2.1.6 - Cell Division, Cell diversity, Cellular Organisation

Question 1

Define ‘cell cycle’ ?

Answer 1

Cell Cycle : The regulated sequence of events that occurs between one cell division and the next

Question 2

What are the three phases of the cell cycle ?

Answer 2

• Interphase
• Mitosis ( nucelar division )
• Cytokinesis (cell division )

Question 3

What happens during interphase ?

Answer 3

• The cell increases in mass and size
• The cell carries out its normal cellular function (eg. synthesising proteins and replicating its DNA ready for mitosis)

Question 4

What are the three phases of interphase ?

Answer 4

• Gv1 phase
• S phase
• Gv2 phase

Question 5

Explain what happens during the Gv1 phase ?

Answer 5

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

Question 6

Explain what happens during the S phase ?

Answer 6

• The DNA in the nucleus replicates
• This results in each chromosome consisting of two identical sister chromatids

Question 7

What happens during the Gv2 phase ?

Answer 7

• Cell continues to grow
• New DNA that has been synthesised is checked and any errors are usually repaired
• Other preparations for cell division are made (eg. production of tubulin protein )

Question 8

Define ‘Mitosis’ ?

Answer 8

Mitosis : The process by which nuclear division occurs resulting in two genetically identical daughter nuclei being produced, genetically identical to the parent cell nucleus

Question 9

What happens during mitosis ?

Answer 9

• Cell growth stops
• NUCLEAR DIVISION

Question 10

What happens during cytokinesis ?

Answer 10

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

Question 11

How odes mass of DNA vary throughout the cell cycle ?

Answer 11

Question 12

How is the cell cycle regulated ?

Answer 12

• Checkpoints
• These are points where genetic information contained within the replicated DNA is checked for any possible errors

Question 13

What are the four different checkpoints within the cell cycle ?

Answer 13

• During Gv1 phase
• During Gv2 phase
• During metaphase

Question 14

Explain what happens at the Gv1 checkpoint ?

Answer 14

• Chromosomes are checked for damage
• If damage is detected then the cell does not advance into the S phase until repairs have been made
• Cell goes into resting phase/ Go phase

Question 15

Explain what happens at the Gv2 checkpoint ?

Answer 15

• Chromosomes are checked to ensure they have been replicated correctly
  -If not, then the cell cycle is delayed until repair are made
• Checks organelle replication/ synthesis and protein synthesis

Question 16

Explain what happens at the metaphase checkpoint ?

Answer 16

• The final check
• This determines whether the chromosomes are correctly attached to the spindle fibres prior to anaphase

Question 17

What are the four main stages of mitosis ?

Answer 17

• Prophase
• Metaphase
• Anaphase
• Telophase

Question 18

Explain what happens during prophase ?

Answer 18

• The nuclear envelope breaks down and the nucleolus disappears
• Chromosomes condense and are now visible when stained
• The chromosomes consist of two identical sister chromatids that are joined together at the centromere
• Centrioles migrate to opposite poles of cell
• Spindle fibres form

Question 19

Explain what happens during metaphase ?

Answer 19

• Chromosomes line up at the equator/ metaphase plate
• Spindle fibres attach to chromsomes/ each sister chromatids

Question 20

What protein is involved in the attachment of spindle fibres to centromeres of chromosomes ?

Answer 20

Kinetochores

Question 21

Explain what happens during anaphase ?

Answer 21

• The sister chromatids divide at the centromere
• Spindle fibres begin to shorten
• The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres

Question 22

Explain what happens during telophase ?

Answer 22

• Chromosomes arrive at opposite poles and begin to decondense
• Nuclear envelopes begin to reform around each set of chromosomes
• The spindle fibres break down
• New nucleoli form within each nucleus

Question 23

Where does mitosis occur in plants ?

Answer 23

• Growth in plants occurs in meristem tissue, found in the shoots and roots of plant
• It is a source of undifferentiated cells for growth, repair of damaged cells/ tissue …

Question 24

Explain how to study mitosis using plant cells ?

Answer 24

• Garlic/ onion root tips are most commonly used since contain cells FREQUENTLY UNDERGOING MITOSIS
• Remove the tips of the roots (about 1cm) and place in a suitable stain
• The stained root tip is gently squashed on a glass slide using a blunt instrument
• Cells undergoing mitosis can be seen, drawn and annotated to show different stages of mitosis

Question 25

What stage of mitosis is portrayed ?

Answer 25

Question 26

What stage of mitosis is portrayed ?

Answer 26

Question 27

What are the limitations of observing mitosis in plant cells ?

Answer 27

- It can be very difficult to distinguish between prophase and telophase in cells - Treatment of cells during slit preparation could alter structure of cells - The size of cells or structures of tissues may appear inconsistent in different specimen slides - Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen

Question 28

Why is mitosis significant ?

Answer 28

- Growth of multicellular organisms - Replacement of cells and repair of tissues - Asexual reproduction

Question 29

Explain how mitosis is significant in the growth of multicellular organisms ?

Answer 29

- Genetically identical daughter cells are produced - Produces many cells/ increases the number of cell for growth

Question 30

Explain how mitosis is significant in the replacement of cells and repair of tissues ?

Answer 30

- Damaged tissues can be repaired by mitosis followed by cell division - As cells are constantly dying they need to be continually replaced by genetically identical cells

Question 31

Define 'asexual reproduction' ?

Answer 31

Asexual reproduction : The production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent

Question 32

Define 'Meiosis' ?

Answer 32

Meiosis : The process by which a diploid cell divides to from four genetically various haploid cells

Question 33

What are the two stages/ divisions in Meiosis ?

Answer 33

- Meiosis l - Meiosis ll

Question 34

What are the different stages involved Meiosis l and Meiosis ll ?

Answer 34

Meiosis l : - Prophase l - Metaphase l - Anaphase l - Telophase l Meiosis ll : - Prophase ll - Metaphase ll - Anaphase ll - Telophase ll

Question 35

Explain what occurs during prophase l ?

Answer 35

- The nuclear envelope breaks down and the nucleolus disintegrates - DNA condenses and becomes visible as chromosomes - DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere - The chromosomes are arranged side by side in homologous pairs/ bivalents - As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur producing recombinants - Centrioles migrate to opposite poles - Spindle fibres form

Question 36

What is a bivalent ?

Answer 36

A pair of homologous chromosomes

Question 37

What is a chiasma ?

Answer 37

The point at which the crossing over occurs of non-sister chromatids

Question 38

Explain what occurs during metaphase l ?

Answer 38

- The bivalents line up along the equator/ metaphase plate - The maternal and paternal can end up facing either poll/ in either daughter cell ; independent assortment - The orientation of each bivalent is independent of another

Question 39

Explain what occurs during anaphase l ?

Answer 39

- The homologous pairs of chromosomes are separated as spindle fibres pull whole chromosomes to opposite ends of the spindle - Spindle fibres shorten - The centromeres do not divide

Question 40

Explain what occurs during telophase l ?

Answer 40

- The chromosomes arrive at opposite poles - Spindle fibres start to break down - Nuclear envelopes form around the two groups of chromosomes and nucleoli reform

Question 41

Explain what occurs during cytokinesis ?

Answer 41

- This is when the division of the cytoplasm occurs - Cell organelles also get distributed between the two developing cells - In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half

Question 42

Explain what occurs during prophase ll ?

Answer 42

- The nuclear envelope breaks down and chromosomes condense - Centrioles migrate to opposite poles of cell - Spindle fibres form

Question 43

Explain what occurs during metaphase ll ?

Answer 43

- Chromsomes/ RECOMBINANTS line up in a single file along the equator of the spindle/ metaphase plate - Independant assortment - spindle fibres attach to chromosomes

Question 44

Explain what occurs during anaphase ll ?

Answer 44

- Centromeres divide and individual chromatids are pulled to opposite poles - This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

Question 45

Explain what occurs during telophase ll and cytokinesis ?

Answer 45

- Nuclear membranes form around each group of chromosomes - Cytoplasm divides as new cell surface membranes are formed creating four haploid cells

Question 46

Why is meiosis significant ?

Answer 46

- Meiosis is involved in sexual reproduction and increases the genetic diversity of gametes ( sex cells ) - Having genetically different offspring can be advantageous for natural selection

Question 47

Explain which mechanisms increasing genetic diversity of gametes produced during meiosis ?

Answer 47

- Crossing over - Independent assortment - Random fusion of gametes

Question 48

Define 'crossing-over' ?

Answer 48

Crossing over : the process by which non-sister chromatids exchange alleles

Question 49

Explain what occurs during crossing over ?

Answer 49

- During meiosis I homologous chromosomes pair up and are in very close proximity to each other - The non-sister chromatids can cross over and get entangled - These crossing points are called chiasmata - The entanglement places stress on the DNA molecules - As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

Question 50

How does crossing over cause genetic variation ?

Answer 50

This swapping of alleles can result in a new combination of alleles on the two chromosomes

Question 51

Define 'independent assortment' ?

Answer 51

Independent assortment : The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs/ recombinants along the equator of the spindle during metaphase I

Question 52

Explain what happens during independent assortment ?

Answer 52

- In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle - Each pair can be arranged with either chromosome on top, this is completely random - The orientation of one homologous pair is independent / unaffected by the orientation of any other pair - The homologous chromosomes are then separated and pulled apart to different poles

Question 53

Explain how does random fertilisation of gametes result in genetic variation ?

Answer 53

This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles