Chapter 14 From DNA to Proteins
I. Beyond Byssus
A. The marine mussel manufactures the ultimate underwater adhesive, and it is a protein.
B. All proteins are synthesized according to instructions provided by DNA.

II. Protein Synthesis
A. The Central Dogma
1. DNA is like a book of instructions written in the alphabet of A, T, G, and C. But merely knowing the letters does not tell us how the genes work.
2. DNA consists of two strands of nucleotides twisted together in a double helix.
a. All DNA is composed of nucleotide subunits utilizing the same four bases but in different sequential order from species to species.
b. In replication, the two strands unwind to serve as templates for assembly of new complementary strands.
3. Each gene is a linear stretch of DNA nucleotides that codes for the assembly of amino acids into a polypeptide chain.
4. The path from genes to proteins has two steps:
a. In transcription, molecules of RNA are produced on the DNA templates in the nucleus.
b. In translation, RNA molecules shipped from the nucleus to the cytoplasm are used as templates for polypeptide assembly.
5. The central dogma is expressed thus:
transcription translation
DNA—————Æ RNA—————Æ proteins
B. Overview of the RNAs
1. Ribosomal RNA (rRNA) combines with proteins to form ribosomes upon which polypeptides are assembled.
2. Messenger RNA (mRNA) carries the “blueprint” for protein assembly to the ribosome.
3. Transfer RNA (tRNA) brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid.

III. Transcription of DNA to RNA
A. How RNA Is Assembled
1. RNA differs from DNA in two ways:
a. RNA uses ribose sugar, not deoxyribose.
b. RNA bases are A, G, C, and URACIL (U).
2. Transcription differs from replication in three ways:
a. Only one region of one DNA strand is used as a template.
b. RNA polymerase is used instead of DNA polymerase.
c. RNA is single stranded; DNA is double.
3. Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) and then moves along to the end of a gene; an RNA transcript is the result.
B. Messenger-RNA Transcripts
1. Only mRNA carries protein-building instructions.
2. mRNA transcripts are modified before leaving the nucleus.
a. The 5' end is capped with a special nucleotide that may serve as a “start” signal for translation.
b. A “poly-A tail” of about 100–200 molecules of adenylic acid is added.
c. Noncoding portions (introns) are snipped out, and actual coding regions (exons) are spliced together to produce the mature transcript.

IV. Translation
A. The Genetic Code
1. Both DNA and its RNA transcript are linear sequences of nucleotides carrying the hereditary code.
2. Every three bases (a triplet) specifies an amino acid to be included into a growing polypeptide chain; this is called the genetic code.
a. Each base triplet in RNA is called a codon.
b. The genetic code consists of sixty-one triplets that specify amino acids and three that serve to stop protein synthesis.
c. AUG (specifies methionine) is the “start” codon.
d. With few exceptions, the genetic code is universal for all forms of life.
B. Codon-Anticodon Interactions
1. Each kind of tRNA has an anticodon that is complementary to an mRNA codon; each tRNA also carries one specific amino acid.
2. After the mRNA arrives in the cytoplasm, an anticodon on a tRNA bonds to the codon on the mRNA, and thus a correct amino acid is brought into place.
3. The first two bases of the anticodon must pair up with the codon by the usual rules of base pairing (A with U and G with C), but there is some latitude in the pairing of the third base (called the wobble effect).
C. Ribosome Structure
1. A ribosome has two subunits (each composed of rRNA and proteins) that perform together only during translation.
2. There are two binding sites for tRNAs (called P and A) and one site for binding mRNA.
D. Stages of Translation
1. In initiation, a complex forms in this sequence: initiator tRNA + small ribosomal subunit + mRNA + large ribosomal subunit.
2. In chain elongation, a start codon on mRNA defines the reading frame; a series of tRNAs deliver amino acids in sequence by codon-anticodon matching; a peptide bond joins each amino acid to the next in sequence.
3. With chain termination, a stop codon is reached and the polypeptide chain is released into the cytoplasm or enters the cytomembrane system for further processing.
4. The three steps just outlined can be repeated many times on the same mRNA because several ribosomes may be moving along the mRNA at the same time (polysome).

V. Mutation and Protein Synthesis
A. Previously, we discussed changes in DNA that result from crossing over and sexual recombination; the changes in number and structure of chromosomes by nondisjunction were also mentioned.
B. A gene mutation is a change in one to several bases in the nucleotide sequence of DNA.
1. Bases can be added, deleted, or replaced.
2. Mutations are rare, chance events caused by mutagens such as viruses, ultraviolet radiation, and chemicals.
3. Examples of spontaneous mutations include sickle-cell anemia (a single base-pair substitution), frameshift mutations (an insertion or deletion that causes the reading to be out of phase), and transposable elements (regions of DNA that “jump” to new locations in DNA).