Reece et al., 9 Ed
1 Chapter 16
The Molecular Basis of Inheritance
I. Chromosome Organization
• Chromosomes contain the cell's genetic information.
• This information is replicated prior to cell division (i.e., in S phase of the eukaryotic cell
• The enormous size of the chromosomes presents a significant obstacle in DNA replication and
nuclear division for eukaryotic cells. The average human chromosome is 1.5 x 10 bp 8
• Chromosome organization evolved in the face of these obstacles
Recall: Eukaryotic chromosomes are composed of chromatin = DNA and proteins
B. Chromatin Structure in a Metaphase Chromosome
• DNA is combined with protein in a precise multilevel system of DNA packing
1) 10 nm fibre - DNA is associated with histones
• Histones have a large number of basic amino acids (i.e., lysine and arginine) that are positively
charged at physiological pH. The histones interact with negatively charged DNA (i.e.,
• DNA wraps approximately twice around a histone complex of 4 types of histones (H2A, H2B,
H3, H4). The resulting beads and adjacent DNA form a nucleosome. DNA between the
nucleosomes is call linker DNA. The amino terminus of each histone (histone tail) extends
outward from the nucleosome.
2) 30 nm fibre – Interactions between the histone tails and the linker DNA and adjacent histones
are involved in the next level of packing.Afifth histone (H1) attaches to the DNA near the
nucleosome and is also involved in this level of packing. The 10 nm fibre is coiled and folded
into the 30 nm fibre.
3) Looped domain - loops of the 30 nm fibre attached to a scaffold of proteins to form loops call
4) Further coiling and folding occurs in a mitotic chromosome
The packing steps are highly specific and precise - genes always end up in the same place in
Chromatin Structure in an Interphase Chromosome
• Much of the chromatin is packed as 30 nm fibre and folded into looped domains although a
discrete scaffold is missing.
• The looped domains appear to be attached to nuclear lamina Reece et al., 9 Ed
• Chromatin of each chromosome occupies a restricted space in the nucleus
• Chromatin fibres of different chromosomes do not become entangled with other chromosomes Reece et al., 9 Ed
II. Historical Perspective
• Heritable material must code for a great deal of heterogeneity and specificity of function
• Up until the 1940s the majority of biochemists suspected that heritable material was protein.
This was partially the case because DNA was relatively uncharacterized
A. The Search for the Identity of Heritable Material
Alarge number of experiments contributed to the discovery that DNA was the heritable material
in cells. Some of the key experiments and observations contributing to the discovery that DNA is
the heritable material in cells are listed below.
1. Griffith experiments with Streptococcus pneumoniae (1928)
S. pneumoniae Heritable traits
• Smooth colonies (S) - polysaccharide capsule & pathogenic to mice
• Rough colonies (R) - no capsule & not pathogenic to mice
Disease was not caused by polysaccharide capsule as dead S cells didn't cause pneumonia
New Experiment (Figure 16.2)
• Heat killed S cells were mixed with live R cells and injected into a healthy mouse
- mouse dies of pneumonia
- isolated live S cells from a dead mouse and the S phenotype was heritable
2. Avery, McCarty, and MacLeod (1944)
• Tested various chemicals (proteins, DNA and RNA) isolated from heat killed S cells of S.
pneumoniae and found only DNA transformed R cells to pathogenic S cells!
3. Hershey and Chase (1952)
T2 bacteriophage (T2 phage for short)
• protein capsid and DNA genome
• T2 bacteriphage turn E. coli cells into T2 phage producing factory (i.e., reprogram host cell to
Hershey and Chase Experiment (Figure 16.4)
i) Tagged phage components by growing E. coli and T2 phage in the presence of either S or P.
3S - labeled proteins (i.e., methionine or cysteine amino acid residues contain sulfur)
3P - labeled DNA (i.e., phosphate is found in the DNA sugar-phosphate backbone)
ii) Used radioactive T2 phage to infect separate batches of nonradioactive E. coli
iii) Shortly after infection cultures were homogenized in a blender and centrifuged. Phage
particles stay in suspension and bacterial cells are found in the pellet. Reece et al., 9 Ed
• With radiolabeled protein – the radioactive label ( S) remained in supernatant
• With radiolabeled DNA – the radioactive label ( P) associated with cell pellet
• Cultured cells infected with P labeled phage produced P labeled phage. This was not the
case with S labeled phage.
4. Observations made by other researchers
• Eukaryotic cells double DNA content before mitosis
• For the same organism, diploid cells have twice as much DNA as gametes (i.e., haploid cells)
5. Erwin Chargaff (1947)
• Determined that DNA composition was species specific - showed molecular diversity
• Amounts of nitrogenous bases differ from species to species
• Ratios of nitrogenous bases vary from one species to another
1) The base composition varies between species
2) Within a species, the number of adenine (A) and thymine (T) bases are equal and the
number of guanine (G) and cytosine (C) bases are equal
B. DNA Structure (Figures 16.5 &16.7)
• Discovered by James Watson and Francis Crick
• Published one page article in Nature inApril 1953
• Their structure for DNA allowed them to predict how DNA replicates
Review Chapter 5 notes on nucleic acids!
III. DNA Replication (Duplication)
• Base pairing enables existing strands to serve as templates for new complementary strands!!
• Each complementary strand contains the information necessary to construct the other strand
• The basic concept DNA replication is shown in Figure 16.9
A. Possible Models for DNA Replication (Figure 16.10)
Meselson and Stahl Experiment (Figure 16.11) Reece et al., 9 Ed
• This experiment was conducted to determine which replication model (i.e., semi-conservative,
conservative or dispersive) was used by cells
1. Cultured E. coli in N medium
2. Transferred to N medium
3. Centrifuged and e