Since we are having a riveting conversation about genetic mutations I figured I'd put up this gem here. I really enjoy science.
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A1876
8.1 Mutations: Types and Causes
The
development and function of an organism is in large part controlled by
genes.
Mutations can lead to changes in the structure of an encoded
protein or to a decrease or complete loss in its
expression. Because a change in the DNA sequence affects all copies of the encoded
protein,
mutations can be particularly damaging to a cell or organism. In contrast, any alterations in the sequences of RNA or
protein molecules that occur during their synthesis are less serious because many copies of each RNA and
protein are synthesized.
Geneticists often distinguish between the
genotype and
phenotype of an organism. Strictly speaking, the entire set of
genes carried by an individual is its
genotype, whereas the function and physical appearance of an individual is referred to as its
phenotype. However, the two terms commonly are used in a more restricted sense:
genotype usually denotes whether an individual carries
mutations in a single
gene (or a small number of
genes), and
phenotype denotes the physical and functional consequences of that
genotype.
Mutations Are Recessive or Dominant
A fundamental genetic difference between organisms is whether their cells carry a single set of
chromosomes or two copies of each
chromosome. The former are referred to as
haploid; the latter, as
diploid. Many simple unicellular organisms are
haploid, whereas complex multicellular organisms (e.g., fruit flies, mice, humans) are
diploid.
Figure 8-1
For a recessive mutation to give rise to a mutant phenotype (more...)
Figure 8-1
.
For a recessive mutation to give rise to a mutant phenotype in a diploid organism, both alleles must carry the mutation
However, one copy of a
dominant mutant
allele leads to a mutant
phenotype.
Recessive mutations result in a loss of function, whereas
dominant mutations often, but not always, result in a gain of function.
Different forms of a
gene (e.g., normal and mutant) are referred to as
alleles. Since
diploid organisms carry two copies of each
gene, they may carry identical
alleles, that is, be
homozygous for a
gene, or carry different
alleles, that is, be
heterozygous for a
gene. A
recessive mutation is one in which both
alleles must be mutant in order for the mutant
phenotype to be observed; that is, the individual must be
homozygous for the mutant
allele to show the mutant
phenotype. In contrast, the phenotypic consequences of a
dominant mutation are observed in a
heterozygous individual carrying one mutant and one normal
allele (
Figure 8-1).
Recessive mutations inactivate the affected
gene and lead to a
loss of function. For instance,
recessive mutations may remove part of or all the
gene from the
chromosome, disrupt
expression of the
gene, or alter the structure of the encoded
protein, thereby altering its function. Conversely,
dominant mutations often lead to a
gain of function. For example,
dominant mutations may increase the activity of a given
gene product, confer a new activity on the
gene product, or lead to its inappropriate spatial and temporal
expression.
Dominant mutations, however, may be associated with a loss of function. In some cases, two copies of a
gene are required for normal function, so that removing a single copy leads to mutant
phenotype. Such
genes are referred to as
haplo-insufficient. In other cases,
mutations in one
allele may lead to a structural change in the
protein that interferes with the function of the wild-type
protein encoded by the other
allele. These are referred to as
dominant negative mutations.
Some
alleles can be associated with both a
recessive and a
dominant phenotype. For instance, fruit flies
heterozygous for the mutant
Stubble (Sb) allele have short and stubby body hairs rather than the normal long, slender hairs; the mutant
allele is
dominant in this case. In contrast, flies
homozygous for this
allele die during
development. Thus the
recessive phenotype associated with this
allele is lethal, whereas the
dominant phenotype is not.
Inheritance Patterns of Recessive and Dominant Mutations Differ
Recessive and
dominant mutations can be distinguished because they exhibit different patterns of inheritance. To understand why, we need to review the type of
cell division that gives rise to
gametes (sperm and egg cells in higher plants and animals). The body (somatic) cells of most multicellular organisms divide by
mitosis (see
Figure 1-10), whereas the
germ cells that give rise to
gametes undergo
meiosis. Like body cells, premeiotic
germ cells are
diploid, containing two of each morphologic type of
chromosome. Because the two members of each such pair of
homologous chromosomes are descended from different parents, their
genes are similar but not usually identical. Single-celled organisms (e.g., the yeast
S. cerevisiae) that are
diploid at some phase of their life cycle also undergo
meiosis (see
Figure 10-54).
Figure 8-2
Meiosis
Figure 8-2
.
Meiosis
A premeiotic
germ cell has two copies of each
chromosome (2
n), one maternal and one paternal.
Chromosomes are replicated during the S phase, giving a 4
n chromosomal complement. During the first meiotic division, each replicated
chromosome (actually two sister
chromatids) aligns at the cell equator, paired with its homologous partner; this pairing off, referred to as
synapsis, permits genetic
recombination (discussed later). One homolog (both sister
chromatids) of each morphologic type goes into one daughter cell, and the other homolog goes into the other cell. The resulting 2
n cells undergo a second division without intervening DNA replication. During this second meiotic division, the sister
chromatids of each morphologic type separate and these now independent
chromosomes are randomly apportioned to the daughter cells. Thus, each
diploid cell that undergoes
meiosis produces four
haploid cells, whereas each
diploid cell that undergoes
mitosis produces two
diploid cells (see
Figure 1-10).
Figure 8-2 depicts the major events in
meiosis.
One round of DNA replication, which makes the cell 4
n, is followed by
two separate cell divisions, yielding four
haploid (1
n) cells that contain only one
chromosome of each homologous pair. The apportionment, or
segregation, of homologous
chromosomes to daughter cells during the first meiotic division is random; that is, the maternally and paternally derived members of each pair, called homologs, segregate independently, yielding
germ cells with different mixes of paternal and maternal
chromosomes. Thus parental characteristics are reassorted randomly into each new
germ cell during
meiosis. The number of possible varieties of meiotic segregants is 2n, where
n is the
haploid number of
chromosomes. In the case of a single
chromosome, as illustrated in
Figure 8-2,
meiosis gives rise to two types of
gametes; one type carries the maternal homolog and the other carries the paternal homolog.
Figure 8-3
Segregation patterns of dominant and recessive mutations (more...)
Figure 8-3
.
Segregation patterns of dominant and recessive mutations
Crosses between genotypically normal individuals (blue) and mutants (yellow) that are
heterozygous for a
dominant mutation (a) or
homozygous for a
recessive mutation (b) produce different ratios of normal and mutant
phenotypes in the F1 generation. Although all the F1 progeny from a cross between a normal individual and an individual
homozygous for a
recessive mutation will have a normal
phenotype, one-quarter of the progeny from the intercross between F1 progeny will have a mutant
phenotype. Observation of
segregation patterns like these led Gregor Mendel (1822  1884) to conclude that each
gamete receives only one of the two parental
alleles, a conclusion known as
Mendels first law.
Now, lets see what
phenotypes are generated by mating of wild-type individuals with mutants carrying either a
dominant or a
recessive mutation. As shown in
Figure 8-3a, half the
gametes from an individual
heterozygous for a
dominant mutation in a particular
gene will have the wild-type
allele, and half will have the mutant
allele. Since
fertilization of female
gametes by male
gametes occurs randomly, half the first filial (F1) progeny resulting from the cross between a normal wild-type individual and a mutant individual carrying a single
dominant allele will exhibit the mu-tant
phenotype. In contrast, all the
gametes produced by a mutant
homozygous for a
recessive mutation will carry the mutant
allele. Thus, in a cross between a normal individual and one who is
homozygous for a
recessive mutation, none of the F1 progeny will exhibit the mutant
phenotype (
Figure 8-3b). However, one-fourth of the progeny from parents both
heterozygous for a
recessive mutation will show the mutant
phenotype.
Mutations Involve Large or Small DNA Alterations
Figure 8-4
Different types of mutations
Figure 8-4
.
Different types of mutations
(a)
Point mutations, which involve alteration in a single
base pair, and small deletions generally directly affect the function of only one
gene. A wild-type
peptide sequence and the mRNA and DNA encoding it are shown at the top. Altered
nucleotides and
amino acid residues are highlighted in green. Missense
mutations lead to a change in a single
amino acid in the encoded
protein. In a nonsense
mutation, a
nucleotide base change leads to the formation of a stop
codon (purple). This results in premature termination of
translation, thereby generating a truncated
protein. Frameshift
mutations involve the addition or deletion of any number of
nucleotides that is not a multiple of three, causing a change in the
reading frame. Consequently, completely unrelated
amino acid residues are incorporated into the
protein prior to encountering a stop
codon. (b) Chromosomal abnormalities involve alterations in large segments of DNA. Presumably these abnormalities arise owing to errors in the mechanisms for repairing double-strand breaks in DNA.
Chromosomes (I or II) are shown as single thick lines with the regions involved in a particular abnormality highlighted in green or purple. Inversions occur when a break is rejoined to the correct
chromosome but in an incorrect orientation; deletions, when a segment of DNA is lost; translocations, when breaks are rejoined to the wrong
chromosomes; and insertions, when a segment from one
chromosome is inserted into another
chromosome.
A
mutation involving a change in a single
base pair, often called a
point mutation, or a deletion of a few
base pairs generally affects the function of a single
gene (
Figure 8-4a). Changes in a single
base pair may produce one of three types of
mutation:
Small deletions have effects similar to those of frameshift
mutations, although one third of these will be in-frame and result in removal of a small number of contiguous
amino acids.
The second major type of
mutation involves large-scale changes in
chromosome structure and can affect the functioning of numerous
genes, resulting in major phenotypic consequences. Such
chromosomal mutations (or abnormalities) can involve deletion or insertion of several contiguous
genes, inversion of
genes on a
chromosome, or the exchange of large segments of DNA between nonhomologous
chromosomes (
Figure 8-4b).
Mutations Occur Spontaneously and Can Be Induced
Mutations arise spontaneously at low frequency owing to the chemical instability of purine and pyrimidine
bases and to errors during DNA replication. Natural exposure of an organism to certain environmental factors, such as ultraviolet light and chemical
carcinogens (e.g., aflatoxin B1), also can cause
mutations.
Figure 8-5
One mechanism by which errors in DNA replication produce (more...)
Figure 8-5
.
One mechanism by which errors in DNA replication produce spontaneous mutations
The replication of only one strand is shown; the other strand is replicated normally, as shown at the top. A replication error may arise in regions of DNA containing tandemly repeated sequences (in this case, GTC) when a portion of the newly synthesized strand (light blue) loops out into a single-stranded form. This slippage displaces the newly synthesized strand back along the
template strand (dark blue), with its 3′ end still paired with the
template. As a result, the DNA-synthesizing
enzymes copy a region of the
template strand a second time, leading to an increase in length of nine
nucleotides (yellow) in this example. A subsequent round of DNA replication results in the production of one normal duplex DNA molecule and one mutant duplex containing the additional
nucleotides.
A common cause of spontaneous
point mutations is the deamination of cytosine to uracil in the DNA double helix. Subsequent replication leads to a mutant daughter cell in which a T·A
base pair replaces the wild-type C·G
base pair. Another cause of spontaneous
mutations is copying errors during DNA replication. Although replication generally is carried out with high fidelity, errors occasionally occur.
Figure 8-5 illustrates how one type of copying error can produce a
mutation. In the example shown, the mutant DNA contains nine additional
base pairs.
In order to increase the frequency of
mutation in experimental organisms, researchers often treat them with high doses of chemical
mutagens or expose them to ionizing radiation.
Mutations arising in response to such treatments are referred to as
induced mutations. Generally, chemical
mutagens induce
point mutations, whereas ionizing radiation gives rise to large chromosomal abnormalities.
Figure 8-6
Induction of point mutations by ethylmethane sulfonate (more...)
Figure 8-6
.
Induction of point mutations by ethylmethane sulfonate (EMS), a commonly used mutagen
(a) EMS alkylates guanine at the oxygen on position 6 of the purine ring, forming
O6-ethylguanine (Et-G), which
base-pairs with thymine. (b) Two rounds of DNA replication of a strand containing Et-G yields a mutant DNA in which a G·C
base pair is replaced with an A·T pair. Cells also have repair
enzymes that can remove the ethyl group from Et-G (
Chapter 12).
Ethylmethane sulfonate (EMS), a commonly used
mutagen, alkylates guanine in DNA, forming
O6-ethylguanine (
Figure 8-6a). During subsequent DNA replication,
O6-ethylguanine directs incorporation of deoxythymidylate, not deoxycytidylate, resulting in formation of mutant cells in which a G·C
base pair is replaced with an A·T
base pair (
Figure 8-6b). The causes of
mutations and the mechanisms cells have for repairing alterations in DNA are discussed further in
Chapter 12.
Some Human Diseases Are Caused by Spontaneous Mutations
Many common human diseases, often devastating in their effects, are due to
mutations in single
genes. Genetic diseases arise by spontaneous
mutations in
germ cells (egg and sperm), which are transmitted to future generations. For example,
sickle-cell anemia, which affects 1 in 500 individuals of African descent, is caused by a single missense
mutation at
codon 6 of the β-globin
gene; as a result of this
mutation, the glutamic
acid at position 6 in the normal
protein is changed to a valine in the mutant
protein. This alteration has a profound effect on hemoglobin, the oxygen-carrier
protein of erythrocytes, which consists of two α-globin and two β-globin subunits (see
Figure 3-11). The deoxygenated form of the mutant
protein is insoluble in erythrocytes and forms crystalline arrays. The erythrocytes of affected individuals become rigid and their transit through capillaries is blocked, causing severe pain and tissue damage. Because the erythrocytes of
heterozygous individuals are resistant to the parasite causing malaria, which is endemic in Africa, the mutant
allele has been maintained. It is not that individuals of African descent are more likely than others to acquire a
mutation causing the sickle-cell defect, but rather the
mutation has been maintained in this population by interbreeding.
Figure 8-7
Role of spontaneous somatic mutation in retinoblastoma, (more...)
Figure 8-7
.
Role of spontaneous somatic mutation in retinoblastoma, a childhood disease marked by retinal tumors
Tumors arise from retinal cells that carry two mutant
Rb−
alleles. (a) In hereditary retinoblastoma, a child receives a normal
Rb+
allele from one parent and a mutant
Rb−
allele from the other parent. A single mutagenic event in a
heterozygous somatic retinal cell that inactivates the normal
allele will result in a cell
homozygous for two mutant
Rb−
alleles. (b) In sporadic retinoblastoma, a child receives two normal
Rb+
alleles. Two separate somatic
mutations, inactivating both
alleles in a particular cell, are required to produce a
homozygous Rb−/
Rb− retinal cell.
Spontaneous
mutation in
somatic cells (i.e., non-germline body cells) also is an important mechanism in certain human diseases, including
retinoblastoma, which is associated with retinal
tumors in children (see
Figure 24-11). The hereditary form of retinoblastoma, for example, results from a germ-line
mutation in one
Rb allele and a second somatically occurring
mutation in the other
Rb allele (
Figure 8-7a). When an
Rb heterozygous retinal cell undergoes somatic
mutation, it is left with no normal
allele; as a result, the cell proliferates in an uncontrolled manner, giving rise to a retinal
tumor. A second form of this disease, called
sporadic retinoblastoma, results from two independent
mutations disrupting both
Rb alleles (
Figure 8-7b). Since only one somatic
mutation is required for
tumor development in children with hereditary retinoblastoma, it occurs at a much higher frequency than the sporadic form, which requires acquisition of two independently occurring somatic
mutations. The Rb
protein has been shown to play a critical role in controlling
cell division (
Chapter 13).
In a later section, we will see how normal copies of disease-related
genes can be isolated and cloned.
SUMMARY
Copyright © 2000, W. H. Freeman and Company
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