The ABCs of DNA — Chromosomes

Diagram of a chromosome: DNA and histones

What is a Chromosome?

When the very long polymeric molecule of DNA is wound around a protein called a histone into a thread like strand, it is called a chromosome. The histone not only supports the DNA strand, but on occasion it can control DNA expression by folding the DNA over a particular gene, physically blocking access to the gene and preventing it from being ‘read’ or transcribed.The DNA of eukaryotes and archae exists as chromosomes.

Not all organisms have chromosomes. In prokaryotes like bacteria the much smaller DNA molecules are circular. Prokaryotes have one large, main circular molecule of DNA and several smaller circular DNA molecules known as plasmids. This DNA material does not exist as a chromosome because no histones are involved.

For most of its existance, eukaryotic DNA does not exist in the characteristic two armed form we have all seen pictures of. Instead, the DNA exists like a pile of yarn that a catnip-inspired kitten has played with for hours. It is only during cell division that the chromosomes organize themselves into an even more tightly wound structure with two chubby arms divided by a constricted connective point called the centromere. This is the form the chromosome takes when it is ready to be copied for cell division. Only when it is in this compact form can DNA be photographed. Scientist lable the long arm on one side of the centromere as the “q” arm and the short arm on the other side of the centromere as the “p” arm.

The Human Karyotype

Human Karyotype: Paired up chromosomes from a human male.

Humans have 23 pairs of chromosomes. One chromosome of each pair came from your father and one from your mother. To produce the human karyotype seen above, the chromosomes were photographed during cell division, the only times they are visible, and after being separated from their duplicated sister chromatid. When the photo was taken, the chromosomes were not neatly lined up in homologous pairs as they are displayed in the above chart but randomly spread throughout the nucleus. The scientists identified each of the pair and cut and paste the photographs to assemble them into the neatly arranged karyotope chart.

When you see photographs of pairs of chromosomes attached at the centromere, such as illustrated both in the top diagram and the photo shown below, it is not your mother’s and your father’s chromosome contribution to you that you are seeing attached together. Instead, these photographs are taken during during cell division and are duplicated chromosomes, called sister chromatids, that will soon go to different new cells. We will discuss this phenominon again when we talk about crossing over, an important event that shuffles the genetic make up of each chromosome during meiosis (which takes place only to make new sex cells such as sperm and eggs) and not during mitosis (which takes place every time you create a new cell in your body.)

Micro-photograph of sister chromatids during cellular reproduction. Each copy carries identical DNA sequences.

 Sister Chromatids vs. Analogous Chromosomes

Sister Chromatids have the exact same sequence of base pairs — baring a replication mistake which is known as either a mutation, a micro deletion or a micro insertion. A micro deletion or insertion involves adding in or leaving out one — or rarely more — DNA bases during chromatid replication. A mutation is where the wrong base is inserted in the sequence by mistake. Anologous pairs — one from each parent — on the other hand, have the same genes and are the same shape with the centromere in the same location, but they are not exactly alike. While the vast majority of the over three billion base pairs in the human genome are exactly the same in all living humans (over 99.9%), a small percentage of these base pairs have some alternative bases that can be found in that location. It is these small  variations which make us all the unique individuals we are! We will talk more about these small coding variations when we discuss SNPs.

Diagram credit: National Institute on Health

Karyotype Micro-photograph credit: Hematogenix

Sister Chromatid Micro-photograph credit: W. W. Norton and Company, Inc.