An Introduction to DNA Transcription

Transcription factors bound to DNA
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DNA transcription is a process that involves transcribing genetic information from DNA to RNA. The transcribed DNA message, or RNA transcript, is used to produce proteins. DNA is housed within the nucleus of our cells. It controls cellular activity by coding for the production of proteins. The information in DNA is not directly converted into proteins, but must first be copied into RNA. This ensures that the information contained within the DNA does not become tainted.

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How DNA Transcription Works

DNA consists of four nucleotide bases that are paired together to give DNA its double helical shape. These bases are: adenine (A)guanine (G)cytosine (C), and thymine (T). Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G). Nucleotide base sequences are the genetic code or instructions for protein synthesis.

There are three main steps to the process of DNA transcription:

  1. RNA Polymerase Binds to DNA
    DNA is transcribed by an enzyme called RNA polymerase. Specific nucleotide sequences tell RNA polymerase where to begin and where to end. RNA polymerase attaches to the DNA at a specific area called the promoter region. The DNA in the promoter region contains specific sequences that allow RNA polymerase to bind to the DNA.
  2. Elongation
    Certain enzymes called transcription factors unwind the DNA strand and allow RNA polymerase to transcribe only a single strand of DNA into a single stranded RNA polymer called messenger RNA (mRNA). The strand that serves as the template is called the antisense strand. The strand that is not transcribed is called the sense strand.
    Like DNA, RNA is composed of nucleotide bases. RNA however, contains the nucleotides adenine, guanine, cytosine, and uracil (U). When RNA polymerase transcribes the DNA, guanine pairs with cytosine (G-C) and adenine pairs with uracil (A-U).
  3. Termination
    RNA polymerase moves along the DNA until it reaches a terminator sequence. At that point, RNA polymerase releases the mRNA polymer and detaches from the DNA.
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Transcription in Prokaryotic and Eukaryotic Cells

While transcription occurs in both prokaryotic and eukaryotic cells, the process is more complex in eukaryotes. In prokaryotes, such as bacteria, the DNA is transcribed by one RNA polymerase molecule without the assistance of transcription factors. In eukaryotic cells, transcription factors are needed for transcription to occur and there are different types of RNA polymerase molecules that transcribe the DNA depending on the type of genes. Genes that code for proteins are transcribed by RNA polymerase II, genes coding for ribosomal RNAs are transcribed by RNA polymerase I, and genes that code for transfer RNAs are transcribed by RNA polymerase III. In addition, organelles such as mitochondria and chloroplasts have their own RNA polymerases which transcribe the DNA within these cell structures.

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From Transcription to Translation

Since proteins are constructed in the cytoplasm of the cell, mRNA must cross the nuclear membrane to reach the cytoplasm in eukaryotic cells. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA into a protein. This process is called translation. Proteins can be manufactured in large quantities because a single DNA sequence can be transcribed by many RNA polymerase molecules at once.