Biology 1090 Textbook Notes Only- Final Review
Chapter 9 (197-214)- DNA and Molecular Structure of Chromosomes
Functions of the Genetic Material
Genetic material performs these three essential functions:
1. The genotypic function, replication. The genetic material must store genetic info and accurately transmit
that info from parents to offspring, generation after generation
2. The phenotypic function, gene expression. The genetic material must control the development of the
phenotype of the organism. That is, the genetic material must dictate the growth of the organism from the
single celled zygote to a mature adult
3. The evolutionary function, mutation. The genetic material must undergo changes to produce variations that
allow organisms to adapt to modifications in the environment so that evolution can occur.
Chromosomes are made of proteins and nucleic acids (DNA and RNA nucleic acids).
Proof that Genetic Information is Stored in DNA
The genetic information of most living organisms is stored in DNA. In some viruses, the genetic info is present in RNA.
DNA is in the chromosomes while RNA and proteins are in the cytoplasm. Most somatic cells of diploid organisms
contain twice the amount of DNA as the haploid germ cells (gametes) of the same species. The molecular
composition of DNA is the same in all cells of the organism but the RNA and proteins are variable. DNA is more
Proof that DNA mediates transformation
When Griffith injected both heat killed Type IIIS bacteria (virulent when alive) and live type IIR bacteria (avirulent)
into mice, and many of them died, and the live type IIIS cells were recovered from their bodies. Transforming
principle- something from the heat killed cells had converted the live type IIIR cells to type IIIS cells. The mice played
no role in the transformation, it is in the DNA.
In separate experiments, highly purified DNA from type IIIS cells was treated with enzymes (1) deoxyribonuclease
which degrades DNA, (2) ribonuclease or (3) protease; the DNA was then tested for its ability to transform type IIR to
IIIS. Only DNase treatment had an effect (eliminated the transforming activity).
Proof that DNA carries the genetic information in bacteriophage T2
Results of Hershey and Chase: genetic information of a particular bacterial virus (bacteriophage T2) was present in
DNA. Viruses are the smallest living organism; only living in a sense that their reproduction is controlled by genetic
information stored in nucleic acids. They can only reproduce with a host (acellular parasites). Dependent on the
metabolic machinery of the host. They showed that the DNA of a virus particle entered the cell where most of the
protein stayed outside. The genetic info necessary for viral reproduction was present in DNA. Their experiments
contained DNA with phosphorous but no sulfur and proteins vice versa. They could then label the phage DNA by
growth in a medium containing radioactive phosphorus or opposite. Thus when put in a blender, the S labeled phage
particles were mixed with E. coli and most of the proteins could be removed without affecting progeny phage
production. DNA however was not subject to removal by shearing. Indicated that DNA of the virus enters the cell
because P was found in the cells.
Proof that RNA stores the genetic information in some viruses
RNA viruses store their genetic information in nucleic acids rather than proteins. Fraenkel-Conrat experiment was
done with tobacco mosaic virus, a small virus composed of a single molecule of RNA encapsuled in a protein coat.
Treated them with chemicals that dissociate the protein coats of the viruses from the RNA moleculs and separated
the proteins from the RNA. They mixed the proteins from one strain with the RNA molecules from another which
they found infective viruses with proteins from one strain and RNA from the other. When tobacco leaves were
infected, the progeny viruses were always pheno and genotypically identical to the parent strain from which the RNA
had been obtained. Thus genetic info is stored in RNA and not protein.
Structures of DNA and RNA
Nature of the Chemical Subunits in DNA and RNA
Nucleic acids are macromolecules composed of a phosphate group and a five carbon sugar (pentose) and a cyclic
nitrogen containing compound called a base. Four different bases: adenosine, guanine, cytosine and thymine (uracil
in RNA). Adenine and guanine are purines and cytosine and thymine are pyrimidines. RNA usually exists as a single
stranded polymer composed of a long sequence of nucleotides. DNA is a right handed double stranded helix
(discovered by Watson and Crick). 5’ to 3’ end. Xray diffraction and concentrations.
Each of the two polynucleotide chains in a double helix consists of a sequence of nucleotides linked together by
phosphodiester bonds, joining adjacent deoxyribose moieties. The two polynucleotide stands are held together in
their helical shape by hydrogen bonding between bases in opposing strands; resulting base pairs are stacked
between the two chains perpendicular to the axis of molecule. A and T and G and C.
Adenine and thymine form two H bonds and guanine and cytosine form 3 H bonds. Hydrogen bonding is not possible
between other base pairs.
Once the sequence of bases in one strand of a DNA double helix is known, the other strand is also known because of
base pairing. The complementarity of the two strands of the double helix makes DNA uniquely suited to store and
transmit genetic information from generation to generation. *
The base pairs are stacked about 0.34nm apart with 10 base pairs per 360 degrees. The sugar-phosphate backbones
of the two complementary strands are antiparallel. Unidirectionally, along a double helix, the phosphodiester bonds
in one strand go from 3’ to 5’ carbon of the adjacent nucleotide and the other is vice versa. This opposite polarity is
Stability of the double helix is a result of the hydrogen bonding and the hydrophobic regions within. Watson and
crick’s DNA is called B-DNA which is the conformation that DNA takes place under physiological conditions (in aq
solutions containing low concentrations of salt). DNA is not a static invariant molecule though and has many forms.
In high concentrations of salts or in the dehydrated state, DNA exists as A-DNA. This DNA is a right handed double
helix but has 11 nucleotides per turn instead. It is also shorter and thicker and DNA molecules never exist as this in
vivo. Left handed double helixes ar Z-DNA. It has 12 nucleuotides per turn with a single deep groove.
DNA Structure: Negative Supercoils In Vivo
All the functional DNA molecules present in living cells display a characteristic that they are supercoiled- supercoils
are introduced into a DNA molecule when one or both strands are cleaved and when the complementary strands at
one end are rotated or twister around each other with the other end held fixed in space and thus not allowed to
spin. This supercoiling causes DNA to collapse into a tightly coiled structure similar to a coiled telephone cord. They
are introduced and removed from DNA by enzymes. It only occurs in DNA with fixed ends that are not free to rotate,
therefore obviously the ends of the circular DNA molecules present in most prokaryotic chromosomes and in
eukaryotic organelles are fixed.
Introduce supercoil by taking a circular DNA molecule, cleaving it then twisting it 360 degrees. If we rotate the free
end in the same direction as the DNA double helix, a positive supercoil will be produced. If we rotate the free end in
the opposite direction, a negative supercoil (left handed) (underwound DNA) will result. DNA molecules of almost all
organisms is negative supercoiled in vivo.
DNA- double helix, with the two strands held together by H bonds between complementary bases (A-T and
Complemntarity of the two strands makes DNA uniquely suited to store and transmit genetic info
The two strands of DNA double helix have opposite polarity
RNA usually exists single standed with uracil
Functional DNA molecules in cells are negatively supercoiled
Chromosome Structure in Prokaryotes and Viruses
In most viruses and prokaryotes, the single set of genes (monoploid) is stored in a single chromosome, which in turn
contains a single molecule of nucleic acid (RNA or DNA). Folded genome- is the functional state of bacterial
chromosome. The DNA molecules in prokaryotic and viral chromosomes are organized into negatively supercoiled
domains. Bacterial chromosomes contain circular molecules of DNA segregated into about 50 domains.
Chromosome Structure in Eukaryotes
Most eukaryotes are diploid, having two sets of genes, one from each parent. DNA is packaged into several
chromosomes and each chromosome is present in two (diploids) or more (polyploids) copies.
Interphase chromosomes are usually not visible, however, chemical analysis ad x-ray diffraction studies of isolated
chromatin (the complex of the DNA, chromosomal proteins and other chromosome constituents isolated from
nuclei) have provided info about structure. When chromatin is isolated from interphase nuclei, the individual
chromosomes are not recognizable. Instead, you see an irregular aggregate of nucleoprotein. Chromatin consists of
DNA and proteins. The proteins are basic proteins called histones and a heterogenous largely acidic group referred
to as nonhistone chromosomal proteins.
Histones play a major structural role in chromatin. Histone types H1, H2a, H2b, H3 and H4 are present in almost all
cell types. In some sperm, histones are replaced by another class of small basic proteins called protamines. DNA and
histones are in equal amounts to eachother. 1:2:2:3:2. Four of the types are specifically complexed with DNA to
produce basic structural subunits of chromatin, small ellipsoidal beads called nucleosomes.
The expose NH3+ groups of arginine and lysine allow histone to act as polycations. The positive charged side groups
on histones are important when interacting with DNA because the negative charged phosphate groups. Histones
important in chromatin structure and DNA packaging. Chemical modifications of histones can alter chromosome
structure which can enhance or decrease the level of expression of genes located in the modified chromatin.
Nonhistone chromosomal proteins are likely candidates for roles in regulation the expression of specific genes or
During metaphase of meiosis and mitosis, the DNA is packaged in a chromosome with a small length. Each
chromosome contains a single, giant molecule of DNA that extends from one end through the centromere all the
way to the other end. It is highly condensed, coiled and folded.
When isolated chromatin from interphase cells is studied, it is found to consist of a series of ellipsoidal beads joined
by thin threads. Further evidence for a regular, periodic packaginh has come from studies of digestion of chromatin
with various nucleases. Partial digestion of chromatin with these nucleases yielded fragments of DNA in a set of
discrete sizes that were integral multiples of the smallest size fragment. Chromatin has a repeating structure and the
bead is packaged in a nuclease resistant form. Nucleosomes are not attacked by nuclease, linkers or interbead
Endonuclease cleaves DNA internally. Nucleosome core- nuclease resistant structure that remains after digestion.
Consists of two molecules of each histone and 146 nucleotide base DNA. The histones protect the segment of DNA in
the nucleosome core from cleavage by endocnucleases.
The complete chromatin subunit consists of the nucleosome core, the linker DNA and the associated nonhistone
chromosomal proteins, all stabilized by the binding of one molecule of histone 1 to the outside of the structure.
Complete nucleosome contains two full turns of DNA superhelix on the surface of the histone octamer. Stabilized by
The basic structural component of eukaryotic chromatin is the nucleosome. The structure of nucleosomes in
transcriptionally active regions of chromatin is known to differ from nucleosomes in transcriptionally inactive
regions. The tails of some of the histone molecules protrude from the nucleosome and are accessible to enzymes
that add and remove chemical groups such as methyl and acetyl groups. The addition of these groups can change the
level of expression of genes packaged in nucleosomes containing the modified histones.
Chapter 9 (197-214)- dna and molecular structure of chromosomes. Genetic material performs these three essential functions: the genotypic function, replication. The genetic material must store genetic info and accurately transmit that info from parents to offspring, generation after generation: the phenotypic function, gene expression. The genetic material must control the development of the phenotype of the organism. That is, the genetic material must dictate the growth of the organism from the single celled zygote to a mature adult: the evolutionary function, mutation. The genetic material must undergo changes to produce variations that allow organisms to adapt to modifications in the environment so that evolution can occur. Chromosomes are made of proteins and nucleic acids (dna and rna nucleic acids). Proof that genetic information is stored in dna. The genetic information of most living organisms is stored in dna. In some viruses, the genetic info is present in rna.