Chapter 4: Cell CycleLong Questions with Answers

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