Introduction to chromosomes

Watch this video for an introduction to chromosomes (2:16)

You have learned about two types of meristems: apical meristems, that result in primary growth, and lateral meristems, like vascular cambium and cork cambium, that result in secondary growth. Meristems are sites of cell division in plants. These new cells may themselves divide once or twice, but then they begin to enlarge and differentiate into cells with specialized function depending upon the type of tissue in which they occur. (Parenchyma cells are an exception because they remain undifferentiated. They can become meristematic and divide to form plant parts like lateral roots, to form interfascicular cambium, and to respond to wounds by filling in space with new cells called callus tissue.)

For a cell to divide into two identical cells, it is critical that all the components of the original cell are duplicated prior to cell division and then distributed between the two sister cells before they separate. It is also critical that the hereditary material, the DNA in the nuclear chromosomes, be exactly duplicated and equally distributed between sister cells. Exact duplication and equal distribution ensure that each cell in the organism has all the genetic instructions it needs to carry on its metabolic processes, and that every cell in the organism has the same instructions.

The type of cell division that takes place in meristems, which is the type where one cell divides into two identical sister cells, is called mitosis.

Mitotic cell division was first observed under light microscopes well over 150 years ago. Microscopists, scientists who use microscopes, noticed dark-staining cell bodies lined up in the middle of a cell that was about to divide. They called these cells chromosomes (based on the Latin for dark staining, “khrôma,” and bodies, “sôma”). Once lined up in the middle of the cell, the chromosomes divided and moved to opposite poles in the cell just prior to the actual division of the cell into two sister cells. It was clear to those scientists that the process of cell division ensured that the chromosomes were specially and carefully handled during cell division. In 1902, Water Sutton, studying grasshoppers, provided proof (optional reading) that chromosomes contained the hereditary material for the organism. That’s why cell division includes such careful division of the chromosomes — the organism needs to ensure that every cell has an exact copy of all of the hereditary material associated with that organism.

What makes up these chromosomes? The short answer is that chromosomes found in the nucleus of plant cells are composed of chromatin (optional reading). Chromatin is made up of DNA wrapped around proteins, called histones. These proteins around which the DNA wraps are called histones. We’ll start with the structure of DNA and build up to a chromosome.

DNA structure

Watch this video for an explanation of DNA structure (4:35)

DNA is a double-stranded chemical polymer (a polymer is several types of molecules bonded together) that looks like a flexible ladder twisted on its long axis. Each side, or single strand of the ladder, is made up of a chain of alternating ribose and phosphate molecules. Ribose is a sugar molecule composed of a ring of five carbons. These molecules are linked to each other by a phosphate molecule made up of one phosphorus atom and two oxygen atoms. This sequence of ribose and phosphate molecules constitutes what is called the DNA’s ribose-phosphate backbone.

The steps, or rungs, of the ladder are constructed from pairs of bases. Four types of these bases compose both strands of DNA. Two of the bases, collectively called purines — Adenine (A) and Guanine (G) — contain two rings of carbon atoms. The other two, collectively called pyrimidines — Cytosine (C) and Thymine (T) — have only one ring of carbon atoms. There is enough space between the sides of the ladder (or ribose-phosphate backbone) to fit two bases with a total of three carbon rings, so each rung of the ladder always has one purine bonded to one pyrimidine.

There isn’t enough space for two purines, and there is too much space for two pyrimidines. And because of the molecular structures of the bases, Adenine (A) always bonds with Thymine (T), because they each can share two hydrogen bonds, and Cytosine (C) always bonds with Guanine (G), because they each can share three hydrogen bonds.

As a consequence, the rungs on the ladder are always made up of AT, TA, GC, or CG. And the sequence or order of As, Ts, Gs, and Cs making up the rungs along one strand of the ladder is the genetic code carried in the DNA. With only rare exceptions, the code is read three-bases at a time and encodes all of the information for a plant’s life cycle.

Nucleosomes

Watch this video for a description of nucleosomes (2:46)

We now have a DNA helix made up of alternating ribose and phosphate rails with rungs containing a purine and a pyrimidine. The DNA helix strand next loops around nucleosomes (optional reading) which are made up of histone proteins (illustration above). The histone proteins are essentially the spools around which the DNA thread wraps, except that DNA doesn’t wrap continuously around one histone protein the way thread wraps around one spool. It makes a couple of loops and then moves on to the next histone protein.

The loops of DNA around these nucleosomes are separated by portions of naked DNA called linker DNA, so the effect is called “beads on a string” (see micrograph below). where the beads are the DNA loops and histone protein, and the string is the strand of linker DNA. This description dates back to the early days of exploring DNA using light microscopy. Chromosomes could be observed with the aid of the microscope and described, but their composition was unknown.

The “beads on a string” structure is further coiled into a 30 nm (nm=nanometer, one billionth of a meter) chromatin fiber, which further folds even more, with the association of other proteins, to form the chromosome.

A chromosome, then, is made up of DNA that has looped around histone proteins, then coiled, then folded. Think of a chromosome as a tightly and carefully packed long thread of DNA that is associated with histone proteins to help with the packing.

Here’s a diagram of this coiling:

Above is a rendering of a micrograph of a chromosome at the cell division stage, when the chromosome is most highly condensed (which is during a phase of mitosis called metaphase, as we will see later) and compactly packed. At this phase there is extensive coiling (DNA coiling is called “condensation”) of the chromatin (DNA + histone proteins). Note that there are two sister chromatids. Just prior to cell division these sister chromatids will separate at the centromere (the constricted spot where they are attached) and move to opposite sides of the cell.

It is helpful to have this detail so you can recognize where the genetic code sits in the DNA molecule, and how DNA is folded up and condensed into a tight package during cell division. When a cell is not dividing, parts of the chromosome relax, unfold, and uncoil so that the DNA base pairs in specific parts of the chromosome that provide the code for specific cellular functions can open up, be copied, and the code can be translated into proteins that do the cell’s metabolic business — a phase called “interphase“). During cell division, however, the package is tightly condensed (metaphase).

The cell cycle

Watch this video about the cell cycle (3:38)

Below is a diagrammatic summary of the cell cycle, the cycle a cell goes through during its lifetime. You’ll see that about 3/4 of the cycle is called Interphase and the other 1/4 is called M (for mitosis). As noted, during interphase the cell isn’t dividing (but may be preparing to divide); the cell divides during M. Interphase is divided into three parts: G1 (the “G” is short for “Gap” or “Growth”), S (S for DNA “Synthesis”), and G2.

The stages in the cell cycle are:

• G1 — cells are enlarging, and some are differentiating into specialized cells like epidermis, collenchyma, sclerenchyma, and cells in the xylem and phloem tissue that will no longer divide. These differentiated cells stay in G1 until death.
• S — chromosomes in cells like apical meristem cells, vascular and cork cambium, or undifferentiated parenchyma cells prepare to divide by replicating their chromosomes. To prepare for division, the cell makes a complete copy of all of its DNA, so in the process of copying the DNA there is DNA Synthesis. Once a chromosome has been replicated, the two copies are called sister chromatids and they are held together at a spot called the centromere. The sister chromatids are exactly alike, down to the exact order of AT, TA, CG, and GC bases in the rungs of the DNA backbone.
• G2 — the cell builds up the chemical machinery that it needs for division.
• M — finally, the cell heads into mitosis (M), addressed next.

Note that the size of the slices of pie in the diagram above are not indicative of the actual time duration.