^ Protein

Information coded in the sequence of base pairs in DNA is passed to molecules of RNA.

Information in RNA is passed to proteins. It never passes from proteins to nucleic acids.

^ Protein

Information coded in the sequence of base pairs in DNA is passed to molecules of RNA.

Information in RNA is passed to proteins. It never passes from proteins to nucleic acids.

12.2 The Central Dogma Information flows from DNA to RNA to proteins, as indicated by the arrows.

RNA differs from DNA

RNA is a key intermediary between DNA and polypeptide.

RNA (ribonucleic acid) is a polynucleotide similar to DNA

(see Figure 3.25), but it differs from DNA in three ways:

► RNA generally consists of only one polynucleotide strand.

► The sugar molecule found in RNA is ribose, rather than the deoxyribose found in DNA.

► Although three of the nitrogenous bases (adenine, gua-nine, and cytosine) in RNA are identical to those in DNA, the fourth base in RNA is uracil (U), which is similar to thymine but lacks the methyl (—CH3) group.

Thymine Uracil

RNA can base-pair with single-stranded DNA. This pairing obeys the same complementary base-pairing rules as in DNA, except that adenine pairs with uracil instead of thymine. Single-stranded RNA can fold into complex shapes by internal base pairing, as we will see later in this chapter.

Information flows in one direction when genes are expressed

Soon after he and Watson proposed their three-dimensional structure for DNA, Francis Crick pondered the problem of how DNA is functionally related to proteins. This led him to propose what he called the central dogma of molecular biology. The central dogma, simply stated, is that DNA codes for the production of RNA, RNA codes for the production of protein, and protein does not code for the production of protein, RNA, or DNA (Figure 12.2). In Crick's words, "once 'information' has passed into protein it cannot get out again."

The central dogma raised two questions:

► How does genetic information get from the nucleus to the cytoplasm? (As we saw in Chapter 4, most of the DNA of a eukaryotic cell is confined to the nucleus, but proteins are synthesized in the cytoplasm.)

► What is the relationship between a specific nucleotide sequence in DNA and a specific amino acid sequence in a protein?

To answer these questions, Crick proposed two hypotheses.

the messenger hypothesis and transcription. To answer the first question, Crick and his colleagues proposed the messenger hypothesis. They proposed that an RNA molecule forms as a complementary copy of one DNA strand of a particular gene. The process by which this RNA forms is called transcription (Figure 12.3). This messenger RNA, or mRNA, then travels from the nucleus to the cytoplasm, where it serves as a template for the synthesis of proteins. Crick's hypothesis has been tested repeatedly for genes that code for proteins, and the answer is always the same: Each gene sequence in DNA that codes for a protein is expressed as a sequence in mRNA.

the adapter hypothesis and translation. To answer the second question, Crick proposed the adapter hypothesis: there must be an adapter molecule that can bind a specific amino acid with one region and recognize a sequence of nucleo-tides with another region. In due course, these adapters, called transfer RNA, or tRNA, were identified. Because they recognize the genetic message of mRNA and simultaneously carry specific amino acids, tRNAs can translate the language of DNA into the language of proteins. The tRNA adapters line up on the mRNA so that the amino acids are in the proper sequence for a growing polypeptide chain—a process called translation (see Figure 12.3). Once again, actual observations of the expression of thousands of genes have confirmed the hypothesis that tRNA acts as the intermediary between the nucleotide sequence information in mRNA and the amino acid sequence in a protein.

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