Download chapter 16 – the molecular basis of inheritance

CHAPTER 16 – THE MOLECULAR BASIS OF INHERITANCE
YOU MUST KNOW:
 The detailed structure of DNA
 The major steps to replication
 The difference between replication, transcription, and translation
 How DNA is packaged into a chromosome
I.
DNA IS THE GENETIC MATERIAL
 There are several famous experiments that proved that DNA is responsible
for inheritance.
A. Bacterial Transformation Experiment (Griffith’s Experiment)
 Griffith used two strains of the Streptococcus pneumoniae bacterium that
causes pneumonia in mammals. One strain was diseases-causing
(pathogenic) while the other was a non-pathogenic strain.
 Griffith could change the harmless strain of the bacterium into a disease
causing strain just by mixing dead harmful bacteria with live harmless
bacteria.
 Transformation – 1. A change in genotype and phenotype due to the
assimilation of external DNA by a cell. 2. The conversion of a normal
animal cell to a cancerous cell.
 Question remains: What is the factor that is responsible for transformation?
B. Avery, McCarty and MacLeod’s Experiments:
 They purified various molecules (proteins, carbohydrates, lipids and
nucleic acids) from heat-killed bacteria and tried to use the purified

molecules to transfer pathogenic characteristics into non-pathogenic
bacteria.
Result: DNA is responsible for inheritance
C. Bacteriophage Experiment (Hershey and Chase Experiment)
 Bacteriophages – viruses that kill bacteria (viruses are mostly composed of
DNA or RNA and proteins)
 In this experiment, T2 phages were used that infect E. coli bacteria
 Results: Phage proteins remained outside of the bacterial cells while
phage DNA entered the cells – radioactive DNA was detected inside of
infected bacteria.
 Conclusion: DNA, not proteins are responsible for inheritance
D. Chargaff’s Experiment:
 He analyzed the base composition of DNA from various organisms.
 Results:
a. DNA composition varies from one species to another – evidence of
molecular diversity among species
b. In each species, the number of adenine bases approximately
equaled the number of thymine bases; the number of cytosine
bases equaled the number of guanine bases. (Chargaff’s rule)
E. X-ray Crystallography (Franklin):
 Her X-ray crystallography photograph of DNA lead to the discovery of the
structure of DNA.
F. Watson and Crick
 Built the first model of the DNA molecule and described the structure of
the molecule.
 They also described the process of DNA replication
II.
DNA REPLICATION:
A. The Basic Ideas on DNA Replication
 Base-pairing rules apply when the DNA bases pair up
 The two strands are complementary, so each strand serves as a template
for ordering nucleotides into a new complementary strand
 The process starts with one double helix and ends up with two DNA
molecules, both double stranded and identical to the parent DNA
 Enzymes link the nucleotides together at their sugar-phosphate groups.
 The replication is semiconservative because every new DNA molecule
contains one new strand and one old strand.
http://www.sumanasinc.com/webcontent/animations/content/meselson.ht
ml
B. A Closer Look at DNA Replication
 DNA replication is extremely accurate and efficient
 Although we know more about DNA replication in prokaryotes than in
eukaryotes, we also know that the two processes are very similar.
 Six major steps of replication:
a. Origins of replication: The site where DNA replication begins. In
prokaryotic cells there is only one origin of replication, in eukaryotic
cells there are hundreds or thousands to speed up replication.
Helicase enzyme is used to untwist the DNA molecule.
b. Initiation proteins recognize the origins of replication and open up
the DNA double helix forming a replication bubble. Than
replication proceeds in both directions until the entire DNA
molecule is copied. Each opened DNA molecule where the
replication takes place forms a replication fork. RNA nucleotides
(primer) are used to mark the start of replication on each DNA
polynucleotide chain.
c. Elongating a new strand: Elongation is catalyzed by enzymes called
DNA polymerases.
d. Individual nucleotides align with complementary nucleotides along
a template strand of DNA. DNA polymerase adds them one by one
to the growing end of the new DNA molecule. DNA polymerize
identifies the starting point by attaching to the prime of the RNA
nucleotides In prokaryotes, there are two different DNA
polymerases while in eukaryotes there are at least 11. The added
nucleotides are actually nucleoside triphosphates (ATP, GTP, TTP,
CTP but with deoxyribose sugar not ribose). When the Pi groups are
broken down of the nucleotides, energy is released. This energy
release fuels DNA replication.
e. Antiparallel elongation: Because the two strands of the DNA
molecule are antiparallel, so they are oriented in the opposite
direction, so the new DNA molecules also have to orient in the
same direction. However, DNA polymerase can attach nucleotides
only to the 3’ end and grows the chain toward the 5’end of the
original chain. The original 3’- 5’ strand is called the leading strand
because the DNA polymerase simply attaches new nucleotides by
using the template of the old DNA chain and forms a new
polynucleotide chain. To elongate the other new strand in the 5’3’ direction, the DNA polymerase works away from the replication
fork, backwards. It forms small segments of the new polynucleotide
chain that is going to be attached together later. These small
segments are called Okazaki fragments. Another enzyme, DNA
ligase, attaches the Okazaki fragments together later.
f. Only one primer is required to start the 3’ end but each Okazaki
fragment requires a primer on the lagging strand. DNA polymerase
I replaces the RNA primers with DNA molecular segments. DNA
ligase joins the sugar-phosphate backbones of the Okazaki
fragments to form a continuous DNA polynucleotide chain.
http://www.wiley.com/college/pratt/0471393878/student/animations/dna_repli
cation/index.html -- more detailed animation of DNA replication
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter14/animations.html -- many
things on DNA replication and the experiments
http://207.207.4.198/pub/flash/24/menu.swf -- DNA replication, also very good
http://www.fed.cuhk.edu.hk/~johnson/teaching/genetics/animations/dna_repli
cation.htm -- basic DNA replication
C. Proofreading and Repairing DNA



In general there is only 1 error out of 10 billion nucleotides when the DNA
molecule is being assembled. But the initial error rate is higher more about
1:100 000 nucleotides. DNA polymerases proofread the DNA molecule as
it is being made and they replace the incorrectly placed nucleotide.
Cells also have special repair enzymes to fix incorrectly paired nucleotides
later – mismatch repair.
Most common factors that can result in the damage of DNA are:
chemicals from metabolic reactions of the cell or from the environment,
radioactive emissions, X-rays, UV light, spontaneous chemical changes of
the DNA molecule.
III.
Replicating the Ends of DNA Molecules:
 Because the end of the DNA molecule on the lagging end runs out of 3’
ends, it cannot be copied. As a result, each repeated round of
replication produce shorter and shorter DNA molecules. The part of
chromosomes that get lost at each DNA replication is called the telomere.
Telomeres do not contain genes, they only have multiple repetitions of
one short nucleotide segments (TTAGGG in humans).
 An enzyme called telomerase catalyzes the lengthening of telomeres in
eukaryotic germ cells with the help of a short RNA segment.