Table of Contents
Overview
Cells do not live forever and cannot simply grow endlessly. To maintain life, organisms must produce new cells. This happens through cell division, which usually includes a preceding nuclear division (division of the genetic material in the nucleus) and a cytoplasmic division (division of the cell body). In eukaryotes, nuclear division occurs in two fundamentally different forms:
- Mitosis – for growth, regeneration, and many forms of asexual reproduction
- Meiosis – for the formation of gametes (sex cells) and spores, and thus for sexual reproduction
Prokaryotes lack a true nucleus, so they divide differently (binary fission). That special case is only briefly contrasted here.
This chapter focuses on what happens specifically with the nucleus and chromosomes during cell division, and how this leads to genetically identical or genetically different daughter cells.
The Cell Cycle
Before a cell divides, it usually passes through a cell cycle: a repeating sequence of growth and division.
The classic eukaryotic cell cycle consists of:
- Interphase (the cell is not dividing visibly)
- G₁ phase (Gap 1) – cell grows, synthesizes proteins and organelles
- S phase (Synthesis) – DNA is replicated; each chromosome is copied
- G₂ phase (Gap 2) – further growth, preparation for division; checking and repairing DNA
- M phase (Mitotic phase) – nuclear division (mitosis or meiosis) plus usually cytokinesis (division of the cytoplasm)
During the S phase, each chromosome is replicated. After replication:
- One chromosome consists of two sister chromatids
- Sister chromatids are held together at a region called the centromere
These sister chromatids are genetically identical copies (barring replication errors). How they are later separated is the core of nuclear division.
Cells can leave the cycle into a G₀ phase (resting state), where they do not divide but may remain metabolically active (for example, many nerve cells).
Chromosomes: Ploidy and Organization
To understand nuclear division, it is important to distinguish:
- Ploidy (set of chromosomes)
- Haploid (n) – one set of chromosomes (e.g., gametes in animals)
- Diploid (2n) – two sets of chromosomes, usually one from each parent (typical of somatic cells in many animals and plants)
In a diploid cell:
- Each chromosome type occurs as a homologous pair (homologous chromosomes)
- Homologous chromosomes carry the same kinds of genes in the same order but can have different variants (alleles)
During mitosis, the ploidy (2n → 2n or n → n) stays the same.
During meiosis, the ploidy is halved (2n → n).
Mitosis: Nuclear Division for Growth and Maintenance
Mitosis ensures that when a eukaryotic cell divides, each daughter cell receives an identical set of chromosomes as the parent cell. It is the basis for:
- Growth of multicellular organisms
- Renewal and repair of tissues
- Many forms of asexual reproduction (e.g., in some plants, unicellular eukaryotes)
Mitosis is usually described in phases, which blend smoothly into each other.
Prophase
- Chromatin (loosely packed DNA-protein complex) condenses into visible chromosomes.
- Each chromosome consists of two sister chromatids.
- The nucleolus becomes less distinct or disappears.
- Outside the nucleus, the mitotic spindle (spindle apparatus) begins to form from microtubules.
- In animal cells, centrosomes move to opposite poles of the cell.
Prometaphase
- The nuclear envelope breaks down, allowing spindle microtubules to interact with chromosomes.
- Microtubules attach to chromosomes at protein structures called kinetochores, located at the centromeres.
- Some microtubules connect to each other from opposite poles, helping to push poles apart.
Metaphase
- Spindle microtubules move chromosomes so that they line up in a single plane in the middle of the cell, the metaphase plate.
- Each chromosome is attached to microtubules from both poles via its kinetochores.
- A checkpoint ensures that all chromosomes are properly attached before the cell proceeds; this helps prevent chromosome mis-segregation.
Anaphase
- The centromeres split; sister chromatids separate.
- Each chromatid is now considered an individual chromosome.
- Shortening of kinetochore microtubules pulls chromosomes toward opposite poles.
- The cell elongates because non-kinetochore microtubules slide past each other.
The key event of anaphase: identical copies of each chromosome are separated, ensuring both poles receive the full genetic information.
Telophase
- Chromosomes arrive at the poles and begin to decondense back into chromatin.
- Nuclear envelopes re-form around each set of chromosomes, creating two nuclei.
- The nucleoli reappear.
- The spindle apparatus disassembles.
Cytokinesis (Cell Division Proper)
Although not part of nuclear division itself, cytokinesis usually follows telophase:
- Animal cells: A cleavage furrow forms; a contractile ring of actin filaments tightens, pinching the cell into two daughter cells.
- Plant cells: Vesicles derived from the Golgi apparatus accumulate at the center, forming a cell plate that grows outward and fuses with the existing cell wall, dividing the cell.
End result of mitosis plus cytokinesis:
- From one parent cell, there are two genetically identical daughter cells (barring mutations), each with the same chromosome number and set as the parent.
Meiosis: Nuclear Division for Sexual Reproduction
Meiosis is a specialized nuclear division that reduces the chromosome number by half. It produces:
- Gametes (sex cells) in animals
- Spores and gametes in plants and many fungi
Key features:
- One round of DNA replication (in S phase)
- Followed by two successive nuclear divisions: meiosis I and meiosis II
- Result: from one diploid (2n) cell → typically four haploid (n) cells
Meiosis has two main purposes:
- Reduction of chromosome number (2n → n)
- Generation of genetic variation via:
- Crossing over (genetic recombination)
- Independent assortment of chromosomes
Overview: Meiosis I vs. Meiosis II
- Meiosis I (reduction division):
- Homologous chromosomes pair and are separated.
- Ploidy is reduced: 2n → n.
- Meiosis II (equational division):
- Sister chromatids are separated, similar to mitosis.
- Ploidy remains n → n.
Meiosis I: Reduction Division
Prophase I
Prophase I is longer and more complex than prophase of mitosis. It includes several substages (names not essential for beginners, but the key events are):
- Chromosome condensation – chromosomes become visible.
- Synapsis – homologous chromosomes (one from each parent) pair up closely along their length, forming bivalents or tetrads (each consists of four chromatids).
- Crossing over:
- Non-sister chromatids of homologous chromosomes exchange corresponding segments.
- Sites of crossing over are visible as chiasmata.
- This recombination shuffles genetic information between maternal and paternal chromosomes.
- The nuclear envelope breaks down; the spindle forms.
Crossing over creates new combinations of alleles and is a major source of genetic variation.
Metaphase I
- Bivalents (paired homologous chromosomes) align on the metaphase plate.
- Each homologous chromosome of a pair is attached to microtubules from only one pole; the other homologous partner is attached to the opposite pole.
- Orientation of each pair is random with respect to poles:
- This independent assortment leads to many possible combinations of maternal and paternal chromosomes in the gametes.
Anaphase I
- Homologous chromosomes separate and move toward opposite poles.
- Sister chromatids remain together; the centromeres do not split yet.
- Because homologous pairs are separated, the chromosome number is halved.
Telophase I and Cytokinesis
- Chromosomes (each still with two sister chromatids) arrive at the poles.
- Depending on species, chromosomes may partially decondense and nuclear envelopes may reform briefly.
- Cytokinesis usually occurs, resulting in two haploid daughter cells.
- Each has one chromosome from each homologous pair, but thanks to recombination and independent assortment, the combinations are unique.
Meiosis II: Equational Division
After a short interphase-like stage (often without another DNA replication), meiosis II resembles a mitotic division, but it starts from haploid cells.
Prophase II
- Chromosomes (still two sister chromatids) condense again if they had decondensed.
- New spindle apparatus forms in each haploid cell.
- Nuclear envelope breaks down if it had re-formed.
Metaphase II
- Chromosomes align individually on the metaphase plate.
- Sister chromatids of each chromosome are attached to microtubules from opposite poles.
Anaphase II
- Centromeres split; sister chromatids separate.
- Each chromatid, now an independent chromosome, is pulled toward opposing poles.
Telophase II and Cytokinesis
- Chromosomes decondense.
- Nuclear envelopes form around each set of chromosomes.
- Cytokinesis divides the cells.
End result:
- From one original diploid cell:
- Four haploid (n) cells, each genetically different from the others and from the parent cell.
Genetic Consequences of Mitosis vs. Meiosis
Mitosis
- Maintains chromosome number: 2n → 2n (or n → n).
- Produces genetically identical daughter cells (clones), except for rare mutations or errors.
- Suitable for growth and maintenance: tissues retain their genetic composition.
Meiosis
- Halves chromosome number: 2n → n.
- Produces genetically diverse haploid cells due to:
- Crossing over in prophase I.
- Independent assortment of homologous chromosomes in metaphase I.
- When two gametes fuse during fertilization, the diploid chromosome number is restored:
$$$$
n \;+\; n \;=\; 2n
$$$$ - This cycle of meiosis and fertilization is fundamental to sexual reproduction and evolutionary adaptation.
Special Case: Nuclear Division Without Cell Division
Sometimes, the nucleus divides without the cell dividing:
- This can lead to multinucleated cells (e.g., in some fungi, muscle fibers, or plant tissues).
- Repeated DNA replication without nuclear division or cell division can produce polyploid cells (more than two chromosome sets: 3n, 4n, etc.), which is especially common and biologically important in plants.
Prokaryotes: Binary Fission (Contrast Only)
Prokaryotes (e.g., bacteria) lack a membrane-bound nucleus and typical chromosomes with histones. Their division process is called binary fission:
- The circular DNA molecule is replicated.
- The two DNA copies attach to different regions of the plasma membrane.
- The cell elongates and then constricts in the middle.
- Two daughter cells form, each with one DNA molecule.
Although simpler and not involving mitosis or meiosis, binary fission has the same basic purpose: accurate distribution of genetic information to daughter cells.
Summary
- Nuclear division ensures that genetic material is accurately copied and distributed to daughter cells.
- Mitosis: one division, no change in ploidy, genetically identical daughter cells; essential for growth, repair, and many forms of asexual reproduction.
- Meiosis: two divisions, reduction of ploidy, genetically diverse haploid cells; essential for sexual reproduction.
- Processes like crossing over and independent assortment in meiosis generate genetic variation, forming the basis for diversity within populations and evolutionary change.