13.3 Mutations

POINT > Define a gene in simple terms POINT > Define and describe genetic mutations POINT > Distinguish between gene and chromosomal mutations POINT > Demonstrate how point mutations can affect zero, one or many amino acids POINT > Identify causes of mutations POINT > Describe how mutations can be harmful, beneficial or have no effect on the cell/organism

POINT > Define a gene in simple terms A gene is a segment of DNA that contains the information to construct one protein Includes exons and introns One DNA molecule (chromosome) has many genes ex. 22,000 human genes (x2) on 46 chromosomes in each cell = ~ 950 genes per chromosome average

POINT > Define a gene in simple terms Each gene codes for one protein that has an important role to play in the cell Recall that the DNA sequence specifies the amino acid sequence in the protein Recall that the order of amino acids in the protein determines how the protein folds up into its functional conformation In order to fulfill its function in the cell, the amino acid sequence of the protein has to be correct! If not...

WB CHECK: find all that are correct A gene a) is a DNA sequence b) is an RNA sequence c) has the information to build one protein d) has the information to build different proteins e) would be found on a chromosome f) is transcribed into mRNA

A mutation is a heritable change to the DNA of an organism A mutation can affect: A single gene (gene mutation) Large portion of a chromosome (chromosomal mutation)

Most gene mutations are point mutations: a change of one nucleotide at a single point in a DNA sequence Usually occurs during DNA replication (DNA polymerase makes a mistake during proof-reading) The mutation gets passed to every cell that develops from that parental cell

WB CHECK: What is a point mutation? When do point mutations usually occur?

POINT > Define and describe genetic mutations Three types of point mutations: 1. Substitution 2. Deletion 3. Insertion

Imagine the following as ‘codons’: THFATCATATETHERAT E

1. Substitution One base gets changed to another base Usually corrected by DNA polymerase, but can be missed (~ 1 per 1,000,000,000)

Substitution THFATCATATETHERAT E T

C T A T C G G A C T G C DNA Template G A U A G C C U G A C G mRNAAmino Acids G G U A G C C U G A C G mRNA with Mutation Amino Acids with Mutation C

Insertion or deletion: a base is either inserted or deleted from the DNA sequence This results in a frameshift mutation: the entire reading frame following that point is thrown off This is much more severe than a substitution, because many amino acids will be altered…

 Frameshift Mutation: THFATCATATETHERAT E

C T A T C G G A C T G C DNA Template G A U A G C C U G A C G C G A U A G C C U G A C G mRNAAmino Acids mRNA with Mutation Amino Acids with Mutation G

C T A T C G G A C T G C DNA Template G A U G C C U G A C G G A U A G C C U G A C G mRNAAmino Acids mRNA with Mutation Amino Acids with Mutation GC STOP

WB CHECK: What are three types of point mutation? Why is an insertion mutation worse than a substitution mutation? Which type of mutation is shown here? ATCCCGGCTA (original sequence) ATCCCGCTA (mutated sequence)

Sometimes entire sections of a chromosome are involved in mutation Chromosomal mutations result in changes to the number or structure of chromosomes May alter location of genes or change how many copies of a gene are present

 Deletion  Duplication  Inversion  Translocation (Original)

1. DNA Replication DNA Polymerase is not perfect, balances speed and accuracy. Most mutations arise here 2. Mutagens: Environmental factors that damage DNA There are two broad categories of mutagens: chemical and physical

2. Mutagens, con’t A. Chemical mutagens: pesticides, tobacco smoke, industrial toxins B. Physical mutagens: radiation, X-rays, UV cause of skin cancers due to sun exposure 3. Biological Factors (viruses, bacteria)

WB CHECK: Give an example of a physical mutagen. What are three examples of chemical mutagens? What causes most mutations?

Once the DNA sequence is altered, the change is passed to every daughter cell If it occurs in a gamete, changes are passed to offspring

Most mutations have little or no effect. Why? Mutations may occur in stretches of DNA that are not genes (this includes most DNA!) If a mutation does occurs in a gene, the amino acid change might not be catastrophic to the protein function

Some mutations are harmful Ex. Mutations to cell cycle control proteins (cyclins) may lead to uncontrolled cell division, i.e. cancer Ex. Altered protein structure: sickle cell disease – A single substitution mutation causes a red blood cell protein to fold up irregularly. This leads to sickle cell

Some mutations are beneficial Ex. Chemical resistance to pesticides and antibiotics (plants, insects, bacteria) Ex. Polyploidy (extra copies of chromosomes) in plants often results in larger, stronger organisms This is a key biological mechanism of evolution!

WB CHECK: Why are most mutations harmless? Give an example of a mutation that is helpful to the organism. What are two diseases caused by mutations?

Summary: Gene & Chromosomal mutations get passed to every daughter cell Mutations can happen during DNA replication or from mutagens in the environment Some mutations are harmful, while some may be beneficial. Most are neutral.

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