DNA Replication (PDF)

DNA Replication Replication occurs in the nucleus of eukaryote, but cytoplasm of prokaryote 2  Concept 16.3 A chromosome consists of DNA packed together with proteins Bacterial chromosome is double-stranded, circular DNA molecule associated with small amount o...

DNA Replication Replication occurs in the nucleus of eukaryote, but cytoplasm of prokaryote 2  Concept 16.3 A chromosome consists of DNA packed together with proteins Bacterial chromosome is double-stranded, circular DNA molecule associated with small amount of protein Eukaryotic chromosomes are linear DNA molecules associated with large amount of protein In a bacterium, DNA is “supercoiled” and found in nucleoid Chromatin (DNA and protein mix) undergoes changes in packing during cell cycle © 2014 Pearson Education Ltd. 16-3  Chromatin Euchromatin and Heterochromatin Most chromatin loosely packed during interphase and condenses prior to mitosis Loosely packed chromatin is euchromatin During interphase centromeres and telomeres highly condensed into heterochromatin Dense packing makes it difficult to express genetic information coded in these regions © 2014 Pearson Education Ltd. 16-4 Semi-conservative and Semi-discontinuous DNA replication Relationship between structure and function is manifest in double helix Watson and Crick noted specific base pairing suggested a possible copying mechanism for genetic material  Figure 16.9-3 Watson and Crick’s semiconservative model of replication predicts when double helix replicates, each daughter molecule will have one parental strand and one newly made strand A T A T A T A T C G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C (a) Parent molecule (b) Separation of (c)“Daughter” DNA molecules, strands each consisting of one parental strand and one new strand Competing models were conservative model (the two parent strands rejoin) and dispersive model (each strand is a mix of old and new) The Semiconservative model for DNA replication  DNA Replication: A Closer Look The copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” A eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied © 2011 Pearson Education, Inc.  Figure 16.12 (a) Origin of replication in an E. coli cell (b) Origins of replication in a eukaryotic cell Origin of Double-stranded Parental (template) strand Origin of replication DNA molecule replication Daughter (new) strand Parental (template) Daughter (new) strand strand Replication Double- fork stranded DNA molecule Replication bubble Bubble Replication fork Two daughter DNA molecules Two daughter DNA molecules 0.25 m 0.5 m  Synthesizing a New DNA Strand Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork Most DNA polymerases require a primer and a DNA template strand The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells © 2011 Pearson Education, Inc.  Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate dATP supplies adenine to DNA and is similar to the ATP of energy metabolism The difference is in their sugars: dATP has deoxyribose while ATP has ribose As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate © 2011 Pearson Education, Inc.  DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end The initial nucleotide strand is a short RNA primer An enzyme called primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template The primer is short (5–10 nucleotides long), and the 3 end serves as the starting point for the new DNA strand © 2011 Pearson Education, Inc.  Antiparallel Elongation The antiparallel structure of the double helix affects replication DNA polymerases add nucleotides only to the free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5to3direction © 2011 Pearson Education, Inc.  Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork © 2011 Pearson Education, Inc.  Figure 16.17 Leading strand 5 DNA pol III 3 Primer Primase 3 5 3 Parental DNA pol III Lagging strand DNA 5 4 DNA pol I DNA ligase 35 3 2 1 3 5 Khatam 16