Chromosomes

Chromosome Definition

Let's begin by stating the definition of a chromosome.

Chromosomes are part of a cell's Genome. Here we will explore how chromosomes function within the cell, the structure of chromosomes, explain genomes and homologous chromosomes, and provide an example of sex chromosomes.

Chromosome Function

Though it is microscopic, DNA is an extremely long, string-like molecule. If all the DNA in just one of your Cells was unfolded and stretched end to end, it would be almost 10 feet (3.05 m) long.

Your body is made up of nearly 37 TRILLION Cells. If all of your DNA was stretched out like that, it would extend for 111 billion kilometers, or 68.9 billion miles. To manage this, your cells wrap up DNA when it is not in use.

Let's look at a simpler analogy:

Chromosome Structure and Location

Prokaryotic Chromosomes

A prokaryotic chromosome is a single, long double-stranded DNA molecule that forms a circular loop. The region of the cell where prokaryotes keep their chromosome is called the nucleoid.

Below is a diagram showing the structure of a prokaryotic cell with the chromosome in the nucleoid (figure 1). Prokaryotes often pick up smaller loops of DNA from their environment, these are called plasmids. These are not essential to the life of the prokaryote, but often hold advantageous traits, like antibiotic resistance, and are one-way beneficial traits are passed between bacterial cells in colonies.

Figure 1. Typical prokaryotic cell showing DNA chromosome and nucleoid, wikipedia.org

Eukaryotic Chromosomes

Eukaryotes have many linear DNA molecules separated into chromosomes. Chromosomes are located in the eukaryotic cell's nucleus, and they are most visible in the cell during cellular division.

Chromosomes are formed during complex hierarchical levels of folding. It starts with a long double strand of replicated DNA, after the S phase in the Cell Cycle. The double helix then wraps around a protein called a histone, very similar to wrapping the string of the kite around its spool, or wrapping beads in a string. This DNA and histone "bead" is called a nucleosome.

The nucleosomes continue to coil up, almost like a slinky. This set of multiple coiled nucleosomes is called chromatin. Chromatin is then wrapped up even further around scaffolding Proteins. Finally, all the chromatin wraps around the scaffolding Proteins, forming a chromosome (Fig. 2).

Each time the DNA wraps itself in this way, the physical length gets shorter and shorter, allowing the DNA to be easily stored in the nucleus and kept safe during Cell Division.

Figure 2. The organizational structure of a chromosome, openstax.org

Looking at the last row above, you'll see the duplicated chromosome. The chromosome is duplicated because chromosomes fold after DNA replication, the S phase of the cell cycle.

Since this occurs after DNA replication, the chromosomes look like little X shapes. Each half of the X is identical, replicated DNA, called a chromatid.

Together in this X shape, they are called sister chromatids because they came from the same "parent" during DNA replication. They are held together by a structure in the middle of the X called a centromere.

During cell division, all the cell's chromosomes line up with their centromeres in the middle of the cell. Spindle fibers attach to the centromere to pull the sister chromatids apart into the two new daughter cells.

Genome and homologous chromosomes

Now, let's elaborate on the genome and learn about homologous chromosomes.

In prokaryotes, this includes their singular circular chromosome and the plasmids they pick up from the environment. In eukaryotes, their genome includes their mitochondrial DNA and all of their nuclear chromosomes.

Each species has a certain number of chromosomes in its nuclei. The average human cell, a somatic cell, has a total of 46, and human gametes (the sex cells such as eggs and sperm) have half that number, 23.

When gametes come together during sexual Reproduction, the fertilized egg will have a full set of matching chromosomes, 23 from the sperm and 23 from the egg.

Because the number of chromosomes differs from species to species, scientists use the letter n to designate the number of chromosomes received from the organism's parent. In the case of humans, n would be 23 chromosomes, and the somatic cell would have 2n, or 46 chromosomes. We also have special terms to designate this: we call cells that have 2n chromosomes diploid, and cells that have just one n chromosomes haploid.

Biologists call the matching set of 2n chromosomes homologous chromosomes.

Genes are the functional unit of a chromosome and each gene holds the necessary information to make proteins for a specific characteristic.

A pair of homologous chromosomes both have genes for hair color, but each chromosome might have a slightly different version, a different trait. Both of the chromosomes have information that codes for hair color, but the chromosome you received from the egg might hold instructions for a dark black hair trait, but the chromosome you received from the sperm might hold instructions for a red hair trait.

Each chromosome in a homologous pair comes from a different parent, and is therefore going to be at least slightly different. Even minor differences in the DNA sequences of a homologous pair can lead to different traits, and the combination of the two together allows for even more variation within a species.

XX chromosomes and XY chromosomes

Each chromosome in a homologous pair is almost exactly the same length and holds genes for the same characteristics. The notable exceptions to this rule are the sex chromosomes: X, and Y.

The X chromosome is much larger than the Y chromosome, and only a small portion of these chromosomes have homologous genes, which are genes for the same characteristics. These genes help to determine the sexual characteristics of females and males.

• Typically, humans with XX chromosomes have female traits and humans that have XY chromosomes have male traits.

Below is an image, called a karyotype, showing the 23 homologous human chromosomes. It depicts each pair and the difference in the length of the X and Y chromosomes (Fig. 3).

Figure 3. A human karyotype shows the 23 pairs of homologous chromosomes, including the difference between X and Y chromosomes. wikipedia.org