Ch. 3 - Cell Division and Chromosome Heredity

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook

Sanders 3rd Edition

Ch. 3 - Cell Division and Chromosome Heredity

Problem 5b

Chapter 3, Problem 5b

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.
Explain why a somatic cell of Geneticus introductus has the same number of chromosomes and the same amount of DNA at the beginning of mitotic prophase as one of these cells does at the beginning of prophase I of meiosis.

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Understand the diploid number (2n = 36) and the DNA content (3 ng in G₁). This means that a somatic cell of Geneticus introductus has 36 chromosomes and 3 ng of DNA in its diploid state during the G₁ phase of the cell cycle.

Recall that during the S phase of the cell cycle, DNA replication occurs. This doubles the amount of DNA in the cell, but the number of chromosomes remains the same because the replicated DNA is still attached to the original chromosome at the centromere. Thus, after S phase, the cell has 36 chromosomes and 6 ng of DNA.

At the beginning of mitotic prophase, the cell has completed DNA replication and is preparing for mitosis. The chromosomes are condensed, and each chromosome consists of two sister chromatids. The cell still has 36 chromosomes and 6 ng of DNA.

In meiosis, the process begins with a diploid cell that has also undergone DNA replication during the S phase. At the beginning of prophase I of meiosis, the cell has the same number of chromosomes (36) and the same amount of DNA (6 ng) as it does at the beginning of mitotic prophase.

The reason for this similarity is that both mitosis and meiosis start with a diploid cell that has undergone DNA replication. The difference between the two processes arises later, during the division stages, where mitosis produces two identical diploid cells, while meiosis produces four haploid cells.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Diploid Number

The diploid number (2n) refers to the total number of chromosomes in a somatic cell, which in this case is 36 for Geneticus introductus. This means that each somatic cell contains two sets of chromosomes, one inherited from each parent. Understanding the diploid number is crucial for comprehending how genetic information is organized and passed on during cell division.

Mitosis vs. Meiosis

Mitosis is the process of cell division that results in two identical daughter cells, each with the same number of chromosomes as the parent cell. In contrast, meiosis is a specialized form of cell division that reduces the chromosome number by half, producing gametes. At the beginning of mitotic prophase, the somatic cell has the same chromosome number and DNA content as it does at the start of prophase I of meiosis, as both processes begin with diploid cells.

DNA Content in Cell Cycle

The amount of DNA in a cell varies throughout the cell cycle. In the G₁ phase, a diploid cell contains a specific amount of DNA (3 ng in this case), which is duplicated during the S phase, leading to double the DNA content before mitosis or meiosis. At the beginning of mitotic prophase and prophase I of meiosis, the DNA content is the same because both processes start with cells that have completed DNA replication, ensuring that the genetic material is accurately distributed.

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Textbook Question

In a test of his chromosome theory of heredity, Morgan crossed a female Drosophila with red eyes to a male with white eyes. The females were produced from Cross A, shown in the Figure below. Predict the offspring Morgan would have expected under his hypothesis that the gene for eye color is on the X chromosome in fruit flies.

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Textbook Question

Cohesion between sister chromatids, as well as tension created by the pull of kinetochore microtubules, is essential to ensure efficient separation of chromatids at mitotic anaphase or in meiotic anaphase II. Explain why sister chromatid cohesion is important, and discuss the role of the proteins cohesin and separase in sister chromatid separation.

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Textbook Question

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.

What amount of DNA is contained in each nucleus at the end of the S phase?

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Textbook Question

The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains 3 ng of DNA in G₁.

Complete the following table by entering the number of chromosomes and amount of DNA present per cell at the end of each stage listed.

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Textbook Question

An organism has alleles R₁ and R₂ on one pair of homologous chromosomes, and it has alleles T₁ and T₂ on another pair. Diagram these pairs of homologs at the end of metaphase I, the end of telophase I, and the end of telophase II, and show how meiosis in this organism produces gametes in expected Mendelian proportions. Assume no crossover between homologous chromosomes.

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Textbook Question

Explain how the behavior of homologous chromosomes in meiosis parallels Mendel's law of segregation for autosomal alleles D and d. During which stage of M phase do these two alleles segregate from one another?

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