Characteristics of RNA
1. A single-stranded RNA fragment is transcribed from one of the two strands of DNA.
2. There are three different functional groups of RNA molecules: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
188 Chapter 7 The Blueprint of Life, from DNA to Protein Regulating the Expression of Genes
1. Protein synthesis is generally controlled by regulating the synthesis of mRNA molecules; mRNA is short-lived because RNases degrade it within minutes.
7.2 DNA Replication
1. DNA replication is generally bidirectional and semiconservative. (Figure 7.4)
2. The DNA chain always elongates in the 5' to 3' direction; the base-pairing rules determine the specific nucleotides that are added. (Figure 7.5)
1. DNA replication begins at the origin of replication. DNA polymerase synthesizes DNA in the 5' to 3' direction, using one strand as a template to generate the complementary strand.
The Replication Fork (Figure 7.6)
1. The bidirectional progression of replication around a circular DNA molecule creates two replication forks; numerous enzymes and other proteins are involved.
7.3 Gene Expression
1. The enzyme RNA polymerase catalyzes the process of transcription, producing a single-stranded RNA molecule that is complementary and antiparallel to the DNA template.
2. Transcription begins when RNA polymerase recognizes and binds to a promoter. (Figure 7.8)
3. RNA is synthesized in the 5' to 3' direction. (Figure 7.9)
4. When RNA polymerase encounters a terminator, it falls off the DNA template and releases the newly synthesized RNA.
1. The information encoded by mRNA is deciphered using the genetic code. (Figure 7.10)
2. A nucleotide sequence has three potential reading frames. (Figure 7.11)
3. Ribosomes function as the site of translation. (Figure 7.12)
4. tRNAs carry specific amino acids and act as keys that interpret the genetic code. (Figure 7.13)
5. In prokaryotes, initiation of translation begins when the ribosome binds to the ribosome-binding site of the mRNA molecule. Translation starts at the first AUG downstream of that site. (Figure 7.14)
6. The ribosome moves along mRNA in the 5' to 3' direction; translation terminates when the ribosome reaches a stop codon. (Figure 7.15)
7. Proteins are often modified after they are synthesized; those that contain a signal sequence are transported to the outside of the cell.
7.4 Differences Between Eukaryotic and Prokaryotic
Gene Expression (Table 7.4)
1. Eukaryotic mRNA is processed; a cap and a poly A tail are added.
2. Eukaryotic genes often contain introns which are removed from precursor mRNA by a process called splicing. (Figure 7.16)
3. In eukaryotic cells, the mRNA must be transported out of the nucleus before it can be translated in the cytoplasm.
Analyzing a Prokaryotic DNA Sequence (Figure 7.17)
1. When analyzing a DNA sequence, the nucleotide sequence of the (+) strand is used to infer information carried by the corresponding RNA transcript; computers are used to search for open reading frames (ORFs).
7.6 Regulating Gene Expression
Principles of Regulation
1. Constitutive enzymes are constantly synthesized.
2. The synthesis of inducible enzymes can be turned on by certain conditions.
3. The synthesis of repressible enzymes can be turned off by certain conditions.
Mechanisms to Control Transcription
1. A repressor is a regulatory protein that blocks transcription. (Figure 7.18)
2. An activator is a regulatory protein that enhances transcription. (Figure 7.19)
The lac Operon As a Model for Control of Metabolic Pathways (Figure 7.20)
1. The lac operon employs a repressor that prevents transcription of the genes when lactose is not available; a derivative of lactose functions as an inducer.
2. Catabolite repression prevents transcription of the lac operon when glucose is available.
7.7 Sensing and Responding to Environmental Fluctuations
1. Two-component regulatory systems utilize a sensor that recognizes changes outside the cell and then transmits that information to a response regulator.
2. Bacteria that utilize quorum sensing synthesize a soluble compound, a homoserine lactone, which can move freely in and out of a cell. Only when that compound reaches a critical concentration does it activate specific genes.
The expression of some genes changes randomly, presumably enhancing the chances of survival of at least a subset of a population under varying environmental conditions.
Review Questions 189
2. Antigenic variation is a routine change in the expression of surface proteins such as flagella, pili, and outer membrane proteins.
3. Phase variation is the routine switching on and off of certain genes.
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