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We are going to continue our discussion of molecular genetics with the topics of transcription and translation.0002
Before we delve into the details, I am going to give you an overview of the process.0011
Recall that the central dogma of molecular biology is the flow of information from DNA to RNA to protein.0015
The genetic information is contained on DNA.0029
A transcript is made of RNA and this is then, transported.0033
A particular type of RNA, mRNA is transported into the cytoplasm, where it is translated into polypeptide and forms of protein.0039
This process, therefore, is called transcription.0050
The use of a DNA template to form RNA, and the process of going from RNA using that RNA transcript to form a protein is translation.0055
When a protein is made based on the information contained in DNA, we say that the gene has been expressed.0071
A person might carry a gene for red hair, and that is just the gene.0079
But the actual protein that makes the hair color red, when that is made, we say the gene is expressed. It is expressed in the form of red hair.0086
Now, looking at these names, transcription and translation, recall that DNA is made from nucleotides. The nucleotide monomers form a polynucleotide.0095
RNA is also made from nucleotide monomers.0108
There are slight differences between DNA and RNA, but the essential code is the same.0112
Therefore, a DNA template is simply copied. It is transcribed to make the RNA.0117
That is much difference between RNA and protein, so with DNA, you are working with nucleotides, with RNA, with nucleotides.0125
With protein, it is an amino acid sequence.0132
In order to go from this one type of code - nucleotides - to another code - amino acid sequence - is actually translation.0135
You are not just copying something. You are actually taking the information and translating it into a different form.0145
We are going to discuss transcription first, and to understand transcription, you need to have RNA structure down.0154
Again, this is a topic that was covered under the nucleic acid and protein lecture earlier on, but I am going to review the essentials right now.0160
Unlike DNA, RNA molecules are single stranded.0169
Another difference between RNA and DNA is that they contain uracil instead of thymine.0173
The essential structure is the same, though.0180
RNA consists of nucleotides. It is a nucleotide sequence, and looking at what a nucleotide is, there is a pentose sugar; so that is a 5-carbon sugar.0182
In the case of RNA, the sugar is ribose.0194
In DNA, this oxygen is gone. It is deoxygenated.0198
It is deoxyribose, so this is ribonucleic acid.0202
The second element is a nitrogenous base.0206
And recall that there are two sets of nitrogenous bases- the pyrimidines, which contain a 6-membered ring, and they are cytosine, thymine and uracil.0210
In RNA, you will find uracil. In DNA, you will find thymine, and cytosine is found in both.0224
For RNA, we are just going to have C and U.0233
The second type of nitrogenous base is the purines.0236
These contain a 6-membered ring fused to a 5-membered ring and consist of G and C, adenine and - excuse me - guanine and cytosine, so C, U, G, C.0243
The other thing to be aware of besides the differences between RNA and DNA is the types of RNA.0267
There are multiple types of RNA. Three main ones we will be focusing on.0275
One is messenger RNA. The other is ribosomal RNA, and the third is transfer or tRNA.0279
Messenger RNA is the type of RNA that is used for translation.0289
It is the transcript to make a protein.0297
Ribosomal RNA is not used to make a protein. The rRNA is itself, the product.0303
Ribosomes are composed largely of ribosomal RNA, so these are component of ribosomes.0309
tRNA or transfer RNA delivers amino acids to the ribosome during translation, and we will be going into detail about all three types of these as we go along.0323
We are going to start out mainly focusing on mRNA. All three of these would be transcribed.0340
DNA would be used as a template to form all three types, but as we talk about transcription, I am going to focus on mRNA.0345
And then, we are going to follow that process of transcription with translation.0352
Transcription is the process through which RNA is synthesized using a DNA molecule as a template.0359
There are three phases. Initiation, elongation and termination are the three phases.0368
We are going to go through each of these phases starting out with initiation.0375
Initiation begins with the binding of RNA polymerase to the promoter region.0379
The region on DNA, where this gets started, is known as promoter.0385
Here, we have the DNA double helix, and you see it is separated out here in order to allow transcription to occur.0392
Transcription for a particular gene occurs using only one of the DNA strands as a template.0401
In this case, here we have DNA, DNA, and then, in brown, it is the RNA; and you can see that this RNA strand is using this DNA as the template.0408
This is the template strand. Another name for template strand, we sometimes call it the sense strand, and the other is the antisense strand.0420
During initiation - I will put this right here - the RNA polymerase binds to the promoter region.0430
Promoter regions are also known as TATA boxes because they have the sequence T-A-T-A.0447
Initially, where this promoter region is, the RNA polymerase binds, is slightly upstream of where the first actual nucleotide will be transcribed.0459
Things start out, RNA polymerase binds to this promoter region, and then, slightly pass that, we will get the actual transcription of RNA.0471
In addition to RNA polymerase, there are other factors that bind to this promoter region.0487
If you take it together, the RNA polymerase plus other proteins known as transcription factors,0493
what you have is something called the transcription initiation complex.0506
And the job of these transcription factors is to help the RNA polymerase bind to the correct region.0516
We have RNA polymerase plus transcription factors. All that binds together to the promoter region to get things started.0525
And it is known as a transcription initiation complex.0531
In eukaryotes, there is actually a different type of RNA polymerase for each of those three types of RNA that I mentioned- tRNA, rRNA and mRNA.0535
We are focusing right now in messenger RNA.0550
The one that is used to transcribe DNA into what will become messenger RNA is known as RNA polymerase II.0551
RNA Pol II transcribes DNA into what is eventually messenger RNA.0559
This has occurred. This binding has occurred.0566
The transcription initiation complex has bound.0569
The next thing that needs to happen is the double helices to unwind.0571
And again, helicases are involved in this unwinding, this separating out so that the RNA can use the template strand.0575
Now, for a particular gene, the same strand is always used as a template.0584
Let's look at this DNA and say that there is a gene here that is being transcribed.0589
Well, that is what is happening, and we see that this is the template strand.0594
There might be another gene over here that needs to be transcribed, and this strand may be the template for that.0599
So, the same template is used for a particular gene, but it might be a different strand that is used for another gene, so that is initiation.0606
The next thing that needs to happen is elongation.0617
The initiation complex has bound. RNA polymerase is ready to go.0619
It is bound to the template strand. These other factors are bound.0623
We found the TATA box. The helix has been separated.0628
What is going to happen is that RNA polymerase is going to proceed in the 5' to 3' direction, just like DNA polymerase.0632
And it is going to form a strand of RNA that is complementary to the template strand.0641
Remember that complementary means that we would have G and C. Those two are complementary.0645
And with DNA, when we would say "OK, A is complementary with T", for RNA we do not have T. We have U.0656
For RNA, it is going to be A, U, G, C as complementary nucleotides.0666
Looking here at what I mean, this is the template strand for this situation.0672
This is the RNA strand. RNA polymerase is going to seed T, and that is going to tell it to add A for the nucleotide in the growing RNA strand.0681
For C, the complementary strand is going to be G. For G on the template, we are going to have C.0692
For A, if this was DNA synthesis, we would have T, but it is not. It is RNA synthesis, so instead, we are going to have U, G, C.0701
A on the DNA gives me U, A, T and so on, and recall that we just produced one strand.0713
We do not need a double helix. RNA is just single-stranded.0725
You will also notice that this is the same sequence almost as the antisense strand.0731
It is complementary to this template strand or sense strand.0736
And it is the same as antisense not a 100% the same because you will see here, I have AA GG CC T.0739
There is no T here. There is U instead, CC and U instead of T and so on.0748
OK, the first step was initiation transcription complex - excuse me - transcription initiation complex bound to the template strand.0756
By this point in your biology education, you already know that the genetic information of all living things on the planet can be found in a lengthy molecule known as deoxyribonucleic acid, or DNA. The proverbial holy grail of scientists who sought to find it throughout history, DNA is the metaphorical “blueprint” from which living things’ bodies and systems are built and run. But the question remains: how does DNA accomplish this? How do we get from genetic blueprint to living, reproducing organism?
The answers to these questions lie in the biological processes of transcription and translation. Through the actions of a few important molecules at two primary sites within the cell (the nucleus and the ribosomes), the genetic information in DNA is copied, read, and used to develop precise proteins with specific functions within the cell.
The majority of genes in your DNA code directly for proteins, though some code for the production of other molecules that aid in later production of proteins. Regardless, your genes tell your cells exactly what to make in a very real, physical way—and that’s pretty mind-blowing.
In this AP Biology Crash Course, we’ll go over the processes by which DNA is replicated and “read” with the help of various other molecules. You’ll need to familiarize yourself with these elements and processes for the AP Biology Exam.
DNA and Friends
Let’s first take a quick look at some of the major players in the replication of genetic material:
Deoxyribonucleic acid is the starting point of the processes of transcription and translation. This is the original piece of genetic material through which all biological processes within an organism are governed. DNA is always found in the form of a double-helix.
Similar to DNA, Ribonucleic acid—RNA—is a vital molecule for the function of living things. RNA is a primary factor in the transfer of genetic information and the synthesis of proteins. Unlike DNA, however, RNA can take a variety of forms and shapes.
• RNA polymerase
RNA polymerase is an enzyme that transcribes DNA and produces a strand of mRNA (essentially the transcribed copy of the DNA).
mRNA stands for “messenger RNA,” and aptly so: mRNA is essentially the messenger molecule that goes between the DNA in the nucleus of the cell and the ribosomes where proteins are synthesized.
tRNA stands for “transfer RNA” and is the link between the mRNA and the amino acids that are formed into proteins. Essentially, the tRNA “reads” the mRNA and “translates” it into a sequence of amino acids.
When one transcribes a written piece, as religious persons like monks may do, he looks at the original work and exactly replicates what is written onto a new set of pages. Similarly, professional audio transcriptionists work with audio recordings to type exactly what is said by those recorded. Transcription, in the case of DNA, is much the same: a molecule known as RNA polymerase transcribes the nucleus-bound DNA exactly, producing a replica mRNA strand that can be transferred out of the nuclear membrane for use in the production of proteins.
It is important to note that the two strands of DNA are made up of one sense strand and one antisense strand. mRNA uses the antisense strand as a template when transcribing the information. Because the nucleotides of nucleic acids only bond in specific pairs, the resulting mRNA strand will be identical to the sense strand of the DNA molecule.
The process of transcription occurs in the following steps:
1. First, RNA polymerase binds to what is known as promoter DNA. This DNA is a sequence that signals the start of genetic information for a particular gene.
2. RNA polymerase unwinds and separates the DNA by creating a structure known as the transcription bubble. This bubble breaks the hydrogen bonds between nucleotides.
3. RNA polymerase adds RNA nucleotides to its “copy” by matching nucleotides to those on the antisense strand.
4. A sugar-phosphate backbone is formed, producing a self-supporting strand of RNA (in the case of protein synthesis, mRNA).
5. The hydrogen bonds between the RNA and DNA break, freeing the new strand (mRNA) from the helix.
6. In cells with nuclei, the RNA may undergo further steps before being moved out of the nucleus. Possible additional steps may include splicing(editing of the sequence), capping(attachment of additional nucleotides to the ends of the strand),or polyadenylation(addition of a tail of adenine bases).
7. The RNA (mRNA) strand is moved out of the nucleus via specialized pores in the nucleus.
Image Source: Wikimedia Commons
Let’s go back to the previous example of transcribing a written piece of literature. Once the new copy of the text has been fully transcribed, what is its next purpose? Most likely, someone else will come along and read the new copy, receiving its knowledge and using it for his own purposes. Similarly, in the process of translation, tRNA “reads” the genetic information copy from the mRNA strand and uses it for the purpose of producing proteins.
The process of translation occurs in three main steps known as initiation, elongation, and termination. Take a look at how these stages work:
1. Upon initiation, the mRNA strand enters the ribosome, allowing tRNA to attach at a region called the start codon. The start codon is simply the first piece of code on an mRNA transcript strand.
2. During elongation, tRNA builds a strand of amino acids by transferring the appropriate amino acid to each tRNA along the transcript. The ribosome moves along the strand to each codon as this occurs, almost like a manufacturing machine.
You could even think of the process as similar to printing from a computer: the ribosome is the printer, the tRNA is the print head, the amino acids are the ink, and the mRNA strand is the document to be printed from.
3. Termination occurs at the end of the process, when the stop codon—or final piece of code—is reached by the ribosome. At this point, the ribosome releases the resulting polypeptide (amino acid chain).
Transcription & Translation AP Biology Exam Review & Practice
Let’s review what we’ve learned in this AP Biology Crash Course so far:
• DNA is the genetic “blueprint” of living organisms and the starting point for all proteins. Its information is copied and transferred into RNA to produce proteins.
• Promoter DNAis a segment of DNA that signals the start of genetic coding for a specific gene. RNA polymerase will attach here at the start of transcription for the gene.
• RNA is an important molecule that comes in various types. With regard to transcription and translation, RNA not only copies and moves genetic information, but also turns that information into the resulting proteins.
• RNA polymerase is the molecule that plays the key role in the transcription process. RNA polymerase attaches to the DNA and makes a copy of the genetic information in the form of an mRNA strand.
• mRNA stands for “messenger RNA,” the copy of DNA information that is moved out of the nucleus to give “instructions” in the process of protein formation.
• tRNA stands for “transfer RNA,” and is the molecule that takes mRNA’s instructions and turns individual amino acids into proteins.
Transcription is the process of making RNA from DNA in order to transfer genetic information out of the nucleus and to the site of protein synthesis (the ribosomes). RNA polymerase “rewrites” the DNA information and creates a new copy in the form of mRNA.
Translation is the process by which RNA is used to make proteins. tRNA “reads” the mRNA strand and “translates” it into a chain of amino acids (a protein).
If you think you’re ready to discuss transcription and translation on the AP Biology Exam, take a stab at this quick practice question:
Q: How are proteins synthesized from genetic information? Describe the processes and the major molecules involved.
A: First, RNA polymerase attaches to the antisense strand at the site of the promoter DNA. The hydrogen bonds between the DNA’s nucleotides break and the helix unwinds, allowing the RNA polymerase to move down the strand, creating a copy of the genetic information in the form of mRNA. This copying of information is the process of transcription. If necessary, the mRNA undergoes capping, splicing, or polyadenylation, and is then moved out of the nucleus via specialized pores.
Once the mRNA reaches the ribosome, the initiation phase of translation begins. tRNA attaches to the first piece of genetic information—the start codon—and begins to assemble amino acids per the mRNA’s genetic instructions. As each piece of the mRNA is “read,” the ribosome moves along the strand and a longer chain of amino acids is created. This is the elongation phase. Finally, when the ribosome has read the entire strand of mRNA and completed the full polypeptide (protein) chain, the process enters the termination phase, at which point the ribosome releases the finished protein. This protein release is the final step of translation.
There you have it: DNA transcription and translation are the two molecular mechanisms by which organisms’ bodies produce new proteins to build real physical components. Do these processes make sense to you? Are there any elements you’re still struggling to understand? Let us know in the comments!
If you’re still trying to wrap your head around the intricacies of DNA, check out our intensive review of DNA for information on its discovery, structure, and functions.
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