ZOOHCC - 501: Molecular Biology (Theory)



Unit 2: DNA Replication












    Bidirectional replication




    Bidirectional replication is the method of DNA replication found in all
    major kingdoms of life. In bidirectional replication, DNA is replicated in
    her two directions simultaneously, resulting in a leading strand (where
    replication occurs faster) and a lagging strand (where replication occurs
    slower). Each of these strand properties is caused by a DNA polymerase and
    the ability to replicate only in the 5' to 3' direction. In the main strand,
    a single DNA polymerase can replicate most of the strand before it
    dissociates (approximately x 1000-5000 bases before being shed from the DNA
    due to its high processivity). However, on the lagging strand, the DNA is
    duplicated into fragments called Okazaki fragments. Each of these fragments
    is later fused by DNA ligase to create a complete, unfragmented
    strand.


    Explanation:



    The  DNA also has two replication forks, which are regions where
    nucleotides are actively added to the growing chain. Prokaryotes have
    circular chromosomes with a single origin of replication (OriC) and a single
    termination site. However, similar to eukaryotes, linear chromosomes have
    multiple origins of replication and two replication forks each, so
    replication is much faster [1]. At all replication origins, replication
    occurs in a bidirectional fashion, resulting in the formation of a
    'replication bubble'. These bubbles grow larger as replication progresses.
    Eventually, two replication forks meet (at opposite ends of the bubble), at
    which point they merge to create a larger bubble. Eventually, all
    replicative bubbles along the chromosome fuse into one large bubble joint
    only at telomeres. These are split to give two identical DNA strands. This
    process continues to generate many DNA strands that are passed on to
    daughter cells.














    Bi-directional replication is the method of DNA replication found in
    organisms of all major kingdoms. In bidirectional replication, DNA is
    replicated in two directions simultaneously, resulting in a leading strand
    (which replicates faster) and a lagging strand (which replicates slower).
    Each of these strand properties is caused by a DNA polymerase and the
    ability to replicate only in the 5' to 3' direction. In the main strand, a
    single DNA polymerase can replicate most of the strand before it dissociates
    (approximately x 1000-5000 bases before being shed from the DNA due to its
    high processivity). However, on the lagging strand, the DNA is duplicated
    into fragments called Okazaki fragments. Each of these fragments is later
    fused by DNA ligase to create a complete, unfragmented strand. The
    chromosome also has two replication forks, which are regions where
    nucleotides are actively added to the growing chain. Prokaryotes have
    circular chromosomes with a single origin of replication (OriC) and a single
    termination site. However, similar to eukaryotes, linear chromosomes have
    multiple origins of replication and two replication forks each, so
    replication is much faster [1]. At all replication origins, replication
    occurs in a bidirectional fashion, resulting in the formation of a
    'replication bubble'. These bubbles grow as replication continues.
    Eventually, two replication forks meet (at opposite ends of the bubble), at
    which point they merge to create a larger bubble. Eventually, all
    replicative bubbles along the chromosome fuse into one large bubble joint
    only at telomeres. These are split to give two identical DNA strands. This
    process continues to generate many strands of DNA that are passed on to
    daughter cells [








    Bidirectional replication is a method of
    DNA
    replication found in organism from each of
    the main kingdoms. Bidirectional replication involves replicating DNA
    in two directions at the same time resulting in a leading strand (were
    replication occurs more rapidly) and a lagging strand (with slower replication).
    The properties of each of these strands is caused by
    DNApolymerase
    and its ability to only replicate in the 5' to 3' direction. In the
    leading strand, a single 
    DNA
    polymerase
    can replicate large portions of the strand (approximately X1000-5000 bases before it falls off the DNA
    due to its high processivity) before dissociating. However, in the lagging strand, the
    DNA
    is replicate in chunks which are called Okasaki fragments.
    Each of 
    these fragments is later fused together by DNA ligase  

    to
    produce the full, unfragmented strand. 
    The
    chromosome also has two replication forks which are the regions where
    nucleotides are actively added to growing strands.
    Prokaryotes have a circular chromosome with a single origin of replication (OriC) and a
    single termination site. However the linear chromosomes,ike those in
    eukaryotes, have several origins of replication and two
    replication forks for each of these, replication therefore occurs much more quickly.
    At all replication origins, replication takes place in a bidirectional
    format
    which results
    in the
    formation of ‘replication bubbles’. These bubbles grow in size as replication continues. Eventually, two replication forks (at each end of a bubble) meet, at which point they
    fuse together producing a larger bubble. Ultimately,  all the replication bubbles along the
    chromosome merge into one large bubble joint only at the telomeres; these split to give two identical strands of
    DNA. This process continues to produce a 
    many strands of DNA which are then passed on to
    daughter cells
     







    Why is DNA replication bi-directional?



    DNA replication is the process of creating new copies of double-stranded
    DNA by synthesizing new DNA strands. DNA replication is a bidirectional
    process because the DNA strands are antiparallel. H. 3'-5' on one strand and
    5'-3' on the other. DNA polymerases synthesize DNA in the 5'-3' direction.
    Synthesis therefore occurs continuously in the 5'-3' direction on the
    template strand, but discontinuously in the 3'-5' direction on the
    non-template strand, giving rise to small fragments of DNA known as Okazaki
    fragments. increase. Bidirectional replication, therefore, involves the
    simultaneous replication of DNA in two directions, resulting in leading and
    lagging strands. DNA replication can be unidirectional or bidirectional. If
    replication is two-way, neither end is stationary, both are moving.