07/12 Cell Reproduction and Meiosis

Why Cells Divide

Cell division enables organisms to grow, replace damaged cells, reproduce, and produce specialized reproductive cells. The two major types of nuclear division are mitosis and meiosis.

Mitosis generally produces two genetically similar cells and maintains the parent cell’s chromosome-set number. Meiosis produces haploid reproductive cells or spores from a diploid precursor, reducing the chromosome number by half and generating genetic differences.

The key distinction is which chromosome structures separate first:

• In mitosis, sister chromatids separate.

• In meiosis I, homologous chromosomes separate.

• In meiosis II, sister chromatids separate.

A useful starting rule is that chromosome behavior, rather than the number of visible chromosomes alone, determines the outcome of each division.

Takeaway: Mitosis preserves chromosome number for growth and repair; meiosis reduces chromosome number and supports sexual reproduction.

Chromosome Organization and Ploidy

Before division, DNA is replicated and condensed into chromosomes. A chromosome is one DNA molecule associated with proteins and containing many genes. A gene is a DNA sequence that contributes information for a functional product or biological characteristic.

A homologous pair contains chromosomes with the same kinds of genes at corresponding locations, called loci. The two homologs may carry different alleles, which are alternative forms of a gene. After replication, each chromosome consists of two sister chromatids joined at a centromere. The centromere also provides a site for spindle attachment through protein structures called kinetochores.

Before DNA replication, one chromosome contains one chromatid. After replication, one chromosome contains two sister chromatids. Although the replicated chromosome contains twice as much DNA, it is still counted as one chromosome until the sister chromatids separate.

Ploidy describes the number of chromosome sets in a cell. Haploid cells have one set, represented as $n$, while diploid cells have two sets, represented as $2n$.

Takeaway: Do not confuse DNA amount, chromosome number, and chromatid number. Replication doubles the number of chromatids but does not initially double the chromosome count.

The Cell Cycle

The cell cycle is the ordered sequence of growth, DNA replication, chromosome segregation, and cell division. It includes interphase and the division phase.

During interphase:

1. In $G_1$ phase, the cell grows, produces proteins and organelles, and performs normal functions.

2. In $S$ phase, DNA is replicated, so each chromosome becomes two sister chromatids.

3. In $G_2$ phase, the cell continues growing, produces proteins needed for division, and checks whether replication was completed accurately.

4. In $G_0$ phase, some cells leave the active cycle temporarily or permanently. Neurons commonly remain in a differentiated, nondividing state.

The $M$ phase includes nuclear division and cytokinesis. Cell-cycle checkpoints can delay division when DNA is damaged, replication is incomplete, or spindle attachments are incorrect. If these controls fail, cells may undergo uncontrolled proliferation.

Cytokinesis divides the cytoplasm. In animal cells, an actin-and-myosin contractile ring produces a cleavage furrow. In plant cells, vesicles form a cell plate that develops into a new cell wall.

Takeaway: Interphase is an active preparation period, not cellular inactivity. DNA replication occurs specifically during $S$ phase.

Mitosis: One Division

Mitosis is a single nuclear division used for growth, tissue repair, replacement of cells, and many forms of asexual reproduction. Its central accuracy requirement is that each daughter nucleus receives one copy of every chromosome.

The stages are conventionally described as follows:

1. Prophase: Chromatin condenses into visible chromosomes, the spindle begins to form, and centrosomes move apart in animal cells.

2. Prometaphase: The nuclear envelope breaks down, and spindle microtubules attach to kinetochores.

3. Metaphase: Chromosomes align at the cell’s equator, with sister chromatids attached to spindle fibers from opposite poles.

4. Anaphase: Cohesin proteins are released, and sister chromatids separate. Each separated chromatid is now an individual chromosome.

5. Telophase: Chromosomes reach opposite poles, decondense, and become enclosed by newly forming nuclear envelopes.

6. Cytokinesis: The cytoplasm divides, producing two daughter cells.

For example, a typical human somatic cell begins with $46$ chromosomes. After DNA replication, it still has $46$ duplicated chromosomes, containing $92$ chromatids. When sister chromatids separate and cytokinesis is completed, each daughter cell has $46$ chromosomes.

Takeaway: Mitosis separates sister chromatids once and normally produces two cells with the parent cell’s chromosome number.

Meiosis: Two Divisions

Meiosis begins with a diploid cell, includes one round of DNA replication, and proceeds through meiosis I and meiosis II. It usually produces four genetically different haploid cells.

Meiosis I is the reduction division. During prophase I, homologous chromosomes pair through synapsis, forming a tetrad containing four chromatids. Non-sister chromatids may exchange DNA at contact points called chiasmata. This crossing over creates chromosomes with new allele combinations.

During metaphase I, tetrads align at the equator. Each homologous pair orients independently of the other pairs. During anaphase I, homologous chromosomes separate while sister chromatids remain joined at their centromeres. The chromosome number is reduced because homologs, rather than chromatids, move to opposite poles. Telophase I and cytokinesis may produce two haploid cells.

Meiosis II resembles mitosis but occurs in haploid cells. There is no second round of DNA replication between the two meiotic divisions.

1. In prophase II, spindle structures form.

2. In metaphase II, chromosomes align individually at the equator.

3. In anaphase II, sister chromatids separate.

4. In telophase II and cytokinesis, four haploid cells are produced.

Takeaway: Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II. This sequence explains both chromosome-number reduction and the formation of four products.

Mitosis and Meiosis Compared

Mitosis and meiosis differ in purpose, chromosome behavior, and the products they form.

• Number of divisions: Mitosis has one nuclear division; meiosis has two.

• DNA replication: Both processes have one round of replication before division begins. Meiosis has no replication between meiosis I and meiosis II.

• Typical products: Mitosis produces two cells; meiosis usually produces four.

• Chromosome number: Mitosis usually maintains chromosome number; meiosis reduces it by half.

• Homologous pairing: Pairing is absent in mitosis and occurs during prophase I of meiosis.

• Crossing over: It is not a normal feature of mitosis but is common during prophase I of meiosis.

• Genetic similarity: Mitotic products are usually genetically similar; meiotic products are genetically different.

• First structures to separate: Sister chromatids separate in mitosis, while homologous chromosomes separate in meiosis I.

A quick decision process is useful: identify whether there is one division or two, determine whether homologs pair, and then ask which structures separate during anaphase. These observations usually reveal whether the process is mitosis or meiosis.

Takeaway: Mitosis is a chromosome-preserving division for cellular maintenance, whereas meiosis is a chromosome-reducing division that produces variation for sexual reproduction.

Sources of Genetic Variation

Genetic variation arises from several mechanisms associated with sexual reproduction and DNA change.

Crossing over occurs when non-sister chromatids of homologous chromosomes exchange DNA segments during prophase I. It is more likely to separate genes that are far apart on the same chromosome than genes that are close together.

Independent assortment results from the random orientation of homologous pairs at metaphase I. For an organism with $n$ chromosome types, independent assortment can produce approximately $2^n$ chromosome combinations in gametes before crossing over is considered. With $23$ chromosome types, this gives more than $8$ million combinations from independent assortment alone.

Random fertilization increases variation because any one gamete can unite with many possible gametes from another individual.

A mutation is a change in DNA sequence. Some mutations have little or no detectable effect, some are harmful, and some can be beneficial in a particular environment. Mutations in cells that contribute to gametes can be inherited, whereas mutations confined to somatic cells are generally not passed to offspring.

Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate correctly. A resulting gamete may receive too many or too few chromosomes, and fertilization involving that gamete can produce an embryo with an abnormal chromosome number.

Takeaway: Crossing over and independent assortment reshuffle existing alleles, random fertilization combines genomes, and mutation changes DNA sequences.

Investigating Cell Division

Chromosome behavior can be investigated by counting cells, modeling chromosome movement, and simulating random orientations.

Estimating the Mitotic Index

Cells in a growing root can be classified as interphase, prophase, metaphase, anaphase, or telophase. The percentage of cells in a stage is calculated as:

A high proportion of cells undergoing division indicates active growth. A large number of cells in interphase does not mean they are inactive, because growth and DNA replication occur during interphase.

Modeling Chromosome Behavior

A model can use two colors of yarn or paper strips to represent homologs and clips or beads to represent centromeres. After replicating each chromosome, model mitosis by separating sister chromatids. Then reset the model and simulate meiosis by pairing homologs, modeling crossing over, separating homologs, and finally separating sister chromatids. Record chromosome number and genetic composition in every product.

Simulating Independent Assortment

Pairs of differently colored cards can represent homologous chromosomes. Randomly orient each pair at metaphase I, record the chromosome combination moving toward one pole, and repeat the trial at least $20$ times. Compare the observed combinations with the predicted value of $2^n$. A small number of trials may not reveal every possible combination.

Takeaway: Models and cell counts are useful only when chromosome number, chromatid number, stage, and sampling limitations are recorded carefully.

• sister chromatid