Transcription of the DNA code into mRNA

The classic view of the central dogma of biology states that "the coded genetic information hard-wired into DNA is transcribed into individual transportable cassettes, composed of messenger RNA (mRNA); each mRNA cassette contains the program for synthesis of a particular protein (or small number of proteins)." Lodish, et al (2000) Molecular Cell Biology, 4th edition

?ani=16&cat=biology ?ani=16&cat=biology /web_tut/21_04/21_04_01a.swf /web_tut/21_04/21_04_01a.swf The role of a particular gene is to produce one enzyme that has a role in a metabolic pathway One gene/one enzyme theory was proven by Beadle and Tatum in the 1930’s Has since been altered since proteins may be made of more than one polypeptide

1. The DNA replicates information in a process that involves many enzymes: replication 2. the DNA codes for the production of messenger RNA (mRNA) during transcription 3. In eucaryotic cells, the mRNA is processed and migrates from the nucleus to the cytoplasm 4. Messenger RNA carries coded information to the ribosomes. The ribosomes “read” this information and use it for protein synthesis. This process is called transcription

Is a carrier of genetic information Contains a ribose sugar rather than a deoxyribose Ribose sugar has a hydroxyl group on its 2´ sugar

Instead of thymine, RNA contains uracil RNA is single stranded

There are three major classes of RNA: Messenger RNA (mRNA) Transfer RNA (tRNA) Ribosomal RNA(rRNA)

Not all genes are transcribed all the time. Genes are controlled so that transcription occurs only when the product of the gene is required There are three phases to transcription: Initiation Elongation Termination

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RNA polymerase binds to the DNA molecule upstream of the gene at the gene’s promoter region A promoter region has a high number of A and T bases DNA strand is unwound exposing the template strand The RNA that is polymerized will be complementary to the template strand

RNA builds the single stranded RNA in the 5´ to 3 ´ direction DNA already transcribed rewinds into the double helix

RNA polymerase reaches the terminator sequence The newly formed RNA disassociates from the DNA template strand RNA polymerase leaves and is free to bind to another promoter region

Before mRNA can be used by ribosomes as a template for building proteins, it must first be processed. Soon after RNA polymerase begins transcription, a methylated cap is added to the 5' end.

Poly A polymerase synthesizes the polyadenylated tail by adding adenine residues to the 3´ end. The poly-A tail makes the RNA molecule more stable and prevents its degradation. The processed mRNA is now ready to undergo splicing in preparation for translation.

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Introns are non-coding RNA sequences that must be removed before translation. The process of removing the intron is called splicing The intron is looped out and cut away from the exons by snRNPs (small nuclear ribonucleoprotein) (snurps) The exons are spliced together to produce the translatable mRNA The mRNA is now ready to leave the nucleus and be translated into protein

There are only 4 nucleotide bases in RNA but 20 amino acids The minimum combination of bases to code for 20 amino acids was a triplet code called a codon

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The genetic code is nearly universal. Almost all organisms build proteins with the same genetic code Fruit fly codon will code for the same amino acid as in a human This is important in cloning

The genetic code is redundant More than one codon can code for the same amino acid Three codons do not code for an amino acid – these are “stop” codons to end protein synthesis Stop codons are: UGA, UAA, UAG

The genetic code is continuous It reads as a series of 3-letter codons without spaces, punctuation or overlap