Transcription and Translation

Overview of Transcription

Transcription is the process by which genetic information encoded in DNA is copied into RNA. This process occurs in three main phases: initiation, elongation, and termination. The resulting RNA transcript serves as a template for protein synthesis during translation.

• Initiation: Transcription machinery assembles at specific DNA sequences (promoters) and begins RNA synthesis.

• Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' to 3' direction.

• Termination: RNA polymerase releases the newly synthesized RNA and detaches from the DNA.

• Example: In humans, transcription produces messenger RNA (mRNA) from protein-coding genes.

Promoters and Regulatory Sequences

Promoters are DNA sequences that signal the start of transcription. They are recognized by transcription factors and RNA polymerase II in eukaryotes. Promoter elements include the TATA box, CAAT box, and GC box.

• TATA box: Located about 25-30 nucleotides upstream of the transcription start site; helps position RNA polymerase II.

• CAAT box: Found further upstream; involved in promoter efficiency.

• Enhancers: Distant regulatory sequences that increase transcription rates by interacting with activator proteins.

• Example: The beta-globin gene contains a TATA box and CAAT box in its promoter region.

Transcription Initiation Complex

Transcription initiation requires the assembly of general transcription factors and RNA polymerase II at the promoter. This complex unwinds DNA and begins RNA synthesis.

• General transcription factors: Proteins that help position RNA polymerase II and initiate transcription.

• Mediator proteins: Bridge regulatory proteins and the transcription machinery.

• Example: The pre-initiation complex includes TFIID, TFIIB, and other factors.

Transcription Elongation and Termination

During elongation, RNA polymerase II synthesizes RNA by adding ribonucleotides complementary to the DNA template. Termination occurs when RNA polymerase II encounters specific sequences signaling the end of transcription.

• Direction: RNA is synthesized in the 5' to 3' direction.

• Template strand: The DNA strand used for RNA synthesis is called the template (antisense) strand.

• Termination: Involves cleavage of the RNA transcript and release from the DNA.

• Example: Polyadenylation signals (AAUAAA) mark the end of many eukaryotic transcripts.

RNA Processing in Eukaryotes

The initial RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA. These modifications are essential for stability, export, and translation.

• 5' Capping: Addition of a 7-methylguanosine cap to the 5' end of pre-mRNA. Functions include protection from degradation and initiation of translation.

• Polyadenylation: Addition of a poly(A) tail (40-250 adenine nucleotides) to the 3' end, enhancing stability and export.

• Splicing: Removal of non-coding introns and joining of exons by the spliceosome. Involves recognition of splice sites (5' GU and 3' AG) and branch point.

• Alternative splicing: Allows production of multiple protein isoforms from a single gene.

• Example: The calcitonin gene undergoes alternative splicing to produce different proteins in thyroid and brain tissues.

Key Steps in RNA Processing

Translation: Protein Synthesis

Translation is the process by which mRNA is decoded to synthesize proteins. It occurs in the cytoplasm and involves ribosomes, transfer RNA (tRNA), and ribosomal RNA (rRNA).

• Initiation: Ribosome assembles at the start codon (AUG) on mRNA.

• Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons.

• Termination: Occurs when a stop codon (UAA, UAG, UGA) is reached; the polypeptide is released.

• Example: The genetic code table below shows codon assignments for amino acids.

Standard Genetic Code Table

Key Terms and Definitions

• RNA polymerase II: Enzyme responsible for synthesizing mRNA in eukaryotes.

• Exon: Coding region of a gene that remains in mature mRNA.

• Intron: Non-coding region removed during RNA splicing.

• Spliceosome: Complex of proteins and RNAs that mediates splicing.

• Codon: Sequence of three nucleotides in mRNA specifying an amino acid.

• Anticodon: Sequence of three nucleotides in tRNA complementary to mRNA codon.

Important Equations and Concepts

• Direction of RNA synthesis:

• Polyadenylation signal sequence:

• Start codon:

• Stop codons:

Summary Table: Steps in Eukaryotic Gene Expression

Additional info: The study notes above expand on the original material by providing definitions, examples, and structured tables for clarity. The genetic code table is condensed for brevity; students should refer to a full codon chart for all assignments.