6 - CELL DIVISION/ DIFFERENTIATION

Question 1

What are the two main phases of the cell cycle?

Answer 1

Interphase, where cells are prepared for division

M phases, where cells undergo cytokinesis and mitosis

Question 2

What is the difference between cytokinesis and mitosis?

Answer 2

Mitosis is a type of nuclear division which results in the creation of 2 new identical daughter cells, whereas cytokinesis is a process of cytoplasmic division which takes place afterwards.

Question 3

Describe the growth phase (G1) of interphase.

Answer 3

• Cells grow and organelles duplicate
• Transcription and translation take place
• Biosynthesis of proteins and enzymes required for S phase takes place

Question 4

Describe the resting phase (G0) with phase G1.

Answer 4

• Trigger at restriction point
• Cells either undergo differentiation, apoptosis (programmed cell death) or sensescence (biological deterioration)
• Some cells may remain in this phase for a long time or indefinitely

Question 5

Describe the synthesis phase (S) of interphase.

Answer 5

• DNA replication takes place
• Chromatids (pairs of chromosome replicates) are produced
• Genes active in all cell types (housekeeping) replicate first
• Genes used in certain/specialised cells are replicated last
• Rapid due to susceptibility to mutations

Question 6

Describe the gap 2 phase (G2) of interphase.

Answer 6

• Cells grow

- Chemicals are released to stimulate proteins, these cause spindles to form and chromosomes to condense in the M phase

Question 7

What are the two main checkpoints of the cell cycle?

Answer 7

The G1/S checkpoint, also called restriction point and the G2/M checkpoint

Question 8

What is their purpose?

Answer 8

• to prevent uncontrolled division that would lead to tumours
• to detect and repair damage to DNA (i.e that caused by UV light)
• ensuring the cycle cannot be reversed
• ensuring the DNA is only duplicated once during each cell cycle

Question 9

What is a spindle?

Answer 9

A slender mass of microtubules

Question 10

Describe the prophase of mitosis.

Answer 10

• chromosomes shorten and thicken (condense)
• the nuclear envelope breaks down
• centrioles (from centrosomes) move to opposite ends of the cells where cytoskeleton fibres extend out from, forming spindle

Question 11

Describe the metaphase of mitosis.

Answer 11

• spindle fibres attach to the centromeres (region in chromosome where chromatids link)
• it manouveures them so they line up at the equator of the cell

Question 12

Describe the anaphase of mitosis.

Answer 12

• centromere of each chromosome splits

- spindle pulls on threads of chromatids to opposite poles of the cell through the use of motor proteins

Question 13

Describe the telophase of mitosis.

Answer 13

• chromosomes are fully seperate and reach opposite poles of cell
• nuclear envelope forms around the 2 new sets of chromosomes, forming 2 nuclei containing genetically identical material
• even distribution of organelles takes place
• mitosis is now complete

Question 14

Compare the formation of animal and plant cells after mitosis is complete.

Answer 14

In animal cells, the plasma membrane folds inwards and nips in the cytoplasm, whereas in plant cells an end plate forms (where the equator of the spindle was), where a new plasma membrane and cellulose cell-wall material are then laid.

Question 15

What are homologous chromosomes?

Answer 15

Matching chromosomes containing the same genes at the same loci but different alleles

Question 16

What are stem cells?

Answer 16

Unspecialised cells whihc hsve the abiloty to become multiple types of cell, controlled by the switching on and off of certain genes.

Question 17

What are totipotent stem cells?

Answer 17

Stem cells which have the potental to turn into any type of cell, inclyding a placental cell as all genes are switched on. They are derived from the small ball of cells formed shortly after the zygote.

Question 18

What are pluripotent stem cells?

Answer 18

Stem cells whch have the ability to become any type of cell except placental cells. They are found past the 8-cell stage, after the zygote has developed into a blastocyst.

Question 19

What are multipotent stem cells?

Answer 19

They are adult cells whic have the ability to become a limited numbet of different cells some genes will be switched off by this point, e.g bone marrow stem cells can only become blood cells.

Question 20

What are the sources of stem cells?

Answer 20

• Spare embryos from IVF that aren’t implanted have stem cells from the inner blastocyst
• Cells removed for pre-implantation genetic diagnosis at 8-cell stage of an embryo
• Umbilical cord blood contains pluripotent cells that can be cultured
• Induced pluripotent stem cells (iPS) developed in labs
• Adult stem cells found in the brain, liver, skin, and bone marrow

Question 21

What are some potential applications of stem cells?

Answer 21

• Replacement of damaged tissue/ faulty alleles, e.g replacing faulty brain cells inthose with Parkinson’s disease, replacing insulin producing tissue in pancreas in those with diabetes, replacing damaged spinal nerves so that limbs can work again
• Bone marrow transplants to therapeutically treat blood and immune disorders
• Drug research, e.g to grow artificial tissue
• Developmental biology by providing insight into embryological development and thus improving medicine

Question 22

How do stem cells differentiate?

Answer 22

• Switching off of some genes
• Greater expression of other genes
• Change shape
• Change proportion of organelles
• Change their contents

Question 23

How do the specialised cells then divide?

Answer 23

Most specialised cells do not undergo mitosis but divide with the help of multipotent stem cells. Exceptions include:

• skin cells that divide finitely by mitosis
• epithelial (lining) cells which need frequent replacing