From DNA to Protein: Transcription and Translation
From DNA to RNA
From DNA to RNA: DNA does not synthesize proteins on its own. First, segments of the DNA called genes are copied into RNA. These RNA copies are used to direct the synthesis of protein. Many identical copies of RNA can be made from the same gene, enabling cells to rapidly synthesize protein.
Differences Between RNA and DNA
The nucleotides in RNA contain ribose sugar; DNA contains the base uracil instead of thymine. RNA is single-stranded and can fold into a variety of shapes and carry out a variety of functions (structural, regulatory, and catalytic).
Transcription
The process by which the DNA nucleotide sequence is copied into RNA is called transcription. The first step begins with opening a portion of the DNA double helix to expose the bases on both of the DNA strands. One of the two strands serves as a template for RNA synthesis. The strand other than the template strand is called the coding strand. The second step occurs when the RNA polymerase attaches to the template strand and synthesizes covalent bonds between the growing RNA transcript and the incoming ribonucleoside triphosphates. RNA polymerase catalyzes bonds within the sugar phosphate backbone. RNA polymerase unwinds DNA up ahead to expose a new region of the DNA template strand. RNA is elongated from the 5’ to 3’ direction ALWAYS. Incoming triphosphates provide the energy for bond formation.
General/Prokaryotic Initiation and Termination
RNA polymerase only latches onto DNA tightly when it has encountered a gene region called a promoter that contains a specific sequence of nucleotides that lies upstream of the starting point of RNA synthesis. In prokaryotes, the sigma factor is responsible for recognizing the promoter sequence on the DNA. Once RNA polymerase is bound to the correct promoter, it must synthesize in the 5’ to 3’ direction using the 3’ to 5’ DNA strand as its template. The orientation of the promoter determines the direction of transcription.
Eukaryotic Initiation and Termination – General Transcription Factors
Differences between prokaryotic and eukaryotic initiation. Eukaryotic transcription requires a large set of accessory proteins to initiate, including the general transcription factors, which must assemble at each promoter. General transcription factors are proteins that assemble on the promoter, position the RNA polymerase, and pull apart the DNA double helix. Various Types of RNA – RNA molecules encoded by genes which direct the synthesis of proteins are called messenger RNAs. In eukaryotes, each mRNA carries information transcribed just from one gene which codes for a single protein. Ribosomal RNAs form the structural and catalytic core of the ribosomes. Transfer RNAs act as adaptors that select specific amino acids and hold them in place for translation. MicroRNAs serve as regulators for gene expression.
From RNA to Processed RNA (Eukaryotes) – RNA Capping and Polyadenylation
Before RNA can be exported to the cytosol, it must go through RNA processing steps such as capping, splicing, and polyadenylation. RNA capping modifies the 5’ end of the RNA transcript. Polyadenylation provides mRNA with a special structure at its 3’ end. These two modifications act as a landing pad for the ribosome and increase the stability of the mRNA molecule. Splicing – Most protein-encoding eukaryotic genes have noncoding sequences called introns. During RNA splicing, the introns are removed and the exons are stitched together. Small nuclear RNAs (snRNAs) are packaged with additional proteins to form snRNPs, which recognize splice-sites and carry out splicing. Post-Processing – Only processed mRNAs are allowed to pass from the nucleus to the cytosol. Mediated by nuclear pore complexes. RNAs that can’t leave are degraded in the nucleus. Each mRNA molecule is degraded into nucleotides by ribonucleases RNAses. Lifespans of mRNA molecules differ, depending on nucleotide sequences and cell type. From RNA to Protein – Decoding mRNA – Conversion of the information from RNA into a protein represents translation. tRNA – Codons on mRNA do not directly recognize the amino acid. Translation of mRNA into protein depends on transfer RNAs that bind to the codon and to an amino acid. Ribosomes – Ribosome is a large complex made of small proteins and RNA molecules called ribosomal RNAs. Start/Stop of Protein Synthesis – Must have a specific starting site. Translation of an mRNA begins with the codon AUG. End of translation is signaled by stop codons in the mRNA. Characteristics of Translation – Multiple ribosomes bind to each mRNA molecule being translated. Protein Degradation/Life – Proteins are broken down by large protein machines called proteasomes. RNA and the Origins of Life – RNA world view states that RNA both stored genetic information and catalyzed chemical reactions in primitive cells. RNA Can Store Information and Catalyze Reactions – Single-stranded RNA contains the info needed to specify the sequence of a complementary polynucleotide which can in turn specify the sequence of the original molecule. RNA Thought to Predate DNA – Ribose readily formed from formaldehyde, a product of experiments simulating primitive Earth conditions. Control of Gene Expression – Prok. – Gene expression is the process by which cells selectively direct the synthesis of many thousands of proteins and RNAs encoded in their genome. Recombinant DNA – Recombinant DNA is the resulting molecules after DNA fragments have been inserted into a vector. Plasmids are circular pieces of DNA commonly used as vectors. Transformation occurs when bacteria take up DNA molecules in their surroundings. cDNA molecules are molecules of complementary DNA that lack introns because they are reverse transcribed from mRNA.