Download Ch. 10- Structure and Analysis of DNA and RNA p. 262-288

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Ch. 10- Structure and Analysis of DNA and RNA
p. 262-288
1. The functions attributed to genetic material are replication, expression, storage,
and mutation. What does each of these terms mean? (p. 263)
Replication: one facet of the cell cycle, a fundamental property of all living
organisms. Once genetic material is replicated, it is divided equally into daughter
cells. During gamete formation, the genetic material is also replicated, but each
cell only gets half the original genetic material.
Expression: complex process; the basis for the concept of information flow
within the cell. First- transcription of DNA, resulting in the synthesis of three types
of RNA- mRNA (translated into proteins), tRNA, rRNA. Translation occurs in
conjunction with rRNA- containing ribosomes and involving tRNA, which acts as
an adaptor to convert the chemical information in mRNA to the amino acids
making up the proteins.
Storage: genetic information that is present as a repository of all hereditary
characteristics of an organism (information may or may not be expressed). Cells
contain a complete complement of DNA, at any given point they express only
part of this genetic potential.
Mutation: (variability) genetic material is the source of newly arising “variability”
among organisms through the process of mutation (a change in the chemical
composition of DNA). If a mutation occurs, the alteration will be reflected during
transcription and translation, often affecting the specified protein. If a mutation is
present in gametes, it will be passed to future generations and, with time, may
become distributed in the population. Genetic variation (including the
rearrangement between and within chromosomes) provides the raw material for
the process of evolution.
2. Contrast the various contributions made to an understanding of transformation by
Griffith, by Avery, and his coworkers, and by Taylor. (p. 264-268)
Griffith- Critical experiment involved an injection into mice of living IIR cells
combined with heat-killed IIIS cells. Neither cells type caused the death in mice
when injected alone, so Griffith expected that the double injection would not kill
them either. After five days all the mice were dead. Analysis of the blood
revealed large numbers of the living type IIIS bacteria! Based on the control, he
knew that a mutation had not occurred- some type of interaction between the
living IIR and heat-killed IIIS must have happened- he called this phenomenon
transformation. He suggested that the transforming principle might be some part
of the polysaccharide (carbohydrate) capsule or some compound required for
capsule synthesis, but it was not the capsule alone. Griffith’s work led other
physicians and bacteriologists to research the phenomenon of transformation.
Avery- (with MacLeod and McCarty)- published work in what is now regarded the
classic paper in the field of molecular genetics. They reported that they had
obtained the transforming principle in a purified state, and that beyond
reasonable doubt; the molecule responsible for transformation was DNA. They
used large quantity of IIIS virulent that were centrifuged, heat-killed, and
homogenized. To solidify their findings, they sought to eliminate all probable
contaminants from their final product. After treatments the transforming activity
still remained. Chemical testing of the final product gave strong positive reactions
for DNA. Their final experiment involved experiments using
crude samples of DNA- digesting enzyme deoxyribonuclease.
Digestion with this was shown to destroy transforming activity
leaving little doubt that the active transforming principle was
DNA.
Taylor: isolated an extremely rough (ER) mutant strain from a
rough (R) strain. This ER strain produced colonies that were
more irregular then the R strain. The DNA from the R
accomplished the transformation of ER to R. The R strain was
shown to also to be able to serve as the DNA donor in
transformation.
3. What observations are consistent with the conclusion
that DNA serves as the genetic material in eukaryotes?
List and discuss them. (p. 271-272)
1. Distribution of DNA- aside from the nucleus, DNA is found
in chloroplasts and mitochondria which perform genetic
functions. A close correlation exists between the amount of
DNA and the number of sets of chromosomes. No such
correlation can be observed between gametes and diploid cells
for proteins- providing circumstantial evidence favoring DNA
over proteins as the genetic material for eukaryotes.
2. Mutagenesis- (Indirect evidence) UV light is capable of
inducing mutations in the genetic material. Bacteria can be irradiated with various
wavelengths of UV light and the effectiveness of each wavelength is measure by the
number of mutations it induces. Both DNA and RNA absorb UV light most strongly at
260 nm which is also the most mutagenic wavelength. Protein most strongly absorbs at
280 nm, but no significant mutagenic effects are observed at this wavelength. 3.
Eukaryotic Data- strongest evidence has been provided by the application of molecular
analysis referred to as recombinant DNA technology. Segments of eukaryotic DNa
corresponding to specific genes are isolated and literally spliced into bacterial DNA.
Such a complex is inserted into a bacterial cell and its genetic expression is monitored.
If a eukaryotic gene is introduced, the presence of the corresponding eukaryotic protein
product demonstrates directly that this DNA is not only present, but functional in a
bacterial cell.
4. What are the exceptions to the general rule that DNA is the genetic material of all
organisms? What evidence supports these exceptions? (p. 272-273)
Some viruses contain an RNA core rather than once composed of DNA. In these
viruses, it would appear that RNA might serve as the genetic material- an
exception to the general rule that DNA performs this function. This was
demonstrated that when purified RNA from tobacco mosaic virus (TMV) was
spread on tobacco leaves, the characteristic lesions caused by TMV would
appear later on the leaves. It was concluded that RNA is the genetic material of
this virus.
5. Describe the chemical structure of the three components of a nucleotide. What
are the two types of nitrogenous bases? What is different about the pentose
sugars of RNA and DNA? (p. 274)
Nucleotides are the building blocks of all nucleic acid molecules. These structural
units consist of three essential components: a nitrogenous base, a pentose
sugar, and a phosphate group. There are two types of nitrogenous bases:
purines (nine-membered double-ringed) and pyrimidines (six-membered single
ring). The pentose sugars found in nucleic acids give them their names.
Ribonucleic acids (RNA) contain ribose, while deoxyribonucleic acids (DNA)
contain deoxyribose. Deoxyribose is missing one hydroxyl group at the C-2’
position compared with ribose. The presences of a hydroxyl group at C-2’
position distinguishes RNA from DNA.
6. Describe the various characteristics of the Watson-Crick double-helix model for
DNA. (p. 278-283)
1. Two long polynucleotide chains are coiled around a central axis, forming a
right-handed double helix. 2. The two chains are antiparallel that is, their C-5’- toC-3’ orientations run in opposite directions. 3. The bases of both chains are flat
structures, lying perpendicular to the axis; they are “stacked” on one another, 3.4
Å (0.34 nm) apart, and are located on the inside of the structure. 4. The
nitrogenous bases of opposite chains are paired to one another as the result of
the formation of hydrogen bonds; in DNA, on A-T and G-C pairs are allowed. 5.
Each complete turn of the helix is 34 Å (3.4 nm) long; thus, 10 bases exist in
each chain per turn. 6. In any segment of the molecule, alternating larger major
grooves and smaller minor grooves are apparent along the axis. 7. The double
helix measures 20 Å (2.0 nm) in diameter.
7. List the three main differences between DNA and RNA.
1. RNA also has as its building blocks nucleotides linked into polynucleotide
chains, the sugar ribose replaces deoxyribose and the nitrogenous base uracil
replaces thymine. 2. Most RNA is usually considered to be single strandedhowever they sometimes fold back on themselves to form double-stranded
regions following their synthesis. Some animal viruses that have RNA as their
genetic material contain it in the form of a double stranded helix. 3. Size and
sedimentation behavior in a centrifuge classify RNA. DNA stores the genetic
information; RNA functions in the expression of that information.
8. What are the three types of RNA molecules? How is each related to the concept
of information flow? (p. 283- 285)
1. Ribosomal RNA (rRNA)- largest of the molecules- constitutes 80% of RNA in
a cell. They are important structural components of ribosomes, which function as
a nonspecific workbench during the synthesis of proteins during translation.
2. Messenger RNA (mRNA)- carry genetic information from the DNA of the gene
to the ribosome, where translation occurs. 3. Transfer RNA (tRNA)- smallest
class of RNA- carry amino acids to the ribosome during translation.
9. What component of the nucleotide is responsible for the absorption of ultraviolet
light? How is this technique important in the analysis of nucleic acids? (p. 285)
UV light and the ring systems of the purines and pyrimidines interact. Any
molecule containing a nitrogenous base can be analyzed using UV light. It is
important in the localization, isolation, and characterization of nucleic acids. IT is
used in conjunction with standard procedures that separate molecules.
10. What is the basis for determining base composition using density gradient
centrifugation? (p. 285-287)
The mixture can be loaded on top of a solution prepared so that a concentration
gradient has been formed from top to bottom. A density gradient is created that
overlaps the densities of the individual components of a mixture of molecules
(usually a heavy metal salt). During centrifugation the molecules migrate until
they reach a point of neutral buoyant density- the force on them is equal and
opposite to the upward diffusion force and no more migration occurs. If DNAs of
different densities are used, they will separate as the molecules of each density
reach equilibrium. The gradient may be fractionated and the components
isolated. This technique provides high resolution in separating mixtures of
molecules varying only slightly in density.
11. What is the physical state of DNA following denaturation? (p. 287-288)
The hydrogen bonds of the duplex structure break, the duplex unwinds, and the
strands separate.