Re: Do chromosomes exist before prophase of the cell cycle?

Hi Emily,

That’s a lot of questions!
Let’s take them in order:

>Chromosomes only become distinct during prophase of the cell cycle;
>does that mean that they do not exist before prophase?

Chromosomes exist at all times in a cell, not just when they are clearly visible during mitosis. During interphase, each chromosome is a long thin strand of DNA, too thin to be seen through a light microscope. During mitosis, each strand of DNA undergoes several steps of packaging which results in the comparatively short and thick condensed version of a chromomsome that is visible under the light microscope.

When chromosomes were first discovered, their function and composition was not understood. Initially, all that was known was that they appeared in the nucleus before cell division, then disappeared afterwards. Some sources still seem to imply that chromosomes are only called chromosomes when they are condensed and visible, but in practice biologists do not generally make that distinction. For example, in baker’s yeast, the DNA never condenses to the point that chromosomes are visible, but everyone agrees that they have chromosomes.

>Does DNA begin to supercoil around its associated histone
>proteins prior to prophase, or just during prophase?

DNA is associated with histones at all times. However, in preparation for mitosis, more histones become associated with the DNA to allow packaging into the fully condensed form.

>What is the physical state of DNA molecules prior to prophase? I've heard
>that DNA molecules are in a diffused state called chromatin in the nucleus
>prior to cell division. But what exactly is chromatin? As I understand it,
>chromatin is a substance of diffuse DNA molecules and histone proteins. Are
>the DNA molecules actually attached to the histone proteins before
>prophase/the beginning of cell division?

Your understanding is basically correct. I’ll add that chromatin can occur as heterochromatin and euchromatin. Heterochromatin has a higher histone content, is more highly condensed, and not highly expressed. Euchromatin contains fewer histones, is less condensed, and the genes in euchromatin are in general more highly expressed. Prior to prophase, some of the DNA is in the form of euchromatin, some is in the form of heterochromatin. As mentioned above, histones are associated with DNA at all times, although the quantity of histones bound to the DNA varies along the length of each chromosome and during the course of the cell cycle.

>Has a DNA-protein complex already been formed at this point?
>Does the DNA-histone complex perhaps form as soon as DNA
>molecules are formed from DNA replication during S phase of interphase?

As replication proceeds, histones must be temporarily removed from the DNA to allow DNA polymerase to replicate the DNA. However, they reassociate with the DNA immediately afterwards, basically as soon as the newly formed strands are exposed.

>When then does the supercoiling of the DNA and histones begin?

The DNA is always supercoiled to some degree. Short regions may be temporarily “relaxed” for replication or gene expression. As described above, newly replicated DNA wraps/coils around histones as it is formed, and those coils are supercoils.

>Also, I've heard conflicting statements about the number of DNA molecules
>in a chromosome. Is it 1 or 2? If it's 2, is it because there is 1 DNA
>molecule in each chromatid of the chromosome?

Before replication, each chromosome is one molecule. After replication, it is a duplicated chromosome made of two sister chromatids, each a separate DNA molecule. The two are held together by protein complexes known as cohesins. So at this point, you would say it is two molecules. Once anaphase is underway, the two chromatids are no longer held together, so each chromosome is once again a single molecule.

There’s a little bit of semantics involved here. Before replication, each chromosome is one double stranded DNA molecule. Biologists routinely use the phrase “double stranded DNA molecule”. Strictly speaking, a single piece of double stranded DNA is two molecules, since the two strands are not covalently attached: they are separate molecules. For the purposes of this explanation, I have referred to a single piece of double stranded DNA as one molecule.

Dr. Alex Brands
Lehigh University