Genetics 450 Chapter 16
Which of these statements about LINEs (long interspersed elements) is FALSE?
They contain a reverse transcriptase-encoding gene flanked by long terminal repeats.
How are short interspersed elements different from long interspersed elements?
They do not encode their own reverse transcriptase.
How are short interspersed elements similar to long interspersed elements?
They replicate prior to relocating within the genome. They are autonomous transposable elements. They only insert into sites with few genes.
Select the typical features of a typical retrovirus.
reverse transcriptase integrase single‑stranded RNA
The two types of bacterial transposons are:
simple and composite.
Genome size frequently correlates with:
the amount of DNA in that genome derived from transposable elements.
Gene therapy can potentially correct medical conditions resulting from a nonfunctioning gene within a cell. Arrange the steps of the procedure for using a virus as a vector in gene therapy in the correct order.
-a functioning gene is inserted into the DNA of an inactivated virus -the viral vector is presented to the affected cells in a specific tissue -a vesicle envelops the vector and moves the vector into the cell -the vesicle breaks down and releases the vector within the host cell -the viral vector injects its DNA into the nucleus of the host cell -the host cell transcribes the introduced gene to make proteins
What is the defining characteristic of retrotransposons?
A retrotransposon is transposed through an RNA intermediate.
How are replicative transposition and conservative transposition similar?
The reaction is initiated by the element-encoded transposase, which cuts at the end of the transposon.
Repression of transposable elements, such as Tc1, was studied in which organism originally?
The repression of transposable elements, such as Tc1, was first investigated in the late 1990s, using the model organism Caenorhabditis elegans.
Which of these statements is TRUE?
There are several families of transposons in maize, and each has its own transposase.
Approximately what share of the human genome is derived from transposable elements?
50 percent Almost half of the human genome is derived from transposable elements. This includes SINEs, LINEs, and DNA transposons.
Why are Ds dependent upon Ac?
Ac encodes an enzyme called transposase that catalyzes the excision of Ds and its insertion in a new location.
Which of these statements is TRUE?
Ac encodes transposase, while Ds does not encode transposase and requires Ac to activate it.
How are Ac in maize and P elements in Drosophila similar?
Both have inverted terminal repeats and encode a single protein called a transposase.
_____ is a genetic element that can jump around the genome or break chromosomes in the presence of _____.
Ds; Ac
The Colorless (C) locus has three alleles: C, c, and CI. What is the order of dominance for these three alleles?
CI > C > c CI is a dominant inhibitor allele that represses pigment production; it is dominant to C. CI is dominant to c, allele for lack of blue pigment
Which describes a non-autonomous transposable element?
Elements can transpose only with the help of an autonomous element elsewhere in the genome.
Which of these steps in the retrovirus life cycle are CORRECT? I. Viral mRNA is transcribed from the integrated proviral DNA. II. Single-stranded viral DNA is integrated into the DNA of the host chromosome. III. Reverse transcriptase synthesizes a second strand from the DNA copy. IV. Capsid breaks down; resolvase synthesizes a DNA copy of the viral RNA.
I and III. Reverse transcriptase synthesizes a second strand from the DNA copy, which is step 3, and step 1 is when viral mRNA is transcribed from the integrated proviral DNA.
Which of these statements is TRUE regarding simple transposons? I. A transposon can jump from a plasmid to a bacterial chromosome but not from one plasmid to another plasmid. II. Simple transposons are composed of short (< 50 bp) inverted repeat sequences that can encompass bacterial genes. III. The mobility of simple transposons is catalyzed by a transposase that is encoded within the transposon itself rather than in an IS element. IV. Simple transposons also encode resolvase, an enzyme that promotes site-specific recombination.
II, III, and IV. A transposon can jump from a plasmid to a bacterial chromosome or from one plasmid to another plasmid
What are the CORRECT steps are involved in the life cycle of a retrovirus? I. Double-stranded viral DNA is integrated into the DNA of the host chromosome. II. Reverse transcriptase synthesizes a second strand from the DNA copy. III. Capsid enters the host cell and leaves its envelope on the membrane. IV. Capsid breaks down; reverse transcriptase synthesizes a DNA copy of the viral RNA. V. Synthesis of new viruses. VI. Viral mRNA is transcribed from the integrated proviral DNA.
III, IV, II, I, VI, & V The process starts with a capsid entering a host cell and leaving its envelope on the membrane, and ends with synthesis of new viruses.
The bacterial genome contains segments of DNA, termed _____, which can move from one position on the chromosome to another.
IS elements The first insertion element isolated from the gal operon was called IS1 for insertion-sequence (IS) element 1.
Why are transposable elements inserted into introns not typically problematic to an organism?
Introns are spliced out of the pre-mRNA.
Which type of transposable elements is MOST abundant in the human genome?
LINEs LINEs make up about 21 percent of the human genome.
The vast majority of these transposable elements are two types of retrotransposons called _____ and _____.
LINEs; SINEs long interspersed elements, or LINEs, and short interspersed elements, or SINEs
What is an example of transposon targeting?
Long interspersed elements, R1 and R2, insert only into genes that produce ribosomal RNA.
P elements are found wild types, and these strains have become known to have _____. The lab strains are said to have _____.
P cytotype; M cytotype
A student is crossing female flies from the wild and male flies grown in the laboratory. In this scenario, the female flies will have a(n) _____ cell type and the male flies will have a(n) _____ cell type.
P; M Wild strains have become known as P strains and lab strains are called M strains. M stands for "maternal" and P for "paternal," symbolizing that dysgenesis occurs when the lab (M) strain is maternal and the wild (P) strain is paternal.
Which mechanism is utilized in order to repress the expression of active transposable elements in eukaryotic genomes?
RNAi
Why do these diploid plants have more DNA, but less organismal complexity compared with humans?
Repetitive DNA from transposons accounts for the majority of these plant genomes.
mechanisms of transposition
Replicative and conservative transposition are the two mechanisms of transposition.
Which is an indicator that all transposable elements have a mechanism for insertion?
Transposable elements are flanked by a target-site duplication.
Which is incorrect about transposable elements?
Transposable elements replicate the genome.
What is a short interspersed nuclear element (SINE)?
a DNA sequence less than 500 base pairs long that is reverse transcribed into the genome
What is an insertion sequence (IS) element?
a small transposable element that only encodes transposase and a regulatory protein
Which statement describes a successful transposable element?
a transposon that increases copy number without harming its host
A(n) _____ is an element that can move itself in the genome and have a transposase gene.
autonomous transposable element
Retrotransposons are a type of transposable element, or transposon. These segments of DNA can duplicate and insert themselves into new locations in the genome. For example, the Alu sequence is about 300 bases long, and it is the most abundant retrotransposon in primate genomes. In fact, it makes up approximately 15% of the human genome. Choose the description that best explains how the Alu sequence can affect the size of the human genome.
can increase the size of the human genome because it adds 300 bases to the genome each time it is duplicated.
Kernel sectors uncovering the recessive alleles at multiple linked loci on maize _____ indicated that the chromosome broke (dissociated) repeatedly in the same position during kernel development.
chromosome 9
Like most other flowered plants, the maize life cycle includes a process called _____, and each pollen grain in the maize contains _____ sperm cell(s).
double fertilization ; two haploid
Which is NOT a safe haven where mobile elements can successfully insert without being negatively selected?
gene regulatory elements
What occurs only when females from lab strains are mated with males derived from natural populations in fruit flies?
hybrid dysgenesis
When P elements are mobilized in P × M crosses, the F1's exhibit _____.
hybrid dysgenesis
Transposons are movable elements of DNA that insert into new positions within the genome. A transposon may disrupt normal gene function, confer an evolutionary advantage, or have no effect on the organism. Select all scenarios in which a transposon has positively contributed to the evolution of the host's genome.
insertion of a transposon encoding a heat shock protein into the genome of a plant acquisition of a transposon carrying a tetracycline resistance gene in a bacterium acquisition of a transposon carrying an adhesin gene into the Lactobacillus casei genome
Why is the heterochromatin of centromeres considered a safe haven in the genome?
it contains very few genes and many regions of repetitive DNA
What is NOT a characteristic of Ty elements?
mobile elements that are flanked by short terminal repeats
When Ac is present on a separate chromosome, the Ds element most often breaks the chromosome, producing an acentric fragment with the CI allele. With only C present, kernels will be _____.
partially unpigmented with blue pigmented sectors. With Ac present, Ds breaks the chromosome, causing blue sectors by uncovering C.
A researcher has isolated a new genetic sequence in rabbits that appears to be composed of transposon relics and that can also generate piRNAs. She has just identified a:
pi-cluster.
Long RNAs are transcribed from the pi-clusters and then processed into short single-stranded _____ of 23-30 nucleotides in length.
piRNAs
When integrated into host chromosomes as double-stranded DNA, the double-stranded DNA copy of the retroviral genome is called a _____.
provirus
Match the phenotype expected in the F1 kernel in scenerio 2 (Chapter 16 hw) if transposition occurs before fertilization
purple kernel
Match the phenotype expected in the F1 kernel in scenerio 1 (Chapter 16 hw) if transposition occurs during late development
purple kernel with yellow variegation
What is the function of piwi-interacting RNAs?
regulate epigenetic and posttranscriptional gene silencing guides the degradation of mRNAs complementary to the piRNA, that is, mRNAs from the transposons that compose the pi-cluster (Figure 16-28). By degrading the mRNAs complementary to transposases, any transposons represented in a pi-cluster are silenced. Second, the piRNA-Piwi complex directs the placement of histone marks on the pi-cluster chromatin to promote the transcription of the long RNAs from it. This latter step enables the production of the piRNAs needed for the first step.
Which term is similar to transpose?
relocation
Which is an example of gene therapy?
replacing a disease-causing allele for a gene with a normal allele
In bacteria, transposition occurs by at least two different mechanisms. Some transposable elements can replicate a copy of the element into a target site, leaving one copy behind at the original site, called _____. In other cases, transposition consists of the excision of the element and its reinsertion into a new site referred to as _____.
replicative transposition; conservative transposition
The cointegrate, the intermediate formed by replicate transposition, is resolved by a recombination-like event catalyzed by the _____ that turns a cointegrate into two smaller circles, leaving one copy of the transposable element in each plasmid.
resolvase
Organisms with big genomes have lots of sequences that resemble _____, whereas organisms with small genomes have many fewer.
transposable elements
DNA transposons encode a _____ that cuts the transposon from the chromosome and catalyzes its reinsertion at other chromosomal locations.
transposase
Which enzyme does Ac encode that catalyzes the excision of Ds from a chromosome and its insertion in a new location?
transposase
Knowledge of the genetics and molecular biology of _____ has enabled scientists to harness them as tools to clone genes and create transgenic organisms.
transposons