Cells spawn copies of themselves in the process called mitosis. A crucial process within mitosis is genetic (chromosomal) replication.
A chromosome is a genetic package in the nucleus of a eukaryotic cell, comprised of DNA base pairs. A DNA base pair comprises 2 complementary nucleobases on opposite sides of a DNA double helix, linked by hydrogen bonds.
DNA is the physical ledger of information which cells use to produce specific proteins and other bioproducts. Behind DNA and all cellular artifacts are unseen energetic quanta which retain knowledge and assist in the illusory fabrication of physicality by localized forces of coherence, which is the ordering principle in Nature. Nature is the exhibition of existence.
A chromosome is a single macromolecule, the largest in Nature. Human DNA has 46 chromosomes. Chromosome 1 in the human body, the biggest, has 10 billion atoms. The average human chromosome has ~140 billion base pairs.
“Most cells in a multicellular organism can be functionally very distinct despite the fact that they share the same genomic information. Cellular specialization during development is based on the ability to establish, maintain, and execute different gene expression programs,” explains German geneticist Axel Imhof.
Mitosis begins with chromosomal DNA base pairs being cleaved in half: the hydrogen bonds holding each ladder rung broken by enzymes. This happens in several sections simultaneously. To keep things tidy, proteins neatly stack the chromosomes in layers, forming multilaminar plates.
Then each half-ladder has its complement rebuilt. The result is 2 DNA ladders: half each of the original nucleotide string. The other side of each ladder is reconstructed.
Then these 2 sets of chromosomes are pulled apart from the center of the cell, in opposite directions, by protein ropes. These microtubules are part of the cytoskeleton, and play numerous roles in cell upkeep, mostly in moving bits about the cell.
Mitosis is an incredibly intricate process: an orchestration of many thousands of proteins moving through space, finding their exact positions at just the right time. While the proteins remember where they are supposed to go, and have an innate affinity to the right location, innumerable decisions must be made for cell division to succeed.
The protein workforce involved in mitosis is extensive and specialized. Certain enzymes supervise other proteins. Enzymes are a class of busybody proteins.
DNA sequences just the beginning of physical genetics. There are chemical signatures – epigenetic marks – which affect whether and how certain DNA are interpreted when it comes time to use genetic information.
These epigenetic marks are chemical artifacts which are distilled from life experiences. Organisms physiologically inherit the knowledge and preferences of their ancestors at the cellular level. Some of this occurs by changes in DNA (genetics), but much information is passed on epigenetically: outside of DNA sequencing.
Epigenetic influences take effect during mitosis. Hence, life experiences are passed on during each cell replication.
In higher organisms, most of the DNA in cells is condensed: densely packed as chromatin, in which DNA is wrapped around packaging proteins known as histones.
The functional accessibility of DNA depends on how it is packaged. Specifically, histones become modified in ways that modulate the accessibility of the DNA in chromatin, thus controlling whether the proteins that are instrumental in expressing genes can bind to the DNA and get the information they need to do their jobs.
The trick in cell inheritance is replicating the many layers of packaging about DNA to precisely define the identities of the different cell types, and have daughter cells remember all that their mother cells impart.
“Chromatin states must be maintained during cell proliferation to uphold cellular identity and genome integrity. Inheritance of histone modifications is central in this process,” explains Danish geneticist Anja Growth.
Chromatin replication is done in stages. First, the basic DNA patterns are copied. New histones for the daughter cell are made. Then chemical epigenetic marks, such as methylation, are transcribed and laid into place. During methylation, enzymes attach methyl groups onto histones: the sturdy proteins which stack and hold DNA in place. “Methylation is established on newly synthesized histones,” notes British geneticist Constance Alabert.
Mitosis takes 15–30 minutes per chromosome. Replication of a human cell DNA takes up to 10 hours. Yet human cells may divide as often as every day.
When the whole DNA molecule has been duplicated, the chromosomes move to opposite ends of the cell. Then cytokinesis kicks in: the cytoplasm of a eukaryotic cell divides; whence 2 cells are constituted.
Ishi Nobu, The Science of Existence, BookBaby (2019).
Ishi Nobu, Unraveling Reality: Behind the Veil of Existence, BookBaby (2019).
Ishi Nobu, “The mechanics of existence,” (10 December 2019).
Constance Alabert et al, “Domain model explains propagation dynamics and stability of histone H3K27 and H3K36 methylation landscapes,” Cell Reports (28 January 2020).