Genetic Information Flow Help
Genetic Information Flow
DNA serves as the main repository of genetic information within a cell. Each strand of the DNA double helix serves as a template for its own replication. This activity precedes all cell division and is thus how genetic information is transmitted to new generations of cells. All cellular RNA molecules are synthesized from DNA templates in a process called transcription. Within a transcriptional unit, only one of the strands of DNA serves as a template for the synthesis of RNA molecules. Different transcriptional units may reside on the same or on different DNA strands. Genes are said to be active, or expressed, when they are being transcribed into RNA. Proteins are only synthesized from mRNA templates by a process called translation.
Other types of RNA molecules are not used to make proteins, serving other cellular functions instead (e.g., ribosomal RNA). This generalized flow of genetic information from DNA to protein is often referred to as the central dogma of molecular biology (see figure below). This oversimplification emphasizes the central idea that DNA does not serve as the direct template for protein synthesis and that nucleic acids are not synthesized from proteins. In the 1990s, the discovery that DNA in telomeric regions of at least some chromosomes could be synthesized from an RNA template served to demonstrate that, in special cases, RNA could be used as a template for DNA synthesis. Furthermore, certain viruses (viruses are not cells) use RNA as genetic material that is copied into a DNA intermediate (reverse transcription) by an enzyme called reverse transcriptase (dashed arrow in figure below). Such viruses are called "retroviruses" because they reverse the cellular dogma that RNA strands are made from DNA templates. Ultimately, proteins and various RNA molecules go on to carry out their designated functions.
The Genetic Code
The genetic code is the set of consecutive triplet nucleotides, known as a codon, that specifies a particular amino acid in a protein. Each protein consists of a certain number of amino acids in a precisely ordered sequence. The blueprint that specifies this amino acid sequence is encoded in the sequence of a region of DNA called a gene. Genes are thus composed of codons. A codon is made up of an adjacent group of three nucleotides in either the DNA or in its mRNA transcript. There are 64 possible codons (see Table 3-1). If the transcription of a functional genetic unit of DNA into mRNA is always read from a fixed position, then the first six codons in one chain of the corresponding DNA might be as follows. (Note: DNA and RN Asequences, including the codons in Table 3-1, areconventionally written starting with the 50 end on the left.)
One of the two DNA strands acts as the template for synthesis of an mRNA strand (see Transcription section later in this chapter). The specific order of these codons is called the reading frame. An open reading frame (ORF) is one that starts with an initiator mRNA codon (usually AUG for the amino acid methionine) followed by codons that specify the remaining amino acid sequence, and ends with at least one mRNA stop codon (UAA, UAG, or UGA).The addition or deletion of a single base (e.g.,G) at the end of the second codon below would shift all other codons one nucleotide out of register and prevent the correct reading of all codons to the right of the eletion or addition. This event results in a mutation. In fact, in this case a different codon followed by a stop codon is created.
By successively adding bases in a nearby region, it should be possible to place the reading frame of the codons back into register, although the codon changes result in different amino acids being placed in the chain (see below).
The genetic code is degenerate because more than one codon exists for most amino acids. The genetic code is generally the same in all organisms, and thus is said to be universal. However, a few codons in some organelle DNAs have different meanings than those in nuclear DNAs. Codons that specify amino acids are called sense codons. Three of the 64 possible three-letter codons do not specify an amino acid. These triplets are called nonsense codons or stop codons, and they serve as part of the translation termination signal. The AUG codon that specifies methionine is referred to as the initiator codon, as well as a sense codon, as it is most likely the first codon in a mRNA message. The mRNA codons for the 20 amino acids are listed in Table 3-2. These codons are conventionally written from the 5' end (at the left) toward the 3' end (at the right) because translation (protein synthesis) begins at the 5' end of an mRNA molecule.
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