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Cell division occurs in two main stages:

Nuclear division - the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis.

Cell division - follows nuclear division and is the process by which the whole cell divides.


What happens before a nucleus divides? Why is this?

It's DNA must be replicated. This is to ensure that all the daughter cells have the genetic information to produce the enzymes and other proteins that they need.


What are the four requirements for semi-conservative replication?

The four types of nucleotide, each with their bases of adenine, guanine and thymine must be present.

Both strands of the DNA molecule must act as a template for the attachment of nucleotides.

The enzyme DNA polymerase is needed to catalyse the reaction.

A source of chemical energy is required to drive the process.


How does semi-conservative replication take place?

The enzyme DNA helical breaks the hydrogen bonds linking the base pairs of DNA.

The double helix separates into two strands.

Each exposed polynucleotide strand then acts as a template to which complementary nucleotides are attached.

Energy is used to activate these nucleotides.

The activated Nucleotides are joined together by the enzyme polymerase to form the missing polynucleotide strands.


Why is the DNA replication called 'semi-conservative replication'?

Half the original DNA has been saved and built into each of the new DNA molecules.


What was the other possible model for replication, other than semi-conservative?

The conservative model.


Describe the conservative model.

This model suggested that the parental DNA remained intact and that a separate daughter DNA copy was built up from new molecules of deoxyribose, organic bases and phosphate. Of the two molecules present, one would be made of entirely new material, while the other would be entirely original material.


Who were the two scientists that proved semi-conservative replication?

Meselsohn and Stahl


Which three facts did Meselsohn and Stahl base their investigation on?

All the bases in DNA contain nitrogen.
Nitrogen has two forms N14 and the isotope N15 (heavier).
Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make.


When centrifuged, DNA containing N14 would separate to nearer the...

Top of the centrifuge tube.


When centrifuged, DNA containing N15 would separate to nearer the...

Bottom of the centrifuge tube.


When (one) Bacterium grown in N15 for many generations is put into N14 and generated once what would the centrifuge tube look like?

There would be extracted DNA in the middle

1. 15/15

2. 15/14 15/14


When (one) Bacterium grown in N15 for many generations is put into N14 and generated twice what would the centrifuge tube look like?

Some lighter DNA at the top and some still in the middle.

1. 15/15
2. 15/14 15/14 (after one generation)
3. 15/14 14/14 15/14 14/14 (after two generations)


What does mitosis produce?

Mitosis produces two daughter nuclei that have the same number of chromosomes as the parent cell and each other.


In which rare event in mitosis will the genetic make up of the two daughter nuclei not be identical to the parent cell's nuclei.

As a result of mutation.


Mitosis is always preceded by a period during which the cell is not dividing. What is this called?



Name each of the phases of mitosis...



What happens in prophase?

The chromosomes become visible and the nuclear envelope disappears. The nucleolus also disappears.


What happens in metaphase?

The chromosomes arrange themselves (line up) at the centre (equator) of the cell. Spindle fibres form.


What happens in anaphase?

Spindle fibres attached to chromatids contract. Each of the two threads of the chromosome (chromatid) migrates to an opposite pole.


What happens in telophase?

Chromatids reach poles and become indistinct (no longer visible). The nuclear envelope reforms. The nucleolus reforms and the spindle disintegrates.


Why is mitosis important in organisms?

It produces daughter cells that are genetically identical to the parent cells.


Why is it important that mitosis makes exact copies of existing cells?

Growth: if an organism is to resemble its parents all the cells that grow form this original cell must possess the same set of genetic information. The cell firstly divides to give a group of identical cells.

Differentiation: these cells change or differentiate, to give groups of specialised cells. These different cell types each I divide by mitosis to give tissues made up of identical cells which perform a particular function. This results in efficient function.

Repair: if cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost.


What type of process is mitosis?



When mitosis is viewed under a microscope the observer gets a snapshot of the process. The number of cells at each stage of mitosis is proportional to...

The time each cell spends undergoing that stage.


Cells undergo a regular cycle of division called...

The cell cycle.


What are the three stages of the cell cycle?

-first growth phase, when the proteins from which cell organelles are synthesised are produced
-synthesis phase, when DNA is replicated
-second growth phase, when organelles grow and divide and energy stores are increased

Nuclear division when the nucleus divides into two (mitosis) or four (meiosis).

Cell division which is the process by which the whole cell divides into two (mitosis) or four (meiosis).


How long is a typical mammalian cell cycle?

24 hours.


What is cancer?

Cancer is a group of diseases caused by a growth disorder of cells. It is a result of damage to the genes that regulate mitosis and the cell cycle. Tumours develop - a group of abnormal cells which constantly expands in size.


Where are common places that cancers can develop.

Lungs, prostate gland, breast and ovaries, stomach.


Drugs used to treat cancer disrupt the cell cycle by:

Preventing DNA from replicating
Inhibiting the metaphase stage of mitosis by interfering with spindle formation


What is the problem with chemotherapy?

Such drugs also disrupt the cell cycle of normal body cells but the drugs are more effective against the rapidly dividing cells (cause of hair loss).