Lecture Chapter 16: The molecular basis of inheritance Flashcards
The ratios of A, T, G, and C were roughly ___ across al organisms. This supports the
1:1
helical model of DNA
X ray diffraction revealed the __ of DNA
alpha helical shape
What is the basic principle?
The idea that the two separate strands of DNA can act as a template
A semiconservative replicated strand of DNA contains
one parent strand and one new strand
How did we know that the semiconservative model of DNA replication was correct
Meselson and Stahl
Bacteria cultured in a Nitrogen isotope and transferred to a lighter isotope
the DNA sample was centrifuged after the first replication, then the second
The DNA separated based on density. First replication completely in middle. Second replication one in middle one at top
How might Meselson and Stahl’s results be different if DNA replication followed a conservative model? A dispersive?
conservative: one at bottom and one top after first, same for second
dispersive: one band throughout
How did Taylor, Woods, and Hughes demonstrate semiconservative replication in eukaryotes?
autoradiography causes appearance of black spots where radioactivity is present
chromatids were only half radioactive after second replication
Compare the origin of replication in eukaryotes vs prokaryotes
eukaryotes: multiple origin sites because more complex and long
prokaryotes: only one origin site/replication bubble
RNA primers ____ with the help of the enzyme ___
initiate DNA replication
primase
Primase generates ___ because
short RNA molecules which will later be replaced with DNA
polymerase needs something to attach to, can’t start from scratch
Helicase is involved in
separating the two strands of DNA
Topoisomerase is responsible for
relieves the strain on DNA strands during replication
Single-strand binding proteins allow
the two strands of DNA to remain separate and not form hydrogen bonds after helicase separates them
DNA polymerase requires
a small amount of DNA to be present before it creates the longer strand
Elongation occurs in what direction?
5’ to 3’ direction
DNA is replicated __ per each cycle
once
Okazaki fragments exist because
DNA replciation needs to occur at every section of DNA, even if it is not in the 5’ to 3’ direction
After the first okazaki fragment is done, DNA polymerase 3
detaches and makes a second okazaki fragment. Then DNA pol 1 removes the RNA primers and replaces with DNA
Ligase is responsible for
attaching okazaki fragments and making a continuous strand
The lagging strand bends and loops in order to
ensure the leading strand stays in one direction
Describe the end replication problem
when the final primer is removed at the 3’ end, DNA polymerase should replace it. But DNA can’t, and it creates an overhang. The second round of replication cuts it off, and the DNA gets shorter and shorter until it gets an essential part and the cell dies.
Telomeres are
special nucleotide sequences at the end of chromosomes
Telomerase is responsible for
catalyzing the end of telomeres in some cells to prevent shortening. Not active in most eukaryotic somatic cells
Describe the bacterial chromosome
double stranded
circular
small amount of protein
the DNA is supercoiled and found in the nucleoid
Describe the eukaryotic chromosome
linear DNA
associated with a large amount of protein
Histones are responsible for
packing chromatin (first level)
Histone methylation leads to
tighter packing
Histone acetylation leads to
looser packing
Heterochromatin genes are __
not expressed
euchromatin genes are ___
expressed
Given a polynucleotide sequence such as GAATTC, explain
what further information you would need in order to
identify which is the 5′ end.
n order to tell which end is the 5′ end, you need to know which end has a
phosphate group on the 5′ carbon (the 5′ end) and/or which end has an —OH
group on the 3′ carbon (the 3′ end)
What role does complementary base pairing play in the
replication of DNA?
Complementary base pairing ensures that the two daughter molecules are exact
copies of the parental molecule. When the two strands of the parental molecule
separate, each serves as a template on which nucleotides are arranged by the base-
pairing rules; they will then be polymerized by enzymes into new complementary
strands.
Identify two major functions of DNA pol III in DNA
replication
DNA pol III covalently adds nucleotides to new DNA strands and
proofreads each added nucleotide for correct base pairing.
What is the relationship between DNA
replication and the S phase of the cell cycle?
In the cell cycle,
DNA synthesis occurs during the S phase, between the G1 and G2 phases of inter-
phase. DNA replication is therefore complete before the mitotic phase begins.
Describe the structure of a nucleosome, the basic unit of
DNA packing in eukaryotic cells.
A nucleosome is made up of eight histone proteins, two each of four different
types, around which DNA is wound. Linker DNA runs from one nucleosome to
the next.
How does euchromatin differ from heterochromatin in
structure and function?
The 10-nm fiber of euchromatin is less compacted during inter-
phase than in mitosis and is accessible to the cellular proteins responsible for
gene expression. In contrast, the 10-nm fiber of heterochromatin is relatively
compacted (densely arranged) during interphase, and genes in heterochroma-
tin are largely inaccessible to proteins necessary for gene expression
Interphase chromosomes appear
to be attached to the nuclear lamina and perhaps also the
nuclear matrix. Describe these two structures.
The
nuclear lamina is a netlike array of protein filaments that provides mechanical
support just inside the nuclear envelope and thus maintains the shape of the
nucleus. Considerable evidence also supports the existence of a nuclear matrix,
a framework of protein fibers extending throughout the nuclear interior.
In his work with pneumonia-causing bacteria and mice,
Griffith found that
(A) the protein coat from pathogenic cells was able to trans-
form nonpathogenic cells.
(B) heat-killed pathogenic cells caused pneumonia.
(C) some substance from pathogenic cells was transferred to
nonpathogenic cells, making them pathogenic.
(D) the polysaccharide coat of bacteria caused pneumonia
C
What is the basis for the difference in how the leading and
lagging strands of DNA molecules are synthesized?
(A) The origins of replication occur only at the 5′ end.
(B) Helicases and single-strand binding proteins work at the
5′ end.
(C) DNA polymerase can join new nucleotides only to the 3′
end of a pre-existing strand, and the strands are antiparallel.
(D) DNA ligase works only in the 3′ –> 5′direction.
C
In analyzing the number of different bases in a DNA sample,
which result would be consistent with the base-pairing rules?
(A) A = G
(B) A + G = C +T
(C) A +T = G + C
(D) A = C
B
The elongation of the leading strand during DNA synthesis
(A) progresses away from the replication fork.
(B) occurs in the 3′ S 5′ direction.
(C) produces Okazaki fragments.
(D) depends on the action of DNA polymerase
D
In a nucleosome, the DNA is wrapped around
(A) histones.
(B) ribosomes.
(C) polymerase molecules.
(D) a thymine dimer.
A
E. coli cells grown on 15 N medium are transferred to 14 N
medium and allowed to grow for two more generations (two
rounds of DNA replication). DNA extracted from these cells
is centrifuged. What density distribution of DNA would you
expect in this experiment?
(A) one high-density and one low-density band
(B) one intermediate-density band
(C) one high-density and one intermediate-density band
(D) one low-density and one intermediate-density band
D
A student isolates, purifies, and combines in a test tube a variety
of molecules needed for DNA replication. When she adds some
DNA to the mixture, replication occurs, but each DNA molecule
consists of a normal strand paired with numerous segments of
DNA a few hundred nucleotides long. What has she probably
left out of the mixture?
(A) DNA polymerase
(B) DNA ligase
(C) Okazaki fragments
(D) primase
B
The spontaneous loss of amino groups from adenine in DNA
results in hypoxanthine, an uncommon base, opposite thymine.
What combination of proteins could repair such damage?
(A) nuclease, DNA polymerase, DNA ligase
(B) telomerase, primase, DNA polymerase
(C) telomerase, helicase, single-strand binding protein
(D) DNA ligase, replication fork proteins, adenylyl cyclase
A
Although the proteins that cause the
E. coli chromosome to coil are not histones, what property
would you expect them to share with histones, given their
ability to bind to DNA
Like histones, the E. coli proteins
would be expected to contain many basic (positively charged) amino acids, such as
lysine and arginine, which can form weak bonds with the negatively charged phos-
phate groups on the sugar-phosphate backbone of the DNA molecule.
