA strand, carrying multiple genes.
c) Single mRNA strand can be translated into many proteins.
d) All of the above are characteristics of polycistronic mRNA.

• A. Only found in prokaryotes.
• B. Single mRNA strand, carrying multiple genes.
• C. Single mRNA strand can be translated into many proteins.
• D. All of the above are characteristics of polycistronic mRNA.

Here’s what students ask on this topic:

What is the difference between transcription and translation in prokaryotes and eukaryotes?

In prokaryotes, transcription and translation occur simultaneously in the cytoplasm, specifically in the nucleoid region. This is because prokaryotes lack a membrane-bound nucleus, allowing ribosomes to bind to mRNA and initiate translation even before transcription is complete. In contrast, eukaryotes have a membrane-bound nucleus where transcription occurs. The mRNA must then be processed (adding a 5' cap, poly-A tail, and splicing out introns) and transported to the cytoplasm or rough endoplasmic reticulum for translation. This separation of processes in eukaryotes results in a more complex and regulated gene expression system compared to the more efficient but simpler system in prokaryotes.

What role do sigma factors play in prokaryotic gene expression?

Sigma factors are essential for initiating transcription in prokaryotes. They bind to specific promoter sequences on the DNA, facilitating the recruitment of RNA polymerase to the transcription start site. Different sigma factors recognize different promoters, allowing the cell to express various genes as needed. Standard sigma factors are used for routine growth, while alternative sigma factors are activated under specific conditions, such as heat shock or nutrient limitation. This system enables prokaryotes to rapidly respond to environmental changes by altering gene expression patterns.

How do prokaryotic and eukaryotic mRNAs differ?

Prokaryotic mRNAs are typically polycistronic, meaning they can carry multiple genes within a single mRNA molecule. They do not contain introns and do not require extensive processing after transcription. In contrast, eukaryotic mRNAs are usually monocistronic, carrying only one gene per mRNA molecule. Eukaryotic mRNAs undergo significant processing, including the addition of a 5' cap, a poly-A tail, and the removal of introns through splicing. These differences reflect the distinct mechanisms of gene expression regulation in prokaryotes and eukaryotes.

What is the significance of simultaneous transcription and translation in prokaryotes?

Simultaneous transcription and translation in prokaryotes significantly enhance the efficiency of gene expression. As mRNA is being synthesized by RNA polymerase, ribosomes can immediately bind to the mRNA and start translating it into proteins. This allows for rapid protein production, which is crucial for prokaryotic cells to quickly respond to environmental changes. This process is possible because prokaryotes lack a membrane-bound nucleus, enabling both transcription and translation to occur in the cytoplasm. This efficiency is a key advantage for prokaryotic organisms, such as bacteria, in adapting to their environments.

What are the differences between monocystronic and polycystronic mRNA?

Monocystronic mRNA, typically found in eukaryotes, carries only one gene per mRNA molecule. This means each mRNA translates into a single protein. In contrast, polycystronic mRNA, commonly found in prokaryotes, carries multiple genes within a single mRNA molecule. These genes are separated by non-coding spacer sequences. Polycystronic mRNA allows for the coordinated expression of multiple genes, often related to a single metabolic pathway or function, enhancing the efficiency of gene regulation in prokaryotes. This distinction reflects the different strategies of gene expression regulation in prokaryotic and eukaryotic cells.