RNA Vs DNARNA, like DNA, is a polymer of three subunits joined by phosphodiester bonds. However, as detailed in the table below, there are key differences in the monomer units for each compound. Show
By Difference_DNA_RNA-DE[CC BY-SA 3.0], via Wikimedia Commons  Fig. 1 – Comparison of DNA and RNA Stages of TranscriptionThe process of DNA transcription can be split into 3 main stages: initiation, elongation & termination. These steps are also involved in DNA replication. InitiationTranscription is catalysed by the enzyme RNA polymerase, which attaches
to and moves along the DNA molecule until it recognises a promoter sequence. This area of DNA indicates the starting point of transcription, and there may be multiple promoter sequences within a DNA molecule. Transcription factors are proteins that control the rate of transcription; they too bind to the promoter sequences with RNA polymerase. Once bound to the promoter sequence, RNA polymerase unwinds a portion of the DNA double helix, exposing the bases on each of the two DNA strands. ElongationOne DNA strand (the template strand) is read in a 3′ to 5′ direction, and so provides the template for the new mRNA molecule. The other DNA strand is referred to as the coding strand. This is because its base sequence is identical to the synthesised mRNA, except for the replacement of thiamine bases with uracil. RNA polymerase uses incoming ribonucleotides to form the new mRNA strand. It does this by catalysing the formation of phosphodiester bonds between adjacent ribonucleotides, using complementary base pairing (A to U, T to A, C to G and G to C). Bases can only be added to the 3′ (three-prime) end, so the strand elongates in a 5’ to 3’ direction. TerminationElongation continues until the RNA polymerase encounters a stop sequence. At this point, transcription stops and the RNA polymerase releases the DNA template. Modified from Calibuon at English Wikibooks [Public Domain], via Wikimedia Commons Fig. 2 – The 3 stages of transcription Pre-translational mRNA processingThe mRNA which has been transcribed up to this point is referred to as pre-mRNA. Processing must occur to convert this into mature mRNA. This includes: 5′ CappingCapping describes the addition of a methylated guanine cap to the 5′ end of mRNA. Its presence is vital for the recognition of the molecule by ribosomes, and to protect the immature molecule from degredation by RNAases. PolyadenylationPolyadenylation describes the addition of a poly(A) tail to the 3′ end of mRNA. The poly(A) tail consists of multiple molecules of adenosine monophosphate. This stabilises RNA, which is necessary as RNA is much more unstable than DNA. SplicingSplicing allows the genetic sequence of a single pre-MRNA to code for many different proteins, conserving genetic material. This process is sequence dependent and occurs within the transcript. It involves:
By Thomas Shafee [CC BY 4.0], via Wikimedia Commons Fig. 3 – Post-transcriptional modification of pre-MRNA By the end of transcription, mature mRNA has been made. This acts as the messaging system to allow translation and protein synthesis to occur. Within the mature mRNA, is the open reading frame (ORF). This region will be translated into protein. It is translated in blocks of three nucleotides, called codons. At the 5’ and 3’ ends, there are also untranslated regions (UTRs). These are not translated during protein synthesis. Clinical Relevance – Phenylketonuria (PKU)PKU occurs due to a single base pair substitution (G to A) in the enzyme phenylalanine hydroxylase. This results in intron skipping, producing unstable mRNA. PKU is one of several genetic conditions tested for in babies via the newborn blood spot (heel prick) test. Individuals with phenylketonuria accumulate phenyalanine in their tissues, plasma and urine. Phenylketones are also found in their urine. This results in intellectual disability, developmental delay, microcephaly, seizures and hypopigmentation. Treatment includes consuming diets low in phenylalanine and avoiding high protein foods such as meat, milk and eggs. By U.S. Air Force photo/Staff Sgt Eric T. Sheler [Public Domain], via Wikimedia Commons Fig. 4 – Heel prick testing for PKU What occurs during the initiation stage of transcription?Step 1: Initiation
Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can ''read'' the bases in one of the DNA strands.
What occurs during the initiation step of transcription quizlet?What happens during the initiation step of DNA transcription? A portion of the DNA unwinds and RNA polymerase attaches to the DNA within the transcription bubble. RNA polymerase moves along the template strand of the DNA creating an mRNA strand.
Which of the following is involved in the initiation of transcription?The promoter is a DNA segment that serves to initiate transcription of a particular gene. They are present upstream near the transcription start sites of genes and provide the binding site for RNA polymerase.
Which event occurs during the initiation stage of translation?During initiation, the small ribosomal subunit binds to the start of the mRNA sequence. Then a transfer RNA (tRNA) molecule carrying the amino acid methionine binds to what is called the start codon of the mRNA sequence. The start codon in all mRNA molecules has the sequence AUG and codes for methionine.
What are the 3 steps of the initiation of translation?Translation of an mRNA molecule by the ribosome occurs in three stages: initiation, elongation, and termination.
What are the 4 steps of transcription in order?The major steps of transcription are initiation, promoter clearance, elongation, and termination.
|
Which of the following events occur during the initiation stage of transcription?
zusammenhängende Posts
Urheberrechte © © 2024 membukakan Inc.
