1.6 Cell division

Question 1

Why do cells divide?

Answer 1

Growth - multicellular organisms increase their size by increasing their number of cells via mitosis

Asexual Reproduction - certain eukaryotic organims my reproduce asexually via mitosis (e.g. vegetative reproduction)

Tissue Repair - replacing dead/damaged cells

Embryonic Development - a fertilized egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo

Question 2

If an organisms grows larger, it needs

Answer 2

more cells, AND cells need to have a copy of the organism’s DNA

Question 3

Cell division allows for __________ of the organisms by producing more cells, and it also allows for more _____________ to occur

Answer 3

growth, cell differentiation

Question 4

Mitosis occurs most frequently in

Answer 4

developing embryos

Question 5

New cells are also needed on a regular basis to ___________ dead, damaged, or infected cells

Answer 5

replace

Question 6

Cell division (specifically through mitosis) is also used in

Answer 6

asexual reproduction (self-replication)

Question 7

Cell cycle

Answer 7

series of events through which cells pass to divide and create two identical daughter cells
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

Question 8

Every eukaryote has genes on…

Answer 8

chromosomes - storage units of DNA in the nucleus

Question 9

Each chromosome has a partner…

Answer 9

(homologous chromosomes) - one from each parent. Both copies are required for the cell to function.

Pair of chromosomes with the same gene sequence, loci, chromosomal length, and centromere location

Question 10

Different species have different chromosome numbers:

Answer 10

Humans = 23 pairs (n = 23)
Diploid number (2n) = 46

Question 11

Gametes

Answer 11

(sex cells - sperm and eggs) are haploid (n).

They have a half set of chromosomes, as they will pair up with the other half in fertilization.

Question 12

centromere

Answer 12

the part of a chromosome that links sister chromatids

Question 13

centrioles

Answer 13

organise spindle microtubules

Question 14

Spindle microtubules

Answer 14

(also referred to as spindle fibres)

Question 15

Sister chromatids

Answer 15

are duplicated chromosomes attached by a centromere

After anaphase when the sister chromatids separate they should then be referred to as daughter chromosomes

Question 16

Why do chromosomes supercoil?

Answer 16

Human cells are on average 10𝝻m in diameter and the nucleus within each is less than 5𝝻m in diameter.
Human chromosomes are 15mm to 85mm (15,000𝝻m to 85,000𝝻m) in length
Chromosomes need to be stored compactly to fit within the nuclei of cells
This problem becomes more acute during mitosis when chromosomes need to be short and compact enough that they can be separated and moved to each end of the cell

Question 17

How chromosomes supercoil?

Answer 17

Strain is places on a DNA helix by overwinding or underwinding of the helix
This causes the DNA molecule to coil back on itself becoming shorter and wider
Note: in eukaryotes, proteins called histones aid the process

Question 18

Interphase

Answer 18

part of the cell cycle that does not involve division; majority of life cycle

Question 19

G1 (Gap 1)

Answer 19

Metabolic reactions
Increase volume of cytoplasm
Organelles produced and replicated
Proteins synthesised

Question 20

S (Synthesis)

Answer 20

DNA replicated

Question 21

G2 (Gap 2)

Answer 21

Metabolic reactions
Increase the volume of the cytoplasm
Organelles produced and replicated
Proteins synthesised

Question 22

G0 (Gap 0)

Answer 22

“Resting phase”
Temporary or permanent phase
Cells that do not divide
Carry out regular functions
Metabolic reactions

Question 23

Prophase

Answer 23

DNA supercoils chromatin condenses and becomes sister chromatids, which are visible under a light microscope

The centrosomes move to opposite poles of the cell and spindle fibres begin to form between them

The nuclear membrane is broken down and disappears

Question 24

Metaphase

Answer 24

Spindle fibres from each of the two centrosomes attach to the centromere of each pair of sister chromatids

Contraction of the microtubule spindle fibres cause the sister chromatids to line up along the centre of the cell.

Question 25

Anaphase

Answer 25

Continued contraction of the microtubule spindle fibres cause the separation of the sister chromatids The chromatids are now referred to as chromosomes Chromosomes move to the opposite poles of the cell

Question 26

Telophase

Answer 26

The chromosomes uncoil de-condense to chromatin (and are no longer visible under a light microscope). Chromosomes arrive at the poles. Microtubule spindle fibers disappear New nuclear membranes reform around each set of chromosomes

Question 27

Mitosis

Answer 27

division of the nucleus

Question 28

Cytokinesis

Answer 28

division of the cytoplasm Occurs concurrently with telophase Different in plant and animal cells

Question 29

Animal Cells

Answer 29

A ring of contractile proteins (microfilaments; actin and myosin) immediately inside the plasma membrane at the equator pulls the plasma membrane inward The inward pull on the plasma membrane produces the characteristic cleavage furrow When the cleavage furrow reaches the centre of the cell, it is pinched apart to form two daughter cells

Question 30

Plant cells

Answer 30

During telophase, membrane-enclosed vesicles derived from the Golgi apparatus migrate to the centre of the cell Vesicles fuse to form tubular structures → merge (with the addition of more vesicles) to form two layers of plasma membrane (i.e. the cell plate) Cell plate continues to develop until it connects to the existing cell’s plasma membrane Vesicles deposit, by exocytosis, pectins and other substances in the lumen between the daughter cells to form the middle lamella (“gluing” to cells together) Both daughter cells secrete cellulose to form their their new adjoining cell walls

Question 31

Mitotic index

Answer 31

ratio between number of cells in mitosis in a tissue and the total number of observed cells mitotic index= # cells in mitosis/total # of cells

Question 32

Cyclins

Answer 32

a family of proteins that control the progression of cells through the cell cycle

Question 33

Cyclin in cell cycle

Answer 33

1. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches it threshold. 2. Cyclins bind to enzymes called cyclin-dependent kinases 3. These kinases then become active and attach phosphate groups to other proteins in the cell. 4. The attachment of phosphate triggers the other proteins to become active and carry out tasks (specific to one of the phases of the cell cycle).

Question 34

Normal cell cycle - role of cyclins

Answer 34

The timing and rate of cell division are important and controlled by proteins and enzymes The cell cycle in eukaryotic cells is driven by a combination of two substances - Cyclins are proteins that bind to enzymes called cyclin dependent kinases - Different cyclin/CDK combinations control different activities during different stages of the cell cycle

Question 35

Progression through parts of the cell cycle are affected in...

Answer 35

various ways by specific cyclins

Question 36

different type of cyclins throughout cell cycle

Answer 36

Cyclin D Triggers cells to move from G0 to G1 and from G1 into S phase Cyclin E prepares the cell for DNA replication in S phase. Cyclin A activates DNA replication inside the nucleus in S phase. Cyclin B promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis.

Question 37

Tumours

Answer 37

abnormal growth of tissue that develop at any stage of the life in any part of the body.

Question 38

A cancer

Answer 38

a malignant tumour and is names after the part of the body where the cancer (primary tumour) first develops.

Question 39

A mutation

Answer 39

a change in an organism’s genetic code. A mutation/change in the base sequence of a certain genes can result in cancer.

Question 40

Mutagens

Answer 40

agents that cause gene mutations. Not all mutations result in cancers, but anything that causes a mutation has the potential to cause a cancer.

Question 41

Mutagens can be:

Answer 41

chemicals that cause mutations are referred to as carcinogens high energy radiation such as X-rays short-wave ultraviolet light Some viruses

Question 42

If a mutation occurs in an

Answer 42

oncogenes it can become cancerous. In normal cells oncogenes control of the cell cycle and cell division.

Question 43

Formation of a tumor

Answer 43

- mutation in a oncogene - malfunction in the control of the cell cycle - uncontrolled cell division - tumour formation

Question 44

Several mutations must occur in

Answer 44

the same cell for it to become a tumour causing cell. The probability of this happening in a single cell is extremely small.

Question 45

Factors (other than exposure to mutagens) that increase the probability of tumour development include:

Answer 45

The vast number of cells in a human body – the greater the number of cells the greater the chance of a mutation. The longer a life span the greater the chance of a mutation.