An Introduction to DNA Transcription

Protein Synthesis
In proteins synthesis, DNA is transcribed to RNA and RNA is translated into a protein.

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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.

Key Takeaways: DNA Transcription

  • In DNA transcription, DNA is transcribed to produce RNA. The RNA transcript is then used to produce a protein.
  • The three main steps of transcription are initiation, elongation, and termination.
  • In initiation, the enzyme RNA polymerase binds to DNA at the promoter region.
  • In elongation, RNA polymerase transcribes DNA into RNA.
  • In termination, RNA polymerase releases from DNA ending transcription.
  • Reverse transcription processes use the enzyme reverse transcriptase to convert RNA to DNA.

How DNA Transcription Works

RNA polymerase
This illustration shows the process of transcription of deoxyribonucleic acid (DNA, blue) to produce a complementary copy of ribonucleic acid (RNA, green). This is done by the enzyme RNA polymerase (purple).  Gunilla Elam/Science Photo Library/Getty Images Plus

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. Initiation: 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.

Transcription in Prokaryotic and Eukaryotic Cells

Protein Synthesis
Colored transmission electron micrograph of deoxyribonucleic acid, (DNA pink), transcription coupled with translation in the bacterium Escherichia coli.

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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.

From Transcription to Translation

Translation
Number 1: Synthesis of mRNA from DNA in the nucleus. 2 The mRNA decoding ribosome by binding of complementary tRNA anticodon sequences to mRNA codons. 3-5 ribosomes synthesize proteins in the cytoplasm.

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In translation, the message coded in mRNA is converted into a protein. 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.

Reverse Transcription

Reverse Transcription
DNA is transcribed and translated to produce proteins. Reverse transcription converts RNA to DNA.

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In reverse transcription, RNA is used as a template to produce DNA. The enzyme reverse transcriptase transcribes RNA to generate a single strand of complementary DNA (cDNA). The enzyme DNA polymerase converts the single-stranded cDNA into a double-stranded molecule as it does in DNA replication. Special viruses known as retroviruses use reverse transcription to replicate their viral genomes. Scientists also use reverse transcriptase processes to detect retroviruses.

Eukaryotic cells also use reverse transcription to extend the end sections of chromosomes known as telomeres. The enzyme telomerase reverse transcriptase is responsible for this process. The extension of telomeres produces cells that are resistant to apoptosis, or programmed cell death, and become cancerous. The molecular biology technique known as reverse transcription-polymerase chain reaction (RT-PCR) is used to amplify and measure RNA. Since RT-PCR detects gene expression, it can also be used to detect cancer and in aid genetic disease diagnosis.