MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1c Meiosis Genetic Variation

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This deck focuses on 1c Meiosis Genetic Variation, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Flashcard 1: What is the term for paired homologous chromosomes during prophase I?

Answer: Tetrad (bivalent). This structure forms when homologs pair, enabling crossing over for genetic recombination.

Flashcard 2: Which stage of meiosis separates homologous chromosomes?

Answer: Anaphase I. During this phase, homologous chromosomes are pulled apart to opposite poles, reducing ploidy.

Flashcard 3: What is the ploidy change when a diploid cell completes meiosis?

Answer: Diploid $2n$ to haploid $n$. Meiosis reduces the chromosome number by half to produce gametes that can restore diploidy upon fusion.

Flashcard 4: What is synapsis in prophase I of meiosis?

Answer: Pairing of homologous chromosomes via the synaptonemal complex. Synapsis facilitates close alignment of homologs, allowing crossing over to exchange genetic material.

Flashcard 5: Which stage of meiosis separates sister chromatids?

Answer: Anaphase II. Sister chromatids separate in this phase, similar to mitosis, to distribute identical DNA copies.

Flashcard 6: What is the primary purpose of meiosis in sexually reproducing organisms?

Answer: Produce haploid gametes and increase genetic variation. Meiosis ensures genetic diversity in offspring by halving chromosome number for fertilization and introducing variation through recombination.

Flashcard 7: Which meiotic division is reductional, and what does that mean?

Answer: Meiosis I; reduces ploidy by separating homologs. Meiosis I halves the chromosome number by segregating homologous pairs into daughter cells.

Flashcard 8: What is the key structural difference between homologous chromosomes and sister chromatids?

Answer: Homologs: maternal vs paternal; sisters: identical copies after S phase. Homologs differ in parental origin and may have allelic variations, while sisters are replication products.

Flashcard 9: If a species has $n=3$, what is the number of possible gamete chromosome combinations from assortment alone?

Answer: $2^3=8$. With three pairs, each can assort independently, resulting in $2^3$ unique gamete genotypes from assortment.

Flashcard 10: What is the DNA replication status between meiosis I and meiosis II?

Answer: No S phase; DNA is not replicated between divisions. Absence of interphase replication preserves the reduced DNA content for haploid gamete production.

Flashcard 11: Which meiotic division is equational, and what does that mean?

Answer: Meiosis II; separates sister chromatids without changing ploidy. Meiosis II divides chromatids equally, maintaining the haploid state established in meiosis I.

Flashcard 12: What is the immediate product of meiosis in humans (starting from one primary germ cell)?

Answer: Four haploid cells (gametes or gamete precursors). Meiosis yields four genetically distinct haploid cells from one diploid precursor for reproduction.

Flashcard 13: What is nondisjunction in meiosis?

Answer: Failure of homologs (I) or sister chromatids (II) to separate properly. Nondisjunction leads to aneuploid gametes by improper segregation during meiotic divisions.

Flashcard 14: If nondisjunction occurs in meiosis I, what gamete types result in terms of $n$?

Answer: Two gametes $n+1$ and two gametes $n-1$. Meiosis I nondisjunction affects both daughter cells, producing equal numbers of hyper- and hypoploid gametes.

Flashcard 15: If nondisjunction occurs in meiosis II, what gamete types result in terms of $n$?

Answer: Two normal $n$, one $n+1$, and one $n-1$ gamete. Meiosis II nondisjunction impacts only one cell, yielding two normal and two aneuploid gametes.

Flashcard 16: What is trisomy in terms of chromosome copy number?

Answer: Three copies of one chromosome (a $2n+1$ condition). Trisomy results from gaining an extra chromosome, often due to nondisjunction, altering the diploid complement.

Flashcard 17: Which two meiotic mechanisms are the major sources of genetic variation in gametes?

Answer: Crossing over and independent assortment. These processes shuffle alleles and chromosome combinations, ensuring genetic diversity in offspring.

Flashcard 18: What is independent assortment in meiosis?

Answer: Random orientation of homologous pairs at metaphase I. This random alignment ensures each gamete receives a unique combination of maternal and paternal chromosomes.

Flashcard 19: What is the formula for the number of chromosome combinations from independent assortment?

Answer: $2^n$ (where $n$ is the haploid chromosome number). The formula accounts for each homologous pair having two possible orientations, yielding exponential combinations.

Flashcard 20: Which specific prophase I substage is characterized by crossing over?

Answer: Pachytene. In pachytene, homologs are fully synapsed, enabling enzymes to mediate recombination events.