What is a Chromosome?

A chromosome is a segment of DNA that holds many genes
Chromosomes artwork. Getty/Science Photo Library - SCIEPRO

Evolution is defined as a change in species over time. The Theory of Evolution is based on the idea of natural selection. Natural selection is the mechanism that drives evolution by allowing individuals in a population with desirable adaptations for that environment to live long enough to pass down those traits to their offspring. However, when Charles Darwin first published this hypothesis, it was unknown exactly how those traits were passed down.

Now, armed with the knowledge of DNA and with an emphasis on microevolution, the Modern Synthesis explains how Genetics is linked to evolution. Perhaps the single most important player in the passing down of traits from one generation to the next is the chromosome. What exactly is a chromosome and how is one made within the eukaryotic cell?

Definition of a Chromosome

The actual definition of a chromosome is a unit of deoxyribonucleic acid (DNA) that contains genes that code for proteins. When those proteins are expressed, a certain phenotype is seen on the individual. However, a chromosome is much more than just a strand of DNA.

Structure of a Chromosome

Cells need to divide for several reasons. Sometimes the cell is becoming too big, other times the body needs more cells to repair or replace other damaged or dead cells. DNA, in its natural state, is a double stranded molecule that is extremely long.

If a cell is going to divide, it needs to make sure it doesn't accidentally chop up the DNA during the division or the new cells will not be able to function properly. This is where the chromosome comes into play.

A chromosome is a condensed form of DNA that ensures it will not be cut during cell division.

The long strands of DNA wrap around a core that consists of proteins called histones. 8 of these histone proteins are arranged in a very specific way so the DNA can wind smoothly around it. Once the DNA is wrapped around the histone core, it is now a chromosome.

Chromosomes have two parts called sister chromatids. Sister chromatids are identical to each other and were formed during DNA replication. They are held together by the centromere. Each sister chromatid will go into a different cell during cell division. This ensures each cell is an exact copy of the original cell and that it will function properly.

In most sexually reproducing organisms, chromosomes are arranged in homologous pairs. One of the chromosomes in the pair came from the organism's mother and one came from its father. These chromosomes hold the information for the same traits and the combination of both determines what phenotype the offspring will show.

Chromosomes and Genes

A gene is a specific sequence of DNA that codes for a trait. Each chromosome contains many genes. Not all genes that are coded for are expressed. There are some that need environmental or other triggers in order to be expressed. Some traits are actually a combination of information from several genes on many different chromosomes.

Chromosomes and Evolution

Even though Darwin had no idea about Genetics and Gregor Mendel first discovered the discipline after Darwin proposed his Theory of Evolution, chromosomes play a major role in the changing of species over time. The mechanism of natural selection does not work if there are not differing traits to choose from in the population. Chromosomes carry this information within the eukaryotic cells and are the physical pieces that get passed down from parents to offspring. Without chromosomes, there would be no way to transfer this type of genetic information from generation to generation.