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Chapter 2: Problem 30

Human cells normally have 46 chromosomes. For each of the following stages, state the number of nuclear DNA molecules present in a human cell: a. Metaphase of mitosis b. Metaphase I of meiosis c. Telophase of mitosis d. Telophase I of meiosis e. Telophase II of meiosis

Short Answer

Expert verified
a. 92 DNA molecules, b. 92, c. 46, d. 46 (per original cell), e. 23.

Step by step solution

01

Understand Chromosome and DNA Molecule Terminology

In a human cell, each chromosome consists of one DNA molecule. During certain stages of cell division, these DNA molecules replicate, resulting in two sister chromatids per chromosome. Each chromatid, though part of the same chromosome, is considered a separate DNA molecule.
02

Metaphase of Mitosis

During metaphase of mitosis, each chromosome is aligned at the equatorial plane of the cell. At this point, DNA replication has already occurred, so each chromosome consists of two joined sister chromatids. Therefore, there are 46 chromosomes and 92 DNA molecules.
03

Metaphase I of Meiosis

In metaphase I of meiosis, homologous chromosomes line up in pairs at the metaphase plate. DNA has replicated during interphase, so each chromosome consists of two sister chromatids. Since there are 23 pairs of homologous chromosomes, this results in 46 chromosomes and 92 DNA molecules.
04

Telophase of Mitosis

During telophase of mitosis, the sister chromatids have been pulled apart to opposite ends of the cell. This results in two nuclei each with 46 chromosomes (since each chromatid is now considered an individual chromosome). Therefore, each nucleus has 46 DNA molecules.
05

Telophase I of Meiosis

In telophase I of meiosis, homologous chromosomes are separated into two different nuclei; each nucleus now has 23 chromosomes. Since DNA replication occurred prior to meiosis I, each of these chromosomes consists of two chromatids, making 46 DNA molecules per initial cell (23 per new daughter cell).
06

Telophase II of Meiosis

In telophase II, sister chromatids are separated into individual chromosomes in four daughter cells. Each of these cells has 23 chromosomes, one chromatid per chromosome. Thus, 23 DNA molecules are present in each germ cell.

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Key Concepts

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

Mitosis
Mitosis is a fundamental process used by cells to divide and replicate. This process is essential for growth, development, and tissue repair in multicellular organisms. It ensures that each new daughter cell receives a complete set of identical chromosomes.
Mitosis consists of several stages: prophase, metaphase, anaphase, and telophase. During these phases, the cell undergoes a series of changes leading to the segregation of genetic material:
  • Prophase: The chromosomes become visible as paired chromatids, and the nuclear membrane begins to disintegrate.
  • Metaphase: Chromosomes line up across the center of the cell, known as the metaphase plate.
  • Anaphase: The sister chromatids separate, moving toward opposite poles of the cell.
  • Telophase: The chromatids reach the poles, nuclear membranes re-form around each set of chromosomes, and cytokinesis usually completes the division into two daughter cells.
Mitosis is crucial for maintaining the same chromosome number from one generation of cells to the next, ensuring that the genetic information is passed down seamlessly.
Chromosome Replication
Chromosome replication is a critical step before both mitosis and meiosis. During the cell cycle, chromosomes are replicated during the S phase.
Each chromosome, originally a single DNA molecule, is duplicated to form two identical sister chromatids.
This replication ensures that, after cell division, each daughter cell possesses a complete set of chromosomes.
  • The replication process is highly accurate, employing various enzymes and proteins to ensure the DNA is copied precisely.
  • Replication Fork: During replication, the DNA double helix unwinds and separates, forming what is known as a replication fork, where new strands are synthesized.
  • Semiconservative Replication: Each new double-stranded DNA consists of one original strand and one newly synthesized strand, preserving genetic information.
Understanding how chromosomes replicate is essential to grasp the mechanics of cell division, whether through mitosis or meiosis.
Sister Chromatids
Sister chromatids are the result of DNA replication and are crucial for accurate genetic distribution during cell division.
Each chromatid in a pair is genetically identical, as they are duplicates of a single original chromosome.
The primary function of these chromatids is to ensure that each daughter cell receives an exact copy of the parent cell's DNA.
  • Centromere: The sister chromatids are held together at a region called the centromere, which plays a vital role during chromosome segregation.
  • During metaphase, sister chromatids align at the cell's equator. Spindle fibers attach to the centromeres, readying them for separation.
  • In anaphase, the chromatids are pulled apart, becoming independent chromosomes, which migrate to opposite poles of the cell.
Understanding the concept of sister chromatids helps in appreciating the precision involved in cell division, reducing errors such as nondisjunction, where chromosomes fail to separate properly.
DNA Separation Stages
DNA separation stages are critical events in both mitosis and meiosis. These stages ensure genetic information is evenly distributed to daughter cells.
Let's take a closer look at the key stages involved in this process:
  • Anaphase of Mitosis: Sister chromatids separate and move to opposite poles, ensuring each daughter cell inherits an identical set of chromosomes.
  • Anaphase I of Meiosis: Homologous chromosomes, each consisting of two chromatids, separate. This division reduces the chromosome number by half, making it essential for sexual reproduction.
  • Anaphase II of Meiosis: Similar to mitosis, the sister chromatids finally separate, ensuring each gamete contains a single set of chromosomes.
Each separation phase is critical for genetic variability and correctness, reducing the chance of genetic disorders due to incorrect chromosome numbers. By understanding these DNA separation stages, one gains insight into how life perpetuates through successive generations.

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