Multiple-Choice Answers

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CHAPTER 2: The Cellular and Molecular Basis of Inheritance

1 Base substitutions:

a True. When a stop codon replaces an amino acid
b True. For example, by mutation of conserved splice donor and acceptor sites
c False. Silent mutations or substitutions in non-coding regions may not be pathogenic
d True. For example promoter mutations may affect binding of transcription factors
e False. Frameshifts are caused by the insertion or deletion of nucleotides

2 Transcription:

a False. During transcription mRNA is produced from the DNA template
b True. The mRNA product is then translocated to the cytoplasm
c True. The mRNA is complementary to the antisense strand
d False. Transcription factors bind to regulatory sequences within the promoter
e True. The addition of the 5′ cap and 3′ poly(A) tail facilitates transport to the cytoplasm

3 The following are directly involved in DNA repair:

a True. The DNA glycosylase MYH is involved in base excision repair (BER)
b True. They incorporate the correct bases
c True. They seal gaps after abnormal base excision and correct base insertion
d False. Splicing removes introns during mRNA production
e False. Ribosomes are involved in translation

4 During DNA replication:

a True. It unwinds the DNA helix
b False. Replication occurs in both directions
c True. These fragments are joined by DNA ligase to form the lagging strand
d False. DNA replication is semiconservative as only one strand is newly synthesized
e False. Uracil is incorporated in mRNA, thymine in DNA

CHAPTER 3: Chromosomes and Cell Division

1 Meiosis differs from mitosis in the following ways:

a True. During human meiosis, the number of chromosomes is reduced from 46 to 23
b False. Early cell divisions in gametogenesis are mitotic; meiosis occurs only at the final division
c True. In meiosis, the two divisions are known as meiosis I and II
d True. The bivalents separate independently during meiosis I, and crossovers (chiasmata) occur between homologous chromosomes
e False. The five stages of meiosis I prophase are leptotene, zygotene, pachytene, diplotene, and diakinesis

2 Chromosome abnormalities reliably detected by light microscopy include:

a True. An extra chromosome (e.g., chromosome 21 in Down syndrome) is easily seen
b True. A missing chromosome (e.g., Turner syndrome in females with a single X) is easily seen
c False. A subtle translocation may not be visible
d False. A small deletion may not be visible
e True. Centric fusion of the long arms of two acrocentric chromosomes is readily detected

3 Fluorescent in situ hybridization using whole-chromosome (painting) or specific locus probes enables routine detection of:

a False. Changes in gene dosage may be identified by comparative genomic hybridization (CGH)
b True. Subtelomeric probes are useful in the investigation of non-specific learning difficulties
c True. Trisomies can be detected in interphase cells
d True. The origin of marker chromosomes can be determined by chromosome painting
e True. Subtle rearrangements can be detected by chromosome painting

4 Chemicals used in the preparation of metaphase chromosomes for analysis by light microscopy include:

a True. Colchicine inhibits spindle formation, thus arresting cells at metaphase
b True. Phytohemagglutinin stimulates cell division of T lymphocytes
c True. Giemsa is used to stain chromosomes a pink/purple color
d False. Quinacrine is a fluorescent stain not visible by light microscopy
e True. Hypotonic saline swells the cells, causing cell lysis and spreading of chromosomes

CHAPTER 4: DNA Technology and Applications

1 The following statements apply to restriction enzymes:

a True. Double-stranded DNA can be digested to give overhanging (sticky) ends or blunt ends
b False. More than 300 restriction enzymes have been isolated from various bacteria
c True. If the mutation creates or destroys a recognition site
d True. DNA digestion by a restriction enzyme is the first step in Southern blotting
e False. They are endonucleases as they digest DNA fragments internally, as opposed to exonuclease digestion from the 5′ or 3′ ends of DNA fragments

2 The following describe polymerase chain reaction (PCR):

a True. Millions of copies of DNA can be produced from one template without using cloning vectors
b False. PCR uses the heat-stable Taq polymerase, because a high denaturing temperature (around 95°C) is required to separate double-stranded products at the start of each cycle
c True. PCR may be used to amplify DNA from single cells (e.g., in preimplantation genetic diagnosis); therefore, appropriate control measures are important to avoid contamination
d False. PCR routinely amplifies targets of up to 1 kb and long-range PCR is limited to around 40 kb
e False. Knowledge of the sequence is required to design primers to flank the region of interest

3 Types of nucleic acid hybridization include:

a True. Southern blotting describes the hybridization of a radioactively labeled probe with DNA fragments separated by electrophoresis
b True. Hybridization between the target and probe DNA takes place on a glass slide
c False. Western blotting is used to analyze protein expression using antibody detection methods
d True. Northern blotting is used to examine RNA expression
e True. DNA fingerprinting employs a minisatellite DNA probe to hybridize to hypervariable DNA fragments

4 The following techniques can be used to screen genes for unknown mutations:

a True. Sequencing can be used to detect known or unknown mutations and will characterize an unknown mutation
b True. SSCP is an inexpensive method for mutation screening although its sensitivity is limited
c True. DHPLC is an efficient method for detecting heterozygous mutations
d False. Oligonucleotide ligation assay is used to detect known mutations as the probe design is mutation specific
e False. Real-time PCR is also used to detect known mutations as the probe design is mutation specific

CHAPTER 5: Mapping and Identifying Genes for Monogenic Disorders

1 Positional cloning uses:

a True. Now that the human genome sequence is complete, it is possible to identify a disease-associated gene in silico
b True. After a gene has been mapped to a region, it can be helpful to check for syntenic regions in animal models
c True. Many genes have been identified through mapping of translocation or deletion break-points
d False. Positional cloning describes the search for genes based on their chromosomal location
e True. A genome-wide scan uses microsatellite markers located throughout the genome for linkage mapping

2 A candidate gene is likely to be a disease associated gene if:

a True. This implies causality
b True. This is strong evidence
c True. This excludes the possibility that a single variant is a marker in linkage disequilibrium rather than a pathogenic mutation
d True. For example, a gene associated with blindness might be expected to be expressed in the eye
e False. A pseudogene does not encode a functional protein and mutations are therefore unlikely to be pathogenic

3 Achievements of the Human Genome Project include:

a False. The draft sequence was completed in 2000, but its publication date was February 2001
b True. Sequencing was finished 2 years ahead of the original schedule
c True. Annotation tools such as Ensembl were developed to assist users
d False. Nearly 1500 have been identified to date
e True. Around 5% of the US budget for the Human Genome Project was devoted to studying these issues

CHAPTER 6: Developmental Genetics

1 In development, HOX genes:

a True. They are important in spatial determination and patterning
b False. Only a few malformation syndromes can be directly attributed to HOX gene mutations at present, probably because of paralogous compensation
c False. They contain an important conserved homeobox of 180 bp
d True. They are probably important only in early development
e True. Where malformation-causing mutations have been identified, different organ systems may be involved, e.g., the hand–foot–genital syndrome (HOXA13)

2 In the embryo and fetus:

a False. This occurs later and is the process of laying down the primary body axis in the second and third weeks
b False. Organogenesis takes places mainly between 4 and 18 weeks’ gestation
c True. The genes in these pathways are expressed widely throughout the body
d False. Somites form in a rostro-caudal direction
e True. When mutated these genes lead to the ulnar–mammary syndrome and Holt-Oram syndrome

3 Concerning development pathways and processes:

a False. It is formed from the first pharyngeal (branchial) arch
b True. Remodeling occurs so that these vessels become the great arteries
c True. This has been established in animals and is proving to be highly likely in humans
d False. Achondroplasia is associated with very specific FGFR3 mutations affecting the membrane-bound part of the protein
e False. These different types of mutation usually cause widely differing phenotypes (e.g., the RET gene)

4 Regarding the X-chromosome:

a True. Sometimes the SRY gene is involved in recombination with the pseudoautosomal regions of X and Y
b False. Not all regions of the X are switched off; otherwise, there would presumably be no phenotypic effects in Turner syndrome
c True. However, only when there is a mutated gene on one X chromosome does this have any consequences
d False. SRY has an important initiating function, but other genes are very important
e True. Some unusual phenomena occur in twins leading to the conclusion that these processes are linked

5 Transcription factors:

a False. They are usually proteins that bind to specific regulatory DNA sequences
b False. They also switch genes on
c True. For example, a leg might develop in place of an antenna
d False. Transcription factors are crucial to normal laterality
e True. The zinc finger motif encodes a finger-like projection of amino acids that forms a complex with a zinc ion

CHAPTER 7: Patterns of Inheritance

1 Concerning autosomal recessive inheritance:

a False. Sex ratio is equal
b False. The risk at the time of conception is image
c True. All people carry mutated genes; cousins are more likely to share a mutated gene inherited from a common grandparent
d True. Affected individuals would have to partner a carrier or another affected person for their offspring to be affected as well
e False. The mechanisms causing Angelman syndrome are varied but autosomal recessive inheritance is not one of them

2 Concerning X-linked inheritance:

a True. A father passes his Y chromosome to his son
b True. He might have affected grandsons through his daughters who are carriers
c False. Although the condition affects females, the males are more severely affected because the female has a normal copy of the gene on her other X chromosome, and X-inactivation means that the normal copy is expressed in about half of her tissues
d True. All daughters of an affected man will be affected, but none of his sons
e False. Germline mosaicism always needs to be considered when an isolated case of an X-linked condition occurs

3 In mitochondrial genetics:

a False. This refers to two populations of mitochondrial DNA, one normal, one mutated
b False. The opposite, probably because they replicate more frequently
c False. Any tissue with mitochondria can be affected
d True. If the affected woman’s oocytes contain only mutated mitochondria
e False. Many mitochondrial proteins are encoded by nuclear genes

4 Concerning terminology:

a False. The same disease caused by different genes—but not necessarily on different chromosomes
b False. The basic pattern of inheritance in pseudodominance is autosomal recessive
c True. A proportion of individuals with the mutated gene show no signs or symptoms
d True. Diseases showing anticipation demonstrate increased severity, and earlier age of onset, with succeeding generations
e False. Not a variation in severity (variable expression), but two or more apparently unrelated effects from the same gene

5 In inheritance:

a True. Both copies of a mutated gene can be passed to a child this way
b True. This explains a proportion of cases of Prader-Willi and Angelman syndromes
c False. Digenic inheritance refers to heterozygosity for two different genes causing a phenotype
d True. This explains presenile baldness and gout
e False. Only a small proportion

CHAPTER 8: Mathematical and Population Genetics

1 In applying the Hardy-Weinberg equilibrium, the following assumptions are made:

a False. The population should be large to increase the likelihood of non-random mating
b True. Consanguinity is a form of non-random mating
c True. The introduction of new alleles introduces variables
d True. In theory, if the population is small and donors are used many times this would introduce a form of non-random mating
e True. Migration introduces new alleles

2 If the population incidence of a recessive disease is 1 in 10,000, the carrier frequency in the population is:

a False.
b False.
c False.
d True. The carrier frequency is double the square root of the incidence
e False.

3 Heterozygote advantage:

a False. It refers to conditions that follow autosomal recessive inheritance
b True. The homozygote may show markedly reduced biological fitness (e.g., cystic fibrosis)
c True. People with sickle-cell trait are more able to remove parasitized cells from the circulation
d True. A process of selective advantage may be at work
e False. The presence of the allele in a population may indeed be a founder effect

4 Polymorphic loci:

a False. The alleles need have frequencies of only 1%
b True. They are crucial to gene mapping by virtue of their co-segregation with disease
c True. Linkage analysis using polymorphic loci may be the only way to determine genetic status in presymptomatic diagnosis and prenatal testing
d False. The association of polymorphic loci with disease segregation is key to calculating a logarithmic of the odds score
e False. They may be important (e.g., blood groups)

5 In population genetics:

a False. The incidence of the disease must also be known
b True. In autosomal recessive disease most of the genes in the population are present in unaffected heterozygotes
c True. In recessive conditions unaffected sibships will not be ascertained
d True. Basically, only males are affected and they always manifest the condition
e False. It is useful only when there is a common ancestor from both sides of the family, (i.e., inbreeding)

CHAPTER 9: Polygenic and Multifactorial Inheritance

1 Concerning autism:

a False. It is a neurodevelopmental disorder and no metabolic abnormalities are found
b False. This would imply autosomal dominant inheritance. The rate is about 20%
c False. Although autism occurs in fragile X syndrome the vast majority of affected individuals do not have this condition
d True. The figure is nearly 3% for full-blown autism and a further 3% for milder features—autistic spectrum disorder
e False.

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