AGRY 320 Pretest – ANSWER KEY
Yes/no answers will not be given full credit. Every answer must be accompanied by an explanation.
1) Cell Cycle
a) Define the following terms: chromosome, maternal, paternal, gamete, somatic cell, haploid, diploid, homologous chromosomes, sister chromatids, centromere, kinetochore.
chromosome: the physical structure, composed largely of DNA, that contains the genes (genetic information) of an organsism
maternal: the genetic material contributed by the mother (in the case of plants, the female sex organ, or stigma)
paternal: the genetic material contributed by the father (in the case of plants, the male sex organ, or anther)
gamete: a haploid sex cell
somatic cell: nonsex cell; all the cells in the body except the sperm in males and the egg in females
haploid: the number of chromosomes in the gamete, symbolized as n.
diploid: the number of chromosomes in somatic cells, symbolized as 2n.
homologous chromosomes: chromosomes that are identical in size, shape, and, accept for allelic differences, genetic composition. For example, chromosome 1 in a male and female from a given organism are considered homologous chromosomes.
sister chromatids: one of the two chromatids of a duplicated chromosome. Sister chromatids are identical in term of size, shape and genetic composition.
centromere: constricted region on the chromosome where spindle fibers are attached during cell division. Also called a primary constriction.
kinetochore: bodies composed of protein found on either side of the centromere that are important in separation of the replicated chromosome during mitosis and meiosis
b) Draw the eukaryotic cell cycle and define what occurs in each stage.
Interphase: consists of G1, S and G2
G1: period of cell growth before the DNA is duplicated
S: period when the DNA is duplicated
G2: period after DNA is duplicated; cell prepares for division
Mitosis: consists of prophase, metaphase, anaphase, telophase and at the end, cytokinesis. In mitosis, chromosomes in somatic cells are replicated and partitioned.
2) Mitosis
a) What is the purpose of mitosis?
The purpose of mitosis is to replicate and partition chromosomes in somatic cells.
b) Draw the stages of mitosis, indicating clearly (and briefly) what occurs at each stage. Use at least two different colored pens (or pencils) to clearly differentiate the maternal and paternal chromosomes. Your diagrams must be hand drawn.
early prophase: The chromatin has started to condense into visible chromosomes. The two larger chromosomes are homologous to each other, as are the two smaller chromosomes. The red chromosomes are of maternal origin, and the green chromosomes are of paternal origin.
late prophase: The chromosomes continue to condense. Spindle fibers composed of microtubules assemble at the centrioles. The two centriole pairs migrate to opposite poles. The nuclear envelope disintegrates.
metaphase: each chromosome is at the spindle equator, with one chromatid facing one pole and the other chromatid facing the other pole.
anaphase: the sister chromatids separate and are now called chromosomes migrate toward the two poles.
telophase: the chromosomes arrive at the poles and begin to decondense. Nuclear envelopes form around each set of chromosomes. The cytoplasm begins to divide.
c) Is there a difference between the number of chromosomes in a gamete and the number of chromosomes in a somatic cell?
Yes. The number of chromosomes in a somatic cell is twice what is found in a gamete.
d) Is there a difference between the genetic information in sister chromatids?
No. Sister chromatids are a results of a chromosome being duplicated exactly.
b) Human somatic cells have 23 pairs of chromosomes. How many sister chromatids are there at the beginning of prophase? metaphase? anaphase? telophase?
at prophase: 92
at metaphase: 92
at anaphase: 92
at telophase: 0
3) Meiosis
a) What is the purpose of meiosis?
To produce gametes (sex cells) having half the number of chromosomes of the parental cell.
b) Draw and clearly indicate the stages of meiosis (both I and II) and briefly describe what occurs at each one. Use at least two different colored pens (or pencils) to clearly differentiate the maternal and paternal chromosomes.
Meiosis I
prophase I: duplicated chromosomes condense and pair with their homologues. Nonsister chromatids in the homologous pairs exchange segments, a process called crossing over.
metaphase I: chromosome pairs align at the spindle equator so that one homologue faces one pole and the other homologue faces the opposite pole.
anaphase I: homologous chromosomes separate from one another and migrate to opposite poles.
telophase I: the chromosomes arrive at opposite poles, and the cytoplasm divides to form two cells.
Meiosis II
prophase II: spindle fibers attach to the duplicated chromosomes and begin moving them to the spindle equator.
metaphase II: chromosomes align at the cell equator with one chromatid facing one pole and the other chromatid facing the opposite pole.
anaphase II: the chromatids separate and are now called chromosomes. The chromosomes migrate toward the two poles.
telophase II: the chromosomes arrive at the opposite poles, and the cytoplasm divides. The two meiotic divisions result in four haploid cells.
c) At what point in meiosis does crossing over occur?
Crossing over occurs at the metaphase stage of meiosis I.
d) Concerning crossing over, answer the following questions: What is it? What is its purpose? What allows it to occur? What would be the consequence of no crossing over?
Crossing over is the physical exchange of genetic information between homologous chromosomes that takes place in meiosis I. It is also referred to as recombination.
The purpose of crossing over is to introduce genetic variability into the population.
Crossing-over can occur due to the arrangement of homologous chromosomes along the spindle equator at metaphase in meiosis I. The homologous chromosomes are lined up on top of each other, which allows for physical contact between DNA that is similar but not identical in content. This contact then permits exchange of DNA between the two homologues.
If crossing-over did not occur, there would be no exchange of genetic information between maternal and paternal chromosomes. This would result in progeny that are genetically identical to one of the parents. If crossing-over had never occurred, then all individuals would be genotypically and phenotypically identical.
4) Relationship between meiosis and mitosis
a) State the similarities and differences between mitosis and meiosis. In your answer address the following: relative number of chromosomes per cell at the various stages, the end result of the two processes, any key differences in positioning of chromosomes. Is mitosis more similar to meiosis I or meiosis II? Why?
- relative numbers of chromosomes at prophase, metaphase, anaphase and telophase of mitosis and metaphase I, prophase I, anaphase I and telophase I of meiosis are the same
- relative number of chromosome at these stages of mitosis and the corresponding stages of meiosis II is double (since in meiosis II there are now two cells carrying the same number of chromosomes)
- end result of mitosis: two diploid daughter cells, identical to each other and with an exact copy of the genetic information from the mother cell
- end result of meiosis: four haploid cells
- key differences in positioning of chromosomes: this occurs in metaphase of mitosis and metaphase I of meiosis. In the former, chromosomes align next to each other and along the equator of the cell. In metaphase I of meiosis, homologous chromosomes align on top of each other.
- mitosis is more similar to meiosis II for two reasons: chromosome alignment along the cell equator is the same and there is also separation of sister chromatids, which does not occur at meiosis I.
5) Multiple choice questions
1. The number of cells and the genetic make up after TelophaseI in the meiotic process are, respectively
a) two cells each one being diploid
b)
two cells each one being
haploid
c) four cells each one being diploid
d) four cells each one being haploid
2. Chromosomes in MetaphaseII (before starting to move towards the position of the centrosomes) have:
a) One chromatid that is paired with the chromatid of the homologous chomosome.
b) Two sister chromatids that are paired with the homologous chromosome.
c)
Two sister chromatids that remain unpaired because the homologous is
not present.
d) Just one chromatid that remains unpaired because the homologous in not present.
3. Which of the following statements is true?
a)
chromosomes duplicate after TelophaseI
and before ProphaseII
b) second division in a meiotic process is a reduction division
c)
Chiasmata become visible in ProphaseI
d) Chiasmata is the physical place where interchange of genetic information between sister chromatids takes place
4. Which of the following is true?
a)
Sister chromatids contain identical genetic information
b) Homologous chromosomes contain identical genetic information
5. The cell cycle can be divided in four phases: G1, S, G2 and M where:
a) G stands for gap meaning that in this phase no cell growth occurs.
b) S stands for synthesis meaning that cell growth and organelle duplication take. place only in this phase.
c) M stands for multiplication meaning that DNA replication takes place in this phase.
d)
None of the above is
correct.
6. Each of the following biological processes takes place mainly in only one of the phases of the cell cycle (G1, S, G2 and M):
(1)-Cell growth and preparation for Mitosis
(2)-Cell division
(3)-DNA replication and chromosome duplication
(4)-Cell growth and organelle duplication
If you have to mention the main biological process associated to G1, S, G2 and M respectively, the right order would be:
a) 4, 1, 3, 2
b) 1, 3, 4, 2
c) 1, 3, 2, 4
d)
4, 3, 1, 2
7. In the cell cycle, the difference between mitosis and meiosis during the interphase period is:
a) there is chromosome duplication in mitosis but not in meiosis
b) the centrosome is not duplicated in mitosis but it is duplicated in meiosis
c) the cell becomes tetraploid before entering meiosis, which is not true for mitosis
d)
none of the above
8. Meiosis can be thought as two consecutive cell divisions. Which of the following is true in Meiosis:
a) MeiosisII is a reduction division, therefore it resembles Mitosis
b) MeiosisI is a reduction division, therefore it resembles Mitosis
c) MeiosisI is a non-reduction division, therefore it resembles Mitosis
d)
MeiosisII is a non-reduction division, therefore it resembles Mitosis