Question

During crossing over in meiosis I, a. homologous chromosomes are not altered. b. homologous chromosomes exchange genetic material. c. chromosomal damage occurs. d. genetic information is lost. e. cytokinesis occurs.

Short answer

The correct answer is b. Homologous chromosomes exchange genetic material during crossing over in meiosis I.

Step-by-step solution

Understanding Meiosis

Understanding the process of meiosis is the first step. Meiosis is a specialized form of cell division that reduces the chromosome number by half. This process creates four daughter cells. Each of these cells only has half the number of chromosomes of the parent cell. In humans, meiosis produces cells with 23 unpaired chromosomes — sperm or egg cells.

Crossing Over in Meiosis I

The crossing over stage occurs in meiosis I, specifically in a phase called prophase I. During crossing over, homologous chromosomes (paired chromosomes from the mother and father) align and exchange segments of DNA. This leads to genetic recombination, which increases genetic diversity in offspring.

Evaluating the Choices

Given our understanding of the process, we can now evaluate the given options in the exercise. Option a. indicates that homologous chromosomes are not altered. This is incorrect because, as mentioned above, they do exchange segments of DNA. Option b. states that homologous chromosomes exchange genetic material. This is accurate and agrees with our understanding of crossing over. Option c. is not correct because crossing over doesn't cause chromosomal damage. It is a normal part of meiosis. Option d. is not correct either. Genetic information is not lost in crossing over, instead, it is rearranged. Option e. suggests that cytokinesis occurs. Although cytokinesis occurs as part of meiosis, it's not specific to crossing over in meiosis I.

Key concepts

Crossing Over

Crossing over is a crucial process that happens during meiosis, specifically in the first phase called prophase I. It involves the exchange of genetic material between homologous chromosomes. Simply put, homologous chromosomes are pairs of chromosomes that contain the same genes but may have different versions of those genes, called alleles. During crossing over, these chromosomes come together and swap sections of DNA. This process is important because it increases genetic variation among offspring. This means that siblings from the same parents can have different combinations of traits, contributing to the diversity within a species. Crossing over is, therefore, a key component in ensuring genetic diversity and the adaptability of organisms over time.

Homologous Chromosomes

The term homologous chromosomes refers to pairs of chromosomes in a cell that have the same structure and sequence of genes. In humans, for instance, one chromosome of the pair comes from the mother, and the other chromosome comes from the father. Both chromosomes in a pair carry the same genes, which are arranged in the same order. However, they might contain different alleles, which are different versions of the same gene.

Homologous chromosomes are critical during meiosis because they undergo pairing and crossing over. This process facilitates the random mixing of genetic material, ensuring that offspring receive a unique combination of genes. This contributes to genetic diversity in populations, which is beneficial for evolution and adaptation.

Genetic Recombination

Genetic recombination is a process that results from crossing over. It refers to the reshuffling of genetic material during meiosis, which produces new combinations of alleles in offspring. When homologous chromosomes exchange genetic material during crossing over, new combinations of parental traits are formed.

This recombination is vital for evolution. It introduces new genetic variations into a population, allowing for traits that may be beneficial for survival and reproduction. In the long run, genetic recombination helps species adapt to changing environments and enhances their ability to survive threats like diseases or changing climates.

Prophase I

Prophase I is the first stage of meiosis I and marks the onset of the process where cells begin to divide to form gametes, which are sperm in males and eggs in females. During this stage, homologous chromosomes pair up in a process known as synapsis. This is an important step as it sets the stage for crossing over.

• Formation of synaptonemal complex: A protein structure forms between homologous chromosomes to hold them together.
• Chromosomes condense: Making them more visible under a microscope and easier to manage during cell division.
• Crossing over occurs: Genetic segments are exchanged between these paired chromosomes.

This phase is crucial for genetic recombination and variability. The changes initiated in prophase I are carried through to the rest of meiosis, eventually leading to the generation of genetically diverse gametes.