AP Biology : Understand causes of genetic variation

Study concepts, example questions & explanations for AP Biology

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Example Questions

Example Question #1 : Understanding Crossing Over

Crossing over occurs during which stage of meiosis?

Possible Answers:

Metaphase I

Anaphase I

Prophase II

Prophase I

Correct answer:

Prophase I

Explanation:

During prophase I homologous chromosomes will line up with one another, forming tetrads. During this lining up, DNA sequences can be exchanged between the homologous chromosomes. This type of genetic recombination is called crossing over, and allows the daughter cells of meiosis to be genetically unique from one another.

Crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.

Example Question #1 : Understanding Crossing Over

What is the evolutionary purpose of cells that undergo crossing over?

Possible Answers:

To produce gametes that are genetically identical

To keep the redundancy of the cell high

To produce two cells instead of one

To increase genetic diversity

To keep mutations from forming

Correct answer:

To increase genetic diversity

Explanation:

Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.

Example Question #3 : Understanding Crossing Over

During which step of cell division does crossing over occur?

Possible Answers:

Metaphase II

Prophase I

Metaphase I

Prophase II

Correct answer:

Prophase I

Explanation:

When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.

By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.

Example Question #4 : Understanding Crossing Over

What structures exchange genetic material during crossing over?

Possible Answers:

Egg and sperm chromosomes

Sister chromatids

Nonsister chromatids

Non-homologous chromosomes

Correct answer:

Nonsister chromatids

Explanation:

During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word "nonsister" implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.

Example Question #5 : Understanding Crossing Over

Crossover of homologous chromosomes in meiosis occurs during which phase?

Possible Answers:

Prophase I of meiosis

Prophase II of meiosis

Anaphase I of meiosis

Anaphase II of meiosis

Correct answer:

Prophase I of meiosis

Explanation:

The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.

Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.

Example Question #6 : Understanding Crossing Over

During crossing over, two homologous chromosomes pair to form which of the following choices?

Possible Answers:

Mitotic Bond

None of these

Tetrad

Base Pair

Chromatid

Correct answer:

Tetrad

Explanation:

The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.

Example Question #7 : Understanding Crossing Over

Chromosomal crossover occurs in which phase of meiosis?

Possible Answers:

Anaphase II

Anaphase I

Prophase I

Prophase II

Metaphase I

Correct answer:

Prophase I

Explanation:

During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.  

Example Question #8 : Understanding Crossing Over

Crossing over is a phenomenon that happens during Meiosis I in the attempt to create genetic diversity. Crossing over typically occurs between which of the following structures?

Possible Answers:

Sister chromatids

Tetrads

Homologous chromosomes

Chromatin

Correct answer:

Homologous chromosomes

Explanation:

Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.

Example Question #9 : Understanding Crossing Over

When in meiosis does crossing over occur?

Possible Answers:

Metaphase I

Telophase I

Prophase I

Interphase

Anaphase I

Correct answer:

Prophase I

Explanation:

Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.

Example Question #1 : Understand Causes Of Genetic Variation

Which of the following affect the frequency of chromosomes of two linked genes crossing over?

Possible Answers:

Size of the gene

Size of the chromosome

Whether the genes are dominant or recessive

Distance between the genes

Correct answer:

Distance between the genes

Explanation:

The recombination frequency depends upon the distance between the genes; a larger distance between the genes increases the probability of crossing over occurring.

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