Asexual Reproduction and Mitosis

1Overview

A single bacterium placed in a nutrient-rich flask becomes a cloudy population in a day. No partner, no courtship and no seeds were needed: the cell simply copied itself and split. At the same time, a human body replaces skin cells every day without any change in its identity. Both facts depend on the same basic skill, copying DNA and dividing a cell, but the cells involved use very different methods. This lesson covers asexual reproduction in its several forms and the eukaryotic cell cycle that includes mitosis. The next lesson covers meiosis and sexual reproduction.

2Asexual and sexual reproduction: the big picture

In asexual reproduction, one parent produces offspring that are genetic copies of itself. In sexual reproduction, two parents each contribute a gamete (a sex cell), and the offspring carry a new combination of genes. The curriculum lists five processes: binary fission, budding and conjugation (usually found in single-celled organisms), mitosis, and meiosis. Mitosis underlies asexual reproduction in eukaryotes and growth in multicellular bodies, and meiosis underlies sexual reproduction.

3Binary fission in prokaryotes

Prokaryotes have a single circular chromosome in the nucleoid. In binary fission, one body separates into two new bodies, and each new cell receives one copy of the DNA. The steps are:

1. The circular chromosome is copied (replicated).
2. The two copies move toward opposite ends of the elongating cell.
3. The cell divides by cytokinesis, building a partition between the two sets of DNA.
4. Two daughter cells result, each with its own chromosome and genetically identical to the parent unless a mutation occurred.

Binary fission is also used by protists, which may divide along a transverse or a longitudinal axis. Mitochondria and chloroplasts divide in a similar way, a clue to their bacterial ancestry (see the previous lesson).

Budding

In budding, a new individual develops as an outgrowth of the parent. The bud grows into a copy of the parent and may detach and live independently or stay attached and form a colony. In yeast, a unicellular fungus, a bulge forms on the cell, the nucleus divides by mitosis, and the bud detaches from the parent. The cell divides unequally: the bud starts smaller than the parent. Hydra, a small freshwater cnidarian, also reproduces by budding, growing a new individual from a specific site on the body.

Other asexual methods in multicellular life

• Fragmentation. A body or fungal hypha breaks into pieces and each piece grows into a new individual. Planarians (flatworms) fission and regenerate missing parts, so each portion becomes a complete animal.
• Spores. Many fungi produce spores by mitosis that are genetically identical to the parent thallus. Spores are carried by wind or animals.
• Strobilation. In some animals, such as tapeworms and the polyp stage of certain jellyfish, regular transverse fission produces a chain of units. These units are only indirectly reproductive, but they show that fission is not limited to single cells.

Careful wording matters on exams. "Asexual" refers to the number of parents and the absence of gametes, not to the simplicity of the organism. A hydra is a multicellular animal with tissues, and yet it can reproduce by budding.

4Conjugation: gene transfer, not copying

Bacteria also exchange DNA through direct contact. In conjugation, a donor bacterium transfers DNA to a recipient. In E. coli, donors carry the F plasmid (fertility factor); donors are called F⁺ and recipients F⁻. The donor builds a sex pilus that attaches to a recipient and retracts, drawing the cells together to form a mating bridge. One strand of the plasmid is nicked and passed through the bridge, each cell makes a complementary strand, and the recipient becomes F⁺ and can donate in turn.

5Chromosomes and the eukaryotic cell cycle

A eukaryotic genome is several linear DNA molecules packaged with proteins. DNA wraps around histone proteins to form nucleosomes, and the resulting chromatin coils further. Chromosomes are most condensed during mitosis. After DNA copying, each chromosome consists of two identical sister chromatids joined at the centromere.

Human body (somatic) cells have 46 chromosomes in 23 pairs; sperm and egg cells have 23. Cells with two sets are diploid (2n) and cells with one set are haploid (1n).

The cell cycle is the ordered series of events from one division to the next. It has two main parts: interphase (growth and DNA copying) and the mitotic phase (mitosis plus cytokinesis).

• G1: the cell grows and builds DNA components, proteins and energy reserves.
• S: DNA is replicated, producing sister chromatids; the centrosome also duplicates.
• G2: the cell adds proteins and energy stores needed to move chromosomes, and duplicates some organelles.
• G0: some cells stop dividing temporarily or permanently, as in mature cardiac muscle and nerve cells.

6Mitosis, phase by phase

Mitosis is the division of the nucleus into two nuclei, each with the same chromosome set as the parent. In the order PMAT:

1. Prophase. Chromosomes condense and become visible. The nuclear envelope breaks down, the nucleolus disappears, centrosomes move toward opposite poles and the spindle starts to form.
2. Prometaphase. Kinetochores form at the centromeres and spindle microtubules from opposite poles attach to them.
3. Metaphase. Chromosomes line up at the metaphase plate, the midline between the poles.
4. Anaphase. Cohesin proteins break down, and sister chromatids separate and move toward opposite poles. The cell elongates.
5. Telophase. Chromosomes reach the poles and decondense, nuclear envelopes reform and the spindle breaks down.

Cytokinesis: dividing the cytoplasm

Cytokinesis usually overlaps with late mitosis. In animal cells, a contractile ring of actin filaments forms a cleavage furrow that deepens until the cell is pinched in two. Plant cells have walls and cannot be pinched. Golgi vesicles gather in the middle and form a cell plate that grows outward and fuses with the existing walls, and a new wall is then built between the daughter cells.

Strengths and limits of asexual reproduction

• Advantages: offspring are copies of a successful parent; reproduction can be very rapid; no partner is needed, so every individual can reproduce.
• Disadvantages: without sex, variation comes only from mutation, so a population may be uniformly vulnerable to a new disease or environmental change.

Comparing division in prokaryotes and eukaryotes

The underlying logic is the same: the genetic information is copied first, and the copies are then distributed so that each daughter cell gets a complete set. The eukaryotic version needs more machinery because there are several chromosomes, and each of them must be sorted accurately.

Using mitosis: growth, repair and the cells that stop dividing

In a multicellular organism, mitosis builds and maintains the body. A fertilized egg divides repeatedly to produce all the body's cells. Later, mitosis replaces worn-out cells and repairs damage, such as skin cells replaced after a scrape. Different cell types divide at different rates. Some cells enter G0 and do not prepare to divide: mature cardiac muscle cells and nerve cells are examples of cells that may remain in G0 permanently, while others enter G0 temporarily depending on conditions such as nutrient availability or growth-factor signals.

Evaluating the evidence: asexual reproduction in the real world

Asexual reproduction is common in unicellular organisms and in fungi, and it appears in some multicellular animals such as hydra and planarians. The textbook point is that nearly all multicellular organisms reproduce sexually, and purely asexual multicellular organisms are exceedingly rare. This pattern raises a question that the next lesson answers: if asexual reproduction is faster and needs no partner, what advantage has kept sexual reproduction so widespread?

7Summary

• Asexual reproduction yields genetically identical offspring through binary fission, budding, fragmentation, spores and mitosis.
• Conjugation moves genes between bacteria without producing new cells.
• The cell cycle consists of G1, S, G2 and M, with cells that stop dividing in G0.
• Mitosis (prophase, metaphase, anaphase, telophase) divides the nucleus; cytokinesis divides the cytoplasm.
• Animal cells divide by a cleavage furrow; plant cells build a cell plate.

?Quick check

Try each question first, then reveal the answer.