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Chapter 5: Problem 11

Which of the following statements are correct? Explain your answers. A. Each eukaryotic chromosome must contain the following DNA sequence elements: multiple origins of replication, two telomeres, and one centromere. B. Nucleosome core particles are \(30 \mathrm{nm}\) in diameter.

Short Answer

Expert verified
Statement A is correct; statement B is incorrect.

Step by step solution

01

Analyze Statement A

A eukaryotic chromosome indeed contains multiple origins of replication, which are necessary for the initiation of DNA replication at various points along the chromosome. Eukaryotic chromosomes also have two telomeres, located at each end of the chromosome, which protect the ends from degradation and prevent them from being recognized as broken DNA. In addition, each chromosome has one centromere, which is crucial during cell division as it serves as the attachment point for spindle fibers. Thus, statement A is correct.
02

Examine Statement B

Nucleosome core particles, which consist of DNA wrapped around a histone octamer, are approximately 11 nm in diameter rather than 30 nm. The 30 nm measurement is typically associated with a higher-order chromatin structure, often referred to as the 30 nm fiber, but not with individual nucleosome core particles. Therefore, statement B is incorrect.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Origins of Replication
Eukaryotic chromosomes are large and complex, which necessitates efficient and multiple points of DNA replication to ensure all genetic material is accurately copied during the S phase of the cell cycle. These points are known as "origins of replication." Each eukaryotic chromosome contains multiple origins of replication to allow replication to proceed in parallel, which helps shorten the overall time required for the entire chromosome replication.
  • Parallel Processing: Multiple origins mean DNA synthesis can proceed simultaneously at various locations along the chromosome.
  • Replication Timing: Not all origins are activated simultaneously; some start earlier or later than others, depending on cell type and environmental conditions.
  • Replication Machinery: Specialized proteins recognize origin sequences to form a complex that unwinds the DNA and begins the replication process.
The strategic location and activation of these origins are vital for the fidelity and efficiency of DNA replication, ensuring that each daughter cell receives a complete set of genetic information.
Telomeres
Telomeres are repetitive DNA sequences located at the ends of eukaryotic chromosomes. They play a crucial role in maintaining chromosome integrity and stability. Without telomeres, chromosomes could fray like shoelaces or be mistaken for broken DNA, leading to genetic instability.
  • Protection Role: Telomeres act as caps that protect the ends of chromosomes from deterioration or from fusion with neighboring chromosomes.
  • Cellular Aging: During cell division, telomeres gradually shorten, which is associated with cellular aging and eventual cell death.
  • Telomerase Function: Some cells produce an enzyme called telomerase, which extends telomeres, helping to maintain length and allowing for additional cell divisions.
Telomeres are thus essential for the health and longevity of cells, and they provide insight into the mechanisms of aging and cancer.
Centromere
The centromere is a specific region of the chromosome that plays a vital role during cell division, specifically during mitosis and meiosis. It serves as the site of attachment for spindle fibers, which are crucial for the correct segregation of chromatids into daughter cells.
  • Structure: Typically located near the middle of the chromosome (though not always), centromeres can vary greatly in size and complexity.
  • Function in Division: The centromere is the anchor for the kinetochore, a structure that allows spindle fibers to attach and pull chromatids apart during cell division.
  • Centromere Position: Its position and structure are essential for the identity and stability of chromosomes.
The centromere ensures that each daughter cell will inherit the correct number of chromosomes, making its function pivotal for genetic stability.
Nucleosome Core Particles
Nucleosome core particles are the basic units of DNA packaging in eukaryotic cells. Each core particle consists of DNA wrapped around a histone octamer, forming a "bead-on-a-string" structure essential for DNA compaction.
  • Structure: Each nucleosome core particle is about 11 nm in diameter, containing DNA wrapped 1.65 times around a histone octamer.
  • Histone Composition: The histone octamer is made up of pairs of histone proteins: H2A, H2B, H3, and H4.
  • Compaction Level: This structure not only helps in compacting the DNA but also plays a role in regulating gene expression, as tightly packed DNA is less accessible to transcription machinery.
Through this nucleosome structure, DNA is efficiently organized and protected within the nucleus, facilitating both dynamic cellular processes and structural stability.

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Most popular questions from this chapter

Define the following terms and their relationships to one another: A. Interphase chromosome B. Mitotic chromosome C. Chromatin D. Heterochromatin E. Histones F. Nucleosome

A compact disc (CD) stores about \(4.8 \times 10^{9}\) bits of information in a \(96 \mathrm{cm}^{2}\) area. This information is stored as a binary code-that is, every bit is either a 0 or a 1 A. How many bits would it take to specify each nucleotide pair in a DNA sequence? B. How many CDs would it take to store the information contained in the human genome?

A. The nucleotide sequence of one DNA strand of a DNA double helix is \(5^{\prime}\) -GGATTTTTGTCCACAATCA-3'. What is the sequence of the complementary strand? B. In the DNA of certain bacterial cells, \(13 \%\) of the nucleotides contain adenine. What are the percentages of the other nucleotides? C. How many possible nucleotide sequences are there for a stretch of single- stranded DNA that is \(N\) nucleotides long? D. Suppose you had a method of cutting DNA at specific sequences of nucleotides. How many nucleotides long (on average) would such a sequence have to be in order to make just one cut in a bacterial genome of \(3 \times 10^{6}\) nucleotide pairs? How would the answer differ for the genome of an animal cell that contains \(3 \times 10^{9}\) nucleotide pairs?

Which of the following statements are correct? Explain your answers. A. \(A\) DNA strand has a polarity because its two ends contain different bases. B. G-C base pairs are more stable than A-T base pairs.

Histone proteins are among the most highly conserved proteins in eukaryotes. Histone H4 proteins from a pea and a cow, for example, differ in only 2 of 102 amino acids. Comparison of the gene sequences shows many more differences, but only two change the amino acid sequence. These observations indicate that mutations that change amino acids must have been selected against during evolution. Why do you suppose that aminoacid-altering mutations in histone genes are deleterious?
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