Transcript The Birth and Death Of Genes
The Molecular Evolution of Gene Birth and Death
Author:
Ann Brokaw AP Biology Teacher Rocky River High School Rocky River, Ohio
The Birth and Death of Genes
To the student:
The following slides provide examples of how genes can be gained and lost through evolution.
Answer the questions on the student handout when prompted.
The Birth of Genes
Mutations are the ultimate source of new genes. A
mutation
is a change in an organism ’s genetic information, or DNA. Mutations occur at random. When mutations occur in genes, they may result in new phenotypes that natural selection acts upon.
Student Handout: Answer Question 1
Heritable Mutations
Mutations can occur at any time and in the DNA of any type of cell. Mutations that occur in somatic (body) cells only impact the individual in which they occur. Mutations that occur in gametes (egg and sperm) can be passed on to the next generation.
Student Handout: Answer Question 2
Types of Mutations
Mutations can occur on a small scale, most often affecting one or two nucleotides of DNA, or can involve large segments of a chromosome or even an entire chromosome.
Regardless of the type, mutations in a gene ’ s DNA can alter the function of the protein encoded by the gene.
Point Mutations
Most mutations involve changes to a single nucleotide or a few nucleotides of an organism ’ s DNA.
Point mutations
are changes in a single nucleotide of the DNA sequence. They include
substitutions
,
insertions
, and
deletions
. Insertions and deletions are not only point mutations; they can involve two or more nucleotides.
Student Handout: Answer Questions 3 - 4
Substitutions
A substitution mutation is the replacement of a single nucleotide with another. It can be classified in one of three ways depending on the effect it has on the protein produced: •
Silent mutations
have no effect on the protein. •
Missense mutations
result in a single amino acid change in the translated sequence. •
Nonsense mutations
result in an amino acid codon being replaced by a “ stop ” codon. Nonsense mutations cause a premature end to translation, resulting in a truncated protein.
Substitution Examples
DNA Wild-type (unmutated)
G-T-C
mRNA Amino acid
C-A-G Glutamine
Silent mutation
G-T-
T
C-A-A Glutamine
Missense mutation
G-T-
G
C-A-C Histidine
Nonsense mutation A
-T-C U-A-G “ Stop ”
Student Handout: Answer Questions 5 - 6
Sickle Cell Anemia
Sickle cell anemia
is a disease caused by a point mutation that affects the shape of red blood cells. People with sickle cell anemia, which is also called sickle cell disease, experience frequent pain, infections, and other symptoms.
Click to view an animation on sickle cell anemia
A.
B.
C.
From the BioInteractive.org website:
http://www.hhmi.org/biointeractive/dna/ DNAi_sicklecell.html
Student Handout: Answer Question 7
Rett Syndrome
Rett syndrome
is a disorder that primarily affects girls and causes severe problems with communication, learning, and coordination. The most common cause of Rett syndrome are mutations in the
MECP2
gene on the X chromosome. This gene provides instructions for making a protein that is critical for normal brain function.
Click to view a lecture clip on Rett syndrome
From the BioInteractive.org website:
www.biointeractive.org/neuroscience/ child_dev+rett.html
A.
A.
B.
B.
C.
C.
D.
D.
Student Handout: Answer Question 8
Insertions and Deletions
Insertion and deletion mutations occur when one or more base pairs are inserted or deleted from the DNA sequence.
Since mRNA is translated three nucleotides at a time, insertions and deletions that do not involve three or multiples of three nucleotides change how all the mRNA downstream of the mutation is translated. These mutations are known as
frameshift mutations
. Frameshift mutations almost always result in a nonfunctional protein.
Student Handout: Answer Question 9
Repeat Expansion
Some protein-coding genes contain sequences of three nucleotides, repeated several times. This type of repeat is called a
trinucleotide repeat
. The number of repeats can increase, or “ expand, ” due to an error during DNA replication. The repeat expansion gives rise to a new allele but can still result in a functioning protein. However, if the number of repeats exceeds a certain “normal” threshold for that gene, the resulting protein will no longer function properly.
Student Handout: Answer Question 10
Spinocerebellar Ataxia Type 1
An example of a disease caused by a trinucleotide repeat expansion is
spinocerebellar ataxia type 1 (SCA1)
. SCA1 is an inherited disease of the central nervous system. The mutations cause malfunctioning of the nerve fibers that carry messages to and from the
Click to view a lecture clip on spinocerebellar ataxia type 1
brain. The overall result is degeneration of the cerebellum, the coordination center of the brain.
From the BioInteractive.org website:
http://media.hhmi.org/hl/03Lect3.html?star
t=12:26&end26:16 Student Handout: Answer Question 11
Are All Mutations
“
Bad
”
?
It is a common misconception that “ all mutations are bad.
” We have already learned that some mutations in genes have no effect, such as silent mutations, whereas others cause disease. But mutations can also be beneficial to an organism.
Here is an example:
View the short film
The Making of the Fittest: Natural Selection and Adaptation
From the BioInteractive.org website:
www.biointeractive.org/shortfilms Student Handout: Answer Question 12
Chromosomal Alterations
Changes to the number or structure of chromosomes can also affect an organism ’s phenotype. Such large-scale changes most often occur during the S phase of interphase, when DNA is replicating; during prophase I of meiosis, when crossing over occurs; or upon exposure to damaging agents, such as radiation.
There are four main types of alterations in chromosome structure: deletions, inversions, translocations, and duplications.
Student Handout: Answer Questions 13 - 14
Types of Chromosomal Alterations
• • • •
Chromosomal deletions
occur when part or all of a chromosome is lost.
Chromosomal inversions
occur when a segment of a chromosome breaks off and reattaches in the reverse orientation.
Chromosomal translocations
occur when a part of a chromosome breaks off and attaches to a nonhomologous chromosome.
Chromosomal duplications
occur when part or all of a chromosome is repeated.
Student Handout: Answer Question 15
Cri-du-chat Syndrome
Cri-du-chat
is a rare disorder caused by a chromosomal alteration on chromosome 5. Infants with this condition often have a high-pitched cry that sounds like that of a cat. A drawing of an affected individual ’ s chromosome 5 is shown below.
Student Handout: Answer Question 16
Chronic Myelogenous Leukemia
Chronic myelogenous leukemia (CML)
is a rare form of cancer caused by a chromosomal alteration that occurs in somatic cells. It is characterized by increased and unregulated growth of myeloid cells in the bone marrow and the accumulation of these cells in the blood.
Click to view a lecture clip on chronic myelogenous leukemia Click to view an animation on CML and Gleevec
From the BioInteractive.org website: http://media.hhmi.org/hl/03Lect2.html?start=32:20 &end36:11 From BioInteractive.org website:
www.biointeractive.org/dna/DNAi_cml_gleevec.html
Student Handout: Answer Question 17
A Closer Look at Duplications
Chromosomal duplications
occur when all or part of a chromosome is repeated, resulting in two copies of a gene or genes. Two genes are said to be
paralogous
if they exist at different chromosomal locations in the same organism and if they arose from a common ancestral gene. Gene duplication can lead to new traits and has played a major role in the evolution and diversification of life.
Student Handout: Answer Question 18
Causes of Gene Duplications
Mechanisms by which complete genes are duplicated include:
Unequal crossing over:
Chromatids from homologous chromosomes are misaligned during crossing over in prophase I of meiosis, causing one chromatid to gain a second set of genes from the homologous chromatid.
Chromosome breaks:
A piece of a chromosome breaks off and becomes attached as an extra segment to another chromosome during meiosis.
Retrotransposition:
mRNAs are reverse transcribed into DNA, and these reverse copies of genes are inserted at random sites in the genome.
Gene Duplication Outcomes
When a gene is duplicated:
one of the two gene copies can lose its function by accumulating mutations over generations.
one of the two gene copies can gain a novel function through subsequent mutation; this can only happen if the original gene duplication event does not have severe effects on the organism and can persist over generations.
the two copies of the gene can partition the total function of the ancestral gene into two unique but related functions for more efficient expression.
Student Handout: Answer Question 20
Duplicated Genes in Animals
Lysozyme is an enzyme in animals that protects against bacterial infection by hydrolyzing the cell walls of bacteria. α-Lactalbumin is a nonenzyme protein that plays a role in mammalian milk production. Both proteins have similar amino acid sequences and three dimensional structures. They are both present in mammals, but only lysozyme is present in birds.
Student Handout: Answer Question 21
Duplicated Genes in Flowering Plants
The flowering plant genes
P1
and
paleoAP3
are involved in stamen development. Scientists have traced the origin of these genes to the duplication of a gene called
MADS.
This duplication took place prior to the evolution of flowering plants.
More recently, the
paleoAP3
gene duplicated again. The new gene evolved a novel function in petal development.
Student Handout: Answer Question 22
Other Examples of Duplicated Genes
The following examples of gene duplications demonstrate the evolutionary significance of these events:
Human olfactory genes
Early mammals carried a single gene for detecting odors; today, humans have 1,000 olfactory receptor genes.
Genes that break down RNA
Most primates have one gene for the pancreatic enzyme RNAse, which breaks down dietary RNA and degrades double-stranded RNA, perhaps as a defense against viruses. The colobine monkey, douc langur, has two genes that produce two RNA-degrading enzymes —RNAse1 and RNAse1B—which work best at different pH levels. Douc langurs primarily eat leaves, and the new gene improved the efficiency of digestion.
Student Handout: Answer Questions 23
The Death of Genes
Duplication can lead to new genes. But genes can also lose their protein-producing abilities or stop being expressed. Such genes are called
pseudogenes
, or “ molecular fossils.
” As species have evolved, diverged, and gone extinct, so too have their genes.
Genes become pseudogenes when they accumulate mutations over long periods of time. Note that this only happens if the mutations do not compromise the organism ’ s survival, or else natural selection would eliminate the mutations.
Student Handout: Answer Questions 24 - 25
Examples of Gene Death
There are several examples of genes losing their functions that demonstrate the evolutionary significance of these events.
Olfactory receptor genes in humans and mice
Mice have about 1,500 olfactory receptor genes, while humans have about 1,000. Only 20 percent of mouse olfactory receptor genes are pseudogenes, compared to about 60 percent in humans.
Myoglobin gene in some icefish species
Myoglobin is an oxygen-binding protein found in muscles, similar to hemoglobin in red blood cells. Myoglobin is absent from muscles, including the heart, of several, but not all, icefish species.
Student Handout: Answer Question 26
Gene Birth and Death in Icefish
Icefish provide an excellent example of both the birth and death of genes.
Click to view the short film The Making of the Fittest:
The Birth and Death of Genes
From the Biointeractive.org website:
www.biointeractive.org/shortfilms Student Handout: Answer Questions 27 - 30
Evolutionary Significance of Gene Birth and Death
“
Natural selection acts only in the moment.
It cannot preserve what is no longer used, and it cannot plan for the future.
”
Dr. Sean Carroll, Into the Jungle
Student Handout: Answer Questions 31 - 34
Resources:
Campbell, Neil A. and Jane B. Reece.
Biology, AP Edition.
Eighth Edition. Pearson Benjamin Cummings Publishing. San Francisco, California. 2008.
Carroll, Sean B. In Cold Blood: The Tale of the Icefish, Chapter 9, Benjamin Cummings, San Francisco, California. 2008.
Into the Jungle
, Deng, Cheng,
et al
. Evolution of an Antifreeze Protein by Neofunctionalization Under Escape from Adaptive Conflict.
Proceedings of the National Academy of Sciences
, Vol. 107, No. 50. December 14, 2010.
Howard Hughes Medical Institute, 2003 Holiday Lectures on Science, Learning from Patients – The Science of Medicine, Chevy Chase, Maryland, 2003.
Howard Hughes Medical Institute,
Making of the Fittest: Natural Selection and Adaptation
, Chevy Chase, Maryland, 2011.
Howard Hughes Medical Institute,
Making of the Fittest: The Birth and Death of Genes
, Chevy Chase, Maryland, 2011.
Howard Hughes Medical Institute, www.BioInteractive.org, DNA animations, Chevy Chase, Maryland, 2011.
Hurles, M. Gene Duplication: The Genomic Trade in Spare Parts.
e206. doi:10.1371/journal.pbio.0020206, 2004.
Raven, Peter H. and George B. Johnson.
Publishing. New York, New York. 2011.)
Biology.
PLoS Biol.
2(7): Ninth Edition. (McGraw-Hill Zhang, Xhang,
Genetics et al
. Gene Duplication Adapts to Changing Environment (online). March 4, 2002.
. Nature