Question

Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the one that is best in each case. Answers can be found here. The event that occurs when homologous chromosomes come into close contact during meiosis I is called (A) crossing-over (B) synapsis (C) tetrad (D) cytokinesis (E) interphase

Short answer

The event in which homologous chromosomes come into close contact during meiosis I is called 'synapsis'.

Step-by-step solution

Identifying the process

Given that we are asked about the event that happens when homologous chromosomes come into close contact during meiosis I. This event occurs during Prophase I.

Understanding the choices

Examine each of the answer choices and their definitions:\n(A) 'crossing-over' is when an exchange of genes between homologous chromosomes occurs.\n(B) 'synapsis' is the pairing of homologous chromosomes.\n(C) 'tetrad' is the group of four chromatids formed by pairing of a pair of homologous chromosomes.\n(D) 'cytokinesis' is the division of the cytoplasm to form two separate daughter cells.\n(E) 'interphase' is the cell prep stage where it grows and copies its DNA in preparation for mitosis.

Matching the definition to the term

Given that during Prophase I, homologous chromosomes come into close contact and pair up, this event is best described as 'synapsis'.

Key concepts

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes that have the same structure and carry the same genes at the same loci, or positions on the chromosome. However, they may have different alleles or versions of those genes.
These chromosomes are essential in ensuring that each daughter cell has a complete set of chromosomes after cell division. During meiosis, which is the process of forming gametes such as sperm and egg cells, homologous chromosomes play a critical role.

• Each parent contributes one chromosome to the homologous pair.
• The two chromosomes in a pair have the same size, length, and centromere position.
• Understanding their function helps us comprehend genetic diversity, as they allow the shuffling of genes during meiosis.

In summary, homologous chromosomes are like partners that come from different sources but have much in common.

Synapsis

Synapsis is a crucial event in meiosis, specifically occurring during the first phase of the process, Prophase I. This is when homologous chromosomes come together, aligning precisely with each other to form pairs. This close pairing is vital for the subsequent genetic recombination and proper segregation of chromosomes.

• Helps ensure genetic variation by facilitating crossing-over, where genetic material is exchanged between homologous chromosomes.
• The synaptonemal complex, a protein structure, forms between paired homologous chromosomes to support this pairing.
• Without synapsis, errors in genetic recombination or chromosome segregation could occur.

Synapsis is fundamental to promoting genetic diversity and ensuring accurate distribution of genetic information.

Prophase I

Prophase I is the initial stage of meiosis I, characterized by several key events critical to the process of genetic reshuffling. This phase involves homologous chromosomes pairing up through synapsis, laying the groundwork for crossing-over to occur, which in turn increases genetic variability.

• Chromosomes condense and become visible under a microscope.
• The nuclear membrane begins to break down, and spindle fibers start to form.
• Crossing-over, facilitated by synapsis, allows for the exchange of genetic material, introducing new genetic combinations.

Prophase I is one of the most complex stages of meiosis, setting up the chromosomal rearrangements that lead to diverse gametes. Its processes are finely tuned to ensure that genetic material is shuffled and passed on correctly.