Chapter 14 Summary
14.1 Genes Code for Proteins
Experiments on metabolic enzymes in the bread mold Neurospora led to the one-
gene, one- enzyme hypothesis. We now know that there is a one- gene, one- polypeptide relationship. Review Figure 14.1a and Figure 14.1b
14.2 Information Flows from Genes to Proteins
The central dogma of molecular biology states that DNA encodes RNA, and RNA encodes proteins. Proteins do not encode proteins, RNA, or DNA.
The process by which the information in DNA is copied to RNA is called transcription. The process by which a protein is built from the information in RNA is called translation. Review Focus: Key Figure 14.2, Activity 14.1
A product of transcription is messenger RNA (mRNA). Transfer RNAs (tRNAs) translate the genetic information in the mRNA into a corresponding sequence of amino acids to produce a polypeptide.
Certain RNA viruses present exceptions to the central dogma. For example, retroviruses synthesize DNA from RNA in a process called reverse transcription.
14.3 DNA Is Transcribed to Produce RNA
In a given gene, only one of the two strands of DNA (the template strand) acts as a template for transcription. RNA polymerase is the catalyst for transcription.
RNA transcription from DNA proceeds in three steps: initiation, elongation, and termination. Review Figure 14.4, Animation 14.1
Initiation requires a promoter, to which RNA polymerase binds. Part of each promoter is the initiation site, where transcription begins.
Elongation of the RNA molecule proceeds from the 5′ to 3′ end.
Particular base sequences specify termination, at which point transcription ends and the RNA transcript separates from the DNA template.
The genetic code is a “language” of triplets of mRNA nucleotide bases (codons) corresponding to 20 specific amino acids; there are start and stop codons as well. The code is redundant (an amino acid may be represented by more than one codon) but not ambiguous (no single codon represents more than one amino acid). Review Investigating Life: Deciphering the Genetic Code, Figure 14.5, Animation 14.2, Activity 14.2
14.4 Eukaryotic Pre-
Unlike prokaryotes, where transcription and translation occur in the cytoplasm and are coupled, in eukaryotes transcription occurs in the nucleus and translation later in the cytoplasm. Review Table 14.2
Eukaryotic genes contain introns, which are noncoding sequences within the transcribed regions of genes. Review Figures 14.6B, 14.7
The initial transcript of a eukaryotic protein-
coding gene is modified with a 5′ cap and a 3′ poly A tail. Review Figure 14.8 Pre-
mRNA introns are removed in the nucleus via RNA splicing. Then the mRNA passes through the nuclear pore into the cytoplasm, where it is translated through ribosomes. Review Figure 14.9, Animation 14.3
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14.5 The Information in mRNA Is Translated into Proteins
See Animation 14.4
During translation, amino acids are linked together in the order specified by the codons in the mRNA. This task is achieved by tRNAs, each of which binds to (is charged with) a specific amino acid and has an anticodon complementary to a specific mRNA codon. Review Figures 14.10, 14.11
The ribosome is the molecular workbench where translation occurs. It has one large and one small subunit, each made of ribosomal RNA and proteins.
Three sites on the large ribosomal subunit interact with tRNA. The A site is where the charged tRNA anticodon binds to the mRNA codon; the P site is where the tRNA adds its amino acid to the growing polypeptode chain; and the E site is where the tRNA is released. Review Figure 14.12
Translation occurs in three steps: initiation, elongation, and termination. The initiation complex consists of tRNA bearing the first amino acid, the small ribosomal subunit, and mRNA. Review Figure 14.13
The growing polypeptide chain is elongated by the formation of peptide bonds between amino acids, catalyzed by rRNA. Review Figure 14.14
When a stop codon reaches the A site, it terminates translation by binding a release factor. Review Figure 14.15
In a polysome, more than one ribosome moves along a strand of mRNA at one time. Review Figure 14.16
14.6 Polypeptides Can Be Modified and Transported during or after Translation
Signal sequences of amino acids direct polypeptides to the cellular destinations, such as organelles. Review Figures 14.17, 14.18
Proteins addressed to the RER bind to a receptor protein in the RER membrane. Review Figure 14.17
Posttranslational modifications of polypeptides include proteolysis, whereby a polypeptide is cut to smaller fragments; glycosylation, whereby sugars are added; and phosphorylation, whereby phosphate groups are added. Review Figure 14.19
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