All cells arise from other cells

Question 1

Interphase

Answer 1

During Interphase the cell increases in mass and size and carries out its normal cellular functions (eg. synthesising proteins and replicating its DNA ready for mitosis)

Interphase consists of three phases:
G1 phase
S phase
G2 phase

Question 2

G1 phase

Answer 2

Cells make the RNA, enzymes and other proteins required for growth

Question 3

S phase

Answer 3

The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)

Question 4

G2 phase

Answer 4

The cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired

Other preparations for cell division are made (eg. the production of tubulin protein, which is used to make microtubules for the mitotic spindle)

Question 5

Prophase

Answer 5

Chromosomes condense and are now visible when stained

The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere

The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)

Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)

The nuclear envelope (nuclear membrane) breaks down into small vesicles

Question 6

Metaphase

Answer 6

Centrosomes reach opposite poles

Spindle fibres (protein microtubules) continue to extend from centrosomes

Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles

Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres

Each sister chromatid is attached to a spindle fibre originating from opposite poles

Question 7

Anaphase

Answer 7

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 8

Telophase

Answer 8

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

Question 9

Cytokinesis

Answer 9

Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells

In animal cells, cytokinesis involves constriction of the cytoplasm between the two nuclei and in plant cells a new cell wall is formed

Question 10

What happens when mitosis goes wrong?

Answer 10

Mitosis is a controlled process

Uncontrolled cell division can lead to the formation of tumours and of cancers

Many cancer treatments are directed at controlling the rate of cell division

Question 11

Binary fission

Answer 11

Replication of the circular DNA and of plasmids

Division of the cytoplasm to produce two daughter cells, each with a single copy of the circular DNA and a variable number of copies of plasmids