Which statement correctly describes the process of crossing over?
A Pieces of a matemal chromatid break off and swap pieces with its sister chromatid, and then the same occurs between the paternal chromatids.
B Two chromatids (one matemal and one paternal) break off at corresponding points on homologous chromosomes. The pieces of chromatid rejoin to the chromosome in each other’s places.
C During replication, a maternal and a paternal chromosome switch places.
D Two alleles exchange positions on a chromosome, creating new recombination frequencies. W
E hile homologous chromosomes are splitting apart and separating to either end of the new nucleus, two chromosomes switch positions and move toward the ends, opposite from where they were originally headed.
The correct answer and explanation is :
The correct statement is:
B. Two chromatids (one maternal and one paternal) break off at corresponding points on homologous chromosomes. The pieces of chromatid rejoin to the chromosome in each other’s places.
Explanation:
Crossing over is a critical process that occurs during meiosis (the process of cell division that produces gametes, such as eggs and sperm) and is crucial for genetic diversity. It takes place during prophase I of meiosis, when homologous chromosomes (chromosomes that have the same genes but may have different alleles, one inherited from each parent) come together in a structure called synapsis.
In crossing over, the chromatids of homologous chromosomes physically exchange segments. Here’s a more detailed step-by-step breakdown of how it happens:
- Chromosome Pairing: Homologous chromosomes, one from the mother and one from the father, align next to each other. This is called synapsis.
- Chromatid Breakage: The chromatids (the individual strands of a chromosome, which are essentially duplicate copies) of the paired homologous chromosomes break at corresponding points. Each break happens at a specific location on both the maternal and paternal chromatids.
- Exchange of Segments: After the breaks, the chromatids exchange segments. Essentially, the broken-off piece of one chromatid (say from the maternal chromosome) reattaches to the other chromatid (the paternal chromosome), and vice versa.
- Rejoining: The broken chromatids rejoin at the points where they were exchanged, creating a new combination of alleles on the chromatids. These new chromatids are called recombinant chromatids.
- Resulting Diversity: This process results in genetic recombination, increasing genetic variation among offspring. The alleles on the resulting chromatids are shuffled, meaning that the gametes produced will carry different combinations of alleles than the parent cells, which is why no two gametes (except in rare cases) are genetically identical.
Crossing over is essential for ensuring that the offspring inherit a mix of genetic material from both parents, contributing to genetic diversity in populations.
Why the other options are incorrect:
- A is incorrect because it refers to sister chromatids (identical copies of a chromosome) instead of homologous chromosomes.
- C is incorrect because crossing over doesn’t involve chromosomes switching places; it’s about the exchange of chromatid segments.
- D is incorrect because crossing over happens between chromatids of homologous chromosomes, not between alleles on a single chromatid.
- E is incorrect because it misrepresents the process, which happens during prophase I and involves chromatid exchange, not chromosomal movement after the homologous chromosomes have already separated.