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Hoofdstuk 6 DNA antwoorden Humane levencycles 1
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- Vrije Universiteit Amsterdam (VU)
De antwoorden van Hoofdstuk 6 DNA Humane levencycles 1
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Hoofdstuk 6: DNA replication, repair and recombinationDNA replication
DNA replication is the process by which a copy of a DNA molecule is made. A random produced,
permanent change in the nucleotide sequence in DNA is called a mutation. Each strand of a DNA
contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its
partner strand. Each strand can be a template: a molecular structure that serves as a pattern to
produce other molecules. For example, one strand of DNA directs the synthesis of the
complementary DNA strand. The ability of each strand of a DNA molecule to act as a template for
producing a complementary strand enables a cell to copy, or replicate. The replication origin is the
nucleotide sequence at which DNA replication is initiated. Here, the initiator proteins pry the two
DNA strands apart, breaking the hydrogen bonds between the bases. Separating a short length of
DNA a few base pairs at a time does not require a large energy input (hydrogen bonds are very weak
on their own), and the initiator proteins can readily unzip short regions of the double helix at normal
temperatures. Y-shaped junction at the site where DNA is being replicated are called replication
forks. Two replication forks are formed at each replication origin. At each fork, a replication machine
moves along the DNA, opening up the two strands of the double helix and using each strand as a
template to make a new daughter strand. The two forks move away from the origin in opposite
directions, unzipping the DNA double helix and copying the DNA as they go (bidirectional).
The movement of the replication fork is driven by DNA polymerase, this enzyme catalyzes the
addition of nucleotides to the 3ʹ end of a growing DNA strand, using one of the original, parental
DNA strands as a template. It uses a telomere to start the catalyzes. The 5ʹ-to-3ʹ direction of the DNA
polymerization reaction poses a problem at the replication fork. The sugar–phosphate backbone of
each strand of a DNA double helix has a unique chemical direction, or polarity, determined by the
way each sugar residue is linked to the next, and the two strands in the double helix are antiparallel;
that is, they run in opposite directions. Therefore, at each replication fork, one new DNA strand is
being made on a template that runs in one direction (3ʹ to 5ʹ), whereas the other new strand is being
made on a template that runs in the opposite direction (5ʹ to 3ʹ). The replication fork is therefore
asymmetrical. Sometimes a figure shows the direction in which the replication fork is moving and
not how its growing. All DNA polymerases add new subunits only to the 3ʹ end of a DNA strand. As a
result, a new DNA chain can be synthesized only in a 5ʹ-to-3ʹ direction. This can account for the
synthesis of one of the two strands of DNA. The other DNA strand that appears to grow in the
incorrect 3’-to-5’ direction are actually made discontinuously and is called the lagging strand
(because the cumbersome backstitching mechanism imparts a slight delay to its synthesis). Short
length of DNA, including an RNA primer, produced on the lagging strand during DNA replication is
called Okazaki fragment. Following primer removal, adjacent fragments are rapidly joined together
by DNA ligase to form a continuous DNA strand. At a replication fork, the DNA strand that is made by
continuous synthesis in the 5′-to-3′ direction, is called the leading strand. The disaster that DNA has
an incorrect base pair can be avoided by 2 processes one of them is proofreading: the process by
which DNA polymerase corrects its own errors as it moves along DNA.
DNA polymerase is started with an enzyme that can begin a new polynucleotide strand simply by
joining two nucleotides together without the need for a base-paired end. This enzyme makes a short
length of a closely related type of nucleic acid and this is called RNA (single-stranded, polynucleotide
composed of covalently linked ribonucleotide subunits) and uses the DNA strand as a template. This
short length of RNA, about 10 nucleotides long, is base-paired to the template strand and provides a
base-paired 3ʹ end as a starting point for DNA polymerase. An RNA fragment serves as a primer for
DNA synthesis, and the enzyme that synthesizes the RNA primer is known as primase.
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