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Genetics: The Science of Heredity Transcription DNA vs.RNA: differences • DNA (deoxyribose nucleic acid) and RNA (ribose nucleic acid) are both nucleotide polymers. These molecules are very similar but there are some distinct differences between them. • Both molecules are helical: – DNA is a double helix – RNA is a single helix. • DNA bases: – Adenine (A), Thymine (T), Cytosine (C) & Guanine (G) • RNA bases: – A, G and C but T is replaced with Uracil (U) • DNA has one less oxygen on the 5 carbon sugar than RNA; thus the difference in their names. Deoxyribose simply refers to a ribose sugar lacking an oxygen molecule. DNA vs. RNA molecular difference • The lack of one oxygen molecule on the DNA 5 carbon sugar RNA types • 1. Ribosomal RNA (rRNA): make up ribosomes • 2. Transfer RNA (tRNA): transport amino acids to ribosomes • 3. Messenger RNA (mRNA): copied from DNA, conveys information from chromosomes to ribosomes DNA vs. RNA: Similarities • Both essential in protein synthesis. – Transcription: DNA is transcribed into Messenger RNA (mRNA). – Translation: mRNA is translated into a polypeptide chain with the aid of Ribosomal RNA (rRNA) and Transfer RNA (tRNA). Transcription Essentials • Transcription occurs in nucleus. • Transcription: production of mRNA copy of the DNA gene. – Think of DNA as instructions to build hardware (proteins), unfortunately, these instructions are in another language and incomprehensible to the workers that will eventually assemble the hardware. This is where mRNA will come into the picture - to provide new instructions that will be used by the workers. Steps of Transciption 1. Initiation: DNA is unzipped and the enzyme RNA polymerase runs along the template strand of the DNA. – The template strand of DNA can be identified by finding the promotor region: nucleotide sequence T A C at the 3’ end (If the strand is written backwards it may look like C A T at the 3’ end). This identifies that strand as the template and the other strand, the information strand, will not be used in this transcription (this does not mean, however, that it may not be used in future transcription processes). 2. Elongation: As the RNA polymerase runs along the DNA template strand it will add the complementary RNA nucleotides to the DNA nucleotides. – This means that G will be paired with C, and visa versa, and A (DNA) will be paired with U (RNA - rather than T in DNA replication) and T (DNA) paired with A (RNA). 3. Termination: Transcription continues until RNA polymerase reaches a DNA region called the termination signal: nucleotide sequence that marks the end of a gene. • When the single helix mRNA strand is complete, RNA polymerase releases the DNA andnew RNA molecule. The DNA will re-zip into the double helix. The Process of Transcription Diagrams of Transcription Processing the Products of Transcription • In eukaryotes, once the mRNA is transcribed it will then be processed. – A cap and tail will be added to the ends of the mRNA strand. – The strand will be spliced. • The introns (non-coding regions) will be removed • The exons (coding regions) will be spliced together – The completed mRNA strand has groups of three nucleotides known as codons (for example, A U G is the codon in mRNA that was transcribed from T A C). These groups of three will code for a particular amino acid in translation (A U G will code for the start amino acid, methionine, in translation).