Question 1

Which of the following amino acids has a positive charge that helps to hold the DNA in contact with the histones?

Accepted Answer

Arginine is a positively charged amino acid that interacts with the negatively charged phosphate backbone of DNA, helping to stabilize its association with histones.

Question 2

While working on Drosophila, you find a new mutant strain with an abnormal histone H3 gene. This novel histone mutant is predicted to cause the nucleosomes to bind an extra 15 bp of DNA compared to wild type. If you digest isolated chromatin with a nuclease to release the core nucleosomes, what size DNA fragments would you expect from wild-type and mutant flies?

Accepted Answer

In wild-type Drosophila, the core nucleosome consists of DNA wrapped around histone proteins. The core particle is typically associated with about 147 base pairs (bp) of DNA. This is the length of DNA that is protected from nuclease digestion when the chromatin is treated with enzymes such as micrococcal nuclease, which preferentially digests the linker DNA between nucleosomes.

If you have a mutant histone H3 that causes nucleosomes to bind an extra 15 bp of DNA, then the mutant nucleosomes would be expected to protect 147 bp (the normal length) + 15 bp (the additional length due to the mutation) from nuclease digestion. Therefore, the size of the DNA fragments you would expect to see after nuclease digestion of the mutant chromatin would be 162 bp.

To summarize:

- Wild-type nucleosomes: ~147 bp DNA fragments
- Mutant nucleosomes: ~162 bp DNA fragments

These sizes represent the DNA that is tightly bound to the nucleosome core and protected from nuclease digestion. The actual size of the fragments could vary slightly due to the presence of linker DNA, which connects adjacent nucleosomes and can vary in length. However, the core protected DNA would be expected to show the sizes mentioned above.

Question 3

How does histone acetylation affect chromatin?

Accepted Answer

Histone acetylation reduces the positive charge on histones, decreasing their affinity for negatively charged DNA, leading to a more open chromatin structure and increased transcription.

Question 4

The agouti locus helps determine coat color in mice, and this phenotype can vary from light to dark between genetically identical individuals. You have discovered a drug that reduces the variation in the agouti phenotype. What is a likely explanation for this drug's mechanism of action?

Accepted Answer

The agouti phenotype variation is often due to epigenetic changes, such as DNA methylation. Inhibiting DNA methyl transferases would reduce methylation, leading to less variation in phenotype.

Question 5

You are studying a small eukaryotic gene of about 2000 bp in length. Estimate how many copies of histone H1 you would find along this region of the chromosome.

Accepted Answer

Histone H1 binds to the linker DNA between nucleosomes. Each nucleosome typically wraps about 147 bp of DNA, and the linker DNA is about 20-80 bp. For 2000 bp, approximately 10 nucleosomes would be present, so about 10 copies of histone H1 would be expected.

Question 6

How many base pairs per turn of the helix would MOST likely correspond to a positively supercoiled DNA molecule?

Accepted Answer

The answer of How many base pairs per turn of...

Question 7

If a bacterial chromosome were inserted into a eukaryotic cell, would it be stable? Would it segregate like eukaryotic chromosomes do during mitosis and meiosis? Why or why not?

Accepted Answer

The answer of If a bacterial chromosome were inserted into...

Question 8

How many complete rotations would MOST likely correspond to a positively supercoiled DNA molecule that is 100 bp in length?

Accepted Answer

A positively supercoiled DNA molecule has more twists than the normal relaxed state. Typically, a relaxed DNA has about 10.5 base pairs per turn, so 100 bp would have about 9-10 turns. A positively supercoiled DNA would have more turns, making 15 a plausible choice.

Question 9

Which of the following descriptions is NOT true of heterochromatin?

Accepted Answer

Heterochromatin is tightly packed and transcriptionally inactive, unlike euchromatin, which makes up most chromosomal material and is where most transcription occurs.

Question 10

When chromatin from any eukaryote is digested with micrococcal nuclease (an endonuclease) and fractionated using electrophoresis, DNA fragments of approximately 200 base pairs in length are observed. Explain this result.

Accepted Answer

The answer of When chromatin from any eukaryote is digested...

Question 11

Describe the structure and packing of a bacterial chromosome.

Accepted Answer

The answer of Describe the structure and packing of a...

Question 12

Which of the following is an example of an epigenetic change in eukaryotes?

Accepted Answer

Epigenetic changes involve modifications that affect gene expression without altering the DNA sequence itself. The addition of methyl groups to cytosines, particularly in the promoter region of a gene, is a common epigenetic modification that can regulate gene expression.

Question 13

The human Y chromosome is about 50 million base pairs long. About how many nucleosomes would you expect to find associated with this chromosome?

Accepted Answer

Each nucleosome is associated with about 200 base pairs of DNA. Therefore, for 50 million base pairs, you would expect 50,000,000 / 200 = 250,000 nucleosomes.

Question 14

How many base pairs per turn of the helix would MOST likely correspond to a negatively supercoiled DNA molecule?

Accepted Answer

The answer of How many base pairs per turn of...

Question 15

Which statement is NOT true of negatively supercoiled DNA?

Accepted Answer

Negatively supercoiled DNA has more than 10 bp per turn of its helix.

Question 16

How many complete rotations would MOST likely correspond to a negatively supercoiled DNA molecule that is 100 bp in length?

Accepted Answer

The answer of How many complete rotations would MOST likely...

Question 17

Which of the following statements is NOT true of bacterial DNA?

Accepted Answer

Bacterial DNA is associated with proteins that help in its compaction, contrary to the statement that it is not attached to any proteins.

Question 18

How many base pairs per turn of the helix would MOST likely correspond to a relaxed DNA molecule?

Accepted Answer

A relaxed DNA molecule typically has about 10 base pairs per turn of the helix, which is the natural state of B-DNA, the most common form of DNA in cells.

Question 19

You are studying a small eukaryotic gene of about 2000 bp in length. Estimate how many copies of histone H4 you would find along this region of the chromosome.

Accepted Answer

The answer of You are studying a small eukaryotic gene...

Question 20

How many complete rotations would MOST likely correspond to a relaxed DNA molecule that is 100 bp in length?

Accepted Answer

A relaxed DNA molecule typically has about 10 base pairs per helical turn, so a 100 bp DNA would have approximately 10 complete rotations.

Quiz 11: Chromosome Structure and Organelle Dna