Problem 8 - Biology of Humans: Concepts, Applications, and Issues

Question

An abnormal number of chromosomes can result during meiosis because of a. crossing over. b. recombination. c. nondisjunction. d. synapsis.

Step-by-Step Solution

Answer

c. nondisjunction.

Step 1: Understand the Context

Chromosomes are structures in cells that contain DNA. During meiosis, chromosomes are duplicated and sorted to create gametes (sperm and egg cells). Errors in this process can lead to an abnormal number of chromosomes.

Step 2: Evaluate Each Option

Assess each option by defining and relating it to the creation of an abnormal number of chromosomes: a. Crossing over: The exchange of genetic material between homologous chromosomes during meiosis. b. Recombination: The process through which genetic material is rearranged. It is related to crossing over. c. Nondisjunction: The failure of chromosome pairs to separate properly during cell division. d. Synapsis: The pairing of homologous chromosomes during meiosis.

Step 3: Identify the Correct Option

Crossing over and recombination (a, b) involve the exchange and rearrangement of genetic material but do not cause an incorrect number of chromosomes. Synapsis (d) is the pairing phase and does not lead to numerical abnormalities. Nondisjunction (c) is the specific failure of chromosome pairs or sister chromatids to separate, leading to an abnormal number of chromosomes.

Step 4: Conclusion

Since nondisjunction directly involves the improper separation of chromosomes, leading to an abnormal number of chromosomes, it is the correct answer.

Key Concepts

chromosomal abnormalities

Chromosomal abnormalities occur when there is a deviation from the normal number or structure of chromosomes. This can result in conditions like Down syndrome, Turner syndrome, and Klinefelter syndrome. These abnormalities are typically present in all cells of the body and have significant impacts on development and health. Chromosomal abnormalities can arise through several mechanisms:

• **Deletion**: Part of a chromosome is missing.
• **Duplication**: A part of a chromosome is duplicated, resulting in extra genetic material.
• **Inversion**: A chromosome segment breaks off, flips around, and reattaches.
• **Translocation**: A segment of one chromosome breaks off and attaches to another.

However, the focus here is on numerical abnormalities, which are often caused by nondisjunction during meiosis.

meiosis errors

Meiosis is a critical process that results in the formation of gametes—sperm and egg cells. It ensures genetic diversity and the correct number of chromosomes in offspring. However, errors can occur during this complex process. One significant type of error is nondisjunction, where chromosomes fail to separate properly.
Nondisjunction can occur in two stages of meiosis:

• Meiosis I: Homologous chromosomes fail to separate, resulting in two cells with an extra chromosome and two without.
• Meiosis II: Sister chromatids fail to separate, leading to similar abnormal gamete formation.

These errors lead to an abnormal number of chromosomes in the resulting gametes, which, upon fertilization, can cause chromosomal abnormalities in the offspring.

gamete formation

Gamete formation, or gametogenesis, is the process through which human reproductive cells are formed. It involves meiosis, which ensures that each gamete contains half the number of chromosomes compared to a normal body cell. In humans, this means gametes contain 23 chromosomes, compared to the 46 found in somatic cells.
There are two main types of gametogenesis:

• **Spermatogenesis**: The formation of sperm cells in males occurs in the testes and results in four viable sperm cells from one precursor cell.
• **Oogenesis**: The formation of egg cells in females happens in the ovaries. It is more complex and results in one viable egg cell and three polar bodies from one precursor cell.

Nondisjunction during these processes can lead to gametes with an abnormal chromosome number. When such gametes participate in fertilization, they lead to zygotes with chromosomal abnormalities, impacting the resulting organism.