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BIOLOGY CH-16 (In 1953, James Watson and Francis Crick introduced an…

- - - - DNA is copied during DNA replication, and cells can repair their DNA.
        
        Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists.
        
        Scientific inquiry
        
        Evidence that DNA can transform bacteria
        
        Evidence that viral DNA can program cells
        
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        Griffith worked with two strains of a bacterium, one pathogenic and one harmless.
        
        The discovery of the genetic role of DNA began with research by Fredrick Griffith in 1928.
        
        When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic.
        
        He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA.
        
        Later work by Oswald Avery, Maclyn McCarty, and Colin Manacled identified the transforming substance as DNA.
        
        Many biologists remained skeptical, mainly because little was known about DNA.
        
        The role of DNA in hereditary was first discovered by studying bacteria and the viruses that infect them
        
        When T. H. Morgan's group showed that genes are located on chromosomes, the two components of chromosomes---DNA--became candidates for the genetic material.
- - - - Getting started
        
        A eukaryotic chromosome may have hundreds or even thousands or origens of replication
        
        Replication proceeds in both directions from each origin, until the entire molecule is copied.
        
        Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble"
        
        Origins of replication
        
        Single-strand binding proteins bind to and stabilize single-stranded DNA
        
        Topoisomerase relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands.
        
        Helicases are enzymes that untwist the double helix at the replication forks.
        
        Sinthesizing a new DNA Strand
        
        The initial nucleotide strand is a short RNA primer
        
        This is synthesized by the enzyme primase.
        
        DNA polymerases require a primer to which they can add nucleotides
        
        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
        
        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.
        
        Enzymes called DNA polymerases catalyze the synthesis of new DNA at a replication fork
        
        Each nucleotide that is added to a growing DNA strand is nucleoside triphosphate
        
        ATP supplies adenine to DNA and is similar to the ATP of energy metabolims
        
        The difference is in their sugars: dATP has deoxyribose while ATP has ribose
        
        As each monomer joins the DNA strand, via a dehydration reaction, it loses two phosphate groups as a molecule of pyrophosphate
        
        Antiparallel Elongation
        
        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
        
        The antiparallel structure of the double helix affects replication
        
        Along one template strand DNA , the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork.
        
        Leading strand
        
        To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication work
        
        The lagging strand in synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase
        
        The DNA replication complex
        
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        T the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating