Cell Division Flashcards
What are the two main checkpoints within the cell cycle?
The G1/S checkpoint (the restriction point)
The G2/M checkpoint
What is the purpose of the checkpoints?
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
Name the phases of the cell cycle
M phase
Gap 0 (G0) phase
Gap 1 (G1) phase
Synthesis (S) phase
Gap 2 (G2) phase
What happens during the M phase?
Cell growth stops
Nuclear division (mitosis) - prophase, metaphase, anaphase and telophase
Cytokinesis
What happens during the G0 phase?
Cells may undergo apoptosis, differentiation or senescence
Cells may remain in this phase indefinitely or for a very long time
What happens during the G1 phase?
Cells grow and increase in size
Transcription of genes to make RNA occurs
Organelles duplicate
Biosynthesis e.g. protein synthesis
What happens during the S phase of interphase?
DNA replicates and when all chromosomes have been duplicated, each one consists of a pair of identical sister chromatids
Why is the S phase of interphase quick?
The exposed DNA base pairs are more susceptible to mutagenic agents so the speed reduces the chances of spontaneous mutations
What happens during the G2 phase of interphase?
Cells grow
What are chromatids?
Replicates of chromosomes
What is mitosis?
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)
Why do all living organisms need to undergo mitosis?
Asexual reproduction
Growth
Tissue repair
What are the main stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
What are the events that occur during prophase?
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
What are the events that occur during metaphase?
The pairs of chromatids attach to the spindle threads at the equator region
They attach by their centromeres
What are the events that occur during anaphase?
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
What are the events that occur during telophase?
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
What process happens once mitosis is complete?
Cytokinesis - division of cell cytoplasm so two cells are produced
How does cytokinesis differ in plant and animal cells?
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
What is meiosis?
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)
What is the significance of meiosis in life cycles?
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.
What are homologous chromosomes?
Matching pairs of chromosomes, one maternal and one paternal, that contain the same genes at the same gene loci
May contain different alleles
How many divisions take place during meiosis?
Two
What are the stages that occur during meiosis?
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
What events occur during prophase 1?
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
What events occur during metaphase 1?
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
What events occur during anaphase 1?
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
What events occur during telophase 1?
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
What events occur during prophase 2?
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
What events occur during metaphase 2?
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
What events occur during anaphase 2?
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
What events occur during telophase 2?
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
How does meiosis produce genetic variation?
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
