How does a cell use the information in its DNA?
To transcribe means "to paraphrase or summarize in writing." The information in DNA is transcribed - or summarized - into a smaller version - RNA - that can be used by the cell. This process is called transcription.
The process in which cells make proteins is called protein synthesis. It actually consists of two processes: transcription and translation. Transcription takes place in the nucleus. It uses DNA as a template to make an RNA molecule. RNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. Translation reads the genetic code in mRNA and makes a protein.
Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to messenger RNA (mRNA). During transcription, a strand of mRNA is made that is complementary to a strand of DNA. Figure below shows how this occurs. You can watch an animation of the process at this link: http://www.biostudio.com/d_%20Transcription.htm.
Overview of Transcription. Transcription uses the sequence of bases in a strand of DNA to make a complementary strand of mRNA. Triplets are groups of three successive nucleotide bases in DNA. Codons are complementary groups of bases in mRNA.
Steps of Transcription
Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in Figure below.
Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can "read" the bases in one of the DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.
Elongation is the addition of nucleotides to the mRNA strand.
Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA.
Steps of Transcription. Transcription occurs in the three steps - initiation, elongation, and termination - shown here.
In eukaryotes, the new mRNA is not yet ready for translation. It must go through additional processing before it leaves the nucleus. This may include splicing, editing, and polyadenylation. These processes modify the mRNA in various ways. Such modifications allow a single gene to be used to make more than one protein.
Splicing removes introns from mRNA (see Figure below). Introns are regions that do not code for proteins. The remaining mRNA consists only of regions that do code for proteins, which are called exons. You can watch a video showing splicing in more detail at this link: http://vcell.ndsu.edu/animations/mrnasplicing/movie-flash.htm.
Editing changes some of the nucleotides in mRNA. For example, the human protein called APOB, which helps transport lipids in the blood, has two different forms because of editing. One form is smaller than the other because editing adds a premature stop signal in mRNA.
Polyadenylation adds a “tail” to the mRNA. The tail consists of a string of As (adenine bases). It signals the end of mRNA. It is also involved in exporting mRNA from the nucleus. In addition, the tail protects mRNA from enzymes that might break it down.
Splicing. Splicing removes introns from mRNA. UTR is an untranslated region of the mRNA.
- Transcription is the DNA → RNA part of the central dogma of molecular biology.
- Transcription occurs in the nucleus.
- During transcription, a copy of mRNA is made that is complementary to a strand of DNA. In eukaryotes, mRNA may be modified before it leaves the nucleus.
Use these resources to answer the questions that follow.
- What is a transcription unit?
- Describe the importance of the template strand.
- What enzyme is used in transcription? What does this enzyme do?
- What are the promoter and initiation site?
- Describe the TATA box.
- How does RNA polymerase bind to the DNA?
- Describe elongation of the RNA during transcription.
- What must happen to the RNA prior to translation? Why?
- Distinguish between an intron and exon.
- What is the role of the GTP cap?
1. What is protein synthesis?
2. Describe transcription.
3. How may mRNA be modified before it leaves the nucleus?