Lec 5 DNA Replication .pdf

Biology Lecture 5 : DNA Replication Central Dogma : DNA RNA Proteins  This unidirectional flow equation represents the Central Dogma (fundamental law) of molecular biology.  This is the mechanism whereby inherited information is used to create actual objects, namely enzymes and structural proteins...

Biology Lecture 5 : DNA Replication Central Dogma : DNA RNA Proteins  This unidirectional flow equation represents the Central Dogma (fundamental law) of molecular biology.  This is the mechanism whereby inherited information is used to create actual objects, namely enzymes and structural proteins.  An exception to the central dogma is that certain viruses (retroviruses) make DNA from RNA using the enzyme reverse transcriptase. Central Dogma : Gene Expression : DNA RNA Proteins  Genes are DNA sequences that encode proteins (the gene product)  Gene expression refers to the process whereby the information contained in genes begins to have effects in the cell.  DNA encodes and transmits the genetic information passed down from parents to offspring. Genetic Code :  The alphabet of the genetic code contains only four letters (A,T,G,C).  A number of experiments confirmed that the genetic code is written in 3-letter words, each of which codes for particular amino acid.  A nucleic acid word (3 nucleotide letters) is referred to as a CODON. Genetic Code :  The alphabet of the genetic code contains only four letters (A,T,G,C).  A number of experiments confirmed that the genetic code is written in 3-letter words, each of which codes for particular amino acid.  A nucleic acid word (3 nucleotide letters) is referred to as a CODON.  So how can 4 bases be translated into a sequence of amino acids when there are 20 possible amino acids to code for? Genetic Code :  If each base coded for one amino acid only 4 amino acids could be sequenced.  If we had pairs of bases coding for amino acids, how many combinations would then be possible?  16 still not enough !  The answer is to use 3 bases to code for each amino acid.  With three bases 64 combinations are possible.  More than enough for the 20 amino acids and start and stop signals.  This is called the Triplet Code. Genetic Code :  Start (AUG) and Stop (UGA) codons initiate and terminate polypeptide sequences DNA Replication :  Replication of the DNA molecule is semi-conservative, which means that each parent strand serves as a template for a new strand and that the two new DNA molecules each have one old and one new strand. DNA Replication : DNA replication requires: A. A strand of DNA to serve as a template B. Substrates - deoxyribonucleoside triphosphates (dATP, dGTP, dCTP, dTTP). A. DNA polymerase - an enzyme that brings the substrates to the DNA strand template B. A source of chemical energy to drive this synthesis reaction. DNA Replication : STEP 1 : Unwinding and Exposing the Strands :  DNA strands are unwound and opened by enzymes called HELICASES  Helicases act at specific places called ORIGINS OF REPLICATION  Synthesis of new DNA strands proceeds in both directions from an origin of replication resulting in a bubble with REPLICATION FORKS at each growing point. DNA Replication : STEP 1 : Unwinding and Exposing the Strands : DNA Replication : STEP 2 : Priming The Strand :  In order to begin making a new strand, a helper strand called a PRIMER is needed to start the strand.  DNA polymerase, an enzyme, can then add nucleotides to the 3‘ end of the primer.  Primer is a short, single strand of RNA (ribonucleic acid) and is complimentary to the DNA template strand.  Primers are formed by enzymes called PRIMASES. DNA Replication : STEP 2 : Priming The Strand : DNA Replication : STEP 3 : Strand Elongation :  DNA Polymerase III catalyses elongation of new DNA strands in prokaryotes  Two molecules of DNA polymerase III clamp together at the replication forks, each acting on 1 of the strands  One strand exposed at its 3' end produces a daughter strand which elongates from its 5' to 3' end and is called the LEADING STRAND.  This strand is synthesized continuously and grows from 5' to 3'. DNA Replication : STEP 3 : Strand Elongation : DNA Replication : STEP 3 : Strand Elongation :  The second daughter strand is called the LAGGING STRAND and is antiparallel to the LEADING STRAND.  It's template is exposed from the 5' to 3' end but it must direct the 5' to 3' synthesis of the lagging strands, since nucleotides are added at the 3' end of the chain.  The lagging strand is constructed in small, backward directed bits consisting of discontinuous sections of 100-200 nucleotides in eukaryotes and 1000-2000 nucleotides in prokaryotes, called OKAZAKI FRAGMENTS. DNA Replication : STEP 3 : Strand Elongation : DNA Replication : STEP 3 : Strand Elongation : When an Okazaki fragment forms:  DNA polymerase I removes the RNA primer and replaces it with DNA adjacent to the fragment.  leaving 1 bond between adjacent fragments missing.  A second enzyme called a DNA LIGASE catalyses the formation of the final bond. DNA Replication : STEP 3 : Strand Elongation :