Cell Biology: Topic 1.6 Cell Division

Question 1

Define Cell cycle

Answer 1

Mitosis is part of a precisely controlled process known as the cell cycle
The cell cycle is the regulated sequence of events that occurs between one cell division and the next
The cell cycle has three phases:
interphase
nuclear division (mitosis)
cell division (cytokinesis)

Question 2

The length of the cell cycle varies depending on:

Answer 2

The environmental conditions, the cell type and the organism

Question 3

Define mitosis

Answer 3

Mitosis is division of the nucleus into two genetically identical daughter nuclei

Question 4

There are four key reasons why a cell may be required to divide by mitosis (GATE)

Answer 4

Growth: Multicellular organisms derive new cells via mitosis to increase in their size
Asexual reproduction- Vegetative propagation in plants occurs via mitotic division
Tissue repair / replacement - Damaged or aged cells replaced with identical healthy ones
Embryonic Development - Zygotes undergo mitosis and differentiate to become embryos

Question 5

This phase of the cell cycle is a continuum of three distinct stages:

Answer 5

G1 – First intermediate gap stage in which the cell grows and prepares for DNA replication
S – Synthesis stage in which DNA is replicated
G2 – Second intermediate gap stage in which the cell finishes growing and prepares for cell division

Question 6

What is interphase, and mention the functions of interphase (DOCTOR mnemonic)

Answer 6

Interphase is an active period in the cell cycle when many metabolic reactions occur

Many events need to occur in interphase to prepare the cell for successful division

These key processes include:

DNA replication – DNA is copied during the S phase of interphase
Organelle duplication – Organelles must be duplicated for twin daughter cells (G1 and G2 primary function)
Cell growth – Cytoplasmic volume must increase prior to division (G1 and G2 primary function)
Transcription / translation – Key proteins and enzymes must be synthesised (G1 and G2 primary function) ( (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle in G2)
Obtain nutrients – Vital cellular materials must be present before division (for example cellulose synthesis in plants carried in vesicles) (G1 and G2)
Respiration (cellular) – ATP production is needed to drive the division process (through duplication of mitochondria in G1 and G2)

Question 7

What is chromatin?

Answer 7

DNA is usually loosely packed within the nucleus as unravelled chromatin
In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated
DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase

Question 8

What is chromosome?

Answer 8

DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling)
DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)
In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery

Question 9

How is chromosome formed from supercoiling

Answer 9

During the second growth phase, G2, the chromatin (elongated DNA and histones) begins to condense. This condensation is
accomplished via a process called supercoiling. First, the DNA wraps around histones to produce nucleosomes. The nucleosomes are further wrapped into a solenoid. Solenoids group together in looped domains, and then a final coiling occurs to produce the chromosome.

Question 10

What are sister chromatids?

Answer 10

As the DNA is replicated during the S phase of interphase, the chromosome will initially contain two identical DNA strands

These genetically identical strands are called sister chromatids and are held together by a central region called the centromere

When these chromatids separate during mitosis, they become independent chromosomes, each made of a single DNA strand

Question 11

Points which identify interphase in a cell diagram

Answer 11

DNA is present as uncondensed chromatin (not visible under microscope)
DNA is contained within a clearly defined nucleus
Centrosomes and other organelles have been duplicated
Cell is enlarged in preparation for division

Question 12

Describe prophase

Answer 12

The chromatin fibres become more tightly coiled to form chromosomes (shorter and wider and visible under microscope)
The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles of the cell
Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
The nuclear envelope (nuclear membrane) disintegrates and nucleolus disappears

Question 13

Describe metaphase

Answer 13

Centrosomes reach opposite poles
Spindle fibres (protein microtubules) continue to extend from centrosomes
sister chromatids line up at the equator of the cell (also known as the metaphase plate) so they are equidistant to the two centrosome poles
Spindle fibres (protein microtubules) reach the sister chromatids and attach to the centromeres
This attachment involves specific proteins called kinetochores
Each sister chromatid is attached to a spindle fibre originating from opposite poles

Question 14

Describe anaphase

Answer 14

The sister chromatids separate at the centromere (the centromere divides in two)
Spindle fibres (protein microtubules) begin to shorten
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Question 15

Describe telophase

Answer 15

Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
The spindle fibres break down
New nucleoli form within each nucleus

Question 16

What is cytokinesis

Answer 16

Follows the nuclear division (mitosis) phase
Once the nucleus has divided into two genetically identical nuclei, the cell divides in two with one nucleus moving into each cell to create two genetically identical daughter cells

Question 17

Explain cytokinesis in animal cells

Answer 17

In animal cells:
After anaphase, microtubule filaments form a concentric ring around the centre of the cell
The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre
When the furrow meets in the centre, the cell becomes completely pinched off and two cells are formed
Because this separation occurs from the outside and moves towards the centre, it is described as centripetal

Question 18

Explain cytokinesis in plant cells

Answer 18

After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell (equatorial plane)
The cell plate is formed from vesicles carrying carbohydrates, lipids and proteins fusing together to create the two plasma membranes within the middle of the cell
After this other vesicles, carrying pectin and cellulose, deposit these substances by exocytosis in the gap between the two new membranes leading to the formation of new cell walls
The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells
Because this separation originates in the centre and moves laterally, it is described as centrifugal

Question 19

What is mitotic index

Answer 19

The mitotic index is the ratio between the number of cells in mitosis and the total number of cells
The mitotic index is a measure of the proliferation status of a cell population (i.e. the proportion of dividing cells)
It also functions as an important prognostic tool for predicting the response of cancer cells to chemotherapy

Question 20

Define cyclins and its importance

Answer 20

Cyclins are a family of regulatory proteins that control the progression of the cell cycle
When each of the different cyclins reach a certain concentration they trigger the next stage of the cell cycle
This ensures key processes (e.g. DNA replication, organelle multiplication and protein synthesis) occur at the correct time

Question 21

Explain the mechanism

Answer 21

When a specific cyclin has reached a certain concentration it will bind with another group of proteins (cyclin-dependent kinases) forming a complex which is activated
This complex phosphorylates (attaches a phosphate) a target protein which activates it, causing it to trigger specific functions (e.g. DNA replication)
Once the specific function is complete the phosphate is released, the cyclin breaks down and the cyclin-dependent kinases become inactive

Question 22

Describe functions of different types of cyclins

Answer 22

There are four different cyclins (D, E, A & B) whose concentrations rise and fall over the cycle:
D – present first, triggers cells to move from G1 to S phase
E – highest concentration at the start of S phase, prepares the cell for DNA replication during S phase
A – highest concentration in G2 phase but activates two different kinases that trigger two processes:
In the S phase, it activates DNA replication
In G2 phase, it prepares the cell for mitosis
B - highest concentration at the beginning of mitosis, promotes the formation of the mitotic spindle

Question 23

How does cancer form?

Answer 23

Cancers demonstrate how important it is that cell division is precisely controlled, as cancers arise due to uncontrolled mitosis
Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells)

Question 24

What causes cancer formation?

Answer 24

Cancers start when changes occur in the genes that control cell division
A change in any gene is known as a mutation
If the mutated gene is one that causes cancer it is referred to as an oncogene

Question 25

What are carcinogens

Answer 25

If mutagens cause cancers they are called carcinogens
Carcinogens are any agents that may cause cancer

Question 26

Types of tumours

Answer 26

Some tumours (such as warts) do not spread from their original site – these are known as benign tumours and do not cause cancer
Some tumours spread through the body, invading and destroying other tissues – these are known as malignant tumours and cause cancer

Question 27

Explain the effect of malignant tumours

Answer 27

Malignant tumours interfere with the normal functioning of the organ/tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels)
Malignant tumour cells can break off the tumour and travel through the blood and/or lymphatic system to form secondary growths in other parts of the body
Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour
Metastasis is very dangerous as it can be very difficult to detect, locate and remove secondary cancers