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Why must DNA be packed into chromosomes during mitotic phase?

Why must DNA be packed into chromosomes during mitotic phase?


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Why does DNA have to be packed into chromosomes? Why can't DNA just divide itself evenly?


A chromosome is simply a length or segment of DNA. Bacteria have few structural proteins on their DNA, and they have one circular chromosome. In humans, before DNA replication, the nucleus contains 46 strands of DNA, i.e. chromosomes (22 chromosomes in two copies and usually two X or one X and one Y for males and females, respectively). All chromosomes are DNA that is bound by histone proteins that are organised in nucleosomes, approximately every 147 base pairs.

What you are referring to is a condensed chromosome in metaphase, which confusingly in humans is actually an X shape consisting of 2 identical chromatids, structurally condensed and joined at the centromere to form a duplicated chromosome.

The duplicated chromosomes must condense and join before the cell can divide because a tangle of 96 chromosomes as chromatin would be almost impossible to organise and separate properly. The joining of identical chromosomes and compacting process allows all the 46 compacted and duplicated chromosomes (i.e. 96 chromatids) to simply align along the spindle apparatus that forms in centre of the cell during mitosis, and have separation of the chromatids during anaphase so that each of the new cells has a full set of 46 chromosomes.


If you mean to say why DNA is in the form of chromosomes, then obviously answer is simple: For Compression and Packaging - Around 2metre long DNA must be compressed (at an unbelievably high ratio) to let it fit in the tiny nucleus (order of micrometers)


Why is DNA tightly packed in a chromosome?

Double-stranded DNA loops around 8 histones twice, forming the nucleosome, which is the building block of chromatin packaging. DNA can be further packaged by forming coils of nucleosomes, called chromatin fibers. These fibers are condensed into chromosomes during mitosis, or the process of cell division.

Also, why is DNA tightly enclosed in the nucleus of our cells? The Nucleus. The nucleus is a membrane-enclosed organelle found in most eukaryotic cells. DNA in the nucleus is organized in long linear strands that are attached to different proteins. These proteins help the DNA coil up for better storage in the nucleus.

Just so, what is the advantage of the highly condensed state of the DNA of mitotic chromosomes?

This state allows gene expression needed during mitosis. The highly condensed state favors delivery of an intact package of DNA to each daughter cell. The condensed state favors DNA repair prior to mitosis.

What is DNA packaged into?

Double-stranded DNA loops around 8 histones twice, forming the nucleosome, which is the building block of chromatin packaging. DNA can be further packaged by forming coils of nucleosomes, called chromatin fibers. These fibers are condensed into chromosomes during mitosis, or the process of cell division.


Why must DNA be packed into chromosomes during mitotic phase? - Biology

Before we discuss mitosis, let&rsquos review the structure of DNA. Chromosomes are packaged by histone proteins into a condensed structure called chromatin. The first level of packaging is represented as the &ldquobeads-on-a-string&rdquo structure. The condensed chromatin is folded and tightly coiled, like a coiled telephone cord, allowing the cell&rsquos DNA to be packed into the nucleus.

Before a cell can divide, it must first replicate its DNA so that each of the two daughter cells will receive a complete copy of the DNA. The two identical chromosomes that result from DNA replication are referred to as sister chromatids. Sister chromatids are held together by proteins at a region of the chromosome called the centromere.

Chromosomes undergo additional compaction at the beginning of mitosis. When fully condensed, replicated chromosomes appear as thick X-shaped structures that are readily observed under the microscope (see figure below). Chromosomes can have 1 or 2 chromatids, depending on whether they have replicated.

CHROMOSOMAL STRUCTURE

A chromatid is a condensed DNA subunit of a chromosome. The two chromatids of a duplicated chromosome are held together at a region of DNA called the centromere (see figure below). Centromeres are the attachment points for microtubules, which are responsible for the guiding the movement of chromosomes during mitosis and meiosis.

Most eukaryotic cells contain two sets of chromosomes, with one set originating from the father and the other from the mother. For example, every human cell has 23 pairs of chromosomes: one chromosome from each pair is inherited from the father (via the sperm), and the other is inherited from the mother (via the egg).

The figure below shows a cell that contains four chromosomes (found as two pairs) the pink chromosomes were inherited from the mother and the blue chromosomes were inherited from the father. Each chromosome contributed by the father has a corresponding chromosome that was contributed by the mother. These corresponding chromosomes, which are alike in structure and size, constitute a homologous pair (also referred to as bivalents). The DNA sequences of homologous chromosomes are usually not exactly identical.

The nuclei of most human cells contain 46 chromosomes. These 46 chromosomes consist of 23 pairs of homologous chromosomes, or homologs, meaning each of these pairs are alike, but not necessarily identical. The 23rd pair of chromosomes in humans determines sex these two chromosomes may be very different from each other, depending on gender (XX produces females, XY produces males). The convention is to describe the chromosome number in humans as 2n = 46 because the cells are diploid, meaning they have two complete sets of chromosomes.


Binary Fission

Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.

The process of binary fission is the most commonly observed mechanism for cell division in bacteria and archaea (at least the culturable ones studied in the laboratory).
One structural feature of relevance to DNA replication and segregation in the bacteria and archaea is that their genetic material is not enclosed in a membrane-bound nucleus, but instead occupies a location, the nucleoid, within the cell. Moreover, the DNA of the nucleoid is associated with numerous proteins that aid in compacting the DNA into a smaller, organized structure. Another organizational feature to note is that the bacterial chromosome is typically attached to the plasma membrane at about the midpoint of the cell (again, there are always exceptions). The starting point of replication, the origin, is close to this attachment site. The replication of the DNA is bidirectional, with replication forks moving away from the origin on both strands of the loop simultaneously. As the new double strands are formed, each origin point moves away from the cell wall attachment toward the opposite ends of the cell. It's is not clear how this occurs- are the origins pushed or pulled apart? What are the motors and filaments that achieve this? Are motors even required?

It is important to note here that the active separation of the recently replicated origins solves the key problem of cell division- how to get one copy of the genome into each daughter cell. The bacterial genome has only one (circular) chromosome, which has only one origin of replication. Separating the newly duplicated origins therefore separates the genomes perfectly- each daughter cell gets a complete set of genes. In contrast, the eukaryotic genome has thousands of origins of replication, located on several chromosomes, each of which carries different parts of the genome. Can you see why this same strategy would not work in eukaryotes?

The formation of a ring composed of repeating units of a protein called FtsZ (a cytoskeletal protein) directs the formation of a partition between the two new nucleoids. Formation of the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell wall materials to the site. Gradually, a septum is formed between the nucleoids, extending from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.

These images show the steps of binary fission in prokaryotes. (credit: modification of work by &ldquoMcstrother&rdquo/Wikimedia Commons)

How does attaching the replicating chromosome to the cell membrane aid in dividing the two chromosomes after replication is complete?

Control of these processes

Not surprisingly, the process of binary fission is strictly controlled in most bacteria and archaea. Somewhat surprisingly, however, while some key molecular players are known, much remains to be discovered and understood about how decisions are made to coordinate the activities.


Why does DNA coil up?

Wrapping DNA around histone proteins is a way to compact and organize the DNA in the nucleus so that it doesn't get hopelessly tangled. Thus DNA is wrapped around histone proteins for at least two reasons: Compaction and storage, and regulation of gene expression.

Subsequently, question is, what is coiled up DNA? In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

Also Know, why must DNA coil up before it divides?

Why DNA must coil up into chromosome structures before it divides? It's more difficult to divide long thin strands of DNA than compacted, smaller chromosome. The chromosome of a prokaryotic cell is a circular DNA double helix, whereas the chromosomes of eukaryotic cells are linear DNA double helices.

Why is DNA packaging so important?

DNA packaging is an important process in living cells. Without it, a cell is not able to accommodate large amount of DNA that is stored inside. Therefore, DNA packaging is crucial because it makes sure that those excessive DNA are able to fit nicely in a cell that is many times smaller.


The eleven stages of the cell cycle, with emphasis on the changes in chromosomes and nucleoli during interphase and mitosis

Since we had subdivided the cell cycle into 11 stages--four for mitosis and seven for the interphase--and since we had experience in detecting DNA in the electron microscope (EN) by the osmium-amine procedure of Cogliati and Gauthier (Compt. Rend. Acad. Sci., 1973276:3041-3044), we combined the two approaches for the analysis of DNA-containing structures at all stages of the cell cycle. Thin Epon sections of formaldehyde-fixed mouse duodenum were stained by osmium-amine for electron microscopic examination of the stages in the 12.3-hr long cell cycle of mouse duodenal crypt columnar cells. In addition, semi-thin Lowicryl sections of mouse duodenal crypts and cultured rat kidney cells were stained with the DNA-specific Hoechst 33258 dye and examined in the fluorescence microscope. The DNA detected by osmium-amine is in the form of nucleofilaments, seen at high magnification as long rows of 11 nm-wide rings (consisting of stained DNA encircling unstained histones). At all stages of the cycle as well as in nondividing cells, nucleofilaments are of three types: 'free,' 'attached' to chromatin accumulations, and 'compacted' in all chromatin accumulations, the form of dense spirals within. At stage I of the cycle, besides free and attached nucleofilaments, compacted ones are observed in the three heterochromatin forms (peripheral, nucleolus-associated, clumped). Soon after the S phase begins, chromatin 'aggregates' appear, which are small at stage II, mid-sized at stage III, and large at stage IV. Chromatin 'bulges' also appear at stage III and enlarge at stage IV, while heterochromatins disappear. At stage V, aggregates and bulges accrete into 'chromomeres,' a process responsible for the apparent chromosome condensation observed at prophase. The chromomeres gradually line up in rows and, at stage VIa (prometaphase), approach one another within each row and coalesce to build up the metaphase chromosomes which are fully formed at stage VIb (metaphase). Daughter chromosomes arising at stage VII (anaphase) are eventually packed into a chromosomal mass at each pole of the cell. During stage VIII (telophase), the chromosomal mass is split into large chunks. In the course of the G1 phase, the chunks thin out to give rise to irregular 'bands' at stage IX, the bands are then cleaved into central and peripheral fragments at stage X, and finally the central fragments are replaced by free nucleofilaments and clumps at stage XI, while the peripheral fragments are replaced by peripheral heterochromatin. The "nucleoli" at stages I-III are associated with stained heterochromatin but otherwise appear as unstained lucent areas, except for weakly stained patches composed of histone-free DNA filaments. During stage IV, nucleoli lose patches and associated heterochromatin, while weakly lucent, pale vesicles appear within nucleoli and in the nucleoplasm. By the end of substage VIa, nucleoli generally disappear, while pale vesicles persist around the chromosomes appearing at substage VIb. At stages VIII and IX, the vesicles seem to become strongly lucent and, at stages IX and X, they associate and fuse to yield homogeneous lucent areas, the 'prenucleolar bodies,' which include histone-free DNA patches. During stage XI, groups of these bodies associate to give rise to nucleoli. In conclusion, the cell cycle DNA changes can be classified into 4 broad periods (Fig. 6): 1) Stage I is a 2-hr long interphase "pause," during which the stained DNA shows no signs of either chromosome condensation or decondensation, while the overall nuclear pattern is similar to that in nondividing cell nuclei. Nucleoli are fully developed. 2) From stage II to VIa, the "chromosome condensation" period extends over about 7 hr, during which the events are interpreted as follows. Throughout the S phase (stages II-IV), newly-synthesized segments of nucleofilaments approach one another, adhere and thus build aggregates and later bulges on nuclear matrix sites. (ABSTRACT TRUNCATED)


10.2 The Cell Cycle

In this section, you will explore the following questions:

  • What processes occur during the three stages of interphase?
  • How do the chromosomes behave during the mitotic phase?

Connection for AP ® Courses

The cell cycle describes an orderly sequence of events that are highly regulated. In eukaryotes, the cell cycle consists of a long preparatory period (interphase) followed by mitosis and cytokinesis. Interphase is divided into three phases: Gap 1 (G1), DNA synthesis (S), and Gap 2 (G2). Interphase represents the portion of the cell cycle between nuclear divisions. During this phase, preparations are made for division that include growth, duplication of most cellular contents, and replication of DNA. The cell’s DNA is replicated during the S stage. (We will study the details of DNA replication in the chapter on DNA structure and function.) Following the G2 stage of interphase, the cell begins mitosis, the process of active division by which duplicated chromosomes (chromatids) attach to spindle fibers, align themselves along the equator of the cell, and then separate from each other.

Following mitosis, the cell undergoes cytokinesis, the splitting of the parent cell into two daughter cells, complete with a full complement of genetic material. In animal cells, daughter cells are separated by an actin ring, whereas plant cells are separated by the cell plate, which will grow into a new cell wall. Sometimes cells enter a Gap zero (G0) phase, during which they do not actively prepare to divide the G0 phase can be temporary until triggered by an external signal to enter G1, or permanent, such as mature cardiac muscle cells and nerve cells.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework, as shown in the tables. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.A Heritable information provides for continuity of life.
Essential Knowledge 3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 3.7 The student can make predictions about natural phenomena occurring during the cell cycle.
Essential Knowledge 3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 3.8 The student can describe the events that occur in the cell cycle.
Essential Knowledge 3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Learning Objective 3.11 The student is able to evaluate evidence provided by data sets to support the claim that heritable information is passed from one generation to another generation through mitosis.

Teacher Support

Discuss with students the difference between diploid and haploid cells. Show students a graphic of the difference.

Discuss with students how in mitosis, the ploidy of the cell remains constant. In a cell culture of human somatic cells, all of the cells will be diploid. In contrast the DNA content, the amount of DNA in a cell culture will change as the cells replicate (undergo S-phase and replicate their DNA). In relative amounts, the initial amount of DNA is considered to be 1x, after S-phase it will be 2x, and so on. More information on the methods used by scientists to track ploidy can be found here.

Introduce mitosis using visuals such as this video.

Students may think that interphase is a resting phase, where no events occur. Remind students that cells are metabolically active in this phase. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.

In addition, students may not realize that the events of mitosis are continuous, and the organization into discrete stages is for convenience. Show students a time lapse video to illustrate this, such as found here.

The stages of the cell cycle can be taught using the images available here.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.35][APLO 2.15][APLO 2.19][APLO 3.11][APLO 2.33][APLO 2.36][APLO 2.37][APLO 2.31]

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 10.5). During interphase , the cell grows and DNA is replicated. During the mitotic phase , the replicated DNA and cytoplasmic contents are separated, and the cell divides.

The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. The cytoplasm is usually divided as well, resulting in two daughter cells.

Interphase

During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G1, S, and G2.

G1 Phase (First Gap)

The first stage of interphase is called the G1 phase (first gap) because, from a microscopic aspect, little change is visible. However, during the G1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus.

S Phase (Synthesis of DNA)

Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase , DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle , the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles , which are at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi.

G2 Phase (Second Gap)

In the G2 phase , the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.

The Mitotic Phase

The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis , or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells.

Link to Learning

Revisit the stages of mitosis at this site.

  1. Colchicine increases inflammation by inhibiting mitosis. Inhibition of mitosis results in decreased white blood count.
  2. Colchicine decreases inflammation by inhibiting mitosis. Inhibition of mitosis results in decreased white blood count.
  3. Colchicine increases inflammation by inhibiting mitosis. Inhibition of mitosis results in increased white blood count.
  4. Colchicine decreases inflammation by inhibiting mitosis. Inhibition of mitosis results in increased white blood count.

Karyokinesis (Mitosis)

Karyokinesis, also known as mitosis , is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus (Figure 10.7).

Everyday Connection for AP® Courses

These budding plants demonstrate asexual reproduction, one of the main purposes of mitosis. The other two purposes are growth and repair.

Which of the following statements best describes the relationship between mitosis and asexual reproduction?

  1. Mitosis is a process that can result in asexual reproduction.
  2. Mitosis is a process that always results in asexual reproduction.
  3. Asexual reproduction is a process that always results in mitosis.
  4. Asexual reproduction is a process that can result in mitosis.

Visual Connection

  1. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus reforms and the cell divide. Cohesin proteins break down and the sister chromatids separate.
  2. The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the cell divides.
  3. The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides.
  4. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divide.

During prophase , the “first phase,” the nuclear envelope starts to dissociate into small vesicles, and the membranous organelles (such as the Golgi complex or Golgi apparatus, and endoplasmic reticulum), fragment and disperse toward the periphery of the cell. The nucleolus disappears (disperses). The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope.

During prometaphase , the “first change phase,” many processes that were begun in prophase continue to advance. The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region (Figure 10.8). The proteins of the kinetochore attract and bind mitotic spindle microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles. Spindle microtubules that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis.

During metaphase , the “change phase,” all the chromosomes are aligned in a plane called the metaphase plate , or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other by cohesin proteins. At this time, the chromosomes are maximally condensed.

During anaphase , the “upward phase,” the cohesin proteins degrade, and the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap.

During telophase , the “distance phase,” the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing into a chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.

Cytokinesis

Cytokinesis , or “cell motion,” is the second main stage of the mitotic phase, during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells. Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.

In cells such as animal cells that lack cell walls, cytokinesis starts during late anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. This fissure, or “crack,” is called the cleavage furrow . The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved in two (Figure 10.9).

In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast (a vesicular structure) at the metaphase plate. There, the vesicles fuse and coalesce from the center toward the cell walls this structure is called a cell plate . As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall (Figure 10.9).

Science Practice Connection for AP® Courses

Activity

  • Use a set of pipe cleaners (or other materials as directed by your teacher) that you can use to model chromosomes during mitosis and meiosis:
    1. Each of the pipe cleaners represents a single, unreplicated chromosome. Each chromosome should differ in size, as they do in most organisms. Assume that your dividing cell contains 3 chromosomes: numbered chromosome 1, 2, and 3.
    2. Using both members of each homologous pair for chromosomes 1–3, model how the chromosomes would appear in a cell that had just finished the S phase of the cell cycle. Once your teacher has approved your model, have one member of your group document the model by photographing or drawing it.
    3. Now, repeat step 2 but show the cell at metaphase during mitosis.
    4. Finally, model the two daughter cells that will result from mitosis. Again, have one member of your group document the model.
    5. Repeat steps 2–5 for both meiosis I and meiosis II. Remember that you should have four daughter cells at the end of meiosis II. Also remember to ask your teacher for approval and document your model before moving on to the next phase of meiosis.
    6. Exchange/ copy all of the drawings or photographs that your group took of your models. As a group or individually (as directed by your teacher) create a report to turn in that labels and explain each picture of your model.
  • An organism’s ploidy count is the total number of chromosome sets contained in each body cell. Most organisms have a ploidy level of 2, meaning that they have two sets of chromosomes due to presence of homologous pairs. However, some plants are multiploid, meaning they can have ploidy levels greater than 2. The table shows possible multiploid levels of some common crop plants.
Common name Multiploid chromosome count Normal chromosome count
Bananas3311
Potatoes4812
Wheat427
Sugar cane8010

Analyze the data with a partner of in a group as directed by your teacher. On a separate sheet of paper, answer the following questions.

  1. How does the multiploid count of the crop plants relate to their normal chromosome count?
  2. Explain the basis for the relationship you described in part a, in terms of what occurs to chromosomes during replication and meiosis.
  3. Give one additional example of a possible multiploid chromosome count for each species in the table above.

A. A comparison of the relative time intervals of mitotic stages can be made by completing the task described. In evaluating each time interval, the problem suggests that you assume that the length of time to complete one cell cycle is 24 hours. How can that assumption be tested?

Suppose that you have a growth chamber in which roots of a newly germinated plant can be examined visually with a lens that provides a magnification from which lengths can be determined with a precision of ± 0.05 mm. The field of view can be rotated so that measurements can be made of both the length and diameter of the growing tip. A large number of growing roots can be studied. Tips can be sampled, sectioned, and examined microscopically with a 25× magnification so that estimates of the diameter and length of cells can be made.

Cells in the growing tip of the root rapidly undergo mitosis, just as the whitefish blastula described in Figure 10.10. With increasing distance from the growing tip, the rate at which mitosis occurs slows until tissue is reached in which the initiation of the cell cycle is delayed.

A. Describe a sequence of measurements that could be used to test the assumption that the cell cycle, once started, has a total time interval of 24 hours. Hint: Rather than counting cells, it might be useful to measure the length of the root tip and the average length of a cell.

B. Using the data obtained from your measurements described in part A, how can the rate of cell division be calculated?

An experiment that is perhaps similar to one you have proposed was conducted previously (Beemster and Baxter, 1998), and the results are shown in the table.

Distance (mm) Per hour
0 0.035 ± 0.01
0.1 0.047 ± 0.005
0.2 0.044 ± 0.01
0.3 0.039 ± 0.01
0.4 0.042 ± 0.01
0.5 0.031 ± 0.005

C. Using these data, estimate the length of time of the cell cycle, including an estimate of precision by calculating the standard deviation.

Growth factors are signals that initiate cell division in eukaryotes. (The data in the table above show that cells in the plant root less than a mm from the root tip are showing a reduction of growth rate.) The interaction of two plant hormones, auxin and brassinosteroids, have been shown [Chaiwanon and Wang, Cell, 164(6), 1257, 2016] to regulate cell division in root tips. Auxin concentrations are higher near the root tip and decrease with distance from the tip. Brassinosteroids decrease in concentration near the root tip. Auxin is actively transported between cells, whereas brassinosteroids have limited transport between cells.

D. Based on these data and the observed distribution of brassinosteroids and auxin in the growing root, predict a mechanism for their interaction and justify the claim that brassinosteroid synthesis is negatively regulated by auxin transported to the cell, and that auxin is positively regulated and amplified.

Think About It

Chemotherapy drugs such as vincristine and colchicines disrupt mitosis by binding to tubulin (the subunit of microtubules) and interfering with microtubule assembly and disassembly. What mitotic structure is targeted by these drugs, and what effect would this have on cell division?

Teacher Support

The first activity is an application of Learning Objective 3.8 and Science Practice 1.2 because students are modelling steps of the cell cycle, including mitosis and meiosis. A variety of materials can be used to represent chromosomes in the model as long as the students can easily distinguish between the three chromosomes (such as by having different-sized pipe cleaners) as well as distinguish between homologs (such as by using two colors of pipe cleaner). Be sure to provide enough chromosomes to represent sister chromatids in both the mitosis and meiosis models. The critical point to stress is that, in modelling mitosis, students should place homologous chromosomes (each with a sister chromatid) above and below each other during metaphase, ensuring a sister chromosome from each homolog enters each daughter cell. Conversely, in metaphase I of meiosis, the homologous chromosomes (each with a sister chromatid) will pair together side-by-side so that each cell only receives one of the two homologs.

The second activity is an application of Learning Objective 3.11 and Science Practice 5.3 because students are using their knowledge of meiosis to explain and predict possible ploidy levels in crop plants. Students should work in pairs or as a group.

An expanded lab investigation for mitosis and meiosis, involving studying onion cells undergoing mitosis (part 2), and karyotype analysis (part 3) is available from the College Board’s ® AP Biology Investigative Labs: An Inquiry-Based Approach in Investigation 7.

Possible Answer

  1. The multiploid count is always a whole-number multiple of the normal chromosome count.
  2. Before meiosis (and mitosis) all of an organism’s chromosomes are replicated before any segregation takes place. Therefore, ploidy levels will always involve whole-number multiples of the original chromosome levels.
  3. Answers will vary but all answers should be whole-number multiples of the normal chromosome numbers.

The Think About It question is an application of Learning Objective 3.7 and Science Practice 6.4 because the student must be able to describe the events that occur in the cell cycle before you can make a prediction about the effects of a disruption in mitosis.

Possible Answer

The mitotic spindle is formed of microtubules. Microtubules are polymers of the protein tubulin therefore, it is the mitotic spindle that is disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during mitosis. The cell will arrest in mitosis and die.

G0 Phase

Not all cells adhere to the classic cell cycle pattern in which a newly formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.

Scientific Method Connection

Determine the Time Spent in Cell Cycle Stages

Problem: How long does a cell spend in interphase compared to each stage of mitosis?

Background: A prepared microscope slide of blastula cross-sections will show cells arrested in various stages of the cell cycle. It is not visually possible to separate the stages of interphase from each other, but the mitotic stages are readily identifiable. If 100 cells are examined, the number of cells in each identifiable cell cycle stage will give an estimate of the time it takes for the cell to complete that stage.

Problem Statement: Given the events included in all of interphase and those that take place in each stage of mitosis, estimate the length of each stage based on a 24-hour cell cycle. Before proceeding, state your hypothesis.

Test your hypothesis: Test your hypothesis by doing the following:

  1. Place a fixed and stained microscope slide of whitefish blastula cross-sections under the scanning objective of a light microscope.
  2. Locate and focus on one of the sections using the scanning objective of your microscope. Notice that the section is a circle composed of dozens of closely packed individual cells.
  3. Switch to the low-power objective and refocus. With this objective, individual cells are visible.

Switch to the high-power objective and slowly move the slide left to right, and up and down to view all the cells in the section (Figure 10.10). As you scan, you will notice that most of the cells are not undergoing mitosis but are in the interphase period of the cell cycle.

Record your observations: Make a table similar to Table 10.2 in which you record your observations.

Phase or StageIndividual TotalsGroup TotalsPercent
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Totals100100100 percent

Analyze your data/report your results: To find the length of time whitefish blastula cells spend in each stage, multiply the percent (recorded as a decimal) by 24 hours. Make a table similar to Table 10.3 to illustrate your data.


Metaphase Chromosome

Sharron Vass , Margarete M.S. Heck , in Encyclopedia of Biological Chemistry , 2004

Reversing the Process: Postsegregation Decondensation

After chromosome segregation and nuclear envelope reformation, the extreme condensation reached during metaphase must be reversed to facilitate transcription and DNA replication in actively cycling cells. As a corollary, terminally differentiated cells that are not transcriptionally active or cycling (e.g., erythrocytes in certain species) have highly condensed nuclei. How the chromatin is decondensed, and how active this process is, remains unclear. DNA topoisomerase II is proteolytically degraded at the end of mitosis, and the mitosis-specific phosphorylation of histone H3 is removed. Indeed, certain proteins essential for DNA replication, though dispersed during metaphase, are assembled back onto chromosomes during anaphase. While correlated, these events are not necessarily causal to the process of chromosome decondensation. Probably other structural proteins (e.g., subunits of the condensin complex) are also degraded or regulated through differential localization or post-translational modification.


Telophase I

Meiosis contains two divisions, both of which contain a telophase stage. During telophase I, the homologous chromosomes get segregated into separate nuclei. Although another division must take place for meiosis to be complete, the cells must still reform the nuclear envelopes, disassemble the spindle fiber microtubules, and go through cytokinesis. The cells then enter a short resting stage, known as interkinesis.

Telophase II

During the subsequent cell division, the sister chromatids of each chromosome are separated. During telophase II, the sister chromosomes are surrounded by new nuclear membranes. Although the two cells created during telophase II come from the same chromosome that has been duplicated, variation can be introduced in the process of recombination, in which parts of homologous chromosomes were exchanged in prophase I. Between the four cells produced at the end of meiosis, the two alleles for each gene can be segregated in many different ways, in combination with alleles for many other genes.


Major Events of Mitosis: Anaphase

Each chromatid has a centromere therefore, in metaphase, the back-and-forth jostling results in chromosomes that are not only lined up in a single plane, but each sister chromatid is aligned opposite one another. This arrangement is well suited for accurate partitioning of the chromatid.

Once the cell senses proper alignment along the metaphase plate, the replicated chromatids separate rapidly, signifying anaphase. Two notable things happen during anaphase: first, the centromeres that hold the chromatids together dissolve, separating the chromatids from each other and second, the newly freed chromatids (now properly called chromosomes) move rapidly toward the poles. Figure 5 shows a cell in anaphase. The separated chromatids (now chromosomes) are stained blue and the fibers that pull the chromosomes to each pole of the cell are stained green.


Figure 5. A cell during anaphase (Click to enlarge). From Wikipedia Commons.


Examiners report

Practically everybody knew the role of helicase in DNA replication. Extremely few could clearly explain the need for mitosis.

The question was often confused with other details of DNA replication, transcription and even translation. Though DNA replication was correctly described as semiconservative, further expansion of that term became muddled. Most knew A-T and G-C base pairing but the idea of complementarity was not always included. Diagrams were drawn but lacked labels and annotations most of the time. Occasionally, candidates mentioned that DNA polymerase was used

Of all the parts in Section B, this one (describe the events of mitosis) was answered best. Many candidates earned close to the maximum number of marks. A few candidates thought that interphase is a part of mitosis.