What are the steps involved in the process of transcription?

Answer 1

1. DNA unwinds/"unzips" as the Hydrogen Bonds Break.
2. The free nucleotides of the RNA, pair with complementary DNA bases.
3. RNA sugar-phosphate backbone forms. (by RNA Polymerase.)
4. Hydrogen bonds of the untwisted RNA+DNA "ladder" break, freeing the new RNA

Answer 2

Transcription is the process by which the genetic information coded in DNA is used to produce RNA. Here are the main steps involved:

1. Initiation: RNA polymerase binds to a specific region of DNA called the promoter.
2. Elongation: RNA polymerase reads the DNA template strand and synthesizes a complementary RNA strand.
3. Termination: RNA polymerase reaches a specific termination sequence on the DNA, releasing the newly formed RNA molecule.
4. Post-transcriptional modifications: The RNA transcript may undergo processing, including splicing and addition of a 5' cap and a poly-A tail.

Answer 3

Transcription starts off by the promoter region of the DNA strand. This is where the TATA box also is and it is the site of the beginning of transcription. There are several transcription factors (TF-IIA, TF-IIB, TF-IID) that bind together on the TATA box. The transcription factors recruit RNA polymerase which will make an mRNA strand off the DNA strand. It will continue to transcribe the DNA strand until it reaches a termination sequence.

Once the mRNA is made, it will be capped on the 5' end, cropped and polyAdenylated to make a poly A tail. It will then be spliced to be ready to be sent out of the nucleus to be translated into a protein.

If you think of a zipper in a way it makes it easier to visualize. The handle part is the RNA polymerase and as you unzip it's making a copy of the original part. Exception being that the A nucleotide gets translated into U. In DNA replication the A would get copied with T.

Answer 4

initiation- RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter

elongation- only one of the unmound DNA strands acts as a template for the RNA synthesis

termination- RNA polymerase continues to elongate until it reaches the terminator, a specific sequence of nucleotides that signals the end of transcription

Answer 5

Transcription is a hugely complicated process and can be described in several levels of detail. Below is probably too much detail for most people, so here is a shorter summary first:

RNA polymerase II from yeast (eukaryotic) is a 12 subunit polymerase that assembles with a number of transcription factors (transcription factors TFII D, A, B, F, E, H and J, in that order) to form the preinitiation complex for the production of a complementary mRNA copy of the DNA antisense strand. Other polymerases have other functions: RNAP I produces rRNA and RNAP III produces tRNA. Initiation occurs at specific sites on the DNA where there are promoter regions such as the TATA box (which is recognised by TFIID's TATA binding protein) and one or more other up or downstream promoter elements. TFIID can have associated factors for recognition of promoter regions other than the TATA box. Elongation occurs when the polymerase matches base pairs of DNA to free ribonucleotides and catalyses the formation of a phosphodiester bond between it and the end of the RNA chain (if there is one) being synthesises. It occurs in the 3'->5' direction. An important obstacle to overcome is "abortive transcripts" where short (up to nine nucleotides long) RNA strands are forced to dissociate due to sterics clashes between the "B finger" of polymerase and the transcribed DNA-RNA hybrid, as well as the difficulty with which very short oligonucleotides (the new RNA strand) can be held in position in the polymerase. The B finger fills the space where the hybrid duplex passes, which is also a binding site for TFIIB, so that past 5 nucleotides long the transcript duplex must compete for space. An abortive transcript is formed when TFIIB remains bound, whereas release from the initiation site will occur if TFIIB is displaced. Abortive transcript frequency is reduced by burying the sites of the first few nucleotides deep in the protein, with various short range interactions, but also one long range electrostatic interaction with an arginine residue. Mutation of that residue resulted in an increase in abortive transcript frequency. If TFIIB is successfully displaced (so no more abortive transcripts are formed), elongation occurs until termination at a site many base pairs later than the end of the gene. Regulation can be achieved through a number of mechanisms, making use of regulators, enhancers, repressors, alteration of nucleosome structure and different transcription factors. Eukaryotes are capable of post translational regulation by regulating the processes of polyadenylation, splicing and capping.

More detailed response: RNA synthesis occurs in a process known as transcription. It is a vital process within cells, as it is required for protein synthesis and ribosome synthesis, as well as in the creation of smaller RNA strands for various functions, such as snRNA for splicing. Genes are organized in operons on prokaryotes, in a highly logical way, so that all related genes are transcribed together. In contrast, eukaryotic gene organization has related genes frequently separated physically (even on different chromosomes), and there are sections of DNA within the gene coding region not used in translation (suggesting the existence of clipping methods - introns and exons in splicing). There is only one RNA polymerase involved in RNA synthesis in E. coli (prokaryotic) cells. On the other hand, there are three different RNA polymerases (I, II and III) used in eukaryotic cells for the same process. For the production of tRNA, RNase P is also required to cleave off the extra nucleotides on the tRNA precursor. Again there are differences in this for eukaryotic and prokaryotic cells, since eukaryotic RNase P requires many more associated proteins. The part of the RNase P molecule that is not the associated proteins (eukaryotes), or the single polypeptide chain for prokaryotes, is actually an RNA chain itself. Other RNases are also used to cleave transcribed RNA strands, though this is much more common in eukaryotes, since prokaryotes begin translation while transcription is still occurring (since it takes place in the cytoplasm rather than the nucleus). For example, RNase III in prokaryotes, which can cleave double stranded regions of RNA (many are known to process precursors to rRNA). In eukaryotes, another protein may be used in transcription - the TATA binding protein - for initiation of transcription, as it helps to melt the TATA region of DNA (sequence TATA followed by a number of A bases), and bends the DNA. It is required by all three RNA polymerases in eukaryotes, but primarily for RNA polymerase II. In a similar way, the sigma factor in prokaryotes is a protein required as a subunit of RNA polymerase (for a holoenzyme) for initiation of transcription, though the sigma factor dissociates from the enzyme soon after the elongation stage of transcription begins. The sigma factors, and its alternative versions, are important in regulating gene expression under different conditions, such as RpoH, which is used in 'heat shock' to transcribe genes required to combat the cause of the heat shock. RNA polymerase, in prokaryotes, requires ATP, CTP, GTP and UTP (short term life of RNA so thymine not important) to work, as well as the Mg2+ divalent cation and the DNA template (with a promoter region and sigma factor). The E. coli polymerase consists of two alpha subunits (40 kDal), a beta subunit (155 kDal), a beta prime subunit (160 kDal) and the sigma factor (often 85 kDal). In most cases of transcription in E. coli, the sigma factor will bind to the -10 and -35 sequences of DNA (relative to the start of the gene being transcribed). The sigma factor (and holoenzyme) is able to find these sites as the entire holoenzyme slides along the DNA molecule, making and breaking weak hydrogen bonds with exposed bases until the promoter sequence is found (which is often TTGACA on the -35 sequence and TATAAT on the -10 sequence). RNA polymerase then unwinds a section of DNA to allow for transcription to begin (about 12-17 base pairs) in a transcription bubble. At either end of the transcription bubble, positive (3' end) or negative (5' end) supercoiling occurs to stabilize the DNA strand after helix twists are removed by RNA polymerase. No primers are required for RNA polymerase to create RNA chains from the DNA template, and like DNA synthesis, transcription occurs in the 5' to 3' direction (as shown experimentally by labeling). RNA polymerase will then synthesise RNA by allowing complementary base pairs to pair with the template strand of the DNA molecule and catalyzing the hydrolysis of the 3' OH group and the phosphate groups of the next ribonucleotide to be added. The newly synthesised RNA forms a hybrid with DNA for eight base pairs (almost a complete turn), before structures in the RNA polymerase enzymes force it to separate. As RNA polymerase moves along the DNA chain, transcription cofactor proteins are able to assist with proof reading abilities of the enzyme. It can pause and backtrack when errors occur, and can also initiate DNA repair pathways if base pairs are mismatched in the DNA (the Mfd cofactor). A termination sequence causes RNA polymerase to stop transcribing DNA into RNA. In intrinsic termination, a G and C rich sequence followed by a number of U residues causes a hairpin bend and a double strand to form, which makes the RNA polymerase pause. The U-A bonds binding the RNA strand to the DNA strand are very weak, and so this can then break off easily from the template and then enzyme. The transcription bubble can then close when the DNA reforms its double helix. Alternatively, rho dependent termination involves another protein. When some RNA molecules were produced in vitro, the RNA strands were longer than those produced in vivo, since the rho protein was not added. The rho protein destabilizes the interaction (and thus unwinds, as helicase does) between the template and the RNA strand, causing it to separate as with rho-independent (intrinsic) termination. Though the eukaryotic method of transcription is similar to this, there are some key differences. One such difference is the location of transcription, since prokaryotic transcription takes place in the cytoplasm (and translation frequently occurs instantly on the transcript), whereas eukaryotic transcription takes place in the nucleus, before the products are edited and transported to the cytoplasm. Perhaps most importantly, eukaryotes use three different RNA polymerases. RNA polymerase I synthesises the precursor to rRNA, polymerase II synthesises precursors to mRNA, and III synthesises tRNA precursors (and small RNA sequences, including the 5S subunit of ribosomes). RNA Pol I is made of twelve subunits, and is the simplest and fastest acting transcription enzyme, since little regulation is required of ribosome synthesis, due to its constant demand in high concentration. A ribosomal initiator element, and an upstream promoter element, bind proteins that help to recruit RNA polymerase I. RNA polymerase III have promoters within the transcribed sequence (downstream). RNA Pol II also contains twelve subunits, but this time requires transcription factors to bind to it to promote transcription of genes, since it is used to create a vast number of different mRNA strands, so strict regulation is required. Upstream of the transcription sequence can be a TATA box and Inr (initiator element), or an Inr and DPE (downstream promoter element). In both cases, there will be enhancer elements many bases upstream from the start site of transcription, to enhance the transcription of a region (enhancers are only active in the cells the genes are required in). General transcription factors are required to activate RNA polymerase II, which are DNA binding proteins. The most common promoter region in eukaryotic DNA is the TATA box aforementioned. Transcription factors bind to these promoter regions, such as TFII. Initially, TFIID binds to the TATA box of the DNA strand, if present, since the TBP is present in the TFIID complex. The shape of TBP causes large changes in the shape of the DNA molecule, unwinding it greatly. This allows TFIIA and then TFIIB (with TFIID) to bind to the promoter region, followed by TFIIF (similar to sigma factor in prokaryotes) with RNA polymerase II, then TFIIE, which enables the polymerase to move along the DNA easily, and TFIIH, which contains a helicase to unwind the DNA and an enzyme region to phosphorylate (initiating elongation) a region of the polymerase II enzyme known as the C terminal domain (which is unique to this form of polymerase II). This forms a complex known as the basal transcription apparatus. Additional transcription factors are used at other promoter sites to selectively stimulate gene transcription and increase the rate of mRNA synthesis. Transcription factors binding to regulatory sequences can also have negative effects, inhibiting transcription. Whereas in E. coli, gene repressor proteins directly compete with RNA polymerase, eukaryotic repressor proteins compete indirectly, though the mechanism is not well understood. It is possible that the activator is blocked, or the activator binds to the respressor, or the repressor affects the TATA sequence so that the activator can't. Prokaryotic regulation makes use of the sigma factor, as discussed previously, and operons. These operons regulate gene expression due to repressor proteins, which bind to the promoter sequence and prevent the sigma factor from binding to it. In certain conditions, this process will be reversed to allow transcription to take place. An example of this is the lac operon, in which absence of lactose allows the repressor to bind, whereas lactose can bind to the repressor and remove it from the promoter region when present, so that it can be metabolized by E. coli. Sometimes a second protein, acting as a sensor, is required in regulation in this way, which then activates the repressor protein or activator protein. In E. coli, and many prokaryotes, mRNA molecules are generally not altered after transcription. However, tRNA and rRNA is (in fact, four of these all come from one single RNA transcript). RNase P in E. coli cleaves tRNA molecules after transcription. Enzymes may modify the RNA produced, by addition of nucleotides or groups (such as methylation). In contrast, (almost) all eukaryotic RNA transcripts are processed in some way. Since RNA polymerase only produces one RNA strand, this must be processed to produce the subunits of ribosomal RNA. First, nucleotides are modified by snoRNPs, then cleaved into the three separate RNA strands required. This generally takes place in the nucleolus. RNA polymerase III produces tRNA, which are processed as in prokaryotes. RNase P cleaves a section off, then a CCA is added. However, unlike prokaryotes, tRNA is often base modified, or spliced (endonuclease and ligase) to remove introns. Once again RNA polymerase II is the complicated one. In eukaryotes, the mRNA produced at first may contain coding regions for a huge number of different genes. By splicing this 'pre-mRNA' with spliceosomes to remove introns, the desired product can be formed. Spliceosomes are snRNPs, and splice at splice sites which are recognized by a specific base sequence. Splicing is catalysed by small RNA sequences present in the nucleus. The upstream exon is first cleaved from the upstream intron, leaving a 3' OH terminus which can attack the phosphodiester bond between the exon and intron downstream of this. The mRNA in eukaryotes may also be capped, by the addition of a 5' to 5' phosphodiester bond and methylated to help stabilize the mRNA molecules. The 3' end is also modified by cleaving the pre-mRNA at an AAUAAA sequence by endonuclease, then addition of poly(A) occurs by polymerase to add a poly(A) tail to the mRNA, as this enhances translation. RNA editing can alter the base sequence of the RNA after transcription, such as apoliprotein B's RNA, which is changed from a cytidine residue to uridine, to change the codon from Gln to stop, leading to a shorter polypeptide for a different function.

Mitochondrial and chloroplast DNA are transcribed in ways specific to these organelles, as they contain their own polymerases. Mitochondrial RNA polymerase resembles bacteriophage RNA polymerase, and the sigma factor of eukaryotic polymerase. Chloroplast RNA polymerase is homologous to prokaryotic polymerase without the sigma factor.

Inhibition of transcription is an important method of destroying prokaryotic pathogens. Highly specific inhibitors can be used as antibiotics, such as rifampicin and actinomycin. Rifampicin blocks the path the RNA/DNA hybrid passes through in transcription, preventing initiation. Many viruses rely on the opposite to transcription - reverse transcription - to replicate, by invading cells and inserting their DNA from reverse transcription of the RNA they carry. This DNA is then transcribed by the cell to produce more viruses. Mutation of any part of transcription, including the post transcriptional processing, could lead to serious diseases in affected cells.

It is possible to go into yet more detail, by examining the structure of the polymerase enzyme and its complex with transcription factors, DNA and RNA. For this level of detail, look up the original publications of the structures on a journal database.

Answer 6

information flows frome genes to polypeptide. first, a sequence of nulceotides in DNA ( a gene) is trscribed into RNA inthe cell's nucleus. then the RNA travels to the cytoplasm where it is translated into the specfic amino acid sequence of a polypeptide.

Answer 7

amino

acid

hydrogen

bond

Answer 8

Typing from a pre-recorded audio file using computer software such as Word.

Answer 9

The main steps are initiation, elongation, and termination.