Module 7 Lesson 3 - 4

Transcription

---

The first step in protein production is copying the information in DNA into messenger RNA. This process is called the transcription. Transcription occurs in the nucleus and is dependent on one type of RNA: mRNA (messenger RNA). After DNA is transcribed in the nucleus, mRNA is transported to the cytoplasm for translation. The DNA molecule remains in the nucleus and the information or code for a gene is copied onto mRNA. Messenger RNA is a single strand of RNA coded from one strand of the DNA molecule called the antisense strand. The antisense strand is referred to as the template because the mRNA strand is built from its sequence of nucleotides. The other non-template strand, which is not copied, is called the sense strand. Consider the figure below to see the difference between the sense and antisense strands.

Why is the template strand called the antisense strand?

The template strand is called the antisense strand and the strand of DNA that is not transcribed is called sense strand. This is because the non-template strand has a similar sequence to the mRNA strand, but thymine takes the place of uracil.

NOTE:  On page 638 of the Inquiry into Biology textbook, the sense and anti-sense strands are incorrectly identified.  

Sense vs. Antisense

DNA Sense Strand	CTT AAC CGC TGT ATT	This is the non-template strand and runs 5' to 3'.
DNA Antisense Strand (template)	GAA TTG GCG ACA TAA	This is the template strand and runs 3' to 5'.
mRNA Strand	CUU AAC CGC UGU AUU	Notice how the mRNA strand sequence is very similar to the DNA sense strand. The only difference is U instead of T. A new base is added to 3' end and runs 5' to 3'.

RNA polymerase is the enzyme that binds to the DNA strand at the promoter region. RNA polymerase opens the DNA double helix and builds a strand of mRNA in the 5' to 3' direction that is complementary to the DNA antisense strand.  When the stop signal is reached, the RNA polymerase detaches from the DNA strand, and the DNA double helix forms again. Read more about transcription in your textbook on pages 636 to 638.

Example

A DNA antisense strand contains the following nucleotide sequence:  3' - TAC TGC CTC CCC ATA AGA ATT - 5'.

What is the nucleotide sequence of the mRNA strand that is transcribed from this DNA template?

Watch and Listen

---

Watch the following Biologix video for a review of the transcription process.