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Samenvatting Biology: A Global Approach Global Edition Chapter 16, 17, 18, 19.1, 19.2, 20
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Chapter 16 Nucleic acids and inheritance16.1 DNA is the genetic material
The Search for the Genetic Material: Scientific Inquiry
Evidence That DNA Can Transform Bacteria
Griffith -> two strains (varieties) of a bacterium, one pathogenic (disease-causing) and one
nonpathogenic (harmless). Found that when pathogenic bacteria killed with heat and then
mixed the cell remains with living bacteria of the nonpathogenic strain, some of the living
cells became pathogenic. Furthermore, this newly acquired trait of pathogenicity was
inherited by all the descendants of the transformed bacteria. Apparently, some chemical
component of the dead pathogenic cells caused this heritable change, although the identity
of the substance was not known.
Griffith called the phenomenon transformation, now defined as a change in genotype and
phenotype due to the assimilation of external DNA by a cell.
Evidence That Viral DNA Can Program Cells
A virus is little more than DNA enclosed by a protective coat, which is often simply protein.
To produce more viruses, a virus must infect a cell and take over the cell’s metabolic
machinery.
Hershey and Chase showed by an experiment that only one of the two components of T2-
phage actually enters the E. coli cell during infection. In their experiment, they used a
radioactive isotope of sulfur to tag protein in one batch of T2 and a radioactive isotope of
phosphorus to tag DNA in a second batch. Because protein, but not DNA, contains sulfur,
radioactive sulfur atoms were incorporated only into the protein of the phage. In a similar
way, the atoms of radioactive phosphorus labeled only the DNA, not the protein, because
nearly all the phage’s phosphorus is in its DNA. In the experiment, separated samples of
nonradioactive E. coli cells were infected with the protein-labeled and DNA-labeled batches
of T2. The researchers then tested the two samples shortly after the onset of infection to see
which type of molecule (protein or DNA) had entered the bacterial cells and would therefore
be capable of reprogramming them.
Hershey and Chase found that the phage DNA entered the host cells but the phage protein
did not. Moreover, when these bacteria were returned to a culture medium and the infection
ran its course, the E. coli released phages that contained some radioactive phosphorus. This
result further showed that the DNA inside the cell played an ongoing role during the infection
process. They concluded that the DNA injected by the phage must be the molecule carrying
the genetic information that makes the cells produce new viral DNA and proteins.
Additional Evidence That DNA Is the Genetic Material
Chargaff analyzed the base composition of DNA from a number of different organisms. In
1950, he reported that the base composition of DNA varies from one species to another.
Chargaff’s evidence of molecular diversity among species, which most scientists had
presumed to be absent from DNA, made DNA a more credible candidate for the genetic
material.
Chargaff also noticed a peculiar regularity in the ratios of nucleotide bases. In the DNA of
each species he studied, the number of adenines approximately equaled the number of
thymines, and the number of guanines approximately equaled the number of cytosines.
Chargaff’s rules:
1. DNA base composition varies between species
2. For each species, the percentages of A and T bases are roughly equal, as are those
of G and C bases.
Building a Structural Model of DNA: Scientific Inquiry
,Images produced by X-ray crystallography are not actually pictures of molecules. The spots
and smudges in the image were produced by X-rays that were diffracted (deflected) as they
passed through aligned fibers of purified DNA. Watson was familiar with the type of X-ray
diffraction pattern that helical molecules produce, and an examination of the photo that
Wilkins showed him confirmed that DNA was helical in shape. The presence of two strands
accounts for the now-familiar term double helix.
In the model Watson constructed the two sugar-phosphate backbones are antiparallel (their
subunits run in opposite directions).
Adenine and guanine are purines, nitrogenous bases with two organic rings, while cytosine
and thymine are nitrogenous bases called pyrimidines, which have a single ring. Pairing a
purine with a pyrimidine is the only combination that results in a uniform diameter for the
double helix.
Each base has chemical side groups that can form hydrogen bonds with its appropriate
partner: Adenine can form two hydrogen bonds with thymine; guanine forms three hydrogen
bonds with cytosine.
Although the base-pairing rules dictate the combinations of nitrogenous bases that form the
“rungs” of the double helix, they do not restrict the sequence of nucleotides along each DNA
strand. The linear sequence of the four bases can be varied in countless ways, and each
gene has a unique base sequence.
16.2 Many proteins work together in DNA replication and repair
The Basic Principle: Base Pairing to a Template Strand
Watson and Crick - DNA replication -> “Our model for deoxyribonucleic acid is a pair of
templates, each of which is complementary to the other. We imagine that prior to duplication
the hydrogen bonds are broken, and the two chains unwind and separate. Each chain then
acts as a template for the formation onto itself of a new companion chain.”
Watson and Crick’s model predicts that when a double helix replicates, each of the two
daughter molecules will have one old strand, from the parental molecule, and one newly
made strand. This semiconservative model can be distinguished from a conservative model
of replication, in which the two parental strands somehow come back together after the
process (parental molecule is conserved). In a third model, dispersive model, all four strands
of DNA following replication have a mixture of old and new DNA.
,DNA Replication: A Closer Look
Getting Started
Origins of replication -> short stretches of DNA that have a specific sequence of nucleotides.
Proteins that initiate DNA replication recognize this sequence and attach to the DNA,
separating the two strands and opening up a replication “bubble”. Replication of DNA then
proceeds in both directions until the entire molecule is copied. In contrast to a bacterial
chromosome, a eukaryotic chromosome may have multiple replication origins. Multiple
replication bubbles form and eventually fuse, thus speeding up the copying of the long DNA
molecules. As in bacteria, eukaryotic DNA replication proceeds in both directions from each
origin.
At each end of replication bubble is a replication fork, a Y-shaped region where the parental
strands of DNA are being unwound. Several kinds of proteins participate in the unwinding.
Helicases are enzymes that untwist the double helix at the replication forks, separating the
two parental strands and making them available as template strands. After the parental
strands separate, single-strand binding proteins bind to the unpaired DNA strands, keeping
them from re-pairing. The untwisting of the double helix causes tighter twisting and strain
ahead of the replication fork. Topoisomerase is an enzyme that helps relieve this strain by
breaking, swiveling, and rejoining DNA strands.
Synthesizing a New DNA Strand
The enzymes that synthesize DNA cannot initiate the synthesis of a polynucleotide; they can
only add DNA nucleotides to the end of an already existing chain that is base-paired with the
template strand. The initial nucleotide chain that is produced during DNA synthesis is actually
a short stretch of RNA. This RNA is a primer and is synthesized by the enzyme primase.
Primase starts a complementary RNA chain with a single RNA nucleotide and adds RNA
nucleotides one at a time, using the parental DNA strand as a template.
Enzymes called DNA polymerases catalyze the synthesis of new DNA by adding nucleotides
to the 3’ end of a pre-existing chain.
Most DNA polymerases require a primer and a DNA template strand, along which
complementary DNA nucleotides are lined up. DNA polymerase III adds a DNA nucleotide to
, the RNA primer and then continues adding DNA nucleotides, complementary to the parental
DNA template strand, to the growing end of the new DNA strand.
Each nucleotide to be added to a growing DNA strand consist of dATP. The only difference
between the ATP energy metabolism and dATP, the adenine nucleotide used to make DNA,
is the sugar component, which is deoxyribose in the building block of DNA but ribose in ATP.
Like ATP, the nucleotides used for DNA synthesis are chemically reactive, partly because
their triphosphate tails have an unstable cluster of negative charge. DNA polymerase
catalyzes the addition of each monomer via a dehydration reaction. As each monomer is
joined to the growing end of a DNA strand, two phosphate groups are lost as a molecule of
pyrophosphate. Subsequent hydrolysis of the pyrophosphate to two molecules of inorganic
phosphate (Pi) is a coupled exergonic reaction that helps drive the polymerization reaction.
Antiparallel Elongation
The two strands of DNA in a double helix are antiparallel, they are oriented in opposite
directions to each other. Therefore, the two new strands formed during DNA replication must
also be antiparallel to their template strands.
The antiparallel arrangement of the double helix, together with a property of DNA
polymerases, has an important effect on how replication occurs. Because of their structure,
DNA polymerases can add nucleotides only to the free 3’ end of a primer or growing DNA
strand, never to the 5’ end. Thus, a new DNA strand can elongate only in the 5’ -> 3’
direction.
Along one template strand, DNA polymerase III can synthesize a complementary strand.
DNA pol III remains in the replication fork on that template strand and continuously adds
nucleotides to the new complementary strand as the fork progresses.
In contrast to the leading strand, which elongates continuously, the lagging strand is
synthesized discontinuously, as a series of segments. These segments of the lagging strand
are called Okazaki fragments.
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