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Chapter 2: Problem 27

During meiosis I, assuming no crossing over, what chromatid combination(s) will be present at the completion of prophase? Draw all possible alignments of chromatids as migration begins during early anaphase.

Short Answer

Expert verified
Answer: Assuming two homologous pairs and no crossing over, there are four possible chromatid combinations at the end of Prophase I: 1. [M1, M2], [P1, P2] 2. [M1, P2], [P1, M2] 3. [P1, M2], [M1, P2] 4. [P1, P2], [M1, M2] During early Anaphase I, these combinations will start migrating towards opposite poles of the cell, maintaining their respective alignments.

Step by step solution

01

Understand Meiosis I and the events of Prophase

During meiosis I, a diploid cell divides into two haploid cells. This process involves two main stages: Prophase I and Anaphase I. In Prophase I, chromosomes replicate and condense, and homologous pairs (a set of one maternal and one paternal chromosome) associate with each other. Since there is no crossing over in this case, the chromatids remain identical. In Anaphase I, homologous pairs separate and migrate towards opposite poles of the cell.
02

Determine the number of possible chromatid combinations at the end of Prophase I

As no crossing over occurs, each homologous pair remains distinct from each other. Considering that there are 2^n possible orientations for homologous pairs at the end of prophase I, where n is the number of homologous pairs, we can calculate the number of possible chromatid combinations. However, since the exact number of chromosomes is not given in this exercise, we will assume a hypothetical situation with two homologous pairs. Therefore, we can compute the possible chromatid combinations as: 2^2 = 4.
03

Draw the possible chromatid combinations at the end of Prophase I

Assuming two homologous pairs, we'll have four possible combinations at the end of Prophase I. Denoting maternal chromosomes as M1 and M2 and paternal chromosomes as P1 and P2, the combinations will be: 1. [M1, M2], [P1, P2] 2. [M1, P2], [P1, M2] 3. [P1, M2], [M1, P2] 4. [P1, P2], [M1, M2]
04

Understand the events during early Anaphase I

In early Anaphase I, the homologous pairs separate and migrate towards opposite poles of the cell. The chromatids do not separate during Anaphase I, which happens in Anaphase II.
05

Draw all possible alignments of chromatids as migration begins during early Anaphase I

Based on the possible chromatid combinations in Step 3, the possible alignments as migration begins during early Anaphase I will be as follows: 1. [M1, M2] [P1, P2] | | v v 2. [M1, P2] [P1, M2] | | v v 3. [P1, M2] [M1, P2] | | v v 4. [P1, P2] [M1, M2] | | v v In each of these four alignments, the chromatids start migrating towards opposite poles of the cell during early Anaphase I.

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Key Concepts

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

Prophase I
Prophase I is the first stage of meiosis I and is particularly crucial because it sets the stage for genetic diversity. During this phase, several critical events occur:
  • Chromosomes condense, becoming visible under a microscope.
  • Each chromosome pairs with its homologous partner, meaning each chromosome aligns perfectly with its matching chromosome from the other parent, forming what's known as a "homologous pair".
  • Without crossing over, the chromatids within these homologous pairs remain identical.
By the end of Prophase I, the cell has laid out its chromosomes in pairs, and each homologous pair consists of four chromatids. This setup is sometimes referred to as a "tetrad" owing to the four chromatids being grouped together.
Anaphase I
During Anaphase I, homologous chromosomes鈥攅ach composed of two sister chromatids鈥攕tart migrating towards opposite poles of the cell. This stage marks the beginning of the division process that will eventually reduce chromosome number by half. Key characteristics during this phase include:
  • Movement: Homologous chromosomes are pulled apart, not the sister chromatids. Each pair moves as a whole unit to different poles.
  • Cell Configuration: The cell starts to elongate as it prepares to divide.
  • Reduction: Critical for ensuring each new cell gets a unique set of chromosomes.
This separation is essential as it ensures that when the first division completes, each resulting cell has half the number of chromosomes, making them haploid. However, these chromosomes still have two chromatids connected by a centromere.
homologous pairs
A homologous pair consists of two chromosomes of the same type, one inherited from each parent. These pairs are crucial during the prophase stage of meiosis I. Here's what to remember about homologous pairs:
  • Composition: Each pair comprises one maternal and one paternal chromosome.
  • Function: They line up along the center of the cell during metaphase, setting the stage for their separation in Anaphase I.
  • Significance: They ensure genetic variation across generations by allowing different combinations of maternal and paternal chromosomes to be passed on.
In meiosis, the alignment and subsequent separation of homologous pairs is key to reducing the chromosome number by half and ensuring genetic diversity.
chromatid combinations
Chromatid combinations are essential considerations during meiosis. Assuming no crossing over, the arrangement of chromatids can still lead to diverse genetic outcomes. Here's how they function in meiosis I:
  • Prophase I: Without crossing over, chromatids within each homologous pair remain identical, and there are several potential arrangements.
  • Possible Arrangements: For instance, with two homologous pairs, there are 2^2, or four, possible chromatid combinations at the end of Prophase I.
  • Anaphase I: These combinations impact how chromatids align as homologous pairs start moving apart during Anaphase I, affecting how genetic information is distributed in the resulting cells.
The distinctive organization and movement of chromatids during meiosis ensure the unique combination of genes in the gametes, setting the stage for variation in offspring.

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Most popular questions from this chapter

What is the probability that, in an organism with a haploid number of \(10,\) a sperm will be formed that contains all 10 chromosomes whose centromeres were derived from maternal homologs?

In this chapter, we focused on how chromosomes are distributed during cell division, both in dividing somatic cells (mitosis) and in gamete- and spore- forming cells (meiosis). We found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, answer the following questions. (a) How do we know that chromosomes exist in homologous pairs? (b) How do we know that DNA replication occurs during interphase, not early in mitosis? (c) How do we know that mitotic chromosomes are derived from chromatin?

If two chromosomes of a species are the same length and have similar centromere placements and yet are not homologous, what is different about them?

Given the end results of the two types of division, why is it necessary for homologs to pair during meiosis and not desirable for them to pair during mitosis?

21. Such individuals therefore ha鈥� # Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome #21. Such individuals therefore have 47 chromosomes. Assume that a mating occurs between a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome? Justify your answer.
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