CHAPTER 14: Genes in Action

Essential Questions:

1.What is a Mutation?

2. What Causes Mutation?

3. What is the origin of genetic differences among organisms?

4. What types of mutations are possible?

5. How can genetic change occur on a larger scale?

6. How will Modern Technology in DNA affect us?

Gene Mutations1. A Mutation is a change in the sequence of DNA bases within a gene

2. What Causes Mutations: • Mutations can be spontaneous or caused by

environmental influences called mutagens. • Mutagens include radiation (X-rays, UV

radiation), and organic chemicals (in cigarette smoke and pesticides).

Origins of Genetic Difference3. MUTATION is the origin differences among organisms

For the most part, genetic differences among organisms originate as some kind of mutation.

A genetic mutant is an individual whose DNA or chromosomes differ from some previous or normal state.

Every unique allele of every gene began as a mutation of an existing gene.

In genetics, a mutation is a change in the structure or amount of the protein produced

Effects of Mutations

–Because of the way DNA is translated, a mutation can have many possible effects.

–A mutation may have no effect, or may harm or help in some way.

–Mutations are noticed when they cause an unusual trait or disease, such as sickle cell anemia.

–Many mutations may go unnoticed.

Mutations• Changes to DNA are called mutations

– change the DNA

– changes the mRNA

– may change protein

– may change trait

DNA TACGCACATTTACGTACG

mRNA AUGCGUGUAAAUGCAUGC

aa aa aa aa aa aa aaprotein

trait

Types of mutations4. Changes to the letters (A,C,T,G bases) in the DNA can result in 2 types of mutation

– Point mutation• change to ONE letter (base) in the DNA

• may cause change to protein, may not

– Frameshift mutation• addition of a new letter (base) in the DNA sequence

• deletion of a letter (base) in the DNA

• Both shift the DNA so it changes how the codons are read

• BIG changes to protein

Types of Point Mutations1. Silent mutation - the change in the codon results in the same amino acid

Ex: UAU UAC both code for tyrosine

2. Nonsense mutation - a codon is changed to a stop codon; resulting protein may be too short to function

Ex: UAC UAG (a stop codon)

3. Missense mutation - involves the substitution of a different amino acid, the result may be a protein that cannot reach its final shape

Ex: Hbs which causes sickle-cell disease

Sickle cell anemia• Hemoglobin protein in red blood cells

– strikes 1 out of 400 African Americans– limits activity, painful & may die young

Normalround cells

Misshapensickle cells

Only 1 out of146 amino acids

Point Mutations • One base change

– can change the meaning of the whole protein

THEFATCATANDTHEREDRATRAN

THEFATCARANDTHEREDRATRAN

THEFATCATENDTHEREDRATRAN

OR

Does this changethe sentence?A LITTLE!

Point Mutations • Silent mutation = no change to protein

AUGCGUGUAUACGCAUGCGAGUGA

MetArgValTyrAlaCysGluStop

AUGCGUGUAUACGCUUGCGAGUGA

MetArgValTyrAlaCysGluStop

Does this changethe protein?Why not?

The code hasrepeats in it!

Mutations as Changes in Results of Genes

–Changes in a DNA sequence may affect the results of genes in many ways.

–A mutation is silent when it has no effect on a gene’s function.

–Point mutations are often silent because the genetic code is redundant (each amino acid has multiple codons).

–A missense or replacement mutation results when a codon is changed such that the new codon codes for a different amino acid.

Point Mutations • Missense mutation = changes amino acid

AUGCGUGUAUACGCAUGCGAGUGA

MetArgValTyrAlaCysGluStop

AUGCGUGUAUACGUAUGCGAGUGA

MetArgValTyrValCysGluStop

Doesthis changethe protein?DEPENDS…

Point Mutations • Nonsense mutation = change to STOP

AUGCGUGUAUACGCAUGCGAGUGA

MetArgValTyrAlaCysGluStop

AUGCGUGUAUAAGCAUGCGAGUGA

MetArgValStop

Really destroyedthat protein!

Frameshift MutationsThe reading frame of a sequence depends on the starting point for reading. An insertion or deletion can shift the reading frame, or cause a frameshift.

In frameshift mutations, the remaining sequence may be “read” as different codons.

An insertion or deletion of many codons is likely to disrupt the resulting protein’s structure and function.

Frameshift Mutations • Add or delete one or more bases

– changes the meaning of the whole protein

THEFATCATANDTHEREDRATRAN

THEFATCANTANDTHEREDRATRAN

THEFATCAANDTHEREDRATRAN

OR

Add one!Delete one!

Does this changethe sentence?A LOT!

Frameshift Mutations • Addition = add one or more bases

AUGCGUGUAUACGCAUGCGAGUGA

MetArgValTyrAlaCysGluStop

AUGCGUGUAUACGUCAUGCGAGUGA

MetArgValTyrValMetArgValA

Does this changethe protein?A LOT!

Frameshift Mutations • Deletion = lose one or more bases

AUGCGUGUAUACGCAUGCGAGUGA

MetArgValTyrAlaCysGluStop

AUGCGUGUAUACGAUGCGAGUGA

MetArgValTyrAspAlaSerGA

Does this changethe protein?A LOT!

Deletion Mutation:Cystic Fibrosis

deletion

Loss of one amino acid!

LARGE-SCALE GENETIC CHANGE

5. How can genetic change occur on a larger scale?

Very large-scale genetic change can occur by misplacement, recombination, or multiplication of entire chromosomes.

Chromosomal Mutations

During meiosis, chromosomes pair up and may undergo crossover. Usually, the result is an equal exchange of alleles between homologous chromosomes.

Errors in the exchange can cause chromosomal mutations.

Changes in Chromosome Structure

Chromosome mutation - a change in chromosome structure 4 Types of chromosomal mutations:

Inversion Translocation Deletion

Duplication

Chromosomal Deletion: Williams Syndrome

• Deletions occur when a single break causes a lost end piece, or two breaks result in a loss in the interior.

• Children have a pixie look (turned-up noses, wide mouth, small chin, large ears)

• Poor academic skills, good verbal and musical abilities. Skin ages prematurely from lack of the gene that controls the production of elastin (also affects cardiovascular health).

Chromosomal Duplication: Dup15

• Duplication results in a chromosome segment being repeated in the same chromosome

• An inverted duplication in chromosome 15 causes Dup 15 syndrome

– Poor muscle tone, mental retardation, seizures, curved spine, and autistic characteristics

Chromosome Translocation : Alagille Syndrome

• Translocation is exchange of chromosomal segments between two, non-homologous chromosomes.

–Ex: Alagille syndrome results from a deletion of chromosome 20 or a translocation that disrupts an allele on chromosome 20.

–Distinctive face, abnormalities of eyes & internal organs, and severe itching.

Chromosome Inversion• Inversion involves a segment of a chromosome being turned

180 degrees

• The reverse sequence of alleles can alter gene activity.

• Crossing-over between inverted and normal chromosomes can cause duplications and deletions

Changes in Chromosome Number • Nondisjunction – occurs when:

– In meiosis I, homologous pair both go into the same daughter cell or in meiosis II, the sister chromatids both go into the same gamete.

• Example: Trisomy 21 Down syndrome is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is associated with growth delays, and mild to moderate intellectual disability.

EFFECTS OF GENETIC CHANGE

•What are the possible effects of mutations? The results of genetic change may be harmful, beneficial, or neutral; most changes are neutral and may not be passed on to offspring.

Most genetic changes will cause no change in the appearance or function of organisms.

•Mutations that occur in gametes can be passed on to offspring, but mutations in body cells affect only the individual in which they occur.

Some Common Genetic DiseasesHuntington’s Disease: Autosomal dominant

Neuromuscular disease; degeneration of muscle, onset in early 30’s.

Cystic Fibrosis: Autosomal Recessive; Defective Protein is made that creates excess mucus; clogs lungs.

Color Blindness: Sex-linked recessiveInablity to distinguish colors (8% of male population)

Hemophilia: Sex-linked recessive Inablity of blood to clot

Muscular Dystrophy: Sex-linked recessiveLoss of muscle that begins in early childhood

Tay-Sach’s Disease : Autosomal RecessiveDegeneration of Central Nervous System; infant mortality

DNA Technology and Modern Medicine

• The Human Genome Project Decoded an entire length of DNA in 2003

• The Genetic Sequencing allows for improvements in the detection and treatment of genetic disease and cancer.

• Gene Therapy is a technology in which good copies of a gene are introduced into those individuals affected by genetic disease