Cell Division

Question 1

include the 3 phases

describe what happens during interphase

Answer 1

period of intense synthesis and growth
* G1: intensive cellular synthesis (organelles grow and replicate – mitochondria, protein synthesis, cell size increases, ATP synthesis)
* S: semi-conservative DNA replication resulting in 2 genetically identical DNA molecules
* G2: intensive cellular synthesis: cell size increases, protein synthesis

Question 2

what does the M phase of mitotic cell cycle consist of?

Answer 2

• Mitosis: Nuclear division
• Cytokinesis

Question 3

prophase

Answer 3

• chromatin condenses to form visible chromosomes, each chromosome appears as two identical sister chromatids joined at the centromere
• centrioles move to opposite poles of the cell
• spindle fibres form and start to attach to the kinetochore complex, on both sides of the chromosome, at the centromere
• nuclear envelope disintegrates
• nucleolus disappears

Question 4

metaphase

Answer 4

• chromosomes align in a single file along the metaphase plate
• pole-to-pole spindle fibres form

Question 5

anaphase

Answer 5

• each centromere divides
• shortening spindle fibres pull the separated chromatids apart, becoming individual chromosomes, centromere-first, towards opposite poles of the cell
• pole-to-pole spindle fibres lengthen to push the poles apart

Question 6

telophase

Answer 6

• chromosomes reach opposite poles of the cell, decondense to form chromatin
• spindle fibres disintegrate
• nuclear envelope reforms around chromatin at each pole
• nucleolus in each daughter cell reappears

Question 7

cytokinesis

Answer 7

• division of cytoplasm
• distribution of cellular organelles between the 2 daughter cells
• may occur during telophase

Question 8

how does mitotic cell cycle ensure integrity of genetic information in daughter cells (DNA synthesis)

Answer 8

• semi-conservative DNA replication requires parental DNA to be the template for making copies of daughter DNA
• DNA replication occurs before the disappearance of the protective nuclear membrane so as to minimise mutations
• DNA replication occurs prior to equal division of genetic material

Question 9

how does mitotic cell cycle ensure equal division of nuclear DNA to daughter cells

Answer 9

• prophase: coiling of long, thin chromatin into thick, condensed chromosomes -> prevent entanglement of chromatin and breakage during separation of DNA
• ensures that each daughter cell will have the complete diploid set of chromosomes (metaphase: alignment of chromosomes in a single file at the equator of the cell & anaphase: division of centromeres, separation of sister chromatids by spindle fibres)
• telophase: daughter chromosomes reach opposite poles before cytokinesis -> formation of nuclear envelope prevents entanglement of chromatin, facilitating cytokinesis

Question 10

significant of mitosis (5 pts)

Answer 10

produces cells that are genetically identical to the parent cell for:
* replacement of worn-out cells
* growth in the number of cells
* regeneration of whole parts of body/organs in some organisms
* renewal and proliferation of stem cells
* asexual reproduction in some organisms

Question 11

prophase I

Answer 11

• condensation of chromatin into visible chromosomes, each chromosome appears as 2 identical sister chromatids joined at the centromere
• homologous chromosomes pair up, forming bivalents during synapsis
• crossing over occurs: exchange of corresponding alleles between non-sister chromatids of homologous chromosomes, leading to new combinatios of alleles in resulting chromatids
• centrioles move to opposite poles of the cell
• spindle fibres form, those from one pole attach to one side of the kinetochore complex at the centromere of one homologue while the other homologue is attached to spindle fibres from the opposite pole
• nuclear envelope disintegrates
• nucleolus disappears

Question 12

metaphase I

Answer 12

• pairs of homologous chromosomes, in the form of bivalents, are arranged along the metaphase plate of cell in one row
• independent assortment occurs: each homologue arranges itself on either side of the plate, independently of the other pairs of homologous chromosomes
• pole-to-pole spindle fibres form

Question 13

anaphase I

Answer 13

• centromeres do not divide
• spindle fibres pull chromosomes, centromere-first, towards opposite poles of the cell. homologous chromosomes separate into two haploid sets, one set at each end of the spindle
• pole-to-pole spindle fibres lengthen to push the poles apart

Question 14

telophase I

Answer 14

• chromosomes arrive at opposite poles, with each still consisting of two sister chromatids
• chromosomes usually decondense to form chromatin
• spindle fibres disintegrate
• nuclear envelope reforms around chromatin at each pole
• nucleolus in each cell reappears
• ploidy in each nucleus is now half of that in the original parent cell
• halving of chromosome number but no halving of DNA

Question 15

interphase II

Answer 15

• no further DNA replication occurs
• replication of centrioles occurs again

Question 16

prophase II

Answer 16

• condensation of chromatin into visible chromosomes, each chromosome appears as 2 identical sister chromatids joined at the centromere
• centrioles move to 2 opposite poles of cell
• spindle fibres form, attach to the kinetochore complex, on both sides of the chrommosome, at the centromere
• nuclear envelope disintegrates
• nucleolus disappears

Question 17

metaphase II

Answer 17

• chromosomes align in a single file along the metaphase plate of cell
• pole to pole spindle fibres form

Question 18

anaphase II

Answer 18

• each centromere divides
• spindle fibres pull the separated chromatids centromere-first, becoming individual chromosomes, towards opposite poles of the cell
• pole to pole spindle fibres lengthen to push the poles apart

Question 19

telophase II

Answer 19

• chromosomes reach poles of cell, decondense to form chromatin again
• spindle fibres disintegrate
• nuclear envelope reforms around chromatin at each pole
• nucleolus in each daughter cell reappears
• amount of DNA in each nucleus is now half of that in the original parent cell

Question 20

how does meiosis increase genetic variation in each gamete

Answer 20

• prophase I: formation of bivalents to allow crossing over -> gives rise to new combination of paternal and maternal alleles in each chromatid, non-genetically identical “sister chromatids” can segregate into different daughter cells at anaphase II
• metaphase I: independent assortment of paired homologues at the metaphase plate -> gives rise to different combinations of maternal and paternal chromosomes in the different gametes formed

Question 21

how does meiotic cell cycle distibute equally half the nuclear DNA to each gamete

Answer 21

• interphase I: DNA replicates only once -> chromosome doubling is immediately followed by two successive nuclear divisions to produce haploid cells
• ensure equal distribution of chromosomes to each daughter cell & ploidy in each of the 2 resultatnt daughter cells is halved (metaphase I: alingment of paired homologues at metaphase plate and anaphase I: segregation of homologues)
• ensure equal distribution of chromatids to each daughter cell and amount of DNA content in each of the 4 resultant daughter cells is half the amount in the original parent cell (metaphase II: alignment of chromosomes at metaphase plate and anaphase II: segregation of non-genetically identical chromatids)

Question 22

significant of meiosis

Answer 22

contributes to genetic variation
* crossing over between non-sister chromatids of homologous chromosomes in prophase I results in new combination of alleles in the daughter cells
* independent assormtnet of homologous chromosomes in metphase I/independent orientaiton of chromatids in metaphase II results in new combination of paternal and maternal chromosomes in gametes

allows for sexual reproduction
* if meiosis did not occur, fusion of diploid gametes would result in the doubling of the chromosomes in each successive sexually-reproduced generation
* meiosis helps to produce haploid gametes which fuse during fertilisation to produce a diploid zygote

Question 23

4 differences between mitosis and meiosis

Answer 23

• homologous chromosome pair up/undergo synapsis to form bivalents in prophase I of meiosis but not in prophase of mitosis
• homologous chromosomes form chiasma during crossing-over in prophase I of meiosis but not in prophase of mitosis
• homologous chromosomes line up at the metaphase plate in pairs in metaphase I of meiosis but chromosomes line up singly in metphase of mitosis
• homologous chromosomes separate in anaphase I of meiosis but sister chromatids are separated in anaphase of mitosis

Question 24

distinguish between gain of function mutation and loss of function mutation

Answer 24

• gain of function mutation required to occur only in one copy of the proto-oncogene for its effect to be seen whereas a loss of funftion mutation has to occur in both copies of the tumour suppressor gene for its effect to be seen
• a gain of function mutation behaves in a dominant manner whereas a loss of function mutation behaves in a recessive manner
• gain of function mutation occurs in proto-oncogenes, converting them into oncogenes, whereas loss of function mutation occurs in tumour suppressor genes in order to cause cancer

Question 25

4 factors that cause mutations

Answer 25

* UV light * Ionising radiation * X-ray * Ethidium bromide (chemical carcinogen)

Question 26

difference between translocation and base substitution mutation

Answer 26

translocation affects a longer sequence of DNA than the single-base substitution

Question 27

describe how cancer cells proliferate and migrate from the primary to secondary site with reference to traits of cancer cells

Answer 27

* cancer cells exhibit lack of density-dependent inhibition * cancer cells able to metastasise, leaving primary site and travel to the secondary site via blood vessels, invading the tissue * cancer cells were able to divide uncontrollably even at a new site

Question 28

describe how dysregulation of the cell cycle checkpoints of cell division may lead to cancer

Answer 28

* when the cell bypasses the G1 checkpoint, it allows cells with DNA mutations to continue dividing * when the cell bypasses the G2 checkpoint, it allows cells with incompletely or incorrectly replicated DNA to continue dividing * when the cell bypasses the M checkpoint, it allows cells to continue dividing although their chromosomes are not properly aligned along metaphase plate * cells accumulate more mutations that may lead to uncontrolled cell division

Question 29

define gene mutation

Answer 29

change in DNA nucleotide sequence such as base substitution, base addition and base deletion

Question 30

explain how chromosome aberrations could arise during mitosis (changing number of chromosomes)

Answer 30

* failure of sister chromatids to separate during anaphase, resulting in non-disjunction * as spindle fibres fail to form properly during prophase/fail to attach properly to kinetochore of chromosomes during prophase * resulting in loss of a copy of a tumour suppressive gene due to cells having fewer chromosomes/resulting in additional copy of proto-oncogene due to extra chromosomes in cells

Question 31

explain how chromosome aberrations could arise during mitosis (changing structure of chromosomes)

Answer 31

* damage can occur to chromosomes due to carcinogens (radiation)/chromosomal end-to-end fusion * deletion results from breakage of a chromosome in which a section of the chromosome is lost, resulting in loss of a copy of tumour suppressor gene in the deleted section * translocation, where a section of a chromosome breaks off and attaches to a non-homologous chromosome. a proto-oncogene may be moved from its normal location in one chromosome to another, and placed under the control of a more active promoter/tumour suppressor gene could be moved and placed under the control of a less active promoter * duplication involves replication of a section of the chromosome resulting in an increased number of copies of a proto-oncogene * inversion occurs when a segment of nucleotide sequences/section of chromosome breaks, is reversed/rotates through 180 degrees and rejoins which results in a change in the nucleotide sequence of a tumour suppressor gene which codes for a non-functional tumour suppressor protein/results in a change in the nucleotide sequence of a proto-oncogene which codes for a hyperactive oncoprotein/oncoprotein resistant to degradation

Question 32

explain how chromosome aberrations that arise during mitosis could lead to the development of cancer cells

Answer 32

* gain-in-function mutation of proto-oncogene leading to increased expression of its gene product which triggers the cell to keep dividing even in the absence of a stimulatory signal (growth factor)/which leads to uncontrolled cell division * loss-of-function mutation of tumour suppressor gene leading to a lack of expression of its gene product * cell cycle is not arested when DNA damage is detected/repair of DNA damage is ipaired/cells with damaged DNA do not undergo apoptosis which allow accumulation of more mutations in a single cell lineage * further mutations may result in the activation of genes producing telomerase * further mutations may result in the activation of genes which stimulate angiogenesis * cells lose density-dependent/anchorage dependence * cancer cells may metastasise (spread to other locations distant from their original site in the body and form secondary tumours)

Question 33

how does down syndrome come about

Answer 33

* non-disjunction/failure of sex chromosomes to separate during anapahse I * formation of gametes/sperm/ovum/daughter cell with both X chromosomes/1 extra sex chromosome * fertilised by another gamete/sperm/ovum with normal number of sex chromosome

Question 34

symptoms of down syndrome

Answer 34

* infertility * 3 sex chromosomes unable to pair up equally and so cannot be segregated to form normal gametes/traits of Klinefelter's syndrome (slow learning)