Unit Three. The Continuity of Life
The Central Dogma, discussed in section 12.1, is the same in all organisms. Figure 12.8 overviews the components needed for the key processes of DNA replication, transcription, and translation and the products that are formed in each. In general, the components are the same, the processes are the same, and the products are the same whether in prokaryotes or eukaryotes. However, there are some differences in gene expression between the two types of cells.
Architecture of the Gene
In prokaryotes, a gene is an uninterrupted stretch of DNA nucleotides whose transcript is read three nucleotides at a time to make a chain of amino acids. In eukaryotes, by contrast, genes are fragmented. In these more complex genes, the DNA nucleotide sequences encoding the amino acid sequence of a polypeptide are called exons, and the exons are interrupted frequently by extraneous nucleotides, “extra stuff’ called introns. You can see them in the segment of DNA illustrated in figure 12.9; the exons are the blue areas and the introns are the orange areas. Imagine looking at an interstate highway from a satellite. Scattered randomly along the thread of concrete would be cars, some moving in clusters, others individually; most of the road would be bare. That is what a eukaryotic gene is like: scattered exons embedded within much longer sequences of introns. In humans, only 1% to 1.5% of the genome is devoted to the exons that encode polypeptides, while 24% is devoted to the noncoding introns.
Figure 12.8. The processes of DNA replication, transcription, and translation.
These processes are generally the same in prokaryotes and eukaryotes.
When a eukaryotic cell transcribes a gene, it first produces a primary RNA transcript of the entire gene, shown in figure 12.9 with the exons in green and the introns in orange. Enzymes add modifications called a 5' cap and a 3' poly-A tail, which protect the RNA transcript from degradation. The primary transcript is then processed. Enzyme-RNA complexes excise out the introns and join together the exons to form the shorter, mature mRNA transcript that is actually translated into an amino acid chain. Notice that the mature mRNA transcript in figure 12.9 contains only exons (green segments), no introns. Because introns are excised from the RNA transcript before it is translated into a polypeptide, they do not affect the structure of the polypeptide encoded by the gene in which they occur, despite the fact that introns represent over 90% of the nucleotide sequence of a typical human gene.
Figure 12.9. Processing eukaryotic RNA.
The gene shown here codes for a protein called ovalbumin. The ovalbumin gene and its primary transcript contain seven segments not present in the mRNA used by the ribosomes to direct the synthesis of the protein.
Why this crazy organization? It appears that many human genes can be spliced together in more than one way. In many instances, exons are not just random fragments, but rather functional modules. One exon encodes a straight stretch of protein, another a curve, yet another a flat place. Like mixing Tinkertoy parts, you can construct quite different assemblies by employing the same exons in different combinations and orders. With this sort of alternative splicing, the 25,000 genes of the human genome seem to encode as many as 120,000 different expressed messenger RNAs. It seems that added complexity in humans has been achieved not by gaining more gene parts (we have only about twice as many genes as a fruit fly), but rather by coming up with new ways to put them together.
Protein synthesis in eukaryotes is more complex than in prokaryotes. Prokaryotic cells lack a nucleus and so there is no barrier between where mRNA is synthesized during transcription and where proteins are formed during translation. Consequently, a gene can be translated as it is being transcribed. Figure 12.10 shows how the ribosomes attach to the mRNA as it is synthesized in prokaryotes. These clusters of ribosomes on the mRNA are called polyribosomes. In eukaryotic cells, a nuclear membrane separates the process of transcription from translation, making protein synthesis much more complicated. Figure 12.11 on the next page walks you through the entire process. Transcription (step 1) and RNA processing (step 2) occur within the nucleus. In step 3, the mRNA travels to the cytoplasm where it binds to the ribosome. In step 4, tRNAs bind to their appropriate amino acids, which correspond to their anticodons. In steps 5 and 6, the tRNAs bring the amino acids to the ribosome and the mRNA is translated into a polypeptide.
Figure 12.10. Transcription and translation in prokaryotes.
Ribosomes attach to an mRNA as it is formed, producing polyribosomes that translate the gene soon after it is transcribed.
Figure 12.11. How protein synthesis works in eukaryotes.
Key Learning Outcome 12.4. The general process of gene expression is similar in prokaryotes and eukaryotes, but differences exist in the architecture of the gene and the location of transcription and translation in the cell.