The Central Dogma: DNA Encodes RNA; RNA Encodes Protein The central dogma (Figure 43) describes the flow of genetic information in cells from DNA to mRNA
to protein, stating that genes specify the sequences of mRNAs, which in turn specify the sequences of proteins.
Figure 43. The central dogma states that DNA encodes RNA, which in turn encodes protein.
The process of copying DNA to mRNA is relatively simple, with one nucleotide added to the mRNA
strand for each complementary nucleotide read in the DNA strand. Because groups of three mRNA nucleotides
correspond to one amino acid in the protein sequence, translation to protein is more complicated. However, as
we will see in the following module, the translation to protein is still systematic, with nucleotides 1–3
corresponding to amino acid 1, nucleotides 4–6 corresponding to amino acid 2, and so on.
Transcription: from DNA to mRNA Both prokaryotes and eukaryotes perform fundamentally the same process of transcription, with the
important difference of the membrane-bound nucleus in eukaryotes. With the genes bound in the nucleus,
transcription occurs in the nucleus of the cell and the mRNA transcript must be transported to the cytoplasm.
The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and
transcription occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcription occurs in
three main stages: initiation, elongation, and termination.
Initiation Transcription requires the DNA double helix to partially unwind in the region of mRNA synthesis. The
region of unwinding is called a
transcription bubble . The DNA sequence onto which the proteins and enzymes
involved in transcription bind to initiate the process is called a
promoter . In most cases, promoters exist
upstream of the genes they regulate. The specific sequence of a promoter is very important because it
determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all
(Figure 44).
