Gene Mutations and Gene Recombination

Section 1: Gene Mutations and Gene Recombination

Organisms undergoing sexual reproduction replicate chromosomes during the formation of gametes, essentially duplicating genetic material DNA. The principle of complementary base pairing ensures the accuracy of DNA replication, maintaining consistent genetic information between parent and offspring. However, can errors occur during the replication of genetic information?

Examples of Gene Mutations Sickle cell anemia (also known as sickle cell disease) is a genetic disorder. In normal individuals, red blood cells are centrally indented discs, whereas in sickle cell anemia patients, red blood cells are crescent-shaped (Figure 5-1, right). These abnormal red blood cells are prone to rupture, causing hemolytic anemia and, in severe cases, death. What is the cause of this disease? Analysis of the hemoglobin molecules in patients' red blood cells revealed amino acid substitutions in the peptide chains of hemoglobin molecules (Figure 5-2).

Substitutions of bases can lead to changes in genes, thereby altering the encoded proteins. So, if there is an addition or deletion of bases in the gene sequence encoding a protein, will it also lead to changes in protein structure and thus changes in traits? The answer is yes. Changes in DNA molecules due to base substitutions, additions, or deletions that alter the gene base sequence are called gene mutations.

If gene mutations occur in gametes, they will follow the laws of inheritance and be passed on to offspring. If they occur in somatic cells, they generally cannot be inherited. However, some plants with mutations in somatic cells can pass them on through asexual reproduction.

Cell Transformation Cancer is one of the most serious threats to human health. Is cell transformation related to gene mutations? Let's take colorectal cancer as an example.

DNA in human and animal cells already contains genes related to carcinogenesis: oncogenes and tumor suppressor genes. Generally, proteins expressed by oncogenes are necessary for normal cell growth and proliferation. If these genes mutate or are overexpressed, leading to excessive protein activity, they can cause cell transformation. Conversely, proteins expressed by tumor suppressor genes can inhibit cell growth and proliferation or promote apoptosis. Mutation of these genes, leading to reduced or lost protein activity, can also cause cell transformation. Compared to normal cells, cancer cells (Figure 5-3) exhibit the following characteristics: unlimited proliferation, significant changes in morphology, reduced substances like glycoproteins on the cell membrane, significantly reduced intercellular adhesion, and easy dispersal and metastasis within the body.

Causes of Gene Mutations How are gene mutations generated? In 1927, American geneticist H.J. Muller (1890-1967) discovered that irradiating fruit flies with X-rays significantly increased the number of mutant offspring. In the same year, scientists irradiated seeds of maize and barley with X-rays and gamma rays, obtaining similar results. Since then, it has been gradually discovered that factors that can induce gene mutations and increase mutation frequency can be categorized into three types: physical factors, chemical factors, and biological factors. For example, ultraviolet light, X-rays, and other radiation can damage DNA inside cells; nitrites, base analogs, and other substances can alter nucleic acid bases; and the genetic material of certain viruses can affect host cell DNA, and so on. However, gene mutations can also occur spontaneously due to occasional errors in DNA replication, even without the influence of these external factors.

Characteristics of Gene Mutations In nature, there are many factors that can induce gene mutations, and gene mutations can also occur spontaneously. Therefore, gene mutations are widespread in the biological world. Because changes in DNA base composition are random and non-directional, gene mutations exhibit randomness and non-directionality. The randomness of gene mutations means that they can occur at any stage of an organism's development, within different DNA molecules inside cells, and at different locations within the same DNA molecule. The non-directionality of gene mutations means that one gene can undergo different mutations, resulting in more than one allele.

In natural conditions, the frequency of gene mutations is very low. It is estimated that in higher organisms, only 1 out of 10^5 to 10^7 reproductive cells undergoes a gene mutation.

Significance of Gene Mutations For organisms, gene mutations may disrupt the coordination between the organism and its current environment, thus being harmful to the organism. However, some gene mutations are beneficial to organisms, such as mutations in plants that confer disease resistance or drought tolerance, and mutations in microorganisms that confer drug resistance. There are also gene mutations that are neither harmful nor beneficial; these are neutral mutations. For example, some gene mutations do not result in new traits, and they belong to neutral mutations.

Gene mutations are a pathway to generating new genes. For the reproduction and evolution of organisms, organisms that have produced new genes may better adapt to environmental changes, open up new living spaces, and thus give rise to new types of organisms. Therefore, gene mutations are the fundamental source of biological variation, providing rich materials for biological evolution.

Gene Recombination The law of independent assortment of genes tells us that during the formation of gametes in organisms through meiosis, with the independent assortment of non-homologous chromosomes, non-allelic genes also assort independently, producing different gametes. Thus, fertilized eggs formed by the combination of male and female gametes may have genotypes different from those of their parents, thereby causing variation in offspring (Figure 5-4). In addition, during the tetrad stage of meiosis, sometimes alleles on homologous chromosomes undergo exchange with non-sister chromatids, resulting in gene recombination on chromatids. Gene recombination refers to the recombination of genes controlling different traits during the process of sexual reproduction in organisms.

What is the significance of gene recombination? Generally, gene recombination during sexual reproduction diversifies the types of gametes produced, thereby producing offspring with diversified gene combinations. Some of these offspring may have gene combinations necessary for adapting to certain changes essential for survival. Therefore, gene recombination is also a source of biological variation and is of significant importance to the evolution of organisms.