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6.3 Transcription and RNA Processing

Syllabus
2025
Topic
6.3
Level
—

Learning objectives

6.3A—Describe the mechanisms by which genetic information flows from DNA to RNA to proteinDescribe the mechanisms by which genetic information flows from DNA to RNA to protein.• The sequence of the RNA bases, together with the structure of the RNA molecule, determines RNA function.- i. Messenger RNA (mRNA) molecules carry inf ormation from DNA in the nucleus to the ribosome in the cytoplasm.- ii. Distinct transfer RNA (tRNA) molecules bind specific amino acids and ha ve anticodon sequences that base pair with the codons of mRNA. tRNA is recruited to the ribosome during translation to generate the primary peptide sequence based on the mRNA sequence.- iii. Ribosomal RNA (rRNA) molecules are functional building block s of ribosomes.• RNA polymerases use a single template strand of DNA to direct the inclusion of bases in the newly formed RNA molecule. This process is known as transcription.• The enzyme RNA polymerase synthesizes mRNA molecules in the 5’ to 3’ direction by reading the template DNA strand in the 3’ to 5’ direction.• In eukaryotic cells the mRNA transcript undergoes a series of enzyme-mediated modifications.- i. The addition of a poly-A tail makes mRNA more stable.- ii. The addition of a GTP cap helps with ribosomal recognition.- iii. The excision of introns, along with the splicing and retention of exons, generates different versions of the resulting mature mRNA molecule. This process is known as alternative splicing.

From a DNA Template to Mature RNA

Transcription transfers genetic information from one DNA template strand into a complementary RNA molecule. The RNA base sequence and the molecule's structure then determine its function in the flow of information from DNA toward protein.

RNA type Information-flow role
Messenger RNA (mRNA) Carries information from DNA in the nucleus to ribosomes in the cytoplasm
Transfer RNA (tRNA) Carries a specific amino acid and uses its anticodon to pair with an mRNA codon
Ribosomal RNA (rRNA) Forms functional building blocks of ribosomes

RNA polymerase uses a single DNA strand as the template. It reads that template in the 3′→5′ direction while adding complementary RNA bases so the RNA grows in the 5′→3′ direction. Thus, template reading and RNA synthesis proceed in opposite directions.

Eukaryotic mRNA modification Consequence
Add a poly-A tail Makes the mRNA more stable
Add a GTP cap Helps ribosomal recognition
Excise introns and splice retained exons Produces a mature mRNA sequence
Retain different exon combinations Produces different mature mRNA versions through alternative splicing

A single initial eukaryotic transcript can be processed with different retained exon combinations. The resulting mature mRNAs have different base sequences, so they can direct production of different protein versions from the same gene.

RNA polymerase does not copy both DNA strands for one transcript, and it does not synthesize RNA 3′→5′. Introns are excised; exons are the sequences spliced together and retained in mature mRNA.