How does DNA write its message?
The process of DNA sending a message to the cytoplasm is called transcription. Transcription means "a written copy." However, DNA can't get out a pen and write a message. It creates an mRNA molecule to carry the message.
DNA is located in the nucleus. Proteins are made on ribosomes in the cytoplasm. Remember that information in a gene is converted into mRNA, which carries the information to the ribosome. In the nucleus, mRNA is created by using the DNA in a gene as a template. A template is a model provided for others to copy.
The process of constructing an mRNA molecule from DNA is known as transcription (Figure below and Figure below). Transcription is similar to DNA replication, except in transcription, single stranded RNA is produced instead of double stranded DNA. In transcription, only one strand of DNA is used as a template. First, the double helix of DNA unwinds and an enzyme, RNA Polymerase, builds the mRNA using the DNA as a template. The nucleotides follow basically the same base pairing rules as in DNA to form the correct sequence in the mRNA. This time, however, uracil (U) pairs with each adenine (A) in the DNA. For example, a DNA sequence ACGGGTAAGG will be transcribed into the mRNA sequence UGCCCAUUCC. In this manner, the information of the DNA is passed on to the mRNA. The mRNA will carry this code to the ribosomes to tell them how to make a protein.
As not all genes are used in every cell, a gene must be "turned on" or expressed when the gene product is needed by the cell. Only the information in a gene that is being expressed is transcribed into an mRNA.
Each gene (a) contains triplets of bases (b) that are transcribed into RNA (c). Every triplet, or codon, encodes for a unique amino acid.
Base-pairing ensures the accuracy of transcription. Notice how the helix must unwind for transcription to take place. The new mRNA is shown in green.
- The process of constructing an mRNA molecule from DNA is known as transcription.
- The base pairing rules ensure that the DNA code is conserved in the sequence in the mRNA.
Use the resources below to answer the questions that follow.
Explore More I
- Transcription and Translation at http://www.youtube.com/watch?v=41_Ne5mS2ls (4:06)
- How is RNA different from DNA? Do you think this difference is important? Why?
- Where is the mRNA made?
- Where does the mRNA go after it is produced?
Explore More II
- Cell Transcription at http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/
- How many regions of a gene are there? What are they called? What is the function of each region?
- Imagine one of these three regions was deleted by a mutation. What would happen during transcription? Explain your answer for each separate region.
- What is the function of RNA polymerase? What kind of molecule is RNA polymerase?
- What is the final product of transcription?
- How does the genetic code in the DNA get passed on to the mRNA?
- What will be the mRNA sequence transcribed from a TTAAACGGCCTA template?