Step 1 of Protein Synthesis Transcription Step 1 of Protein Synthesis

Molecular Components of Transcription RNA polymerase is responsible for separating the DNA strands and synthesizing the mRNA from a gene on the template strand. The stretch of DNA that is transcribed into RNA is called a transcription unit. Assembly of nucleotides only occurs in the 5’3’ direction. (“downstream”) The “start” sequence of nucleotides is called the promoter. The “stop” sequence of nucleotides is called the terminator. Prokaryotes only have 1 type of RNA polymerase, whereas eukaryotes have 3.

Synthesis of an RNA Transcript Three stages: initiation, elongation, and termination Initiation Promoter sequence serves as a binding site for RNA polymerase, determines where transcription starts, and indicates which strand of the DNA is used as the template. Transcription factors: a collection of proteins that mediate the binding of RNA polymerase and the initiation of transcription. Transcription Initiation Complex: transcription factors and RNA polymerase TATA box: a crucial promoter nucleotide sequence containing TATA that is upstream from the transcriptional start point. All of these features increase the cell’s control of the process.

Synthesis of an RNA Transcript Elongation As RNA polymerase adds nucleotides to the 3’ end of the growing RNA molecule, the DNA helix reforms behind it and the RNA peels away from the template. Rate – about 60 nucleotides per second in eukaryotes! Multiple RNA polymerases can be transcribing a single gene at once (like trucks in a convoy), which helps the cell make the encoded protein in large amounts.

Synthesis of an RNA Transcript Termination Prokaryotes Transcription proceeds through a terminator sequence in the DNA The transcribed terminator (RNA sequence) causes the polymerase to detach from the DNA and release the transcript, which is ready for immediate use as mRNA Eukaryotes Pre-mRNA is cleaved from the growing RNA chain while RNA polymerase II continues to transcribe the DNA. A sequence called the polyadenylation signal sequence is transcribed which adds AAUAAA to the pre-mRNA. Transcription ends when the polymerase eventually falls off of the DNA Pre-mRNA is then modified during RNA processing…

Eukaryotic cells modify RNA after transcription Alteration of mRNA ends A modified guanine nucleotide is added to the 5’ end of the pre- mRNA: 5’ cap. At the 3’ end, between 50-250 adenines are added: poly-A tail. Functions of these modifications: Facilitate export from the nucleus Protect mRNA from degradation by hydrolytic enzymes Help ribosomes attach to the 5’ end of the mRNA

Eukaryotic cells modify RNA after transcription Split Genes and RNA Splicing RNA splicing: a cut-and-paste job on the RNA molecule that is initially synthesized Introns (intervening sequences): noncoding segments of nucleic acid that are cut out of the RNA Exons (expressed sequences): coding segments of nucleic acid that are spliced back together to form the mRNA snRNPs (small nuclear ribonucleoproteins) and other associated proteins form a spliceosome that cuts out the introns and pastes the exons back together.

Eukaryotic cells modify RNA after transcription Split Genes and RNA Splicing Ribozymes RNA molecules that function as enzymes Sometimes the intron RNA functions as a ribozyme and catalyzes its own excision! The Functional Importance of Introns Play regulatory roles in the cell (control gene activity) Splicing is necessary for the passage of mRNA from the nucleus to the cytoplasm A single gene can encode more than one kind of polypeptide, depending on which segments are treated as exons: alternative RNA splicing Several domains (discrete structural & functional regions of a polypeptide) can be encoded on the same gene… exon shuffling for a variety of polypeptides