Transcript 11.3 Section Objectives – page 296

1. Using the table on Pg. 292, write the amino acid sequence that would
be made according to the codons on the mRNA chain.
2. Why do you think this exact base sequence is important?
Section Objectives:
• Categorize the different kinds of mutations that can
occur in DNA.
• Compare the effects of different kinds of mutations
on cells and organisms.
Mutations
• Any change in DNA sequence is called a mutation.
• Mutations can be caused by errors in replication,
transcription, cell division, or by external agents.
Mutations that can be passed on
• Mutations can affect the reproductive cells of an
organism by changing the sequence of nucleotides
within a gene in a sperm or an egg cell.
• If this cell takes part in fertilization, the altered gene
would become part of the genetic makeup of the
offspring.
Mutations in reproductive cells
• A mutation may produce a new trait or it may result in
a protein that does not work correctly.
• Sometimes, the mutation results in a protein that is
nonfunctional, and the embryo may not survive.
• In some rare cases a gene mutation may have
positive effects.
Mutations in body cells
• What happens if powerful radiation hits the DNA of a
nonreproductive cell, a cell of the body such as in skin,
muscle, or bone?
• If a body cell’s DNA is changed, this mutation would
not be passed on to offspring.
• However, the mutation may cause problems for the
individual.
Mutations in body cells
• Damage to a gene may impair the function of the cell.
• When that cell divides, the new cells also will have the
same mutation.
• Some mutations of DNA in body cells affect genes that
control cell division.
• This can result in the cells growing and dividing
rapidly, producing cancer.
Types of Mutations
• A point mutation is a change in a single base pair in
DNA.
THE DOG BIT THE CAT.
THE DOG BIT THE CAR.
• A change in a single nitrogenous base can change the
entire structure of a protein because a change in a
single amino acid can affect the shape of the protein.
mRNA
Normal
Protein
Stop
Replace G with A
mRNA
Point
mutation Protein
Stop
• An example of a point mutation, is sickle-cell disease.
• Sickle-cell diseases cause the red blood cells to be
deformed. The result is that they get stuck in the
blood vessels, depriving tissues of oxygen, causing
strokes, and blood clots.
Types of Mutations
A point mutation takes place when a single nitrogenous
base (letter) is changed at one point.
THE DOG BIT THE CAT.
THE DOG BIT THE CAR.
What would happen if a single nitrogenous base
(letter) is deleted?
THE DOG BIT THE CAT.
THD OGB ITT HEC AT.
Types of Mutations
• A mutation in which a single base is added or deleted
from DNA is called a frameshift mutation because it
shifts the reading of codons by one base.
Deletion of U
Types of Mutations
• The new sequence with the deleted base would be
transcribed into mRNA.
• But then, the mRNA would be out of position by one
base.
• This mutation would cause nearly every amino acid in
the protein after the deletion to be changed.
Deletion of U
Frameshift mutations
• Frameshift mutations can cause severe genetic
diseases like Cystic Fibrosis and Lou Gehrig’s
Disease.
• Lou Gehrig’s Disease is a progressive neurological
disease. Affects nerve cells used for voluntary muscle
control
Lou Gehrig’s Disease, or Amyotrophic Lateral Sclerosis
(ALS) is caused by a frameshift mutation on
chromosome 21.
With ALS, there is a deletion of a Cytosine on this gene.
Every other base moves down, making all the amino acids
incorrect after that.
Human Karyotype
Cystic Fibrosis
• Cystic fibrosis (CF) is one of the most common fatal
genetic disorders among Americans.
• It affects the lungs and digestive tract
• Due to a defective protein in the plasma membrane,
cystic fibrosis results in the formation and
accumulation of thick mucus in the lungs and digestive
tract.
Cystic Fibrosis is caused by a frameshift mutation on
chromosome 7.
Three nitrogenous bases are deleted to cause this
mutation in the amino acid sequence.
Chromosomal Alterations
• Changes may occur in chromosomes as well as in
genes.
• Sometimes during mitosis or meiosis chromosomes
break and then rejoin incorrectly, or just a piece
breaks off for good.
• Structural changes in chromosomes are called
chromosomal mutations.
Causes of Mutations
• Some mutations seem to just happen, perhaps as a
mistake in base pairing during DNA replication.
• These mutations are said to be spontaneous.
• However, many mutations are caused by factors in the
environment, such as radiation, chemicals, and even
high temperatures.
• Any agent that can cause a change in DNA is called a
mutagen.
Repairing DNA
• Enzymes proofread the DNA and replace incorrect
nucleotides with correct nucleotides.
• These repair mechanisms work extremely
well, but they are not perfect.
• The greater the exposure to a mutagen such as UV
light, the more likely is the chance that a mistake will
not be corrected.
Question 1
Any change in DNA sequences is called a
_______.
A. replication
B. mutation
C. transcription
D. translation
The answer is B.
Question 2
Which is more serious, a point mutation or a
frameshift mutation? Why?
Answer
A frameshift mutation is more serious than a
point mutation because it disrupts more codons
than a point mutation.
Question 3
Why are chromosomal mutations rarely passed on to the
next generation?
Answer
Few chromosomal changes are passed on to the
next generation because the zygote usually
dies. If the zygote survives, it is often sterile
and incapable of producing offspring.