The information contained within the DNA, in the form of a nucleotide sequence, is responsible for the production of the phenotype of an individual. The dna contains specific sequenmces of nucleotides, called genes, that code for specific chasracteristics. The basic concept of how this occurs was described by Francis Crick as the central dogma of mollecular biology and genetics; that a gene codes for a specific intermediate mollecule, that in turn codes for the synthesis of a specific protein.
Thus the first step in expressing a eukaryotic gene is producing the intermediate mollecule, a process called transcription. The DNA cannot be decoded by the cell directly to prodice a phenotype. The process of decoding goes through intermediate mollecules, called RNAs. Thus transcription is the copying of the nucleotide sequence information from a DNA strand into an RNA strand.
The DNA sequences are transcribed into mRNA by DNA dependent RNA polymerases, they are DNA dependant since they use DNA as a template. There are four phases to transcription; RNA polymerase binding, initiation, elongation, and termination. All four stages are highly regulated by the cell.
Transcription of a gene initiates via a region of DNA to the coding region, called a promoter. A promoter consists of specific short DNA sequence motifs. It defines the site of transcript initiation, and the direction of transcription, i.e., which DNA strand is used as the template. It was found that E.coli promoters consist of two main DNA sequence elements; -35 and 10 regions. The first nucleotide in the RNA transcript is defined as +1, and the 10 and 35 DNA elements are measured relative to it .
[...] Eukaryotic promotors are functionaly similar to those in E.coli but are more variable, as wiith promoters for rna pol II, and can consist of twon sequence ellements, the UPE and the CORE, as found in promotors for rna pol I. Promoters for rna pol II are internall. Gernall transcription factors have an anologous function to sigma subunits, in eukaryotic cells. GTFs are mostlly rna polymerase specific multi protein complexes. The only diference between the transcription processes is that in eukaryotic cells during elongation the GTF is jetisoned and replaced by an elongation factor complex which aids processivity of the p;olymerase. [...]
[...] In the cytoplasm the mrna mollecule travels to the r.e.r, where it is read at the attatched ribosomes, and a polypeptide chain is sysnthesised from amino acids attatched to tRNA mollecules thhat are broguht together as the mrna runs between the two subunits of the ribosome. This process is called translation. A ribosome is a ribonucleoprotein particle comprising a small and a large sububit. Rna constitutes approx 2/3 of the mass of a ribosome, and it is currently believed that rna plays a major role in ribosome function, with pproteins playing a minor role. [...]
[...] Gene regulation cellular control of amount and timing of changes to appearance of functional product of a gene (majority of regulated gene products are proteins) Any step of gene expression can be modulated Regulation needed to allow cell to express proteins when needed, at an appropriate level Allows arrangements of cell types with same genome into different structures in a complex pattern Regulation responds to dynamic and complex cues that emerge from, and control, complex developmental pathways Density of packing of DNA in chromatin is indicative of the frequence of transcription Histone responsible for amount of super coiling of DNA affect histones, affect DNA structure Acetylation by acetyl transferase enzymes, e.g. [...]
[...] Rna polymerase uses the antiisense strand as a template to porduce rna, thus rna has t5he same sequence as that of the sense strand. The sense strand for one gene may be antisense for another The rna polymerase forms a transcripotion bubble around the dna duplex. A short region of the dna duplex qithin the holoenzyme complex is meltted to produce an “open complex”. The transcription buibble disrupts 12 dna base pairs and exttends from the region to the start site +1. [...]
[...] There is a tunnel at this site that passes thorugh to the back of the ribosome that the polypeptide passes through as it s synthesised. The trna mollecules cycle through the diifferent sites. The cyclce begins with a peptidyl trna in the p site and an aminoacyl trna binding in the a site. With both sites occupied a new peptide bond is formed. The tRNAs and the mrna are then translocated through the action of elongation factor G which requires the hydrolysis of GTP to GDP. [...]
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