Chapter 4Cell Cycle

DNA replication: the DNA of each chromosome is copied, so each chromosome comes to consist of two sister chromatids. The chromosome number stays the same.

Anaphase. Spindle fibres pull the sister chromatids towards opposite poles.

In prophase I, non-sister chromatids of homologous chromosomes exchange segments at chiasmata. This creates new combinations of genes on chromosomes, so each gamete is unique and offspring differ from their parents.

They attach to the centromeres (kinetochores) of chromosomes, line them up at the equator in metaphase, and pull the sister chromatids to opposite poles in anaphase.

Animal cells: a furrow forms at the equator and a contracting ring of microfilaments pinches the cell in two. Plant cells: Golgi vesicles fuse in the middle to form a phragmoplast (cell plate) that grows outward.

Mitosis keeps the chromosome number the same (2n → 2n); meiosis halves it (2n → n).

Each spindle fibre pulls a single chromosome to its pole, so homologous chromosomes separate: one of each pair goes to each pole. Two haploid sets form; each chromosome still has two sister chromatids.

It duplicates; the two centrosomes move to opposite poles and make the spindle fibres (mitotic spindle) that move the chromosomes.

The two identical copies of a chromosome, made in S phase and joined at the centromere. In meiosis they separate in anaphase-II.

Animals like the sea star regenerate lost parts by forming new cells through mitosis in the cells of the remaining parts; mitosis gives identical cells, so the new part is like the old.

Cells that stop dividing and perform their specific functions are in G0. Neurons stay in G0 indefinitely, liver and kidney cells temporarily; epithelial cells do not enter G0.

About 200 trillion cells (the book's figure), all formed from a single zygote by millions of mitotic divisions with equal distribution of genetic material.

The series of events in a eukaryotic cell from its formation to its division into two daughter cells. Main phases: interphase and M (mitosis) phase.

G1 (first gap), S (synthesis) and G2 (second gap).

The cell grows further, makes proteins for division, checks and repairs DNA damage, and reorganizes its contents for mitosis.

The cell grows, works, replicates its DNA and prepares for division; it lasts about 90% of the cycle.

Division of a cell into two daughter cells with the same number of chromosomes as the parent; in somatic cells of eukaryotes.

The division of prokaryotes into two identical cells; its events differ from mitosis.

Karyokinesis is division of the nucleus; cytokinesis is division of the cytoplasm.

Chromatin condenses into chromosomes (two chromatids each); nuclear envelope and nucleolus break down; centrosomes move apart making spindle fibres.

The plate formed when chromosomes attached to spindle fibres arrange themselves along the equator of the cell.

Special proteins at the centromere where spindle fibres attach.

Plant cells have no centrosome; their spindle forms by aggregation of spindle fibres present in the cytoplasm.

New nuclear envelope forms around each set of chromosomes, nucleolus reforms, and chromosomes unfold into chromatin.

Growth, cell replacement, regeneration and asexual reproduction.

By budding: mitosis forms a mass of cells (bud) on its surface, which grows into a new Hydra.

Masses of cells formed by uncontrolled division when genes regulating mitosis mutate. Benign tumors stay in place; malignant tumors (cancer) invade other tissues.

The spreading of cancer cells from the original tumor to other tissues.

Cell division in which each daughter cell receives half the number of chromosomes of the parent; a diploid cell forms four haploid cells.

Diploid cells have chromosomes in homologous pairs (2n); haploid cells have half the number, no pairs (n).

Synapsis is the pairing of homologous chromosomes in prophase I; each pair (four chromatids) is a tetrad.

X-shaped structures where non-sister chromatids are zipped together; each is a site of crossing over.

Tetrads align along the equator; two spindle fibres from both poles attach with one pair of homologous chromosomes.

In both, sister chromatids separate and the chromosome number does not change in that division.

Gametes are haploid; their fusion at fertilization restores the diploid number in the zygote.

Failure of homologous chromosomes or sister chromatids to separate in meiosis, giving gametes with abnormal chromosome numbers.

Gametes in animals; spores in flowering plants.

The events of the cell cycle are controlled by special genes. At checkpoints (e.g. in G2, where the cell checks and repairs DNA damage) a damaged cell is stopped or repaired.

So checkpoint mistakes allow faulty cells to multiply instead of being repaired or destroyed.

Skin cells keep dividing

Skin (epithelial) cells are constantly worn away, damaged and dying; the book says epithelial cells do not enter G0 and continue to divide throughout life, replacing lost cells and healing wounds.

Nerve and muscle cells stop

Neurons remain in G0 for indefinite periods. Their complex shapes and fixed connections (long axons, networks of synapses) store information and coordinate the body; dividing would break these connections. Muscle cells are packed with contractile proteins arranged for contraction, and division would disturb this structure.

So cells that must be replaced keep dividing, while highly specialized cells that work as a fixed network leave the cycle.

Prophase

Chromatin condenses into thick chromosomes, each of two sister chromatids at one centromere. The nuclear envelope and nucleolus break down. The centrosome duplicates and the two migrate to opposite sides, making spindle fibres (mitotic spindle). Plant cells form the spindle without centrosomes.

Metaphase

Spindle fibres attach at the centromere (kinetochores); two fibres from opposite sides bind each chromosome. Chromosomes line up along the equator: the metaphase plate.

Anaphase

Spindle fibres pull towards the poles; sister chromatids separate and two similar sets move to the poles.

Telophase

New nuclear envelope forms around each set; nucleolus reforms; chromosomes unfold into chromatin.

Cytokinesis

The cytoplasm divides (furrow in animals, cell plate in plants), giving two identical daughter cells.

Cytokinesis is the division of cytoplasm, after karyokinesis.

In animal cells

A furrow develops in the cell membrane at the equator. Beneath it the cytoplasm has a ring of microfilaments. The ring contracts and the furrow moves inward, pinching the parent cell into two.

In plant cells

The Golgi apparatus makes vesicles which move to the middle and fuse to form a plate called phragmoplast (cell plate). The plate grows outward and its membranes fuse with the cell membrane, giving two daughter cells. A new wall forms in the plate.

Reason for the difference: the rigid plant cell wall cannot be pinched inward, so the division is built from the inside out.

Errors

If chromatids fail to separate or the genes regulating mitosis mutate, cells may get wrong chromosome numbers or divide uncontrollably, forming tumors (benign or malignant).

Prophase-I

Chromatin condenses into chromosomes of two sister chromatids. Homologous chromosomes pair (synapsis) to form tetrads. Non-sister chromatids are zipped at chiasmata, where crossing over exchanges segments, recombining genes. Nucleoli disappear, nuclear envelope breaks, centrioles go to opposite poles and make spindle fibres.

Metaphase-I

Tetrads align along the equator (metaphase plate); fibres from both poles attach with one homologous pair.

Anaphase-I

Homologous chromosomes separate: one of each pair goes to each pole, giving two haploid sets; each chromosome still has two sister chromatids.

Telophase-I

Spindles disappear; nuclear envelope and nucleolus reform; chromosomes uncoil; cytokinesis gives two haploid daughter cells.

1. Maintains the chromosome number

In animals, cells of reproductive organs undergo meiosis to make haploid gametes. Male and female gametes join in fertilization to make the zygote, restoring the original number; the zygote grows by mitosis. Without meiosis the number would double every generation.

In flowering plants, cells in flowers undergo meiosis to make haploid spores; these grow into a generation that makes gametes by mitosis; gametes fuse to form the zygote with the full set.

2. Brings genetic diversity

Crossing over creates new gene combinations; every gamete is unique, so the zygote differs from both parents. This diversity helps populations survive changes.

The series of events in a eukaryotic cell from its formation to its division into two daughter cells: interphase (about 90% of the time) and M phase.

G1

Growth phase: proteins and organelles made; cell grows; enzymes for DNA replication made.

S

DNA replicated; each chromosome now has two sister chromatids; number unchanged.

G2

Further growth; proteins for division; DNA damage checked and repaired; contents reorganised.

G0

Cells exit the cycle to do their jobs: neurons indefinitely, liver and kidney cells temporarily; epithelial cells never enter G0.

Prophase-II: nucleoli and nuclear envelope disappear; chromatin condenses; spindle fibres form. Metaphase-II: fibres attach to kinetochores; chromosomes align at the equator. Anaphase-II: sister chromatids pulled to opposite poles. Telophase-II: chromatin forms again; nuclear envelope and nucleolus reform; cytokinesis. Four haploid cells result.

Like mitosis

Sister chromatids separate; no pairing; no crossing over; no change in chromosome number during the division.

Unlike mitosis

It starts with haploid cells; no DNA replication before it; daughter cells are not identical to the original parent because of crossing over in meiosis I.

Similarities

Differences