By Erica Brown March 2007
Updated by Ashley Port Thompson 2015
Tags: basics, chromosomes
The Link Between DNA, Chromosomes and Genes
How do chromosomes, DNA and genes all fit together? To figure out the puzzle, let’s start with the most basic piece: DNA
DNA stands for deoxyribonucleic acid. It is the molecule that contains the genetic instructions to construct and maintain a living organism. One molecule of DNA is made up of a sugar group (deoxyribose), a phosphate group and a base. There are four bases: adenine, thymine, cytosine and guanine (ATCG). The way these bases are strung together creates a ‘code’ that provide instructions for how our body grows and functions. DNA is found in the nucleus of every cell of your body– except red blood cells, because they have no nucleus.
The DNA molecule is like a twisted ladder. Each step of the ladder is made up of two molecules called bases. Two bases joined together are called base pairs. To form a base pair, A joins with T or C joins with G.
We can use the analogy of recipe books to better understand the relationship between DNA molecules (made up of nucleotides), genes and chromosomes. One nucleotide (A, T, G or C) can be thought of as a letter of the alphabet. The letters group together to form words (codons). A gene can be thought of as a recipe, and a chromosome would be like a recipe book, consisting of many recipes. A set of chromosomes (the genome) would be represented by a library of recipe books.
Our chromosomes are found in the nucleus of each of our body cells. We have 46 of them, matched up into 23 pairs. Egg and sperm cells, however, have only 23 chromosomes; and when they come together to make a baby, he or she will get 46.
The first 22 chromosome pairs (called ‘autosomes’) are the same in men and women. The sex chromosomes make up the 23rd pair. Generally, females have two X chromosomes, while males have one X and one Y chromosome.
Scientists in the lab are able to look inside the nucleus and make a picture of our chromosomes called a karyotype. They match up the chromosomes into pairs and line up the pairs from biggest to smallest. The picture below shows what these matched-up chromosomes look like under the microscope after they have been released from actively dividing cells, fixed and stained.
When laboratory technicians first look at chromosomes under the microscope, the chromosomes are all jumbled together and not found neatly lined-up and paired-up, as in the picture below.
In the jumble of chromosomes, technicians must look closely at the structure of each chromosome to figure out which ones match up. The following features of a chromosome are helpful in sorting out the chromosomes and are show on a diagram on the next page:
- The centromere– central part of the chromosome that pinches inwards
- The ‘p’ arm– the shorter part of the chromosome
- The ‘q’ arm– the longer section of the chromosome
- The telomeres– the ends of the chromosomes
- The banding– the pattern of light and dark stripes that run along the length of the chromosomes when the chromosomes have been stained with a special dye. The bands are numbered to aid in locating specific genes, as shown for chromosome 12 in the picture below. Banding is used to distinguish chromosomes of the same size. Plus, changes in this pattern may be a clue that there is missing or extra genetic material in the chromosome.
Plant & Animal DNA
We now know humans have their DNA packaged into chromosomes, but what about other animals and plants?
It turns out that both plants and animals also have their DNA packaged into chromosomes. All normal members of a plant or animal species have the same number of chromosomes; however, the number of chromosome pairs differs among different species.
For example, in plants the number of chromosome pairs varies from just two pairs of chromosomes in some flowering plants to hundreds of pairs of chromosome in some ferns (1).
- The garden onion has 8 pairs of chromosome.
- The potato has 24 pairs of chromosome.
- Wheat has 21 pairs of chromosome.
- Rice has 12 pairs of chromosome.
Similarly, different animal species also have different numbers of chromosome pairs.
- Dogs have 39 pairs of chromosomes.
- Horses have 32 pairs.
- Cats have 19 pairs.
- Frogs have 13 pairs.
- Mosquitos have 3 pairs (see the image below).
Chromosomes of Aedes aegypti - species of mosquito
Codon - Three adjacent bases (letters) in mRNA that “code” for a particular amino acid in protein translation. There are 64 possible 3-letter combinations of the bases but only 20 amino acids, so several of the codons code for the same amino acid.
Chromosome - A long, twisted and folded-up piece of DNA. Each species has a characteristic number of chromosomes. Humans, for example, have 46, 23 contributed by each parent.
Chromatin - A complex of macromolecules found in cells, consisting of DNA, protein and RNA. The primary functions of chromatin are 1) to package DNA into a smaller volume to fit in the cell, 2) to reinforce the DNA macromolecule to allow mitosis, 3) to prevent DNA damage, and 4) to control gene expression and DNA replication.
DNA - DNA stands for deoxyribonucleic acid. It is the molecule that contains the genetic instructions to construct and maintain a living organism.
Eukaryote – Single-celled or multicellular organisms that contain a nucleus and organelles bound within membranes.
Gene - The fundamental unit of inheritance. Genes encode messages for the synthesis of proteins and functional RNAs. Genes help determine an organism’s appearance, its metabolism and may interact with the environment to influence its behaviour.
Histone – A group of structural proteins that act as spools around which DNA winds.
Karyotype - The characteristic set of chromosomes for a particular species. Chromosome number, shape and size all determine the species’ karyotype. For example, humans have 46 chromosomes: two pairs each of chromosomes 1-22 and a pair of sex chromosomes.
Nucleosome - A basic unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around eight histone protein cores. This structure is often compared to thread wrapped around a spool.
A sub-cellular structure in eukaryotic cells with specialized function. These membrane-bound compartments are sometimes compared to organs in the human body where each system has a different job. Examples include mitochondria, Golgi complex, endoplasmic reticulum, lysosome, peroxisome and the nucleus.
Ribonucleic Acid (RNA) - A long nucleic acid molecule found in the nucleus and cytoplasm of a cell. Similar to DNA, RNA consists a sugar-phosphate backbone with nitrogenous bases. RNA differs from DNA by having a different sugar in its backbone (ribose instead of deoxyribose); having uracil as a base instead of thymine; and functioning as a single-stranded molecule instead of a double-stranded helix. One function of RNA is to convey genetic information, encoded by DNA, to the protein synthesis machinery. This process is known as translation and involves three types of RNA that work together to achieve this task: mRNA, rRNA and tRNA.
- Griffiths et al. An Introduction to genetic analysis.7th ed. New York: W.H. Freeman and Company. 1999. (from NCBI Books)
What is a chromosome?
Genes vs. DNA vs. Chromosomes