5. The mitotic cell cycle

Question 1

How do cells divide?

Answer 1

All cells arise from existing cells by division.
Division occurs in two main stages:
1. Nuclear division
There are two types of nuclear division:
* Mitosis
 Results in two daughter nuclei that are identical to parent nuclei.
 Diploid

• Meiosis
   Four daughter nuclei with half the number of chromosomes as parent nucleus.
   Haploid
   Nuclei have a different genetic composition to parent one.

2. Cytokinesis (cell division)
   Whole cell divides, cytoplasm is shared out.

Question 2

What are the three stages of the mitotic cell cycle (overview?)

Answer 2

1. Interphase
   1.1 G1 phase
   1.2 Synthesis
   1.3 G2 phase
2. Mitosis
   2.1 2.1
   2.2 Prophase
   2.3 Metaphase
   2.4 Anaphase
   2.5 Telophase
3. Cytokinesis

Question 3

How long is a cell cycle typically?

Answer 3

Typically, cell cycle of mammalian cell is 24 hours.

Question 4

Describe interphase?

Answer 4

• Longest phase of cell cycle.
• Sometimes known as resting phase as no division is occurring.
• Intense period of chemical activity.
• G1, S and G2 phases.
• G1: Proteins (enzymes) and organelles are synthesised, and biochemicals produced. Great quantity of mRNA for polypeptide synthesis at ribosomes (preparing for transcription and translation).
• S: DNA is replicated. After replication each interphase chromosome is composed of two identical sister chromatids.
• G2: Further protein synthesis. Energy stores increased. Replication of centriole pair completed in animal cells. Large store of microtubules (9 + 2 microtubule arrangement see Chapter 1). for formation of spindle. Centriole replication complete.

Question 5

Note: During cell cycle both mass of DNA and mass of cell as whole changes.

Answer 5

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Question 6

Describe the concept of “checkpoints” during the mitotic cell cycle?

Answer 6

As cell goes through mitotic cell cycle it has to reach and pass ‘quality control’ checkpoints which check for any kind of defects. G1: If DNA is damaged the cell cycle will slow so that it can be repaired. If DNA can’t be repaired cell death is triggered. This system is not 100% foolproof, and where DNA has not been checked properly sometime mutations can occur or lead to tumour formation.

Question 7

Describe the structure of chromosomes:

Answer 7

• Only visible when a cell is dividing – otherwise just see a mass of darkly staining chromatin.
• When first visible appear as long, thin threads around 50 um.
• Two identical sister chromatids joined by a centromere in the middle.
• Capped with telomeres.
• Highly coiled and folded, held in place by histone proteins.
• Made mainly up of:
• Histone proteins (70%)
• DNA (15%), about 2m each cell
• Chromatin (complex formed from histone proteins and DNA, beaded appearance).

Question 8

Nucleosome

Answer 8

portion of DNA that is: 146 base pairs wrapped around histone proteins.

Question 9

Why do telomeres prevent the loss of genes from the ends of chromosomes?

Answer 9

• Telomeres protect genes further along chromosome as they “cap” the end and do not contain genetic information.
• Means that during cell division when replication enzymes cant reach end of chromosome telomere section is eroded and no actual genetic information is lost.
• Length of telomere determines the life span of a cell/cellular aging. The longer a telomere is, the more a cell can divide, more a cell can divide and replicate, the longer it lives.
• Telomeres can be lengthened in dividing cells through enzyme telomerase.
• Telomerase is not present in non-dividing cells.

Question 10

Telomere

Answer 10

a sequence of nucleotide bases repeated many times at the end of a chromosome (a “cap” of sorts).

Question 11

Note: Cancer cells produce more telomerase so that cells can divide more rapidly and live longer (tumor is uncontrolled cell division).

Answer 11

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Question 12

Karytotype

Answer 12

photographical organisation of chromosomes cut out and pasted into logical format, showing full diploid set.

Question 13

Uses of mitosis (overview)?

Answer 13

• Growth of multicellular organisms
• Cell replacement
• Tissue repair
• Asexual reproduction

Question 14

How does mitosis facilitate growth of multicellular organisms?

Answer 14

Cells divide by mitosis to produce new cells of the same kind. In any given cell entire genome is present but only certain gene(s) being expressed. Some cells can differentiate to become specialized cells e.g., xylem cells in plants and muscle cells in animals. All cells must be replicated to be genetically identical so that new cell can perform sam function and have same structure. Mitosis therefore facilitates the growth of a multicellular organism through increasing the number of cells.

Question 15

Why can mitosis be said to produce “genetically identical” but not “identical” cells?

Answer 15

During cytokinesis cytoplasm and organelles may be unequally distributed.

Question 16

How does mitosis facilitate cell replacement?

Answer 16

Allows new cells to be produced to replace damaged or “dead” cells. E.g., a red blood cell only has a life span of 120 days.

Question 17

How does mitosis facilitate the repair of tissues?

Answer 17

Allows genetically identical new cells to be produced that perform same structure and function as damaged tissue so that tissue can continue to function effectively. Replaces damaged cells.

Question 18

How does mitosis facilitate asexual reproduction?

Answer 18

Mitosis is how certain organisms carry out asexual reproduction, and in doing so create “clones” of themselves. In eukaryotes mitosis IS asexual reproduction.
Advantages of this:
* Parents well adapted to current environment so conditions for offspring should be well-suited and favorable too – as long as conditions do NOT change. E.g., a disease could wipe out whole population at once because no genetic variation.
* Rapid form of reproduction – large numbers can be built up quickly to form a population. The whole area can be colonized by this species and species can gain competitive advantage – especially for light by this means.

Question 19

Note: if offspring is not genetically identical could be due to a mutation.
Note: bacteria do not produce by mitosis but binary fission.

Answer 19

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Question 20

Adult stem cells

Answer 20

undifferentiated, continuously dividing cells that occur in animal tissues. E.g., found in inner lining of small intestine, in skin, in lining of gas exchange system, in bone marrow.

Question 21

How do daughter stem cells work?

Answer 21

The genetically identical undifferentiated daughter cells of stem cells can divide to produce even more adult stem cells. When necessary a daughter cell can differentiate and become specialised for a specific role and therefore serve as a replacement for damaged cells.

Question 22

Note: adult stem cells can only specialise/develop into cell type/types in the specific region they are located. They are therefore not completely undifferentiated.

Note: stem cells cannot divide indefinitely.

Note: Embryonic stem cells are completely undifferentiated and can divide an infinite number of times.

Answer 22

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Question 22

Where are stem cells found?

Answer 22

• Adult stem cells found in mature organisms.
• Embryonic stem cells occur at earliest stage of development of an embryo.

Question 23

How does mitosis work in malignant and benign tumours?

Answer 23

Malignant – faster rate of division, when malignant tumour cells are carried into bloodstream they can invade/infiltrate other healthy tissues and form secondary or metastatic tumours.

Benign – grow in a contained area (encapsulated) with a slower rate of division. Do not infiltrate surrounding tissue.

Question 24

How does mitosis enable tumour formation and how are tumours formed?

Answer 24

• Formation of tumour occurs through uncontrolled cell division, which can be caused by the damage of genes that regulate mitosis.
• Proto-oncogenes can mutate and become oncogenes. This causes the cell to continue to grow and divide.
• Tumour suppressor genes (regulate cell division) can become switched off as a result of mutation.
• Common mutagens – radiation. (check with chapter 6).
• Checkpoints that regulate and control mitosis do not function properly in tumour cells.
• Tumour cells do not undergo apoptosis (programmed cell death) and can live indefinitely.
• Cells become less differentiated with each division. Lose ability to function properly.
• All these factors combine to cause group of abnormal cells to form. This is called a tumour.
• Healthy cells stop functioning when coming into contact with cells, known as contact-inhibition. Tumour cells are not affected by contact-inhibition so they keep growing and expanding.
• Tumours can be cancerous (malignant) or benign. Can develop in any part of body.
• Can occur in plants too.

Question 25

Contact-inhibition

Answer 25

when healthy cells come into contact with other cells and stop functioning.

Question 26

Apoptosis

Answer 26

programmed cell death.

Question 27

Brief description of mitotic cell stages?

Answer 27

1. Prophase – chromosomes become visible and nuclear envelope disassembles.
2. Metaphase – chromosomes arrange themselves at equator (also known as metaphase plate) of cell.
3. Anaphase – sister chromatids move to opposite poles, thus becoming known as daughter chromosomes (because now separate entities).
4. Telophase – nuclear envelope reforms.

Question 28

What happens during prophase?

Answer 28

• Chromosomes first become visible as long thin threads.
• Chromosome condensation occurs (chromosomes shorten and thicken, DNA molecule in each chromatid coils and supercoils).
• Animal cells: two pairs of centrioles separate and each pair moves to opposite poles.
  Note: two pairs of centrioles make up a centrosome.
• From each pair of centrioles microtubules develop and form spindle fibres. Spindle fibred span cell from pole to pole.
  Note: collectively microtubules are known as spindle apparatus/spindle.
• Nucleolus disappears and nuclear envelope disassembles by breaking up into vesicles. This leaves chromosomes dispersed at random free in cytoplasm of cell.
• Chromosomes become attached to microtubules of spindle fibres by centromere and are moved towards the equator (latter half of prophase).

Question 29

Coiling

Answer 29

refers to the wrapping of DNA around histone proteins to form nucleosomes and the subsequent organization into higher-order structures like the 30 nm chromatin fiber.

Question 30

Supercoiling

Answer 30

refers to the additional twisting of the DNA double helix beyond its normal helical structure, leading to either over-winding (positive supercoiling) or under-winding (negative supercoiling), resulting in a more compact structure, which is crucial for the efficient condensation of chromosomes during prophase in mitosis.

Question 31

What happens during metaphase?

Answer 31

• Spindle is fully formed.
• Chromosomes are arranged at spindle equator (get pulled by spindle fibres attaching to centromere and REMAIN attached in metaphase).
• Sister chromatids remain attached to one another.

Question 32

What happens during anaphase?

Answer 32

• Centromeres divide into two and spindle fibres joined to either side of (now divided) centromere contract, pulling sister chromatids apart and to opposite poles of cell.
• Chromatids are now known as daughter chromosomes.
• A lot of energy is needed for this process and is provided by mitochondria gathered around spindle fibres.
  Note: some chemicals aiming to stop mitosis could target and destroy the spindle fibres. This would ensure that the chromosomes remain at the equator unable to reach the poles.

Question 33

What happens during telophase?

Answer 33

• Daughter chromosomes reach their poles and become longer and thinner (opposite of chromosome condensation), eventually becoming chromatin again.
• Spindle disassembles.
• Nuclear envelope reassembles (the vesicles which were formed in prophase fuse back together again).
• Nucleolus reforms.
• Two separate nuclei can now be seen.
  Note: cytokinesis frequently begins before the end of telophase.

Question 34

What happens during cytokinesis in animal cells?

Answer 34

• Cleavage furrow forms and cytoplasm constricts to pinch the cell into two new cells.
• Cell organelles are shared out between two cells (however this does not always happen equally so even though cells will be genetically identical they might not always be identical).

Question 35

What happens during cytokinesis in plant cells?

Answer 35

• No cleavage furrow forms.
• Organelles shared out.
• Vesicles from Golgi body bring materials to form a cell plate across the equator of parent cell from centre outwards.
• Cellulose laid down along this dividing plate to form a cell wall.

Question 36

What are the differences of nuclear and cell division between plant and animal cells?

Answer 36

Animals:
* Stem cells able to divide by mitosis.
* Stem cells occur where there is a requirement for growth, tissue repair and cell replacement.
* Mitosis occurs in nucleus of cell (until nucleus in dissolved).

Plants:
* Mitosis occurs in specialized tissue called meristematic tissue.
* Plant meristems occur in growing regions – e.g., root and shoot tips, in cambium of stems and roots.
* Plants do not have centrioles. Instead they have MTOCs/microtubule organizing centres that perform the same role.

Question 37

Cambium

Answer 37

– tissue layer containing and instrumental in producing meristematic cells (plant version of stem cells, also unspecialized). Located between xylem and phloem.

Question 38

Clarification of all those words starting with C but sound the same in this chapter:

Answer 38

Centrioles – structure made from “9+2” microtubule triplets arrangement. Centrioles make up centrosomes. Centrosomes act as animal cell MTOC because play a role in organizing microtubules. Centrioles produce spindle fibres that attach to centromere on chromosome. These fibres pull the centromere and sister chromatids apart to form daughter chromosomes.
Centromeres – specialized structures on chromosome appearing during cell division as region holding two sister chromatids together.
Centrosomes – Contains centrioles, animal cells primary microtubule organizing centre. Facilitates organization of spindle poles during mitosis.
Chromosomes – contains DNA. 23 pairs in each animal cell.
Chromatids – refers to sister chromatids that become daughter chromosomes after being pulled apart. One short arm and one long arm when drawing.
Chromatin – tightly coiled combination of DNA and histone proteins. Made up of nucleosomes.

Question 39

How does the kinetochore work?

Answer 39

Kinetochores are large proteins assemblies that are present in the centromere of chromosomes and attach to spindle fibres of microtubules. They therefore help in chromosomes segregation during cell division in metaphase and anaphase.

Question 40

How to observe mitosis in prepared slides?

Answer 40

• Onion, garlic and broadbean commonly used for these preparations because
  A) Their roots grow quickly.
  B) The number of chromosomes per cell is low enough to view the individual chromosomes.
• Slides will generally be sections of root tips. Root tips have meristematic region where mitosis occurs. At end of root tip there is a protective cap – no mitosis takes place there.
• Root cap cells appear more rounded than meristematic cells (cells undergoing mitosis and differentiating into e.g., xylem or phloem cells).
• A stain must be used to stain DNA so chromosomes will be visible.