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Download L2 - DNA Replication and Transcription
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The Central Dogma: DNA Replication and Transcription Importance of DNA So, why is DNA important? • The long strands of DNA in the nucleus of a cell contains the genetic information for every characteristic of an organism. • We now know that the characteristics of an organism are dependent upon the proteins that the organism synthesizes. • Therefore, DNA must hold the information needed to synthesize the proteins of living things. Importance of DNA, cont. The Central Dogma • DNA does not direct the synthesis of carbohydrates, lipids or other non-protein molecules essential for life; however, these other materials are manufactured by the cell through reactions made possible by the specificity of enzymes (proteins) produced under the direction of DNA. • The accepted principle stating that genetic information contained in DNA molecules is copied into daughter cells as well as transferred to RNA molecules that direct the synthesis of protein molecules in living organisms is called The Central Dogma. • The process outline follows: • A gene is defined as a segment of a DNA molecule that directs the synthesis of one kind of protein molecule. 1. Replication (copying) of DNA • Human DNA encodes approximately 35,000 genes. 3. Translation (“change language”) of RNA to synthesize proteins primary structure The Flow of Genetic Information According to The Central Dogma • Genes are segments of DNA that contain information necessary for the synthesis of proteins according to the following sequence: For the passing of genetic information to future generation cells For the production of proteins 2. Transcription of DNA (rewriting) to RNA DNA Replication • Because DNA contains the genetic information for living things it is important that an exact copy can be made to pass on to the next generation of cells. • Watson and Crick proposed along with their model of DNA a model for the replication of DNA molecules in the nucleus called semiconservative replication. • Semiconservative replication - new DNA molecule is formed which has one strand from the parent nucleic acid along with a new complementary strand. 1 DNA Replication, cont. DNA Replication, cont. • Once the replication fork is formed (1), base paring of new nucleotides form new DNA strands under the direction of DNA polymerase (2). Occurs in three steps: Step 1: Unwinding of the DNA by helicase and formation of a replication fork or replication bubble. Step 2: Synthesis of DNA strands and broken segments, called Okazaki fragments, by DNA polymerase between replication forks. • Notice that since the new DNA strands must form from the 5’ to 3’ end, one strand is duplicated uninterrupted while the other is formed in small fragments (Okazaki Fragments). Step 3: Joining the Okazaki fragments by DNA ligase. DNA Replication, cont. •The fragments are then joined together the DNA ligase enzyme (3). DNA Replication, cont. • DNA replication usually occurs at multiple sites within a molecule, and the replication is bi-directional from these sites. •The result is two daughter DNA molecules with identical base sequences; each also having one of the two original DNA strands Transcription • DNA replication ensures that DNA molecules can be duplicated for the passing of genetic material from cell to cell. However, within the cell, DNA’s primary role is to code for protein production. • Transcription is the process where the RNA polymerase enzyme catalyzes the synthesizes mRNA from a gene sequence of DNA. • Transcription differs for prokaryotic and eukaryotic cells. Transcription, cont. • Prokaryotic transcription occurs in three steps: 1. Unwinding of DNA helix 2. Synthesis of mRNA starting at an initiation sequence in the 5’ to 3’ direction, moving along the DNA strand in the 3’ to 5’ direction, until RNA polymerase reaches a termination sequence. (U replaces T on the RNA strand as it is transcripted from DNA) 3. Separation of mRNA and rewinding of DNA helix 2 Transcription, cont. Transcription, cont. • Prokaryotic vs. eukaryotic transcription. DNA in prokaryotic cells codes directly for amino acid sequencing DNA in eukaryotic cells contains regions of bases that do not code for proteins called introns. The sections that do code for amino acids are called exons. It is still unclear what the purpose for introns due to the fact they are cut out during the formation of mRNA. Transcription, cont. • Transcription in eukaryotic cells first produces hnRNA, or heterogeneous nuclear RNA. The hnRNA then undergoes a series of enzymecatalyzed reactions that cut and splice the hnRNA to produce mRNA containing only the series of bases that code for amino acids The Genetic Code • The nucleosides of the mRNA molecule must somehow contain a code to direct the attachment of the 20 alpha amino acids in sequence to form proteins. • Since there are only 4 bases in RNA (remember, U replaces T during transcription) there must be more than one base to code for one amino acid. • It was discovered that three nucleotide bases code for one amino acid. Each sequence of 3 nucleotide bases is called a codon. The Genetic Code, cont. • On the DNA molecule ATG, transcribed as AUG on mRNA, codes for the initiation of a protein sequence. AUG is the codon for methionine which is usually cleaved after protein synthesis. • Most amino acids are represented by more than one codon, any of which will signal the addition of the amino acid to the protein chain. • Termination is coded by UAA, UAG, and UGA. • Code is considered universal for all organisms. The Genetic Code, cont. Summary: 3 The Genetic Code, cont. •3 bases / codon •4 bases •Therefore: 43 = 64 possible combinations VIDEO 4