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:
• DNA
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.
• RNA
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
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
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.
Transcription
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.
Translation
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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