AP Biology 5.1-5.2: Meiosis and Genetic Diversity

Meiosis

Meiosis is a nuclear division process that reduces a diploid cell to haploid cells. It includes one round of DNA replication followed by two successive nuclear divisions.

Why must meiosis reduce chromosome number before sexual reproduction?

Gametes must be haploid so that fusion of two gametes restores the diploid chromosome number. Without reduction, chromosome sets would double in every generation.

Haploid cell

A cell with one complete set of chromosomes, represented as $n$. Gametes are typically haploid.

Diploid cell

A cell with two complete sets of chromosomes, represented as $2n$. The two sets consist of homologous chromosomes, with one chromosome of each pair inherited from each parent.

Homologous chromosomes

Chromosomes that form a matched pair and contain the same genes at corresponding locations, although they may carry different alleles. They are not necessarily genetically identical.

What happens to chromosome number and DNA content during meiosis?

DNA is replicated once during S phase, but the nucleus divides twice. Meiosis I reduces chromosome number from diploid to haploid, while meiosis II separates sister chromatids without further reducing ploidy.

What are the products of meiosis in a typical diploid organism?

One diploid starting cell usually produces four genetically distinct haploid cells. In animals, these cells can develop into gametes.

What occurs during interphase before meiosis?

G1 supports growth, S phase replicates each chromosome, and G2 prepares the cell for division. The chromosome replication occurs only once before meiosis I.

Sister chromatids

The two DNA copies produced when one chromosome replicates during S phase. They remain joined at the centromere by cohesin proteins until they separate during anaphase II.

What is the central distinction between meiosis I and meiosis II?

Meiosis I separates homologous chromosomes, whereas meiosis II separates sister chromatids. Thus, meiosis I is the reductional division; meiosis II is analogous to mitosis.

Synapsis

Synapsis is the close pairing and precise alignment of homologous chromosomes during prophase I. The paired homologs are held together by a synaptonemal complex.

Synaptonemal complex

A protein structure that aligns homologous chromosomes during prophase I and helps coordinate genetic exchange between nonsister chromatids.

Tetrad

A paired set of homologous chromosomes in prophase I, containing four chromatids total: two sister chromatids from each homolog.

Crossing over

Crossing over is the reciprocal exchange of equivalent DNA segments between nonsister chromatids of homologous chromosomes during prophase I. It produces recombinant chromosomes with new allele combinations.

Chiasma

A visible connection between homologous chromosomes that marks a site where crossing over occurred. Chiasmata help hold homologs together until they separate in anaphase I.

Why does crossing over increase genetic variation?

It combines DNA segments from homologs originally inherited from different parents. The resulting recombinant chromatids contain allele combinations that were not present on either original chromosome.

What is the chromosome arrangement during metaphase I?

Tetrads align at the metaphase plate, with each homolog oriented toward an opposite spindle pole. The orientation of each tetrad is random and independent of the others.

Independent assortment

Independent assortment is the random orientation and separation of homologous chromosome pairs during meiosis I. It creates different combinations of maternally and paternally inherited chromosomes in haploid products.

How many chromosome combinations can independent assortment produce?

For a diploid organism with $n$ chromosomes in each haploid set, independent assortment can produce $2^n$ combinations. For humans, $2^{23}$ gives more than eight million combinations before considering crossing over.

How do crossing over and independent assortment work together to generate variation?

Crossing over reshuffles DNA within homologous chromosomes, while independent assortment reshuffles whole homologs among gametes. Together they make meiotic products genetically diverse.

What happens during prometaphase I?

The nuclear envelope breaks down, and spindle microtubules attach to kinetochores at the centromeres. Each homolog in a tetrad attaches to microtubules from an opposite pole.

Kinetochore

A multiprotein complex assembled at a chromosome's centromere that attaches the chromosome to spindle microtubules and helps direct chromosome movement.

What separates during anaphase I, and what remains joined?

Homologous chromosomes separate and move to opposite poles. The sister chromatids of each chromosome remain joined at their centromeres.

Why are the cells produced by meiosis I haploid even though their chromosomes still have two chromatids?

Each cell receives only one chromosome from every homologous pair, so it has one chromosome set. A chromosome's ploidy contribution depends on the number of homologs, not on whether each chromosome has one or two chromatids.

What happens during telophase I and cytokinesis?

Chromosomes reach opposite poles, and nuclear envelopes may reform around each haploid set. Cytokinesis then divides the cell into two haploid cells, each containing chromosomes made of two sister chromatids.

Interkinesis

Interkinesis is a possible brief interval between meiosis I and meiosis II. It does not include an S phase, so the chromosomes are not replicated again.

What occurs during prophase II?

A spindle forms in each haploid cell, chromosomes condense, and any nuclear envelope that reformed after meiosis I breaks down. No DNA replication occurs before prophase II.

What occurs during metaphase II and anaphase II?

In metaphase II, chromosomes align individually at the equator. In anaphase II, sister chromatids separate and move to opposite poles; each separated chromatid is then considered an individual chromosome.

What happens during telophase II and cytokinesis?

Chromosomes arrive at opposite poles, nuclear envelopes reform, and chromosomes may decondense. Cytokinesis produces four separate haploid cells, each with one chromatid per chromosome.

What is the final chromosome state after meiosis II?

Each of the four resulting cells is haploid and contains one copy of each chromosome, with each chromosome consisting of a single chromatid.

Reductional division

A division that reduces chromosome sets from two to one. Meiosis I is reductional because homologous chromosomes, rather than sister chromatids, are separated.

How does meiosis differ from mitosis in number of divisions and products?

Mitosis has one nuclear division and usually produces two genetically similar cells with the original ploidy. Meiosis has two nuclear divisions and usually produces four genetically distinct haploid cells.

How does chromosome pairing differ between mitosis and meiosis?

Homologous chromosomes pair, undergo synapsis, and may cross over in meiosis I. Homologous chromosomes generally do not pair with each other during mitosis.

Why is meiosis II considered similar to mitosis?

Both processes align duplicated chromosomes individually and separate sister chromatids. The key difference is that meiosis II begins with haploid cells, whereas mitosis may occur in cells of different ploidies.

What is the role of cohesin during meiosis?

Cohesin holds sister chromatids together after DNA replication. This cohesion persists through meiosis I and is released so sister chromatids can separate during anaphase II.

What must occur for homologous chromosomes to separate properly in meiosis I?

Homologs must pair and become connected through chiasmata formed by crossing over. At least one chiasma per chromosome is generally needed to maintain the proper connection for segregation.

How does cytokinesis differ between animal and plant cells during meiosis?

Animal cells typically divide through a cleavage furrow produced by an actin ring. Plant cells form a cell plate from Golgi-derived vesicles, which develops into a separating cell wall.

What is the relationship between fertilization and meiosis in a sexual life cycle?

Meiosis produces haploid gametes, and fertilization fuses two haploid gametes to form a diploid zygote. Alternating reduction and restoration of chromosome number prevents chromosome sets from increasing each generation.