Answers to Concept Checks
- 1. b
- 2. Bacterial DNA is not complexed to histone proteins and is circular.
- 3. b
- 4. d
- 5. A chromosome that loses its centromere will not segregate into the nucleus in mitosis and is usually lost.
- 6. d
- 7. a
- 8. Many modern protists are hosts to endosymbiotic bacteria. Mitochondria and chloroplasts are similar in size to eubacteria and have their own DNA, as well as ribosomes that are similar in size and shape to eubacterial ribosomes. Antibiotics that inhibit protein synthesis in eubacteria also inhibit protein synthesis in mitochondria and chloroplasts. Gene sequences in mtDNA and cpDNA are most similar to eubacterial DNA sequences.
- 9. c
- 10. a
WORKED PROBLEMS
Problem 1
A diploid plant cell contains 2 billion base pairs of DNA.
- a. How many nucleosomes are present in the cell?
- b. Give the number of molecules of each type of histone protein associated with the genomic DNA.
Solution Strategy
What information is required in your answer to the problem?
The number of nucleosomes per cell and the numbers of each type of histone protein associated with the DNA.
What information is provided to solve the problem?
The cell contains 2 billion base pairs of DNA.
For help with this problem, review:
The Nucleosome in Section 11.1.
Solution Steps
Each nucleosome encompasses about 200 bp of DNA: from 145 to 147 bp of DNA wrapped around the histone core, from 20 to 22 bp of DNA associated with the H1 protein, and another 30 to 40 bp of linker DNA.
Recall: The repeating unit of the chromosome is a nucleosome, which consists of DNA complexed to histone proteins.
- a. To determine how many nucleosomes are present in the cell, we simply divide the total number of base pairs of DNA (2 × 109 bp) by the number of base pairs per nucleosome:
Thus, there are approximately 10 million nucleosomes in the cell.
- b. Each nucleosome includes two molecules each of H2A, H2B, H3, and H4 histones. Therefore, there are 2 × 107 molecules each of H2A, H2B, H3, and H4 histones. Each nucleosome has associated with it one copy of the H1 histone; so there are 1 × 107 molecules of H1.
Problem 2
Suppose that a new organelle is discovered in an obscure group of protists. This organelle contains a small DNA genome, and some scientists are arguing that, like chloroplasts and mitochondria, this organelle originated as a free-living eubacterium that entered into an endosymbiotic relation with the protist. Outline a research plan to determine if the new organelle evolved from a free-living eubacterium. What kinds of data would you collect and what predictions would you make if the theory were correct?
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Solution Strategy
What information is required in your answer to the problem?
A research plan with the types of data you would collect and predictions.
What information is provided to solve the problem?
A new organelle is discovered.-
The organelle contains a small DNA genome.
-
The organelle might have evolved from an endosymbiotic relationship.
For help with this problem, review:
Sections on Endosymbiotic Theory, The Mitochondrial Genome, and The Chloroplast Genome in Section 11.4.
Solution Steps
We should examine the structure, organization, and sequences of the organelle genome. If the organelle shows only characteristics of eukaryotic DNA, then it most likely has a eukaryotic origin but, if it displays some characteristics of eubacterial DNA, then this supports the theory of a eubacterial origin.
Recall: The endosymbiotic theory proposes that organelles evolved from eubacteria.
We could start by examining the overall characteristics of the organelle DNA. If it has a eubacterial origin, we might expect that the organelle genome will consist of a circular molecule and will lack histone proteins. We could compare the DNA sequences found in the organelle genome with homologous sequences from eubacteria and eukaryotic genomes. If the theory of an endosymbiotic origin is correct, then the organelle sequences should be most similar to homologous sequences found in eubacteria.