Question 1

Methylation and acetylation are common changes made to histone H3, and the specific combination of these changes is sometimes referred to as the "histone code." Which of the following patterns will probably lead to gene silencing?

Accepted Answer

Lysine 9 methylation is commonly associated with gene silencing. The other options listed are generally associated with gene activation or no effect on gene expression.

Question 2

Which of the following best describes the mechanism by which chromatin-remodeling complexes "loosen" the DNA wrapped around the core histones?

Accepted Answer

Chromatin-remodeling complexes use energy derived from ATP hydrolysis to change the relative position of the DNA and the core histone octamer, thus loosening the DNA's grip on the histone proteins. This allows access to DNA-binding proteins and gene transcription factors.

Question 3

Hershey and Chase used radiolabeled macromolecules to identify the material that contains heritable information.What radioactive material was used to track DNA during this experiment?

Accepted Answer

Hershey and Chase used radioactive phosphorus (P-32) to label DNA because DNA contains phosphorus in its phosphate backbone, but no sulfur, which is found in proteins. This allowed them to distinguish DNA from proteins and demonstrate that DNA is the genetic material.

Question 4

Most eukaryotic cells only express 20-30% of the genes they possess.The formation of heterochromatin maintains the other genes in a transcriptionally silent (unexpressed) state.Which histone modification is associated with the formation of the most common type of heterochromatin?

Accepted Answer

H3 lysine 9 methylation is associated with the formation of the most common type of heterochromatin. This modification is catalyzed by enzymes known as histone methyltransferases, which add methyl groups to the lysine 9 residue of histone H3. This modification is also recognized by other proteins, which recruit additional factors to further condense the chromatin and maintain transcriptional repression.

Question 5

Many of the breakthroughs in modern biology came after Watson and Crick published their model of DNA in 1953.However, chromosomes were identified earlier.In what decade did scientists first identify chromosomes?

Accepted Answer

Scientists first identified chromosomes in the 1880s through the use of microscopy. Friedrich Miescher had discovered DNA in 1869, but the connection between DNA and chromosomes was not fully understood until later.

Question 6

Which of the following is NOT a chemical group commonly found on core histone N-terminal tails for chromatin regulation?

Accepted Answer

Sulfhydryl groups are not commonly found on core histone N-terminal tails for chromatin regulation. The four commonly found chemical groups are methyl, phosphoryl, acetyl, and ubiquitin.

Question 7

The N-terminal tail of histone H3 can be extensively modified, and depending on the number, location, and combination of these modifications, these changes may promote the formation of heterochromatin.What is the result of heterochromatin formation?

Accepted Answer

Heterochromatin formation results in gene silencing.

Question 8

How do changes in histone modifications lead to changes in chromatin structure?

Accepted Answer

They help recruit other proteins to the chromatin.

Question 9

Stepwise condensation of linear DNA happens in five different packing processes.Which of the following four processes has a direct requirement for histone H1?

Accepted Answer

The formation of the 30-nm fiber directly requires histone H1. The other three processes (formation of "beads-on-a-string", looping of the 30-nm fiber, and packing of loops to form interphase chromosomes) do not have a direct requirement for histone H1.

Question 10

The core histones are small, basic proteins that have a globular domain at the C-terminus and a long, extended conformation at the N-terminus.Which of the following is NOT true of the N-terminal "tail" of these histones?

Accepted Answer

The N-terminal tail of histones is not specific to binding DNA in a sequence-specific manner. Rather, it interacts with DNA non-specifically and helps to pack the DNA tightly into the nucleosome core. The tail is subject to various post-translational modifications, such as acetylation, methylation, and phosphorylation, which can affect gene expression and other DNA-related processes. The N-terminal tail also extends out of the nucleosome core, allowing for interactions with other proteins involved in chromatin organization and gene regulation.

Question 11

In the 1940s, proteins were thought to be the more likely molecules to house genetic information.What was the primary reason that DNA was not originally believed to be the genetic material?

Accepted Answer

DNA was considered too simple due to its composition of only four types of nucleotides, which seemed insufficient to encode the complex information required for life's diversity, unlike proteins which have 20 different amino acids.

Question 12

Fred Griffith studied two strains of Streptococcus pneumonia, one that causes a lethal infection when injected into mice, and a second that is harmless.He observed that pathogenic bacteria that have been killed by heating can no longer cause an infection.But when these heat-killed bacteria are mixed with live, harmless bacteria, this mixture is capable of infecting and killing a mouse.What did Griffith conclude from this experiment?

Accepted Answer

Griffith concluded that the heat-killed pathogenic bacteria "transformed" the harmless strain into a lethal one. This transformation suggested that genetic material from the dead pathogenic strain had been taken up by the live harmless strain, changing its phenotype and making it pathogenic. This experiment provided evidence for the concept of bacterial transformation and the idea that genetic material can be transferred between cells.

Question 13

Although the chromatin structure of interphase and mitotic chromosomes is very compact, DNA-binding proteins and protein complexes must be able to gain access to the DNA molecule.Chromatin-remodeling complexes provide this access by

Accepted Answer

Chromatin-remodeling complexes use the energy of ATP hydrolysis to move nucleosomes, allowing DNA-binding proteins and protein complexes to access the DNA molecule. Choice A is incorrect because chromatin-remodeling complexes do not recruit other enzymes. Choice B is incorrect because chromatin-remodeling complexes do not modify histone tails, although histone acetyltransferases do modify histone tails. Choice D is incorrect because chromatin-remodeling complexes do not denature the DNA molecule.

Question 14

The inactivation of one X chromosome is established by the directed spreading of heterochromatin.The silent state of this chromosome is __________ in the subsequent cell divisions.

Accepted Answer

The statement mentioned that the inactivation of one X chromosome is established, which means it has already completed. Therefore, the correct option is (D) maintained, as the silent state of the chromosome is maintained in the subsequent cell divisions.

Question 15

Using terms from the list below, fill in the blanks in the following brief description of the experiment with Streptococcus pneumoniae that identified which biological molecule carries heritable genetic information.Some terms may be used more than once.
$\begin{array}{lll}
\text { carbohydrate } & \text { lipid } & \text { R-strain } \
\text { DNA } & \text { nonpathogenic } & \text { RNA } \
\text { identify } & \text { pathogenic } & \text { S-strain } \
\text { label } & \text { purify } & \text { transform }
\end{array}$

Cell-free extracts from S-strain cells of S.pneumoniae were fractionated to __________ DNA, RNA, protein, and other cell components.Each fraction was then mixed with __________ cells of S.pneumoniae.Its ability to change these into cells with __________ properties resembling the __________ cells was tested by injecting the mixture into mice.Only the fraction containing __________ was able to __________ the __________ cells to __________ (or __________) cells that could kill mice.

Accepted Answer

The answer of Using terms from the list below, fill...

Question 16

You are a virologist interested in studying the evolution of viral genomes.You are studying two newly isolated viral strains and have sequenced their genomes.You find that the genome of strain 1 contains 25% A, 55% G, 20% C, and 10% T.You report that you have isolated a virus with a single-stranded DNA genome.Based on what evidence can you make this conclusion?

Accepted Answer

Double-stranded genomes have equal amounts of A and T (and G and C) due to base pairing rules, where A pairs with T and G pairs with C. The unequal distribution of nucleotides in the viral genome suggests it is single-stranded, as this rule does not apply.

Question 17

Match between columns Premises: The polarity of a DNA strand results from the polarity of the nucleotide subunits. Hydrogen bonds between each nucleotide hold individual DNA strands together.
There are five different nucleotides that become incorporated into a DNA strand.
DNA molecules, like proteins, consist of a single, long polymeric chain that is assembled from small monomeric subunits.

Accepted Answer

The Answer of False and True is...

Question 18

Several experiments were required to demonstrate how traits are inherited.Which scientist or team of scientists first demonstrated that cells contain some component that can be transferred to a new population of cells and permanently cause changes in the new cells?

Accepted Answer

Griffith's experiment in 1928 demonstrated the phenomenon of transformation, showing that a component from dead bacteria could permanently change the traits of living bacteria.

Question 19

Several experiments were required to demonstrate how traits are inherited.Which scientist or team of scientists obtained definitive results demonstrating that DNA is the genetic molecule?

Accepted Answer

Hershey and Chase used radioactive labeling of DNA and proteins to show that only the DNA entered bacterial cells and was responsible for genetic inheritance. This experiment definitively demonstrated that DNA, not proteins, is the genetic molecule.

Question 20

The octameric histone core is composed of four different histone proteins, assembled in a stepwise manner.Once the core octamer has been formed, DNA wraps around it to form a nucleosome core particle.Which of the following histone proteins does NOT form part of the octameric core?

Accepted Answer

The answer of The octameric histone core is composed of...

Quiz 5: DNA and Chromosomes