1.6 Cell Division

Question 1

Reasons for cell division

Answer 1

• Smaller cells are more efficient
• Growth - needs more cells
• More cells = more cell differentiation (more specialised)
• Replace dead/damaged/infected cells
• Asexual reproduction (self-replication)

Question 2

Mitosis

Answer 2

The division of the nucleus into two genetically identical daughter nuclei.

Question 3

Cell Cycle

Answer 3

Interphase

Mitosis

Cytokinesis

Interphase and cytokinesis is not part of mitosis.

Question 4

Interphase

Answer 4

• Cells spend most of their time in this phase

Interphase is a very active phase of the cell cycle with many processes occurring in the nucleus and cytoplasm.

• e.g. processes = protein synthesis, DNA replication and production or mitochondria and/or chloroplasts
• 3 stages: Growth 1, Synthesis, Growth 2

Question 5

Chromosomes in Mitosis

Answer 5

Division of the diploid eukaryotic nucleus.

Chromosomes condense by supercoiling during mitosis.

• during mitosis, chromosomes condense into visible structures due to a process called supercoiling
• very long and thin -> must condense and coil around histone proteins making the chromosome much shorter and fatter
• helps regulate transcription

Question 6

Cytokinesis

Answer 6

Division of the cytoplasm to form two daughter cells.

Cytokinesis occurs after mitosis and is different in plant and animal cells.

Plant cells - different due to the cell wall - a new cell wall is formed across the equator of the cell with plasma membrane on both sides

Animal cells - membrane is pulled inwards around the equator, once the invagination reaches the centre pinches off and two new cells are formed

Question 7

Steps of mitosis

Answer 7

Prophase

Metaphase

Anaphase

Telophase

Question 8

Prophase

Answer 8

• chromosomes become shorter and more condensed (supercoiling)
• chromosomes replicate
• nuclear membrane degenerates
• centrosomes move towards poles as spindle fibres develop

Question 9

Metaphase

Answer 9

• spindle fibres are attached to the centromeres of the chromosomes; the other end is attach to the poles of the cell
• chromosomes move towards the equator of the cell and line up along the metaphase plate

Question 10

Anaphase

Answer 10

• pairs of sister chromatids are pulled apart by the spindle fibres towards the poles
• chromatids are now considered chromosomes.
• chromosomes move to the poles as a result of the shortening of the microtubule
• cell now has two genetically identical nuclei at each end of the cell

Question 11

Telophase

Answer 11

• nuclear membranes begin to form around each set of chromosomes
• chromosomes begin to uncoil to form chromatin again
• cell elongates and prepares for cytokinesis

Question 12

Mitotic Index

Answer 12

number of cells containing visible chromosomes (in mitosis) divided by the total number of cells in field of view

of cells in mitosis/total # of cells

Question 13

Cyclins

Answer 13

Cyclins are a family of proteins that help regulate the cell cycle (ensure that tasks are performed at the correct time + cell moves to the next stage when appropriate)

Question 14

How Cyclins work

Answer 14

1. bind to enzymes called cyclin-dependent kinases
2. kinases become active and attach phosphate groups to other proteins
3. attachment of phosphate triggers other proteins to become active and carry out tasks specific to one of the phases in the cycle

Question 15

Types of cyclin in the human body

Answer 15

There are 4 types of cyclin in human cells.

• levels of cyclin rise and fall
• when these cyclin levels reach a threshold concentration, the cell progresses to the next stage of the cell cycle

Therefore, cyclins control the cell cycle and ensure cells divide when new cells are needed (and not at other times)

Question 16

Tumors

Answer 16

Mutagens, oncogenes and metastasis are involved in the development of primary and secondary tumours.

• tumours - result of uncontrolled cell division; can occur in any organ or tissue
• primary tumours - localised do not move to other part of your body
• secondary tumours - cancer cells that detach and move elsewhere into the body
• cancer - diseases due to secondary tumours
• Metastasis - movement from a primary tumour to set up secondary tumours in other parts of the body
• Cancer - caused by genetic abnormalities due to a variety of different sources called carcinogens or due to inheritance or errors in DNA replication.