12.3 Organization of DNA in the cell

The Genome

The cell’s entire genetic content is its genome. Genomes consist of one or more chromosomes. Each chromosome is a single, double-stranded molecule of DNA. Prokaryotes generally have a single circular chromosome. Eukaryotes generally have multiple linear chromosomes that are enclosed within a membrane-bound nucleus. Each eukaryotic species has a characteristic number of chromosomes per cell.  For example, humans have 46 chromosomes per cell. Eukaryotic genomes also include mitochondrial DNA and/or plastid DNA. These organelles have their own DNA because they were originally derived from free-living bacteria.

Chromatin and Chromosomes

Chromatin is the material that makes up a chromosome. It consists of DNA and proteins. The major proteins in chromatin are histone proteins, which function to package and condense the DNA molecule. Each chromosome contains one or two double-stranded DNA molecules.  The word chromosome is composed of two parts: “chromo” meaning colored or stained and “some” meaning object or body. It is important to recognize that chromosome refers to a complete object. A single chromosome may contain thousands of genes.

Organization of DNA in a chromosome
Chromatin is the material that makes up a chromosome. It consists of DNA and protein. The major proteins in chromatin are called histones. They act as packaging elements for the DNA. (Figure by the National Human Genome Research Institute is in the public domain).

When a eukaryotic cell is actively undergoing cell division, the chromatin is tightly packaged into a compact chromosome structure. When a cell is not actively dividing, the chromatin is more relaxed and spread out so that gene expression can take place.

 

Chromatin and Chromosomes
Each chromosome in this figure has been labeled with fluorescent dye. At the top, the cell nucleus shows chromatin in its relaxed conformation. At the bottom, chromatin is shown in its tightly packaged, chromosome conformation. The chromosomes are labeled and placed in pairs. Humans have 23 pairs of chromosomes, for a total of 46. (Figure by the National Human Genome Research Institute is in the public domain).

Genes

A gene is defined as a sequence of DNA that codes for a functional product. Many genes contain the information to make protein products. Other genes code for RNA products. On each chromosome, there are thousands of genes that are responsible for determining the genotype and phenotype of the individual.  The human genome contains about 3 billion base pairs and has around 20,000 genes that code for proteins.

 

A gene can code for either a protein product or a non-coding RNA
Genes can encode either proteins or non-coding RNA molecules that have a function in the cell. (Created using BioRender.com by Melissa Hardy and used under a Creative Commons Attribution-NonCommercial license.)
Text adapted from OpenStax Biology 2e and used under a Creative Commons Attribution License 4.0.

[1]

  1. Protein structure PDB ID: 1GPL Withers-Martinez, C., Carriere, F., Verger, R., Bourgeois, D., Cambillau, C. (1996). A pancreatic lipase with a phospholipase A1 activity: crystal structure of a chimeric pancreatic lipase-related protein 2 from guinea pig. Structure 4:1363 ↵
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