Chromosome Mechanics


Chromosome mechanics describes the processes of chromosome replication and interactions during somatic cell division and gametogenesis. Mitosis can be divided into two phases consisting of nuclear and cytoplasmic division ultimately producing identical diploid somatic cells. Segregation during mitosis is largely dependent on chromosomal attachment to the mitotic spindle with a fully developed kinetochore structure. This is in contrast to meiosis where cells undergo two rounds of nuclear division to produce haploid germ cells. During this process, homologous chromosomes pair at the cellular midline allowing for exchange of deoxyribonucleic acid (DNA) between parental alleles and stabilisation of the chromosome structures during separation. Aberrations occurring in either mitosis or meiosis can result in numerical abnormalities (from nondisjunction events) which often result in nonviable offspring. However, there are some aneuploidies that are viable with trisomy 21 being the most common example of this. Similarly, unrepaired DNA damage resulting in chromosomal breakage and structural rearrangements. These large‐scale genetic changes reduce the genetic efficacy of a cell ultimately leading to cancer or cell death. In summary, mitosis and meiosis are integral parts of the cellular life cycle and both processes are essential at maintaining genomic stability within an organism.

Key Concepts

  • The cell cycle comprises two main phases: mitosis, where cell division occurs, and interphase, which includes DNA replication and the expression of many essential proteins. Interphase can be further divided into G1, S and G2.
  • Cyclin‐dependent kinases regulate progression through the cell cycle at three major checkpoints: G1, G2 and a spindle checkpoint during mitosis.
  • Mitosis is the process of cell division resulting in two identical diploid cells and can be divided into prophase, metaphase, anaphase and telophase.
  • Meiosis is the process of cell division that results in germline cells where following one round to DNA replication, the cell undergoes two cell divisions.
  • Meiosis in males results in four identical haploid cells, while meiosis in females produces a single mature ovum and three polar bodies.
  • Aberrations in either mitosis or meiosis can result in nondisjunction resulting in aneuploidy.
  • Defects in DNA repair mechanisms can also result in chromosome damage or breaking which will negatively impact the process of cell division.

Keywords: mitosis; meiosis; recombination; nondisjunction; spermatogenesis; oogenesis; homologous recombination; DNA damage

Figure 1. The stages of mitosis during the normal cell cycle. Cells enter mitosis from a resting state into interphase, the stage in which DNA (deoxyribonucleic acid) is replicated. Chromosomes (shown in green) become visible in early prophase and continue to condense through late prophase. During prophase, the spindle is formed by the two centrioles, and in late prophase, the nuclear membrane breaks down. Replicated chromosomes align on the equatorial plane during metaphase, sister chromatids disjoin during anaphase and cell division is completed in telophase, leaving two identical daughter cells that return to interphase.
Figure 2. The regulation of mitosis (M) and the interphase phases of Gap 1 (G1), Synthesis (S) and Gap 2 (G2) depends on numerous checkpoints. Mitotic cyclin‐dependent kinases (Cdks) and their associated cyclins (Cyc) regulate progression from one phase of mitosis to another. Cdk2/CycE control progression from G1 to S; CycA,B/Cdk1 from S to G2. APC/C regulates chromosome segregation through degradation of proteins such as CycB that permits cytokinesis to progress. Mitotic checkpoint genes such as BUB and MAD produce proteins that inhibit the activity of APC/C.
Figure 3. Sister chromatids are held together by cohesins to ensure the proper orientation of the chromosomes on the spindle and their separation at anaphase. Release of the cohesin complexes in the arms is accomplished by phosphorylation, while those at the centromeres require the action of separase, which is activated after its inhibitor, securin, is degraded.
Figure 4. The stages of meiosis during gametogenesis. Cells enter prophase I after DNA replication, although the chromosomes appear as single strands. Homologues begin to pair in zygotene, with chiasmata (points of recombination or crossing over) becoming evident as chromosomes continue to condense during pachytene. Further condensation and separation cause individual chromatids to become distinct during diplotene. A second decondensation occurs in the female during dictyotene that lasts for decades. Recondensation and movement of the chiasmata towards the chromosome ends take place during diakinesis. Paired homologues, each with two chromatids, line up on the equatorial plate during metaphase, and homologues disjoin towards the two poles during anaphase I. Telophase I (not shown) is generally very short and is punctuated by completion of cell division. Chromosome behaviour during the second phase of meiosis is strikingly similar to that in mitosis except that there are only 23 chromosomes present, each composed of two chromatids that disjoin at anaphase II to form haploid gametes.
Figure 5. Meiotic nondisjunction results in trisomy (47 chromosomes) or monosomy (45 chromosomes) after fertilisation of the aneuploid gamete with a normal gamete. (a) Normal segregation (disjunction) of one pair of acrocentric chromosomes. One chromosome has satellites on the short arm to distinguish the two members of the pair. These represent a heterozygous marker near the centromeres. (b) Nondisjunction at meiosis I (MI). Note that the marker satellites are heterozygous in the gametes. (c) Nondisjunction at meiosis II (MII). Note that the marker satellites are homozygous in the gametes.


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Further Reading

Campbell NA and Reece JB (2010) Biology, 9th edn. Menlo Park, CA: Benjamin Cummings. ISBN‐13: 978-0321558237.

Lodish H, Scott MP, Matsudaira P, et al. (2013) Molecular Cell Biology, 7th edn. New York: WH Freeman & Co. ISBN 9781464102288.

Moore KL and Persaud TVN (2013) The Developing Human, Clinically Oriented Embryology, 9th edn. Philadelphia, PA: WB Saunders. ISBN‐13: 978-143772002.

Pines J, Hyman A and Yanagida M (2015) Mitosis, 1st edn. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. ISBN‐13: 978-1621820154.

Stein GS and Pardee AB (eds) (2004) Cell Cycle and Growth Control: Biomolecular Regulation and Cancer, 2nd edn. Hoboken, NJ: John Wiley‐Liss. ISBN‐13: 978-0471250715.

Vogel F and Motulsky AG (2010) Human Genetics. Problems and Approaches, 4th edn. Berlin: Springer. ISBN‐13: 978-354037653.

Zhong A, Tan FQ and Yang WX (2016) Chromokinesin: kinesin superfamily regulating cell division through chromosome and spindle. Gene 16: 30392–30394.

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LeClair, Renée J, and Best, Robert G(Nov 2016) Chromosome Mechanics. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001441.pub3]