Gene Expression PDF

GENE EXPRESSION  Gene expression is the process by which the information encoded in a gene is used to either, make RNA molecules that code for proteins or, to make non-coding RNA molecules that serve other functions. Gene expression acts as an “on/off switch” & a “volum...

GENE EXPRESSION  Gene expression is the process by which the information encoded in a gene is used to either, make RNA molecules that code for proteins or, to make non-coding RNA molecules that serve other functions. Gene expression acts as an “on/off switch” & a “volume control” Gene expression acts as an “on/off switch” to control when and where RNA molecules and proteins are made and as a “volume control” to determine how much of those products are made. The process of gene expression is carefully regulated, changing substantially under different conditions. The RNA and protein products of many genes serve to regulate the expression of other genes.  Gene Expression: Reading the genetic code Gene expression is the process the cell uses to produce the molecule it needs by reading the genetic code written in the DNA. To do this, the cell interprets the genetic code, and for each group of three letters it adds one of the 20 different amino acids that are the basic units needed to build proteins. Non-coding RNA molecules  RNA types & functions Types of RNAs Primary Function(s) mRNA - messenger translation (protein synthesis) regulatory rRNA - ribosomal translation (protein synthesis) t-RNA - transfer translation (protein synthesis) hnRNA - heterogeneous nuclear precursors & intermediates of mature mRNAs & other RNAs scRNA - small cytoplasmic signal recognition particle (SRP) tRNA processing snRNA - small nuclear mRNA processing, poly A addition snoRNA - small nucleolar rRNA processing/maturation/methylation regulatory RNAs (siRNA, miRNA, etc.) regulation of transcription and translation, other?? 7 Transcription is the first step in gene expression It involves copying a gene's DNA sequence to make an RNA molecule.  DNA → Protein Coding strand: 5'-ATGATCTCGTAA-3‘ ↓ Template strand: 3'-TACTAGAGCATT-5‘ ↓ RNA transcript: 5'-AUGAUCUCGUAA-3‘ ↓ Polypeptide: Met-Ile-Ser-STOP  Eukaryotic vs. Prokaryotic Transcription In eukaryotes, transcription and translation occur in separate compartments. In bacteria, mRNA is polycistronic; in eukaryotes, mRNA is usually monocistronic. – Polycistronic: one mRNA codes for more than one polypeptide – moncistronic: one mRNA codes for only one polypeptide 3 RNA polymerases in eukaryotes, 1 in prokaryotes. “Processing” of mRNA in eukaryotes, no processing in prokaryotes  Differences Prokaryotic Transcription Eukaryotic Transcription Transcription and translation occur Transcription and translation don’t simultaneously occur simultaneously. occurs in the cytoplasm occurs in the nucleus and translation occurs in the cytoplasm. RNAs are released and processed in the RNAs are released and processed in the cytoplasm nucleus RNA polymerases are a complex of five RNA polymerases are a complex of 10 polypeptides. -15 polypeptides. Doesn’t require any proteins or other Requires proteins known as factors for the initiation of transcription transcription factors for the initiation. Transcription Key points  During transcription, the information encoded in DNA is used to make RNA. RNA polymerase synthesizes RNA, using the antisense strand of the DNA as template by adding complementary RNA nucleotides to the 3′ end of the growing strand. RNA polymerase binds to DNA at a sequence called a promoter during the initiation of transcription. Genes encoding proteins of related functions are frequently transcribed under the control of a single promoter in prokaryotes, resulting in the formation of a polycistronic mRNA molecule that encodes multiple polypeptides. Eukaryotes have monocistronic mRNA, each encoding only a single polypeptide  Unlike DNA polymerase, RNA polymerase does not require a 3′-OH group to add nucleotides, so a primer is not needed during initiation. Termination of transcription in bacteria occurs when the RNA polymerase encounters specific DNA sequences that lead to stalling of the polymerase. This results in release of RNA polymerase from the DNA template strand, freeing the RNA transcript. Eukaryotic primary transcripts are processed in several ways to generate a mature mRNA molecule that can be transported out of the nucleus and protected from degradation.  Prokaryotic Ribosomes (a) Bacterial cell Figure 13.13  Eukaryotic Ribosomes Figure 13.13  Bacterial promoters Transcription start +1 UP element -35 element -10 element (Pribnow box) +1 pre –10 element Most bacterial promoters have –35 and –10 elements Some have UP element Some lack –35 element, but have extended –10 region  RNA processing in prokaryotes *In prokaryotes, there is a little or no processing of mRNA transcripts. *It is degraded very rapidly from 5′ end. *To rescue from degradation it is translated before being finally transcribed. *Ribosomes assemble on unfinished mRNA and first cistron (protein coding region) can be translated very soon. *The internal cistrons are partially protected by stem-loop structure formed at 5′- and 3′- ends.  Shine–Dalgarno (SD) sequence The Shine–Dalgarno (SD) sequence is a ribosomal binding site in bacterial and archaeal messenger RNA, generally located around 8 bases upstream of the start codon AUG. The RNA sequence helps recruit the ribosome to the messenger RNA (mRNA) to initiate protein synthesis by aligning the ribosome with the start codon.  Eukaryotic mRNA processing Eukaryotic cell must transport protein-encoding RNA molecules to the cytoplasm to be translated. The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated to prevent its degradation. Eukaryotic mRNA maturation create a molecule with a much longer half-life than a prokaryotic mRNA. Eukaryotic mRNAs last for several hours, whereas the typical E. coli mRNA lasts no more than five seconds.  mRNA Processing in Eukaryotes The primary transcript (also called pre-mRNA) is first coated with RNA- stabilizing proteins to protect it from degradation while it is processed and exported out of the nucleus.  Steps involved in Processing 1. The first type of processing begins while the primary transcript is still being synthesized; a special, 5′ cap, is added to the 5′ end of the growing transcript. 2. Once elongation is complete, another processing enzyme then adds a string of approximately 200 adenine nucleotides to the 3′ end, called the poly- A tail. This modification further protects the pre- mRNA from degradation and signals to cellular factors that the transcript needs to be exported to the cytoplasm 3. The removal of intervening sequences that do not specify the appropriate amino acids. Processing of ribosomal RNAs Prokaryotic cells contain three rRNAs (16S, 23S, and 5S), which are formed by cleavage of a pre-rRNA transcript. Eukaryotic cells (e.g., human cells) contain four rRNAs. One of these (5S rRNA) is transcribed from a separate gene; the other three (18S, 28S, and 5.8S) are derived from a common pre-rRNA. Following cleavage, the 5.8S rRNA (which is unique to eukaryotes) becomes hydrogen-bonded to 28S rRNA.