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MAstering Biology Exam 3

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During _____ both the contents of the nucleus and the cytoplasm are divided.
the mitotic phase
During _____ the cell grows and replicates both its organelles and its chromosomes.
interphase
Which of the following is a function of the S phase in the cell cycle?
The synthesis of sister chromatids
The copying of chromosomes occurs during which of the following phases of the cell cycle?
S phase
For the first several divisions of early frog embryos, cells proceed directly from the M phase to the S phase and back to M without gaps. Which of the following is likely to be true about dividing cells in early frog embryos?
The cells get smaller with each generation.Without gap phases, the cells have no opportunity to grow.
True or false? The M phase is characterized by the replication and division of a cell's chromosomes.
False
Cells replicate their chromosomes during the S phase and divide partition their chromosomes during the M phase.
If an organism normally has 34 chromosomes, how many molecules of DNA should there be in the G1 phase of the cell cycle?
34 There is one DNA molecule per chromosome.
Which of the following events would cause the cell cycle to arrest?
Poor nutrient conditions. Poor nutrient conditions would arrest the cell cycle at the G1 checkpoint.
DNA replication produces two identical DNA molecules called _____ which seperate during mitosis
sister chromatids
After Chromosomes Condense the____ is the region where the identical DNA molecules are most tightly attached to each other
Centromeres
during mitosis microtubules attach to chromosomes at teh
kinetochores
in dividing cellsmost of the cells growth phase occurs during
interphase
the ______ ia a cell structure consisting of microtubules, which forms during early mitosis and plays a role in cell division
mitotic spindles
during interphase most of the nuclus is filled with a complex of DNA and protien in a dispersedform called ____
Chromatin
In Most eukaryotes division of the nucleous is followed by ____when the rest of the cell divides
cytokinesis
the ______ are the organizing center for micro tubules involved in separating chromosomes during mitosis.
centrosomes
Many organisms contain cells that do not normally divide. These cells exit the cell cycle before the
G1 checkpoint
Once a cell passes the G1 checkpoint
it usually completes the cell cycle--that is, it divides.
The first step in preparing for division
is to replicate the cell's DNA in the S phase.
In the G2 phase
the centrosome replicates
In early M phase
the centrosomes move away from each other toward the poles of the cell, in the process organizing the formation of the mitotic spindle.
At the end of the M phase when mitosis is complete...
the cell divides (cytokinesis), forming two genetically identical daughter cells
Are sister chromatids present in all or part of this phase
S G2 Begining of M propase early anaphase
Is the DNA condensed in allor part of this phase
end of M late anaphase telophase Begining of M propase early anaphase
Does the cell contain X2 the amount of DNA in this phase as it did in the G1 Phase
S g2 end of M late anaphase telophase Begining of M propase early anaphase
In prophase,
the microtubules of the spindle apparatus begin to assemble from individual tubulin subunits. As the identical chromatids of each pair of sister chromatids condense during this stage, they are held together by cohesin proteins.
Prometaphase
is marked by fragmentation of the nuclear envelope, expansion of the spindle into the nuclear region, and attachment of some spindle fibers to the chromosomes via the kinetochores.
Metaphase
marked by the alignment of chromsomes along the metaphase plate, is brought about by kinetochores aligning and then remaining motionless relative to the poles of the cell
In anaphase
the cohesin proteins are cleaved, and the kinetochores move toward the poles of the cell, separating the sister chromatids
As telophase proceeds
the kinetochore microtubules of the spindle disassemble. As the chromosomes reach the poles of the cell, the nuclear envelopes of the two new daughter nuclei form.
Mitotic spindles
Prior to metaphase, the mitotic spindle is constructed by lengthening microtubules that extend from each centrosome.
In metaphase, the kinetochore microtubules have attached each pair of sister chromatids, and the nonkinetochore microtubules overlap extensively at the metaphase plate.
During anaphase, the kinetochore microtubules shorten as the chromosomes move toward the poles of the cell. At the same time, the nonkinetochore microtubules lengthen and push past each other, elongating the cell.
By the end of telophase, all the microtubules associated with the mitotic spindle have disassembled.
cohesins
The inhibitor does not affect the cleavage of____ the attachment of the chromosomes to the kinetochore microtubules, or the elongation of the cell due to the nonkinetochore microtubules(the proteins that hold the sister chromatids together),
the inhibitor only affects
The inhibitor only affects the motor protein that pulls the chromosome along the kinetochore microtubule in anaphase.
will the chromosomes reach the poles of the cell?
Thus, in the treated cell, the sister chromatids can still separate at the beginning of anaphase due to the fact that the cell is elongating (the centrosomes at the poles of the cell are moving farther apart) and the kinetochore microtubules still connect the chromosomes to the centrosomes. However, because the chromosomes cannot move along the kinetochore microtubules, they will never reach the poles of the cell.
Nucleoli are present during
interphase
Cytokinesis often, but not always, accompanies _____.
telophase
Chromosomes become visible during _____.
prophase
Centromeres divide and sister chromatids become full-fledged chromosomes during _____.
anaphase sister chromatids separate and daughter chromosomes migrate to opposite poles.
During telophase
both nuclear envelopes and nucleoli re-form.
Spindle fibers attach to kinetochores during _____.
prometaphase
During prophase a homologous pair of chromosomes consists of _____.
two chromosomes and four chromatids
Which event does not take place before the start of mitosis?
The nuclear envelope disintegrates.
True or false? Mitosis takes place during M phase, which is the longest phase of the cell cycle
False
Which term describes two recently replicated DNA strands that are joined together just before cell division?
sister chromatids
During which stage of mitosis are chromosomes lined up in the center of the cell?
Metaphase
Which stage of mitosis is characterized by the disintegration of mitotic spindles and the formation of two new nuclear membranes?
Telophase
To which part of the centromere do mitotic spindle fibers attach during prometaphase?
Kinetochore
During which substage of mitosis do sister chromatids break apart and start moving to opposite poles of the cell?
Anaphase
Mitosis is responsible for what key process in multicellular eukaryotes?
growth
reproduction
wound repair
During what phase of the cell cycle does the DNA become replicated?
s
In eukaryotic cells, chromosomes are composed of _____.
DNA and protiens
Researchers pulsed rapidly dividing cultured cells for 30 minutes with radioactive thymidine. The cells were then exposed to a solution containing non-radiolabeled thymidine. Cells were analyzed at 2-hour intervals. At the 2-hour time point, no cells appeared to be dividing. Only after 4 hours did some labeled cells appear to be in M phase. This result can be explained in the following way:
There seems to be a gap or a lag in the cell cycle, between the synthesis of DNA and cell division.
Scientists isolate cells in various phases of the cell cycle. They find a group of cells that have 1½ times more DNA than do G1 phase cells. The cells of this group are _____.
In the S phase of the cell cycle.
The first gap in the cell cycle (G1) corresponds to _____.
normal growth and cell function
The microtubule-organizing center found in animal cells is an identifiable structure present during all phases of the cell cycle. Specifically, it is known as which of the following?
centrosome
In human and many other eukaryotic species' cells, the nuclear membrane has to disappear in order for what to take place?
attachment of mitotic spindle to kinetochores
Following the attachment of spindle microtubules to kinetochores, chromosomes are moved around by _____.
elongation and shortening of microtubules
How might spindle microtubules assist in the process of splitting centromeres?
creating tension by pulling toward opposite poles
Some cells have several nuclei per cell. How could such multinucleated cells be explained?
The cell underwent repeated mitosis but cytokinesis didnt occur
FtsZ is a bacterial cytoskeletal protein that forms a contractile ring involved in bacterial cytokinesis. Its function is analogous to _____.
The cleavage furrow of eukaryotic animal cells (a)
Once researchers understood that chromosomes are moved by the spindle microtubules, the next question they wanted to answer is how the microtubules function to bring about this process. They used fluorescent labels to make the chromosomes and the microtubular structures fluoresce. When anaphase began (centromeres split), they photobleached a section of microtubules. As chromosomes moved toward the poles of the daughter cells, the photobleached sections of the microtubules remained stationary. This result suggests that _____.
the microtubules elongate and shorten at their kinetochore end
In the process of chromosome separation, how do microtubules maintain contact with the kinetochores and shorten at the same time?
Motor proteins move chromosomes down the microtubular structures of the mitotic spindle.
Proteins that degrade cyclin are activated by events that MPF initiates. Why is this important for cell-cycle regulation?
It sets up an oscillation in cyclin concentration.
MPF, or mitosis-promoting factor, consists of two important cell cycle regulatory proteins called _____.
cyclin and cyclin-dependent kinase (CDK)
Nerve cells lose their ability to undergo mitosis. Instead, they are permanently stuck in _____.
G0
Once a cell completes mitosis, molecular division triggers must be turned off. What happens to MPF during mitosis?
Cyclin is degraded; the concentration of cyclin-dependent kinase remains unchanged, but without cyclin, MPF is not formed.
Which of the following is not an effect of mitosis-promoting factor (MPF) involved in moving a cell into M phase?
degradation of cyclin-dependent kinase
The M-phase checkpoint is designed to make sure all chromosomes are attached to the mitotic spindle. If this fails to happen, in which stage of mitosis would the cells be most likely to arrest?
Metaphase
Which statement best explains why some changes in DNA sequence do not change the corresponding protein?
The genetic code is redundant
What is the likely effect of a silent mutation on an organism?
Neutral
Which statement is most accurate concerning how a compound that blocks RNA synthesis will affect information transfer from DNA to proteins?
Both transcription and translation will be inhibited.
How many amino acids would be encoded in the sequence 5′ AUGUUACGGAAU 3′ by a non-overlapping and maximally overlapping code?
4 and 10 One of the possibilities that researchers interested in the genetic code considered was that the code was overlapping, meaning that a single base could be part of up to three codons.
Which of the following is an important exception to the central dogma of molecular biology?
Information can flow from RNA back to DNA
How can a molecule with these characteristics hold all the information required to build and maintain a cell?
The information in DNA is in a code form that is based on the sequence of bases.
How did experiments with Neurospora crassa mutants support the one-gene, one-enzyme hypothesis? They supported an important prediction of the hypothesis:
Losing a gene (via mutation) resulted in loss of an enzyme.
What is a start codon?
A mRNA codon that signals the beginning of translation
Which of the following describes an important experimental strategy in deciphering the genetic code?
examining the polypeptides produced when RNAs of known sequence were translated
Why does a deletion of three adjacent bases within a protein-coding sequence usually have a less severe effect than a single-base deletion mutation?
because deletions of three adjacent bases do not change the reading frame
What does the one-gene, one-enzyme hypothesis state?
A single gene codes for a single protein.
A translocation is ______.
a chromosome-level mutation
A mutation that has no effect on fitness is a ______.
neutral mutation
A point mutation that results in the substitution of one amino acid for another within a protein is a ______.
missense mutation
What is the relationship between chromosomes, DNA, and genes?
Genes are coded by DNA and found within chromosomes.
Mutations that occur in somatic cells
are likely deleterious if caused by chromosome loss
When an amino acid is specified by more than one codon, what is usually shared by the set of codons that specify this amino acid?
the first and second bases
The redundancy of the genetic code is a consequence of ______.
Having more codons than amino acids, There are 64 codons but only 20 amino acids.
According to the original central dogma, what macromolecule or phenomenon is directly responsible for an organism's phenotype?
proteins (that carry out the majority of cell functions
The process of using the information in mRNA to synthesize a protein is ______.
translation
The idea that the sequence of bases in DNA specifies the sequence of bases in an RNA molecule, which specifies the sequence of amino acids in a protein, is _______.
the central dogma
What are the steps (in the correct order) that link a change in the base sequence of a gene to a change in the phenotype of an organism like a mouse or a human?
This is the central dogma, which summarizes the flow of information in cells from DNA (genes) to proteins. For genes that encode RNAs that do not function as mRNAs, only the DNA—>RNA step occurs.
Beginning within the nucleus, the first step leading to the synthesis of a polypeptide is _____.
transferring of information from DNA to messenger RNA. Transcription is the first of the two main steps of protein synthesis.
Srb and Horowitz showed that ______.
mutations of a single gene resulted in defects of one and only one enzyme. ONE GENE ONE ENZYME HYPOTHESIS
A knock-out allele is ______.
a mutated form of a gene that does not make a functioning product. Knock-out alleles are also known as null or loss-of-function alleles.
Polypeptides are assembled from _____.
amino acids
RNA processing converts the RNA transcript into _____.
mRNA
When is a single base substitution mutation least likely to be deleterious?
when the single base change results in a codon that specifies the same amino acid as the original codon
It is a frameshift mutation.
...
How might a single base substitution in the sequence of a gene affect the amino acid sequence of a protein encoded by the gene, and why?
Only a single amino acid could change, because the reading frame is unaffected.
A translocation is _____.
a chromosome-level mutation
A point mutation that results in the substitution of one amino acid for another within a protein is a _____.
missense mutation
Once researchers identified DNA as the unit of inheritance, they asked how information was transferred from the DNA in the nucleus to the site of protein synthesis in the cytoplasm. What is the mechanism of information transfer?
b. Messenger RNA is transcribed from a single gene and transfers information from the DNA in the nucleus to the cytoplasm, where protein synthesis takes place
Which of the following molecules is not required for translation?
tRNA
rRNA
mRNA
DNA
.DNA
Genotype is to _____ as phenotype is to _____.
DNA base sequence; amino acid sequence
There are three stop codons (nonsense codons); all remaining codons specify addition of an amino acid (sense codons). How many sense codons are there?
61
Which of the following observations may have resulted in the hypothesis that a codon is made up of three bases?
A codon of 3 bases in length, from 4 different bases, would code for a maximum of 64 different amino acids.
Codons, the three base sequences that code for specific amino acids, are part of _____.mRNA
rRNA
protein
tRNA
.mRNA
In the Morse code, a series of dots and dashes code for letters of the alphabet. How is this analogous to the genetic code?
The bases of DNA code for the more complex amino acid sequence of the various proteins found in cells.
Which of the following is not synthesized from a DNA template?
Amino acids
Which of the following molecules must move across the membrane after being synthesized from a DNA sequence and is then translated by a ribosome?
mRNA
In the process of transcription, _____.
RNA is synthesized
What molecule serves as a link between the information-containing macromolecule, DNA, and protein synthesis?
mRNA
Which of the following findings violated the central dogma?
the discovery of RNA viruses that synthesize DNA using reverse transcriptase
True or false? A codon is a group of three bases that can specify more than one amino acid.
False A codon is a group of three bases that can specify only one amino acid.
Which mutation(s) would not change the remainder of the reading frame of a gene sequence that follows the mutation(s)?
One addition and one deletion mutation. This combination results in no net change in the number of bases, so the reading frame would eventually be restored.
If the sequence ATGCATGTCAATTGA were mutated such that a base were inserted after the first G and the third T were deleted, how many amino acids would be changed in the mutant protein?
2 The second and third codons in the new sequence are different from the original codons.
If a mutated DNA sequence produces a protein that differs in one central amino acid from the normal protein, which of the following kinds of mutations could have occurred?
An addition mutation and a deletion mutation. If the mutations occur within the same codon, only that codon (amino acid) will be altered.
How many amino acids could be specified by a four-base genetic code?
4 × 4 × 4 × 4 = 256 is the number of different "words" possible for a code of four bases if the "words" are four bases long.
The redundancy of the genetic code is a consequence of _____.
having more codons than amino acids
A null allele is _____.
a mutated form of a gene that does not make a functioning product
Knockout mice have been genetically altered to knock out specific genes. How are these mice most often used in research?
to determine the role of proteins coded for by those genes that are knocked out, To determine the role of proteins coded for by those genes that are knocked out (b)
Which answer best explains why errors in transcription might lead to changes in a cell's phenotype?
Changes in mRNA base sequence might result in changes in amino acid sequences of proteins.
A cell's phenotype is the result of physical traits that are the product of proteins it produces. Errors in transcription of DNA to mRNA would change the mRNA sequence and might change the amino acid sequence of protein(s).
The idea that the sequence of bases in DNA specifies the sequence of bases in an RNA molecule, which specifies the sequence of amino acids in a protein, is _____.
Central Dogma
When fruit fly embryos first begin to develop, a large cell is generated that contains over 8000 nuclei that are genetically identical with one another. What is most likely responsible for this result?
The embryo passes through multiple rounds of the cell cycle, but cytokinesis does not occur during M phases.
Which statement about the daughter cells following meiosis and cytokinesis is correct?
They are genetically different from each other and from the parent cell.
dentify the events in the cell cycle that must be completed successfully for daughter cells to share an identical complement of chromosomes.
the sister chromatids of each replicated chromosome must be partitioned in anaphase and fully separated into daughter cells by cytokinesis
the spindle apparatus must connect with the kinetochores of each sister chromatid in prometaphase
all the chromosomes must be replicated during the S phase
If a pulse-chase assay were performed with radioactive thymidine on an asynchronous culture, what percentage of mitotic cells would be radiolabeled after 9 hours?
0%
In multicellular organisms, nondividing cells stay in G0 phase.For the cell, why is it better to be held in G1 rather than S, G2, or M phase?
G1 cells have not replicated their DNA in preparation for division.
Under normal conditions, what happens to the cell cycle if the chromosomes fail to separate properly at anaphase?
The cycle would arrest in M phase, and cytokinesis would not occur.
After the S phase, what comprises a single chromosome?
two sister chromatids
What evidence suggests that during anaphase, kinetochore microtubules shorten at the kinetochore?
Daughter chromosomes were observed to move toward the pole faster than do the marked regions of fluorescently labeled kinetochore microtubules.
What major events occur during telophase of mitosis?
The chromosomes end up at opposite ends of the cell, and two nuclear envelopes form around them.
Which type of molecule catalyzes the phosphorylation of other proteins to start M phase?
cyclin-dependent kinases
What would happen if the kinase that adds the inhibitory phosphate to Cdk were defective?
M phase would begin prematurely.Inhibitory phosphorylation of Cdk normally occurs in interphase, when cyclin levels are increasing, and inhibits the Cdk subunit of MPF from prematurely catalyzing the phosphorylation reactions that trigger M phase
What role(s) do gap phases play in the cell cycle?
They allow cells to replicate organelles and manufacture additional cytoplasm. G1 and G2 phases allow the cell to complete these steps so that daughter cells produced in M phase will be normal in size and function.
How would the removal of growth factors affect asynchronous cultures of (1) normal human cells and (2) most human cancer cells?
Arrest in G1; (2) no effect Growth factors are responsible for allowing normal, but not most cancerous, cells to pass through the G1 checkpoint.
The defining characteristic of a malignant tumor is that _____.
its cells can spread to other parts of the body and produce secondary tumors
What two types of defects does a cancerous cell possess?
Cancerous cells possess defects that make proteins required for cell growth active and tumor suppressor genes inactive.
Two important checkpoints that regulate the cell's progression through the cell cycle occur in the _____ and _____ phases of the cell cycle.
G1, G2 Both of these phases contain a major checkpoint.
MPF, or mitosis-promoting factor, consists of two important cell-cycle regulatory proteins called _____.
cyclin and cyclin-dependent kinase (CDK
Proteins that degrade cyclin are activated by events that MPF initiates. Why is this important for cell-cycle regulation?
It sets up an oscillation in cyclin concentration. The oscillation in cyclin concentration results in a corresponding oscillation in MPF (cyclin-Cdk complex) activity, which drives the orderly progression between M phase and interphase
Which answer correctly identifies a cell-cycle checkpoint with a criterion for passing it?
G2 checkpoint: Chromosomes have replicated successfully.G2 occurs after S and before M phase, so it is advantageous for the cell to have a checkpoint assessing chromosome replication here.
The division of the cytoplasm following mitosis is called _____.
cytokinesis
In which mitotic phase do the chromosomes condense and does the mitotic spindle begin to form?
prophase These are both changes that must occur before the chromosomes can be separated properly.
In which mitotic phase are the sister chromatids separated and pulled to opposite poles?
anaphase
Which answer correctly associates a phase of mitosis with what happens to chromosomes during that phase?
Anaphase: Sister chromatids separate.During anaphase ("against-phase"), replicated chromosomes split into two identical sets of unreplicated chromosomes that are pulled to opposite poles of the cell.
Which statement is correct concerning the relationship between chromosomes and either genes, chromatin, or sister chromatids?
Each replicated chromosome consists of two sister chromatids.Replication of a chromosome's single DNA double helix produces two identical DNA copies called sister chromatids.
What is the difference between a chromatid and a chromosome?
A chromatid is one half of a replicated chromosome, whereas a chromosome consists of DNA wrapped around proteins in a highly organized manner.