Gene Duplication: Definition & Mechanism

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

Organisms require a huge variety of proteins to function, and they must have the genes to make the proteins that determine the traits necessary for life. Learn about the role that gene duplication plays in cells, then test your knowledge with a quiz.

Exploring Gene Duplication Through Keratin

Hair and nails are both comprised of a protein called keratin. It is also the protein used to make horns, feathers, and wool. Even your skin and eyes have a thin layer of keratin. How does a protein like this have such diverse roles in different animals or in even different parts of the body?

Figure 1: Gene duplication results in a gene being copied in an organisms genome.
gene duplication

Each of the different types of keratin found throughout the body are expressed by different genes that are duplicates of each other. Gene duplication happens when an extra copy of a gene is made in an organism's genome. In some cases, the duplication leads to the gain of a new function, but in other cases, protein function is lost, as shown in Figure 1.

Sometimes, gene duplication is beneficial to the organism and may eventually lead to the development of a new species. The various types of keratin in the body are the result of duplications of a single gene. Over time, as species diversified, new genes for keratin with different functions arose in different species. Other times, gene duplication can have negative effects on individuals, like those seen with Down syndrome and other chromosomal disorders. Keep reading to learn how gene duplications arise in the genome.

Gene Structure

Figure 2: The basic structure of a gene.
Gene Structure

Before discussing how genes are duplicated, let's review the essential components of a gene (Figure 2). The instructions to make the protein are found in the coding region of the gene. Promoters are sequences of DNA that ensure the gene gets copied into RNA in order to be converted into a protein. If a sequence does not have a promoter, the DNA sequence will not be expressed as a protein (trait) even if it codes for one.

How Do Genes Get Duplicated?

Gene duplication can be DNA- or RNA-based. Gene duplication that is DNA-based happens during any process in which DNA is replicated. For this lesson, we will focus on DNA replication during meiosis (formation of egg and sperm cells). When a gene is duplicated in the genome of an egg or sperm cell, the organism's offspring will inherit the duplicated gene. We will then discuss RNA-based gene duplication, which happens when an mRNA is turned back into DNA and then reinserted into the genome.

DNA-Based Gene Duplication

Unequal Crossing-Over

Crossing-over takes place during the first half of meiosis when all of the chromosomes are lined up at the center of the cell during metaphase I.

Figure 3: Crossing-over recombines genes on chromosomes.
crossing-over

As shown in Figure 3, when homologous chromosomes are joined together, they may exchange genes with one another in a process called homologous recombination, or crossing-over. In Figure 3, the homologous chromosomes shown in orange (maternal) and yellow (paternal) exchange the genes labeled y and z.

Figure 4: Uneven crossing-over results in chromosomes with a disproportionate number of genes.
uneven crossing-over

Normally, crossing-over results in each chromosome ending up with all the genes that are supposed to be found on that chromosome. In order for this to happen, the chromosomes have to be aligned correctly, as shown in Figure 4a. Ideally, this alignment would happen in coding regions of genes to ensure that each cell will end up with a normal chromosome. Unfortunately, chromosomes can align at any DNA sequence with enough similarity, and homologous recombination can occur anywhere. As shown in Figure 4b and 4c, this results in the generation of chromosomes with an abnormal number of genes. The misalignment can result in complete (4b) or partial (4c) gene duplication.

Chromosome Duplication

Chromosomal duplication is another way genes become duplicated. This process is exactly what it sounds like: It results in the duplication of a partial or full chromosome. Chromosome duplication can happen during both stages of meiosis as chromosomes are being separated.

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