The Central Dogma of Molecular Biology

Objectives:

 

The dogma is the framework for understanding the transfer of information from DNA to RNA then to protein.

DNA is "transcribed" into RNA (rewritten, like typing up your notes since DNA and RNA are such similar languages) and then "translated" into the different language of proteins. Most genes contain the information needed to make functional molecules called proteins. A few genes produce other molecules that help the cell assemble proteins.

This journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps:

  1. Transcription: The process of rewriting the DNA code into RNA language which occurs in the nucleus.
  2. Translation: The process of using the RNA code as a template for translation into a protein which occurs in the cytoplasm.

Together, transcription and translation are known as gene expression: the process by which information from a gene is used in the synthesis of a functional gene product. During transcription, the genetic information stored in DNA is transferred to a similar molecule called ribonucleic acid or RNA. Both RNA and DNA are chains of nucleotides; however, their nitrogen bases have slightly different chemical properties. The nitrogen base thymine (T) does not exist in RNA, so uracil (U) fills its place. See Table 1 for more information about the differences between DNA and RNA molecules. 

There are also three different types of RNA, each serving a different role in gene expression (see Table 2.) The type of RNA that contains the information for making a protein is called messenger RNA or mRNA because it carries the information, or message, from the DNA out of the nucleus into the cytoplasm. During translation, the mRNA interacts with a specialized complex called a ribosome, partially composed of ribosomal RNA or rRNA which "reads" the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for one particular amino acid (see Table 3). Amino acids are the building blocks of proteins. A type of RNA called transfer RNA or tRNA brings the specifically coded-for amino acids to the ribosome, one at a time. Protein assembly continues until the ribosome encounters a "stop" codon that codes for no amino acid.

 

Central dogma of molecular biology
Figure 1. Shows the central dogma of molecular biology. DNA is replicated to make more DNA (shown by the circular arrow at the top), transcribed into RNA (represented by the diagonal black arrow going from DNA-upper right to RNA-lower left) and then translated into a protein (represented by a black arrow going left to right from RNA to the protein). In special cases (represented by dashed lines), reverse transcription results in DNA from RNA, or RNA is replicated.

 

Table 1. Differences between DNA and RNA
DNA RNA
Double-helix Single stranded
Deoxyribose sugar Ribose sugar
The nitrogen base Thymine (T) pairs with Adenine (A) The nitrogen base Uracil (U) pairs with Adenine (A)

 

 

Table 2. Descriptions of Types of RNA
Messenger RNA (mRNA) Carries DNA transcript to ribosome
Transfer RNA (tRNA) Transfers the necessary amino acids to the ribosome
Ribosomal RNA (rRNA) Coils up to make part of the ribosome

 

 


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