How is RNA Transcribed from DNA Chapter 14.1 How is RNA Transcribed from DNA AP Biology Fall 2010

Three Classes of RNA It takes three classes of RNA to synthesize proteins Messenger RNA (mRNA) Ribosomal RNA (rRNA) Transfer RNA (tRNA)

Three Classes of RNA Messenger RNA (mRNA) Ribosomal RNA (rRNA) Carries the “blueprint” to the ribosome Ribosomal RNA (rRNA) Combines with proteins to form ribosomes upon which polypeptides are assembled Transfer RNA (tRNA) Brings the correct amino acid to the ribosome and pairs up with an mRNA code for that amino acid

Nature of Transcription RNA differs from DNA RNA uses ribose sugar, not deoxyribose RNA bases are A, G, C, and U (uracil)

Nature of Transcription Transcription differs from DNA replication in three ways: Only one region of one DNA strand is used as a template RNA polymerase is used instead of DNA polymerase The result of transcription is a single-stranded RNA

Nature of Transcription

Transcription Transcription begins when RNA polymerase binds to a promoter region (a base sequence at the start of a gene) Then moves along to the end of a gene Copies from 3 prime to 5 prime end of DNA molecule Builds from 5 prime to 3 prime end of RNA molecule After free ribonucleotides are complementary bonded to the template, an RNA transcript is created

Direction of Transcription

Transcription Transcription ends when RNA polymerase reaches “the end” signal RNA transcript is then released

Finishing Touches on mRNA Transcripts Newly formed mRNA is an unfinished molecule, not yet ready for use mRNA transcripts are modified before leaving the nucleus

Finishing Touches on mRNA Transcripts The 5’ end is capped with a modified guanine that serves as a “start” signal for translation The cap will also help bind the mRNA to a ribosome A “poly-A tail” of about 100-200 molecules of adenine ribonucleotides is added to the 3’ end

Finishing Touches on mRNA Transcripts Noncoding portions (introns) are snipped out, and actual coding regions (exons) are spliced together to produce the mature transcript Both are transcribed before transcript reaches cytoplasm Alternative splicing of exons mixes up different parts of the same gene Resulting in different proteins that increases the cell’s capacity to make diverse proteins One gene can specify two or more proteins that differ slightly in form and function