Transcription is the synthesis of an RNA copy of a segment of DNA. RNA is synthesized by the enzyme RNA polymerase.
Transcription and Translation in Cells
In a prokaryotic cell,transcription and translation are coupled; that is, translation begins while the mRNA is still being synthesized. In a eukaryotic cell, transcription occurs in the nucleus, and translation occurs in thecytoplasm.
Because there is no nucleus to separate the processes of transcription and translation, when bacterial genes are transcribed, their transcripts can immediately be translated.
[pic]Transcription and translation are spatially and temporally separated in eukaryotic cells; that is, transcription occurs in the nucleus to produce a pre-mRNA molecule.
The pre-mRNA is typically processedto produce the mature mRNA, which exits the nucleus and is translated in the cytoplasm.
Different Genes for Different RNAs
There are 4 types of RNA, each encoded by its own type of gene. Thegenomic DNA contains all the information for the structure and function of an organism. In any cell, only some of the genes are expressed, that is, transcribed into RNA.
There are 4 types of RNA,each encoded by its own type of gene:
• mRNA - Messenger RNA: Encodes amino acid sequence of a polypeptide.
• tRNA - Transfer RNA: Brings amino acids to ribosomes during translation.
• rRNA -Ribosomal RNA: With ribosomal proteins, makes up the ribosomes, the organelles that translate the mRNA.
• snRNA - Small nuclear RNA: With proteins, forms complexes that are used in RNA processing ineukaryotes. (Not found in prokaryotes.)
Basic Structure of a Protein-Coding Gene
A protein-coding gene consists of a promoter followed by the coding sequence for the protein and then a terminator.
[pic]The promoter is a base-pair sequence that specifies where transcription begins.
The coding sequence is a base-pair sequence that includes coding information for the polypeptide chain specified by...