Gene Expression - BE101 Lecture 8 PDF

Gene Expression With Anne Leung  Transcription making an RNA copy of a gene's DNA sequence. To make proteins that are used to make up living organisms, we first have to access our DNA. Our DNA is located in the nucleus however, so we have to transcribe it onto RNA. Why go through trans...

Gene Expression With Anne Leung  Transcription making an RNA copy of a gene's DNA sequence. To make proteins that are used to make up living organisms, we first have to access our DNA. Our DNA is located in the nucleus however, so we have to transcribe it onto RNA. Why go through transcription and not make proteins from DNA directly? DNA is huge compared to RNA. It is more convenient and manageable to use RNA. Imagine DNA is a big recipe book. If we’re sharing just one recipe from it, we make a copy on a page, instead of carrying the whole book around all the time. Why go through transcription and not make proteins from DNA directly? DNA is valuable. It is why it is protected by a nucleus. Making RNA from transcription means there is less chance of the DNA getting damaged from moving around in the cell and being accessed. RNA can be recycled and replace. Also more efficient because cells can produce multiple copies of mRNA from a single DNA template. Transcription also helps cells to control gene expression, ie. decide which genes to activate or deactivate. By making RNA copies, the cell can control how much of a protein is made, and it can also modify or destroy the RNA when it's no longer needed. This flexibility in controlling gene expression is essential for cell function and differentiation.  How to actually make mRNA Transcription converts the genetic information stored in DNA into a messenger RNA (mRNA). Steps in Transcription 1. Initiation 2. Elongation 3. Termination Transcription is initiated at a region of the gene called the promoter sequence. This is where the enzyme, Initiation RNA polymerase, binds to and unwinds the DNA. The transcript is synthesized in a 5'-to-3' direction. Terminator Promoter DNA RNA RNA polymerase Can be called a transcription enzyme. Proteins known as transcription factors, also help RNA polymerase bind to the promoter. Together they form the transcription initiation complex. Elongation RNA polymerase moves along the DNA strand in the 3' to 5' direction, so it can synthesize the RNA in the 5' to 3' direction. The RNA polymerase reads the DNA template and builds an RNA strand by adding complementary RNA nucleotides (A, U, C, G) to the growing mRNA strand. Terminator Promoter DNA RNA Termination RNA polymerase reaches a termination signal that tells it to stop and releases the strand. RNA polymerase dissociates from the DNA template, and the DNA rewinds back into its double helix structure.  Post Modifications to make mature mRNA 5' Capping: A modified guanine nucleotide is added to the 5' end of the mRNA. This cap protects the mRNA and assists in its recognition by the ribosome during translation. Splicing: mRNA contain introns, which are removed. The remaining exons are joined together. 3' Polyadenylation: A poly-A tail is added to the 3' end of the mRNA. This protects the mRNA and helps its export from the nucleus to the cytoplasm.  Translation Turning mRNA into proteins Steps in Translation 1. Initiation 2. Elongation 3. Termination Initiation The small subunit of the ribosome attach to the mRNA at the 5’ cap in the cytoplasm. The ribosome scans the mRNA to find the start codon. Now the first tRNA and the large subunit can join. Nucleotides are read in groups of three, called codons. Elongation Transfer RNA (tRNA) bring amino acids to the ribosome. Each tRNA has an anticodon that pairs with a codon on the mRNA. Peptide bonds are formed between the amino acids. A closer look at the Ribosome Termination Ribosome finds a stop codon. Stop codons don’t code for any amino acids, so there’s no matching tRNA to bring in an amino acid for these codons. Release factor enters the ribosome and translation is terminated. The polypeptide chain gets released. Now this protein can undertake any modifications needed.