Question

During meiosis, the crossover occurs between (a) Sister chromatids of homologous chromosomes (b) Non-sister chromatids of homologous chromosomes (c) Sister chromatids of non-homologous chromosomes (d) Non-homologous chromatids of homologous chromosomes

Short answer

The crossover in meiosis occurs between non-sister chromatids of homologous chromosomes (option b).

Step-by-step solution

Understanding Chromatids and Chromosomes

Chromatids are two exact copies of DNA that make up a duplicated chromosome, while chromosomes are DNA molecules with part or all of the genetic material of an organism. After replication, a single chromosome is composed of two sister chromatids where each contains an exact replica of the DNA of the other. Homologous chromosomes, on the other hand, are the pair of chromosomes, one from each parent, that have the same gene sequences, allowing them to pair up during meiosis. Non-homologous chromosomes are chromosomes carrying different genes from each parent; these do not pair during meiosis.

Understanding Crossover (Recombination)

The crossover is a crucial mechanism that occurs during the prophase I of meiosis. It is a process where the genetic material gets exchanged between a pair of homologous chromosomes, contributing to the genetic variation in the offspring. This reciprocal exchange occurs between non-sister chromatids of homologous chromosomes.

Identifying the Correct Option

Based on the understanding of crossover in meiosis, the correct option is (b) Non-sister chromatids of homologous chromosomes. This is where the exchange of genetic materials takes place during the crossover.

Key concepts

Chromatids and Chromosomes

When delving into the world of genetics, understanding the structure and function of chromatids and chromosomes is fundamental. A chromosome is a long DNA molecule that encapsulates part of the genetic blueprint of an organism. For humans, a typical body cell contains 46 chromosomes, grouped into 23 pairs.

In the process of cell division, chromosomes replicate, and each chromosome then consists of two identical structures called sister chromatids, connected at a point known as the centromere. These sister chromatids carry exactly the same genetic information, serving as duplicates for the process of cell division. During the division of sex cells, or meiosis, these sister chromatids are equally divided to give rise to gametes with half the number of chromosomes found in body cells, which is why they're called haploid cells.

Understanding that each sister chromatid eventually becomes a separate chromosome upon cell division helps explain how genetic materials are passed from parents to offspring, maintaining genetic consistency and also allowing for variation through processes like crossing over, which we'll explore later.

Homologous Chromosomes

The concept of homologous chromosomes is another critical piece of the genetic puzzle. These chromosomes are two individual chromosomes that pair up during meiosis. One comes from the mother (maternal) and the other from the father (paternal). While each carries different versions of genes, known as alleles, they have genes at identical locations (loci), which makes them a matching pair.

During meiosis, these homologous pairs line up in the center of the cell before being separated into different cells. This alignment and subsequent separation ensure that each gamete has just one set of chromosomes, thereby playing a key role in reducing the chromosome number by half. This process also presents the stage for genetic recombination due to the crossover event between non-sister chromatids, which will be discussed in more detail next.

Remarkably, it is the pairing of these homologous chromosomes that distinguishes meiosis from mitosis, the other type of cell division, which involves non-sexual cells and does not involve pairing of homologous chromosomes.

Genetic Recombination

Genetic recombination, often referred to as crossover, is a cornerstone of genetics, being a critical source of genetic variation amongst sexually reproducing organisms. It occurs during the prophase I stage of meiosis when homologous chromosomes are tightly paired up.

Crossover is an essential process where non-sister chromatids—one from each homolog—exchange genetic segments. It's akin to shuffling a deck of cards; this mechanism mixes maternal and paternal genetic material to produce new, unique combinations in gametes. Because of this, siblings with the same biological parents can have different genetic make-up.

This swapping of genetic segments contributes to the diversity and adaptability of a species. The crossover ensures that traits from both parents are distributed in various ways, setting the stage for natural selection. The correct answer for the exercise, involving the exchange between non-sister chromatids of homologous chromosomes, highlights the precise manner in which this genetic shuffling occurs to produce variation in offspring.