Cell Division

Question 1

What are the two main checkpoints within the cell cycle?

Answer 1

The G1/S checkpoint (the restriction point)

The G2/M checkpoint

Question 2

What is the purpose of the checkpoints?

Answer 2

To prevent uncontrolled division that would lead to tumours

To detect and repair damage to DNA

To ensure that the cycle happens in the correct sequence and cannot be reversed

To ensure the DNA is only duplicated once each cell cycle

Question 3

Name the phases of the cell cycle

Answer 3

M phase

Gap 0 (G0) phase

Gap 1 (G1) phase

Synthesis (S) phase

Gap 2 (G2) phase

Question 4

What happens during the M phase?

Answer 4

Cell growth stops

Nuclear division (mitosis) - prophase, metaphase, anaphase and telophase

Cytokinesis

Question 5

What happens during the G0 phase?

Answer 5

Cells may undergo apoptosis, differentiation or senescence

Cells may remain in this phase indefinitely or for a very long time

Question 6

What happens during the G1 phase?

Answer 6

Cells grow and increase in size

Transcription of genes to make RNA occurs

Organelles duplicate

Biosynthesis e.g. protein synthesis

Question 7

What happens during the S phase of interphase?

Answer 7

DNA replicates and when all chromosomes have been duplicated, each one consists of a pair of identical sister chromatids

Question 8

Why is the S phase of interphase quick?

Answer 8

The exposed DNA base pairs are more susceptible to mutagenic agents so the speed reduces the chances of spontaneous mutations

Question 9

What happens during the G2 phase of interphase?

Answer 9

Cells grow

Question 10

What are chromatids?

Answer 10

Replicates of chromosomes

Question 11

What is mitosis?

Answer 11

A type of nuclear division that maintains the chromosome number; each new daughter cell contains the same genetic information as the parent cell (they are genetically identical)

Question 12

Why do all living organisms need to undergo mitosis?

Answer 12

Asexual reproduction

Growth

Tissue repair

Question 13

What are the main stages of mitosis?

Answer 13

Prophase

Metaphase

Anaphase

Telophase

Question 14

What are the events that occur during prophase?

Answer 14

Chromosomes that have replicated during the S phase of interphase, now shorten and thicken

Nuclear envelope breaks down

The centriole divides and two new daughter centrioles move to oppose poles of the cell

Tubulin threads form a spindle between the two centrioles

Question 15

What are the events that occur during metaphase?

Answer 15

The pairs of chromatids attach to the spindle threads at the equator region

They attach by their centromeres

Question 16

What are the events that occur during anaphase?

Answer 16

The centromere of each pair of chromatids splits

Motor proteins pull each sister chromatid of a pair, in opposite directions, towards opposite poles

Because their centromere goes first, the chromatids, now called chromosomes, assume a V shape

Question 17

What are the events that occur during telophase?

Answer 17

The separated chromosomes reach the poles

A new nuclear envelope forms around each set of chromosomes

The cell now contains two nuclei each genetically identical to each other and to the parent cell from which they arose

Question 18

What process happens once mitosis is complete?

Answer 18

Cytokinesis - division of cell cytoplasm so two cells are produced

Question 19

How does cytokinesis differ in plant and animal cells?

Answer 19

In animal cells, the plasma membrane folds inwards and ‘nips in’ the cytoplasm

In plant cells, an end plate forms where the equator of the spindle was, and new plasma membrane and cellulose cell wall material are laid down on either side along this plate

Question 20

What is meiosis?

Answer 20

A type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell (haploid gametes)

Diploid (two complete sets of chromosomes) to haploid (one set of chromosomes)

Question 21

What is the significance of meiosis in life cycles?

Answer 21

It increases genetic variation as it involves the combining of genetic material from two unrelated individuals from the same species by fertilisation. This variation within a population increases its chance of survival when the environment changes.

Question 22

What are homologous chromosomes?

Answer 22

Matching pairs of chromosomes, one maternal and one paternal, that contain the same genes at the same gene loci

May contain different alleles

Question 23

How many divisions take place during meiosis?

Answer 23

Two

Question 24

What are the stages that occur during meiosis?

Answer 24

In the first meiotic division, the four stages are prophase 1, metaphase 1, anaphase 1 and telophase 1

In the second meiotic division, the four stages are prophase 2, metaphase 2, anaphase 2 and telophase 2

Cytokinesis occurs at the end of the second division

Question 25

What events occur during prophase 1?

Answer 25

The chromatin condenses and each chromosome supercoils The nuclear envelope breaks down and spindle threads of tubulin protein form from the centriole in animal cells The chromosomes come together in their homologous pairs - each member of the pair consists of two chromatids Crossing over occurs where non-sister chromatids wrap around each other and may swap sections so alleles are shuffled

Question 26

What events occur during metaphase 1?

Answer 26

The pairs of homologous chromosomes, still in their crossed over state, attach along the equator of the spindle by their centromeres The homologous pairs are arranged randomly, with the members of each pair facing opposite poles (independent assortment) The way they line up in metaphase determines how they will segregate independently when pulled apart in anaphase

Question 27

What events occur during anaphase 1?

Answer 27

The members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin threads of the spindle The centromeres do not divide, and each chromosome consists of two chromatids The crossed-over areas separate from each other, resulting in swapped areas of chromosome and allele shuffling

Question 28

What events occur during telophase 1?

Answer 28

In most animal cells, two new nuclear envelopes from around each set of chromosomes, and the cell divides by cytokinesis Each new nucleus contains half the original number of chromosomes, but each chromosome consist of two chromatids In plant cells, the cell goes straight from anaphase 1 into prophase 2

Question 29

What events occur during prophase 2?

Answer 29

If the nuclear envelopes have reformed, then they now break down The chromosomes coil and condense, each one consisting of two chromatids The chromatids of each chromosome are no longer identical, due to crossing over in prophase 1 Spindles form

Question 30

What events occur during metaphase 2?

Answer 30

The chromosomes attach by their centromere to the equator or the spindle The chromatids of each chromosome are randomly arranged The way that they are arranged will determine how the chromatids separate during anaphase

Question 31

What events occur during anaphase 2?

Answer 31

The centromeres divide The chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubulin threads of the spindle, towards opposite poles The chromatids are therefore randomly segregated

Question 32

What events occur during telophase 2?

Answer 32

Nuclear envelopes form around each of the four haploid nuclei In animals, the two cells now divide to give four haploid cells In plants, a tetras of four haploid cells in formed

Question 33

How does meiosis produce genetic variation?

Answer 33

Crossing over during prophase 1 shuffles alleles Independent assortment of chromosomes in anaphase 1 leads to random distribution of maternal and paternal chromosomes of each pair Independent assortment of chromatids in anaphase 2 leads to further random distribution of genetic material Haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of the same species