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M15. Change in the base sequence of DNA can lead to the alteration or absence of proteins, and to the appearance of new characteristics in the descendants.


Student Outcome: M15.1

Know that changes in the DNA sequence are called ‘mutations’.


  • How Do Mutations Occur?

Copying errors are introduced when DNA replicates itself.

Everyone acquires some changes to their DNA during the course of their lives. These changes occur in a number of ways. Sometimes there are simple copying errors that are introduced when DNA replicates itself. (Every time a cell divides, all of its DNA is duplicated so that the each of the two resulting cells have a full set of DNA.) Other changes are introduced as a result of DNA damage through environmental agents including sunlight, cigarette smoke, and radiation. Our cells have built in mechanisms that catch and repair most of the changes that occur during DNA replication or from environmental damage. As we age, however, our DNA repair does not work as effectively and we accumulate changes in our DNA.


"There is a general belief that DNA is 'rock solid' -- extremely stable," says Brandt Eichman, associate professor of biological sciences at Vanderbilt, who directed the project. "Actually DNA is highly reactive."

On a good day about one million bases in the DNA in a human cell are damaged. These lesions are caused by a combination of normal chemical activity within the cell and exposure to radiation and toxins coming from environmental sources including cigarette smoke, grilled foods and industrial wastes.



Here is a video showing what happens when a mutation occurs:


Germline mutations are what cause diseases to run in families.

Some of these changes occur in cells of the body — such as in skin cells as a result of sun exposure — but are not passed on to children. But other errors can occur in the DNA of cells that produce the eggs and sperm. These are calledgermline mutations and can be passed from parent to child. If a child inherits a germline mutation from their parents, every cell in their body will have this error in their DNA. Germline mutations are what cause diseases to run in families, and are responsible for the kind of hereditary diseases covered by Genetic Health.


  • What Kind of Mutations Are There?


A gene is essentially a sentence made up of the bases A, T, G, and C that describes how to make a protein. Any changes to those instructions can alter the gene's meaning and change the protein that is made, or how or when a cell makes that protein. There are many different ways to alter a gene, just as there are many different ways to introduce typos into a sentence. In the following examples of some types of mutations, we use the sentence "The fat cat ate the wee rat" as a sample gene:


    • Point Mutation

A point mutation is a simple change in one base of the gene sequence. This is equivalent to changing one letter in a sentence, such as this example, where we change the 'c' in cat to an 'h':


Original The fat cat ate the wee rat.
Point Mutation The fat hat ate the wee rat.


    • Frame-shift mutation

Frame-shift mutations. In a frame shift mutation, one or more bases are inserted or deleted, the equivalent of adding or removing letters in a sentence. But because our cells read DNA in three letter "words", adding or removing one letter changes each subsequent word. This type of mutation can make the DNA meaningless and often results in a shortened protein. An example of a frame-shift mutation using our sample sentence is when the 't' from cat is removed, but we keep the original letter spacing:


Original The fat cat ate the wee rat.
Frame Shift The fat caa tet hew eer at.


    • Deletion

Mutations that result in missing DNA are called deletions. These can be small, such as the removal of just one "word," or longer deletions that affect a large number of genes on the chromosome. Deletions can also cause frameshift mutations. In this example, the deletion eliminated the word cat.


Original The fat cat ate the wee rat.
Deletion The fat ate the wee rat.


    • Insertion

Mutations that result in the addition of extra DNA are called insertions. Insertions can also cause frameshift mutations, and general result in a nonfunctional protein.


Original The fat cat ate the wee rat.
Insertion The fat cat xlw ate the wee rat.


    • Inversion

In an inversion mutation, an entire section of DNA is reversed. A small inversion may involve only a few bases within a gene, while longer inversions involve large regions of a chromosome containing several genes.


Original The fat cat ate the wee rat.
Insertion The fat tar eew eht eta tac.


    • DNA expression mutation

There are many types of mutations that change not the protein itself but where and how much of a protein is made. These types of changes in DNA can result in proteins being made at the wrong time or in the wrong cell type. Changes can also occur that result in too much or too little of the protein being made.


Source: http://www.genetichealth.com/G101_Changes_in_DNA.shtml



Read about the terrible consequences of having one base change - it is a long read, but rivetting, nonetheless.  It is from the New Yorker and it is about Lesch-Nyhan syndrome. Read it here.


Read here what happens when the DNA needs repair and the remarkably diverse methods of repair used by even the simplest of cells - bacteria.


Student Outcome: M15.2

Know that the mutation rate can be increased by radiation, mutagenic chemicals, and heat.




*The Stability of DNA*

Because of its double helix structure and general lack of reactivity, DNA is a relatively stable molecule. The most reactive part of DNA is its nitrogenous bases, but they are sheltered within the double helix and are partially neutralized through their hydrogen bonding. Mutations, or changes in the structure of DNA, can occur, though. Chemicals, heat, ultraviolet light, and radiation can all sometimes act as mutagens, potentially causing cancer or cell death. Though some mutations prove harmless or even beneficial, the vast majority are harmful. To guard itself against such damage, then, organisms have developed systems to protect themselves.


In many eukaryotes, ourselves included, DNA molecules are tightly wound around nucleosomes, spheres of the protein called histone. This positively charged protein forms an ionic bond with the negative backbone of DNA, and in the process shields the molecule.


Source: http://library.thinkquest.org/20465/mutation.html



Student Outcome: M15.3

Explain how inheritable mutations can lead to changes in the characteristics of the descendants.


  • The effects of mutations

Since all cells in our body contain DNA, there are lots of places for mutations to occur; however, some mutations cannot be passed on to offspring and do not matter for evolution. Somatic mutations occur in non-reproductive cells and won't be passed onto offspring. For example, the golden color on half of this Red Delicious apple was caused by a somatic mutation. Its seeds will not carry the mutation.


The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations.


  • Effects of germ line mutations

A single germ line mutation can have a range of effects:


  1. No change occurs in phenotype. You've probably heard of "junk DNA," DNA that doesn't seem to do anything. Mutations in junk DNA get passed on to offspring, but as far as we know, have no obvious effect on the phenotype.
  2. Small change occurs in phenotype. A single mutation can cause a change in eye colour.
  3. Big change occurs in phenotype. Some really important phenotypic changes, like DDT resistance in insects are sometimes caused by single mutations. A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals — and it doesn't get more negative than that.


Source: http://evolution.berkeley.edu/evolibrary/article/0_0_0/mutations_05


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