Question

Explain how alteration in DNA sequences contribute to variation that can be subject to natural selection. (Please use these terms in your answer)

Chromosome number

Chromosomal human disorders: down syndrome, turner syndrome

Natural selection

Horizontal acquisitions  

Naked DNA

Viral transmission

Conjugation

Transposition

Viruses recombination

Meiosis(in various organisms)  

Answer 1

There are many ways of alteration in DNA sequences which contribute to variation and therefore cause evolution. These alteration can be either due to an increase or decrease in the number of chromosomes or exchange in the nucleotides sequence of the DNA itself.

The change in the number of chromosomes is lethal in case of human beings. If there are three copies of a chromosome, then it is called as trisomy. For example patau syndrome which is caused due to the presence of three copies of chromosome number 18. Monosomy is the presence of only single copy of a chromosome. For example, Turner syndrome which is caused due to the single copy of X chromosome. These abnormal chromosome number in human being arise during gamete formation. During gamete formation in meiosis, nondisjunction events can result in an increase or decrease in the copy number of a chromosome and therefore causing genetic disorders.

Nondisjunction occurs during meiosis during anaphase 1 when homologous chromosomes fail to separate from each other or during anaphase 2 when sister chromatids fail to separate from each other. Apart from nondisjunction, meiosis is considered to be a very important type of cell division because it is responsible for the variation in between progenies of same parent and also between progeny and parent. Independent assortment which occurs during metaphase 1 of meiosis 1 and crossing over which occurs during prophase 1 of meiosis 1 are equally responsible for contributing towards variations. In the end, the random fertilization of gametes from two different parents help not only in the Restoration of normal chromosome number in the zygote, but also is responsible for the variation seen in the zygote from the parents.

Monosomy and trisomy Lethal in animals and human beings but they are not Lethal in plants. Instead an increase in the number of chromosomes in plant increasing the size of the plant and this is economically very important. It is because it give us more produced per hectare in case of Agricultural crops. This is called as polyploidy.

Besides chromosomal rearrangement, mobile genetic elements called transposons are present in Eukaryotic genome. They can move from one place to another and can cause chromosomal rearrangement or change the architecture of the gene. If they get inserted into a gene, then the function of the gene can be increased or decreased. Their movement is called as transposition.

Apart from eukaryotes like plants animals and human being, bacteria also shows primitive type of genetic recombination thereby giving rise to variations.

Conjugation is the transfer of genes between bacteria which requires a direct physical contact between the participating bacteria. Genes located on the plasmid DNA are transferred from one bacteria to the other.

Transformation is the ability of a bacterial cell to pick up naked DNA from the environment and integrate it into its own genome thereby resulting in the formation of new proteins.

Viruses also play an important role in causing variations between bacteria. Viruses can infect bacteria and then integrate their genome into it. This is called as transduction. In this way viruses direct bacterial cells to prepare viral particles.

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