Mitosis (1.6)

Question 1

Why do cells need to divide?

Answer 1

Reproduction, tissue repair, growth, and/or to maintain the efficiency of the transfer of substances in and out of the cell.

Question 2

What happens when there is no uncontrolled reproduction of cells?

Answer 2

Creation of tumors.

Question 3

What is the result of mitosis?

Answer 3

Creation of two genetically identical cells.

Question 4

What makes reproduction of cells susceptible to errors, and what can it lead into?

Answer 4

Since cell division is a multi-phase process, many things can go wrong. If this happens, the cells may die or signal the start of a tumor.

Question 5

The cell cycle is divided into three main stages, what are they?

Answer 5

Interphase

Mitosis

Cytokinesis

Question 6

Describe Interphase

Answer 6

The most active as well as the longest phase of the cell cycle. Cells will spend most of their life within this stage undergoing common cell processes such as metabolism, endocytosis, exocytosis, and using and obtaining nutrients.

Question 7

What organelles are involved in Interphase?

Answer 7

Nucleus and Cytoplasm

Question 8

What phases does it encompass?

Answer 8

G1 (Gap 1), S (synthesis), and G2 (Gap 2).

Question 9

Highlight the activities that take place during G1

Answer 9

Location: Cytoplasm
* The cell grows and functions normally undergoing everyday processes.
* Rapid protein synthesis takes place allowing the cell to grow in size.
* Proteins required for DNA synthesis (the next phase) are made.
* Mitochondria and chloroplasts (in the case of plant cells) are replicated. This also continues in the S phase.

Question 10

Highlight the activities that take place during S

Answer 10

Location: Nucleus
* The amount of DNA doubles as DNA replication takes place.
* The genetic material is duplicated but no chromosomes are formed yet.

Question 11

Highlight the activities that take place during G2

Answer 11

Location: Cytoplasm
* Protein synthesis occurs to produce the proteins needed for cell division, such as microtubule proteins that will make up a mitotic spindle. The cell is actively preparing for cell division.

Question 12

Define Mitosis

Answer 12

The division of the nucleus into two genetically identical daughter nuclei. It involves the separation of sister chromatids into individual chromosomes which are then distributed among the daughter nuclei. the division of the nucleus. This involves separating the DNA that was replicated in the S phase.

Question 13

Define Cytokinesis

Answer 13

Once mitosis has been completed, with the formation of two nuclei with identical sets of chromosomes, the cell enters cytokinesis. In cytokinesis, the cytoplasm of a parental cell is divided between the two daughter cells. (though it often starts in telophase).

Question 14

What are cyclins?

Answer 14

Cyclins are a family of proteins that control the progression of cells through the cell cycle.

Question 15

How do cyclins regulate the cell cycle?

Answer 15

Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches a certain concentration. Cyclins bind to enzymes called cyclin-dependent kinases (CDKs) and activate them. The activated CDKs then attach phosphate groups (phosphorylation) to other proteins in the cell. The attachment of phosphate triggers the other proteins to become active and carry out tasks; in this case, specific to the phases of the cell cycle.

Thus, cyclins and CDKs regulate the cell cycle. Depending on the presence and action of these proteins, the cell cycle can be fast or slow, and it may even stop altogether. Mutations in genes that code for cyclins can lead to problems regulating the cell cycle, which may lead to cancer.

Question 16

Talk about the Cyclin levels at different stages of the cell cycle.

Answer 16

G1 phase
During this phase cyclin D (red line) levels gradually increase.

S phase
Cyclin E (purple line) is instrumental to DNA replication and also promotes centromere duplication.

G2 phase
Cyclin A (blue line) helps induce DNA replication.

Mitosis
Cyclin B (yellow line) is essential for the formation of mitotic spindles and the alignment of chromatids.

Question 17

Explain the relationship between DNA in mitosis

Answer 17

In eukaryotic cells, this is done through the formation of chromosomes. Eukaryotic chromosomes consist of DNA which is tightly wound around proteins called histones. Histones are basic (alkaline) proteins that form part of nucleosomes. Many nucleosomes are coiled together in a specific pattern to form a structure called a chromosome. During interphase, chromosomes are unpacked, so that protein synthesis and replication can take place. DNA is only visible during mitosis.

DNA Double Helix -> Nucleosomes -> Cylindrical coil -> Chromatin Fiber -> Sister Chromatids together (chromosomes)

Question 18

How many phases does mitosis have? what are they?

Answer 18

4, Prophase, Metaphase, Anaphase, Telophase

Question 19

What happens in prophase?

Answer 19

DNA supercoils causing the chromatin to condense.
Nucleolus disappears.
Nuclear membrane disintegrates.
Spindle fibers (made of microtubules) start to form (and are completely formed by the end of prophase).
Centrioles (absent from plant cells) move to opposite poles.

Question 20

What happens in metaphase?

Answer 20

Spindle fibres bind to the centromere of sister chromatids and cause their movement towards the equatorial plate.
Sister chromatids are aligned at the equatorial plate at the end of metaphase.

Question 21

What happens in anaphase?

Answer 21

Sister chromatids are separated (now known as chromosomes) and pulled to opposite poles by the spindle fibres.

Question 22

What happens in telophase?

Answer 22

The chromosomes have reached the poles.
A nuclear membrane starts to reform at each pole.
A nucleolus appears in each new nucleus.
The spindle fibres disintegrate.
The cell elongates in preparation for cytokinesis.
In some cases, the invagination of the membrane is also visible (marking the beginning of cytokinesis).

Question 23

What is mitotic index?

Answer 23

the ratio of the number of cells in a population undergoing mitosis (prophase, metaphase, anaphase or telophase) to the total number of visible cells.

Question 24

How do u calculate the mitotic index?

Answer 24

Mitotic index= (P+M+A+T)/Total cells

Question 25

What is the flaw with mitotic index

Answer 25

you can only give an estimate of the mitotic index as it is sometimes hard to differentiate between cells at prophase and interphase.

Question 26

Why is mitotic index important?

Answer 26

The mitotic index is important because it indicates how many cells in a tissue are dividing at a given time. In a tumor, where cell division is uncontrolled, the mitotic index is higher than in normal tissue. We can therefore use the mitotic index to predict how quickly cancer could spread and the likely outcome in reducing cell proliferation of any treatment, such as chemotherapy.

Question 27

What happens during cytokinesis in animal cells?

Answer 27

A ring of protein (microfilaments) located immediately beneath the plasma membrane at the equator pulls the plasma membrane inward.

The inward pull on the plasma membrane produces the characteristic cleavage furrow.

When the cleavage furrow reaches the centre of the cells, it is pinched apart to form two daughter cells.

Question 28

What happens during cytokinesis in plant cells?

Answer 28

In plants Golgi apparatus forms vesicles that consist of material to build a new cell wall. Vesicles merge and form the cell plate. The cell plate grows and divides into two daughter cells.

Question 29

What is tumorigenesis?

Answer 29

Tumorigenesis is the formation of a tumor (or several of them), which is defined as a mass of cells that divide uncontrollably. There are two types of tumors: benign and malignant tumors. It should be noted that not all tumors lead to cancer. Cancer is caused by a malignant tumor.

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A benign tumor is usually localized and does not spread to other parts of the body. Most benign tumors respond well to treatment.

A malignant tumor is a cancerous growth that is often resistant to treatment. It may spread to other parts of the body and sometimes recur after it has been removed.

Question 30

What is mutation and how does it relate to cancer?

Answer 30

A mutation is a change in an organism’s genetic code. A change in the base sequence of a certain gene can result in tumor formation. However, some parts of a gene do not code for anything, so a mutation in these areas will not affect the organism. So, not all gene mutations lead to uncontrolled cell division.

Question 31

What are mutagens? Cite examples

Answer 31

These are agents that cause gene mutations. Although not all mutations result in cancers, anything that causes a mutation has the potential to cause cancer. Mutagens can be:

Chemicals that cause mutations are referred to as carcinogens, such as asbestos or dioxin.
High-energy radiation, such as X-rays.
Short-wave ultraviolet light.
Some viruses such as hepatitis B.

Question 32

Define Oncogene

Answer 32

An oncogene is a gene that has undergone a mutation that will contribute to the development of a tumor. In their normal, non-mutated state, oncogenes are termed proto-oncogenes. These proto-oncogenes then assist in the regulation of cell division.

Question 33

Differentiate primary and secondary tumors

Answer 33

Once abnormal cell division has started at a particular place in the body, a malignant primary tumor begins to form. If left untreated, this may follow a particular development pathway to form secondary tumors. We say that the tumor has metastasized. The following steps are involved:

Cancerous cells detach from the primary tumor.
Some cancerous cells gain the ability to penetrate the walls of lymph or blood vessels and so circulate the body.
The circulating cancerous cells invade tissues at different locations and develop, by uncontrolled cell division, into secondary tumors.

Question 34

Define metastasis

Answer 34

Metastasis is the movement of cells from a primary tumor to other parts of the body where they develop into secondary tumors.