The cell cycle and its phases
Read this section in the notesHow long does the cell cycle take in humans and in yeast?
A typical human cell in culture divides about once in 24 hours, while yeast can complete the cycle in about 90 minutes. The actual division takes only about an hour in a 24-hour cycle, and the rest, more than 95 per cent, is interphase. Cell cycle length therefore varies with the organism and the cell type.
Interphase: G1, S and G2
Read this section in the notesIs interphase a resting phase?
No. Interphase is the busiest preparatory part of the cycle, in which the cell grows and copies its DNA, and it makes up more than 95 per cent of the cycle. It runs through G1, S and G2. The visible division, the M phase, is short in comparison, so the cell is not resting during interphase.
Does the chromosome number double in the S phase?
No. In S phase the amount of DNA doubles, from 2C to 4C, but the chromosome number stays at 2n. Each chromosome now consists of two chromatids joined at one centromere. In animal cells the centriole also duplicates in the cytoplasm during this phase.
Quiescent stage and which cells divide
Read this section in the notesWhat is G0 phase and is the cell dead in it?
G0, the quiescent stage, is a phase that cells enter from G1 when they stop dividing. The cells are not dead, as they stay metabolically active, but they do not multiply unless called upon to do so. Heart cells are an example of cells that remain in G0 in the adult body.
In which cells does mitosis occur?
In animals, mitosis occurs in diploid somatic cells, with male honey bees as an exception because they are haploid and still divide by mitosis. In plants, it occurs in both haploid and diploid cells. Mitosis is called equational division because the daughter cells have the same chromosome number as the parent.
Prophase and metaphase
Read this section in the notesWhat happens in prophase of mitosis?
In prophase the chromosomes condense and become visible, the centrioles move to opposite poles and the spindle starts to form. By the end of prophase the Golgi complex, endoplasmic reticulum, nucleolus and nuclear envelope are no longer visible. The cell then enters metaphase, where the chromosomes align.
Why is metaphase the stage used to study chromosome morphology?
At metaphase the chromosomes are fully condensed and lined up on the equatorial plate, so their shape and number are easiest to see. Each chromosome shows two chromatids joined at a centromere carrying disc-shaped kinetochores, and spindle fibres attach there. Karyotypes are therefore usually prepared from cells arrested at this stage.
Anaphase and telophase
Read this section in the notesWhat happens during anaphase of mitosis?
The centromeres split and the sister chromatids separate, each now a chromosome in its own right, and move toward opposite poles. The centromere leads and the arms trail behind. Anaphase gives each pole an identical set, and then telophase begins with the chromosomes decondensing and the nuclear envelope re-forming.
Cytokinesis
Read this section in the notesWhat is the difference between karyokinesis and cytokinesis?
Karyokinesis is the division of the nucleus, and cytokinesis is the division of the cytoplasm that follows it. If karyokinesis occurs without cytokinesis, a multinucleate syncytium results, as in the liquid endosperm of coconut. Animal cells divide by a furrow, while plant cells divide by a cell plate.
How is cytokinesis different in plant and animal cells?
An animal cell divides by furrowing, where the plasma membrane constricts and pinches the cell in two. A plant cell cannot do this because of its rigid wall, so a cell plate forms in the centre and grows outwards until it meets the side walls. The cell plate eventually becomes the middle lamella.
Significance of mitosis
Read this section in the notesWhat is the significance of mitosis?
Mitosis keeps the chromosome number constant, so daughter cells are genetically identical to the parent. It allows growth by increasing cell number, replaces cells lost to wear and tear, and helps repair tissues. It also maintains the nucleo-cytoplasmic ratio. In meristems it keeps adding cells throughout a plant's life.
Meiosis and its key features
Read this section in the notesHow is meiosis different from mitosis?
Meiosis has two divisions after a single round of DNA replication and gives four haploid cells, while mitosis has one division and gives two diploid cells. Meiosis involves pairing of homologous chromosomes and crossing over. It occurs in cells that form gametes or spores, and it is called a reductional division.
Prophase I
Read this section in the notesWhat is the correct order of the substages of prophase I?
The order is leptotene, zygotene, pachytene, diplotene and diakinesis. A simple way to remember what happens: pairing at zygotene, crossing over at pachytene, chiasmata visible at diplotene, and terminalisation at diakinesis. After that, the nuclear envelope breaks down and the cell moves to metaphase I.
In which stage does crossing over occur?
Crossing over occurs in pachytene of prophase I, between non-sister chromatids of homologous chromosomes, at recombination nodules, and it is catalysed by the enzyme recombinase. Pairing begins earlier, in zygotene, with synapsis. The chiasmata seen later in diplotene are only the visible results of crossing over.
What are chiasmata and when are they visible?
Chiasmata are the X-shaped structures that mark the sites where non-sister chromatids have crossed over. They become visible in diplotene, after the synaptonemal complex dissolves and the homologues begin to separate. In diakinesis they move towards the chromosome ends, called terminalisation. Diplotene can last for months or years in the oocytes of some vertebrates.
Metaphase I to interkinesis
Read this section in the notesWhat is the difference between anaphase of mitosis, anaphase I and anaphase II?
In mitotic anaphase and in anaphase II, the centromeres split and sister chromatids separate. In anaphase I the centromeres remain intact and whole homologous chromosomes are pulled apart, with each still having two chromatids. So anaphase I is the one that reduces the chromosome number to half.
Is DNA replicated during interkinesis?
No. Interkinesis is the short stage between meiosis I and meiosis II, and no DNA replication takes place in it. Meiosis has only one S phase before the two divisions, which is the reason the final cells are haploid. The cells at the end of meiosis I are a dyad, and meiosis II ends with a tetrad of four.
Significance of meiosis
Read this section in the notesWhat is the significance of meiosis?
Meiosis keeps the chromosome number constant over generations in sexually reproducing organisms by halving it in the gametes, so that fertilisation restores the diploid number. It also increases genetic variability through crossing over and independent assortment of homologues, which provides material for evolution.