21.5 RNA and Transcription A typical ribosome consists of a small subunit and a large subunit. The subunit shapes shown contain both protein and rRNA. Learning Goal Identify the different types of RNA; describe the synthesis of mRNA.

RNA and DNA RNA makes up most of the nucleic acid found in the cell. transmits genetic information from DNA to operate the cell. has several types as shown below. differs from DNA in the following ways: The sugar in RNA is ribose; the sugar in DNA is deoxyribose. The base uracil replaces thymine. RNA molecules are single stranded; DNA molecules are double stranded. RNA molecules are much smaller than DNA molecules.

Types of RNA There are three major types of RNA: 5% of RNA is messenger RNA (mRNA), which carries genetic information from DNA to the ribosomes. 15% of RNA is transfer RNA (tRNA), which translates the genetic information in mRNA into the amino acid sequence for the protein. 80% of RNA is ribosomal RNA (rRNA), which is the most abundant type of RNA; it is combined with proteins to form ribosomes.

tRNA A typical tRNA molecule can be drawn as a two-dimensional cloverleaf. shows more twists that illustrate the L-shape of tRNA in the three-dimensional model. has an acceptor stem at the 3′ end with the nucleotide sequence ACC where an enzyme attaches an amino by forming an ester bond with the free — OH group. contains an anticodon, which is a series of three bases that complements three bases on mRNA.

tRNA Structure (a) A two-dimensional model of tRNA is represented as a cloverleaf. (b) The three-dimensional model of RNA has more twists that shows its L-shape.

RNA and Protein Synthesis In the nucleus, genetic information for protein synthesis is copied from a gene in DNA. makes mRNA in a process called transcription. The mRNA molecules move out of the nucleus into the cytosol, where they bind with the ribosomes. The tRNA molecules convert the information in the mRNA into amino acids in a process called translation.

RNA and Protein Synthesis The genetic information in DNA is replicated in cell division and used to produce messenger RNA that codes for amino acids used in protein synthesis at the ribosomes.

Transcription: Synthesis of mRNA In transcription, a section of DNA containing the gene unwinds. RNA polymerase uses the DNA template strand to form the new mRNA using bases that are complementary to the DNA template. An mRNA is synthesized using complementary base pairing, with uracil (U) replacing thymine (T). The newly formed mRNA moves out of the nucleus to ribosomes in the cytoplasm. Core Chemistry Skill Writing the mRNA Segment for a DNA Template

Transcription: Synthesis of mRNA DNA undergoes transcription when RNA polymerase makes a complementary copy of a gene using the 3′ to 5′ DNA template strand.

mRNA Processing: Exons, Introns In eukaryotes, DNA contains exons that code for proteins and introns that do not code for proteins. a pre-RNA is formed that includes the noncoding introns. the noncoding introns are removed. the exons are joined to form mRNA, which goes to the ribosomes with the information for the synthesis of protein.

mRNA Processing: Exons, Introns A pre-mRNA, containing copies of the exons and introns from the gene, is processed to remove the introns to form a mature mRNA that codes for a protein.

Regulation of Transcription Regulation of mRNA syntheses is controlled at the transcription level by a specific mRNA that is synthesized when the cell requires a particular protein. The transcription of a gene requires the RNA polymerase to bind to DNA in plants and animals. A group of protein complexes called a transcription factor must recognize and bind to promoter segments. Other proteins, called activators, bind with the transcription factor complex to increase the rate of RNA transcription.

Regulation of Transcription Transcription factors at the promoter region bind RNA polymerase to DNA, which activates the transcription of a gene.

Study Check What is the sequence of bases in mRNA produced from a section of the template strand of DNA that has the sequence of bases 3′ C T A A G G 5′? 1) 5′ G A T T C C 3′ 2) 5′ G A U U C C 3′ 3) 5′ C T A A G G 3′

Solution What is the sequence of bases in mRNA produced from a section of the template strand of DNA that has the sequence of bases 3′ C T A A G G 5′? 2)