Biology Chapter 17: Transcription and Translation

Chapter 17: Transcription, RNA Processing, and Translation

• The information in genes directs the synthesis of RNAs and proteins.
• This chapter examines how information in DNA is transcribed into RNA and how that RNA is processed and translated into proteins.

17.1 An Overview of Transcription

• RNA polymerases synthesize an RNA version of the instructions stored in DNA.
• They use ribonucleoside triphosphates (NTPs).
• RNA polymerases match complementary bases to one DNA strand.
• One strand of DNA serves as a template (non-template/coding strand matches the mRNA sequence except U for T).

Bacterial and Eukaryotic Transcription Differences

• Bacteria have one RNA polymerase, eukaryotes have at least three distinct types (I, II, and III).
• RNA polymerases perform template-directed synthesis in the 5' → 3' direction.
• Unlike DNA polymerases, RNA polymerase doesn't require a primer to begin transcription.

17.2 RNA Processing in Eukaryotes

• In bacteria, transcription produces fully functional RNAs.
• In eukaryotes, the initial product of transcription is an immature primary transcript (pre-mRNA) which needs RNA processing.
• Primary transcripts must undergo RNA processing before translation.

17.4 The Structure and Function of Transfer RNA

• Transfer RNA (tRNA) is the adapter molecule used in translation.
• An aminoacyl tRNA is a tRNA molecule linked to its amino acid.
• Amino acids are transferred from tRNAs to the growing polypeptide chain.

The Structure of tRNA

• tRNA molecules are relatively short (75-95 nucleotides).
• They form secondary structures (stem-and-loop) by folding.
• A CCA sequence at the 3' end serves as the amino acid binding site.
• The loop at the opposite end contains the anticodon - a sequence of three nucleotides that base-pairs with the mRNA codon.

How Are Amino Acids Attached to tRNAs?

• ATP is required to attach tRNA to amino acids.
• Aminoacyl-tRNA synthetases attach amino acids to tRNAs.
• There are 20 amino acids, and each has a specific aminoacyl-tRNA synthetase.

How Many tRNAs Are There?

• There are 61 codons but about 40 tRNAs in most cells.
• Crick proposed that the anticodon's third position allows for nonstandard base pairing, allowing one tRNA to read more than one codon.

17.5 Ribosome Structure and Function in Translation

• Ribosomes contain many proteins and ribosomal RNA (rRNA).
• Ribosomes can be separated into two subunits: small (holds mRNA) and large (where peptide bonds form).
• During translation, three tRNAs line up within the ribosome.

Ribosome Structure and Function in Translation (more detail)

• tRNAs fit into three sites on the ribosome (A, P, and E). Each tRNA is bound to corresponding mRNA codons.
• A site (acceptor or aminoacyl): tRNA carries amino acid
• P site (peptidyl): holds growing peptide chain
• E site (exit): tRNA without amino acid exits the ribosome.
• The ribosome synthesizes proteins in a three-step sequence (Arrival of aminoacyl tRNA, Peptide bond formation, Translocation).
• Amino acids are always attached to the carboxyl end (C-terminus) of the growing polypeptide.
• Translation has three phases: initiation, elongation, and termination.

Initiating Translation

• The initiation phase of translation starts near the AUG start codon.
• In bacteria, the small ribosomal subunit binds to the ribosome binding site (Shine-Dalgarno sequence) on mRNA.
• About 6 bases upstream from the start codon.
• Mediated by initiation factors.
• The first tRNA is called initiator tRNA (carries a modified methionine, f-Met, in bacteria).
• Steps of Translation Initiation (mRNA binds to small ribosomal subunit, Initiator tRNA binds, large ribosomal subunit binds).

Elongation: Extending the Polypeptide

• At the start of elongation, the initiator tRNA is in the P site, and the E and A sites are empty.
• An aminoacyl tRNA binds to the codon in the A site.
• Amino acids at the P and A sites are in the ribosome's active site.
• The amino acid on the P-site tRNA is transferred to the amino acid on the A-site tRNA.
• This translocation process repeats, moving the ribosome along the mRNA.

Is the Ribosome an Enzyme or a Ribozyme?

• The active site of the ribosome is entirely ribosomal RNA.
• Ribosomal RNA catalyzes peptide bond formation, showing the ribosome is a ribozyme.
• This supports the RNA world hypothesis.

Moving Down the mRNA

• Translocation occurs when the ribosome moves one codon toward the 3' end of the mRNA.
• Elongation factors help with ribosome movement.
• Translocation accomplishes these three things:
  • The uncharged tRNA moves to the E-site and exits the ribosome.
  • The tRNA attached to the growing protein moves to the P-site.
  • The A-site is now available to accept a new aminoacyl tRNA.

Terminating Translation

• Termination occurs when the A site encounters a stop codon.
• A release factor enters the A site.
• It resembles tRNAs, hydrolyzes the bond linking the P-site tRNA to the polypeptide chain, and thus terminates translation.
• The newly synthesized polypeptide, tRNAs, and ribosomal subunits separate from the mRNA.

17.3 An Introduction to Translation

• The sequence of mRNA bases is translated into an amino acid sequence.
• Translation is a complex series of steps carried out by ribosomes, mRNA, and tRNAs.

17.1 An Overview of Transcription: Additional Details

• The non-template/coding strand is used for comparison to mRNA
• Transcription is the synthesis of RNA from a DNA template.
• RNA polymerase opens DNA helix to form a transcription bubble.
• NTPs pair with complementary DNA bases for polymerization to initiate.

Gene Expression in Eukaryotes

• Eukaryotic gene expression is more complex, involving multiple steps, including RNA processing.
• mRNA processing in eukaryotes involves: capping the 5' end, splicing to remove introns, and adding a poly(A) tail at the 3' end
• mRNA is processed in the nucleus, and translation occurs in the cytoplasm.
• Polypeptides are modified after translation (post-translation) by various processes.

Polypeptide Folding and Chemical Modifications

• Folding determines protein shape and function.
• Molecular chaperones guide protein folding.
• Sugars or lipids may be added to proteins.
• Enzymes may add phosphate groups to alter protein activity.

Table Summary (17.1)

• Presents a comparison of transcription, RNA processing and translation in prokaryotes and eukaryotes highlighting key differences in the number of RNA polymerases, promoter structure, proteins associated with promoters, and the complexities of RNA processing.