Question

Cross-like configuration when non-sister chromatids of a bivalent comes in contact during the first meiotic division are (a) Chiasmata (b) Bivalents (c) Chromomeres (d) Centromeres

Short answer

The correct choice is (a) Chiasmata. It refers to the points where non-sister chromatids remain in contact during crossover in the first division of meiosis.

Step-by-step solution

Understanding Meiosis

Meiosis is a type of cell division that results in four daughter cells, each with half the number of chromosomes of the parent cell, as in the production of gametes. It consists of two stages: Meiosis I and Meiosis II.

Non-sister chromatids and crossover

At the beginning of meiosis, chromosomes replicate and align in pairs. Each pair is made of four chromatids. Non-sister chromatids refer to the chromatids that belong to different homologous chromosomes within the pair. Crossover is the process by which these non-sister chromatids exchange segments of their DNA. This results in new combinations of genes, increasing genetic diversity.

Identifying Chiasmata

During this crossover event, chromatids remain in contact at certain points. These points of contact are known as Chiasmata. Therefore, Chiasmata are a cross-like configuration when non-sister chromatids of a bivalent come in contact during the first meiotic division.

Eliminating other options

Knowing what Chiasmata are allows us to eliminate the other choices. Bivalents refer to the paired homologous chromosomes. Chromomeres are the bead-like condensations along the chromosomes that can be seen during certain stages of cell division. Centromeres are the region on a chromosome where the spindle fiber attaches during cell division.

Key concepts

Meiosis

Meiosis is a special form of cell division necessary for sexual reproduction. It is crucial in halving the chromosome number from diploid to haploid, ensuring that when two gametes fuse during fertilization, the resulting offspring has the correct number of chromosomes. Meiosis consists of two sequential processes: Meiosis I and Meiosis II. Each has unique stages comparable to those in mitosis, but with important differences that lead to genetic variation among the resulting cells.

During Meiosis I, homologous chromosomes (one set from each parent) are separated, and during Meiosis II, the sister chromatids (the duplicated chromosomes) are divided. This two-part division results in four genetically distinct haploid gametes, each with half the chromosome count of the original cell. In organisms like humans, meiosis is the bedrock of genetic diversity, allowing for a mix of genetic traits from both parents.

Non-sister chromatids

When we talk about non-sister chromatids, we delve into the nuances of the chromosomal structures involved in meiosis. Each chromosome, after replication, consists of two identical sister chromatids held together at the centromere. During prophase I of meiosis, pairs of homologous chromosomes (each consisting of a pair of sister chromatids) come together in a pairing called synapsis. This forms a tetrad, where the chromatids involved are not genetically identical — hence named non-sister chromatids. These chromatids are key players in the crossover process.

Non-sister chromatids from homologous chromosomes can exchange genetic material during crossover, creating new combinations of genes which were not originally present on either parent's chromosome. This exchange is fundamental for genetic recombination and contributes to the genetic uniqueness of individuals.

Crossover

Crossover is an event of high significance within meiosis, occurring during prophase I. It is the process in which non-sister chromatids of homologous chromosomes exchange segments of DNA. This 'genetic handshake' allows a physical exchange of genetic information, and is facilitated by enzymes that cut and rejoin the DNA molecules. The crossover results in chromosomes that are not identical to the original or the parent's chromosomes.

Crossover contributes to genetic diversity by shuffling alleles, which are different versions of a gene, between chromosomes. This reshuffling means that the gametes produced will have unique combinations of alleles, which, when combined with another gamete from a different individual, can result in offspring with varied traits.

Genetic diversity

Genetic diversity is the cornerstone of evolutionary mechanisms, providing a population with a palette of traits that can be selected for or against by environmental pressures. Within the context of meiosis, genetic diversity is achieved through two primary mechanisms: Independent assortment and crossover.

Independent assortment refers to the way chromosomes are randomly distributed to the daughter cells during meiosis I, resulting in a variety of possible genetic outcomes. Add to this the crossover of non-sister chromatids, and you have a robust process for creating a vast range of genetic combinations within the gametes. These diverse combinations ensure no two gametes are the same genetically, leading to the potential for unique traits and increased adaptability for populations over time.