Gene Expression: Transcription and Translation in Eukaryotes

Gene Expression: Transcription and Translation

Eukaryotic Transcription

Transcription is the process by which the genetic information encoded in DNA is copied into messenger RNA (mRNA). In eukaryotes, this process is complex and involves multiple steps and regulatory mechanisms to ensure accurate gene expression.

Key Terms and Definitions

• Transcription: The synthesis of RNA from a DNA template.

• Gene: A DNA segment that codes for a functional product, typically a protein.

• Transcriptional Unit: The stretch of DNA transcribed into an RNA molecule.

• Promoter: A DNA sequence upstream of the gene that signals the start of transcription and binds transcription factors and RNA polymerase.

• Start Point: The specific nucleotide where RNA synthesis begins.

• Terminator: A DNA sequence signaling the end of transcription.

• Transcription Factors: Proteins that help RNA polymerase bind to the promoter and initiate transcription.

• TATA Box: A conserved promoter sequence crucial for forming the transcription initiation complex.

• RNA Polymerase II: The enzyme that synthesizes pre-mRNA in eukaryotes.

• Polyadenylation Signal Sequence: A sequence in the pre-mRNA that signals for the addition of the poly(A) tail.

• Pre-mRNA: The initial RNA transcript before processing.

• RNA Processing: Modifications of pre-mRNA, including capping, polyadenylation, and splicing.

• 5’ Cap: A modified guanine nucleotide added to the 5’ end of pre-mRNA for stability and ribosome recognition.

• 3’ Poly(A) Tail: A string of adenine nucleotides added to the 3’ end for stability and export from the nucleus.

• Exons: Coding regions of a gene that remain in mature mRNA.

• Introns: Non-coding regions removed during RNA splicing.

• RNA Splicing: The removal of introns and joining of exons in pre-mRNA.

• Spliceosome: A complex of proteins and RNAs that carries out splicing.

• Ribozyme: An RNA molecule with catalytic activity, sometimes involved in splicing.

• Alternative RNA Splicing: The process by which different combinations of exons are joined to produce multiple mRNAs from one gene.

Gene Structure and Function

• Gene: Contains regulatory and coding regions. The transcriptional unit is the DNA sequence transcribed into RNA. The promoter is where transcription factors and RNA polymerase bind. The start point is where RNA synthesis begins. The terminator signals the end of transcription.

Initiation of Transcription

• Transcription factors recognize and bind to the promoter (often at the TATA box).

• RNA polymerase II binds to the promoter, forming the transcription initiation complex.

• Initiation begins at the start point.

Template vs. Coding Strand

• The template strand of DNA is transcribed; the coding strand is not.

• Only a small region of the DNA strands separates during transcription.

RNA Polymerase II vs. DNA Polymerase III

• Both synthesize nucleic acids in the 5’ to 3’ direction.

• RNA polymerase II does not require a primer; DNA polymerase III does.

• RNA polymerase II synthesizes RNA; DNA polymerase III synthesizes DNA.

Direction of mRNA Synthesis

• mRNA is synthesized in the 5’ to 3’ direction.

Termination of Transcription

• In eukaryotes, RNA polymerase II transcribes a polyadenylation signal, then proteins cut the pre-mRNA free.

• In prokaryotes, transcription ends at a terminator sequence.

RNA Processing in Eukaryotes

• Two major steps: addition of the 5’ cap and 3’ poly(A) tail, and RNA splicing.

• RNA processing does not occur in prokaryotes because transcription and translation are coupled and there are no introns.

Exons vs. Introns

• Exons encode the amino acid sequence of the polypeptide.

• Introns are non-coding and removed during splicing.

Advantages of Introns

• Allow for alternative RNA splicing, increasing protein diversity.

• Exons can be mixed and matched to create different proteins from one gene.

Eukaryotic Translation

Translation is the process by which the sequence of an mRNA is decoded to build a polypeptide, or protein. This process occurs in the cytoplasm and involves ribosomes, tRNAs, and various enzymes.

Key Terms and Definitions

• Translation: The synthesis of a polypeptide using the information in mRNA.

• mRNA: Messenger RNA, carries genetic information from DNA to ribosomes.

• Polypeptide: A chain of amino acids that folds into a protein.

• Ribosomes: Complexes of rRNA and proteins that facilitate translation.

• tRNA: Transfer RNA, brings amino acids to the ribosome and matches them to the mRNA codon.

• Codon: A three-nucleotide sequence in mRNA that specifies an amino acid.

• Anticodon: A three-nucleotide sequence in tRNA complementary to the mRNA codon.

• Wobble: Flexibility in base pairing at the third position of the codon, allowing some tRNAs to pair with more than one codon.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches the correct amino acid to its tRNA.

• A Site: Ribosomal site that holds the tRNA carrying the next amino acid.

• P Site: Holds the tRNA with the growing polypeptide chain.

• E Site: Exit site for tRNAs after they have delivered their amino acid.

• Initiator tRNA: The tRNA that carries the first amino acid (methionine) and initiates translation.

• Translation Initiation Complex: The assembly of the small ribosomal subunit, mRNA, initiator tRNA, and large ribosomal subunit.

• Peptide Bond: The bond formed between amino acids during protein synthesis.

• Translocation: The movement of the ribosome along the mRNA.

• Release Factor Protein: Protein that binds to the stop codon and terminates translation.

• Untranslated Regions (UTRs): Regions of mRNA that are not translated into protein but have regulatory roles.

tRNA Function and Structure

• tRNA brings specific amino acids to the ribosome during translation.

• The 3’ end of tRNA attaches to an amino acid.

• The anticodon pairs with the complementary mRNA codon.

Multiple tRNAs and Wobble

• Cells have multiple tRNAs for each amino acid due to the redundancy of the genetic code.

• The anticodon region varies to match different codons for the same amino acid.

• Wobble allows some tRNAs to recognize more than one codon, increasing efficiency.

Aminoacyl-tRNA Synthetase

• Enzyme that attaches the correct amino acid to its tRNA.

• There are 20 different synthetases, one for each amino acid.

Ribosome Structure and Function

• Composed of a large and small subunit.

• Has binding sites for mRNA and three tRNAs (A, P, E sites).

• A site: Accepts incoming aminoacyl-tRNA.

• P site: Holds tRNA with the growing polypeptide.

• E site: tRNA exits the ribosome.

Initiation of Translation

• The small ribosomal subunit binds to the mRNA at the 5’ cap.

• The initiator tRNA binds to the start codon (AUG).

• The large ribosomal subunit joins to form the complete initiation complex.

Elongation and Translocation

• mRNA moves through the ribosome 5’ to 3’.

• tRNAs enter the A site, transfer their amino acid to the growing chain in the P site, and exit via the E site.

• Peptide bonds are formed between amino acids.

Termination of Translation

• When a stop codon is reached, a release factor binds to the A site.

• This causes the polypeptide to be released and the ribosome to dissociate.

Translation of mRNA

• Only the coding region (between start and stop codons) is translated into amino acids.

• UTRs are not translated but may regulate translation and mRNA stability.

Summary Table: Comparison of Transcription and Translation

Feature	Transcription	Translation
Template	DNA	mRNA
Product	pre-mRNA (eukaryotes), mRNA (prokaryotes)	Polypeptide
Main Enzyme	RNA Polymerase II	Ribosome
Direction of Synthesis	5’ to 3’	5’ to 3’ along mRNA
Initiation Site	Promoter (TATA box)	Start codon (AUG)
Termination Signal	Terminator sequence or polyadenylation signal	Stop codon and release factor

Example: Alternative RNA Splicing

• A single gene can produce multiple proteins by including or excluding different exons during RNA splicing.

• This increases the diversity of proteins without increasing the number of genes.

Additional info: In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm. In prokaryotes, both processes are coupled in the cytoplasm due to the absence of a nuclear envelope.