Cell Biology - Outcome 4 Flashcards
Cell Replication and Division
what two processes must a cell undergo to grow and divide?
- replication: doubling of chromosome complement to create 2 copies of each piece of DNA
- mitosis: nuclear division that results in each daughter cell receiving full chromosome complement
what are the phases of the cell cycle?
- G1
- S
-G2 - M
what phases are known as interphase?
- G1
- S
- G2
what do the phases of the cell cycle depend one to move from one phase to the next?
cyclin-dependent kinases (CDKs)
what do CDKs do?
activate other proteins by adding phosphate groups (phosphorylation)
how do CDKs become active?
they must bind to cyclins
what happens in the G1 phase?
- as level of cyclin increases, it increasingly binds to CDK
- signals cell to prepare for chromosomes for replication
- cell will be growing
what happens during G0 phase?
- resting stage (temp or perm)
- not actively growing and dividing but they are often still highly metabolically active carrying out cellular functions
- phase is brought about by absence of cues for growth and division - genes controlling mitosis are actually repressed
which cells temporarily enter G0?
- lymphocytes which remain in that phase until they encounter an antigen
- when they do - they re-enter G1, becoming self-replicating colonies
what happens during the S phase?
- synthesis of DNA
- occurs at slightly different times within each individual cell
- certain points in the cell cycle some cells will be 4n whilst some are still 2n (diploid)
- after replication two sister chromatids are held together in a dyad by centromeres, until separated by mitosis
what happens during the Gap 2 phase (G2)?
- the level of cyclin E falls, while cyclin B begins to rise
- centrosomes (including centrioles) also duplicate - the centrioles have a significant role to play during mitosis
what is mitosis?
the division of the nucleus
what does the M stage involve?
- nuclear division and cytokinesis
- triggered by mitogens
- formation of M-phase promoting factor which is a complex of Cdks and cyclins
what are the 4 phases of mitosis?
- prophase
- metaphase
- anaphase
- telophase
what happens during prophase?
- begins with m-phase promoting factor
- the chromosomes shorten and condense
- nuclear membrane disintegrates
- centrioles move apart to opposite poles of the nucleus
- bundles of fibre protein fibres called mitotic spindles grow out from centrioles
- two kinetochores appear on centrosomes of each chromosome
- mitotic spindle fibres attach to kinetochores
what happens during metaphase?
- chromosomes still attached by spindle fibres to poles
- line up across centre line of cell (metaphase plate)
what happens during anaphase?
- m-phase promoting factor activates anaphase promoting factor (APC/C) by phosphorylation to begin anaphase
- spinde fibres contract and pairs of chromosomes now become separate and migrate to opposite poles of cell, dragging with it one of chromatids from each chromosome
- APC/C also switches on synthesis of G1 cyclin - initiates next cell cycle
what happens during telophase?
- with newly separated chromatids (chromosomes) clustered at each end of cell - nuclear membrane forms around each cluster
- chromosomes relax from highly condensed form to become fine, thread-like chromatin
what happens during cytokinesis?
- separation of the cytoplasm
- belt of protein actin forms around middle of cell between reformed nuclei
- tightens until it pinches the cell into 2 daughter cells
what are the 2 types of cell cycle checkpoints?
- DNA damage checkpoint
- spindle checkpoints
when do DNA damage checkpoints occur?
during G1, S and G2 phase (interphase)
when do spindle checkpoints occur?
during M phase
what happens during DNA damage checkpoint in G1?
- p53 protein holds cell in G1 until DNA damage is repaired - prevents replication of damaged DNA
- if damage is irreparable, protein can induce apoptosis- cell will commit suicide
what happens during DNA damage checkpoint in G2?
- G2 checkpoint monitors Okazaki fragments on lagging strand of replicated DNA
- cell will not move to M phase until all fragments have been incorporated into continuous DNA strand by removal of RNA primers and closing of gaps by ligase
- ensures DNA has been replicated successfully before mitosis occurs
what happens during spindle checkpoints?
- assess formation of spindles
- holds cell in M phase - prevents cytokinesis
- checks made on attachment of fibres to kinetochores and which direction fibres pull chromosomes
- apoptosis can be triggered if damage is severe
what can the human chromosome complement be represented by?
a karyotype
what are gonads
ovaries or testes
what are special diploid cells in the gonads called?
gamete mother cells
what is meiosis?
a process that has 2 stages and produces 4 haploid gametes
what happens in meiosis I?
- diploid gamete mother cell becomes 2 haploid daughter cells
- mitosis occurs
what happens in prophase I in meiosis?
- homologous chromosomes pair up to form bivalents
- each chromosome consists of a chromosome of pairs chromatids - each bivalent consists of four chromatids with 2 centromeres
- spindle fibers emerge from centrioles and kinetochores form on chromosomes
- one kinetochore forms on each chromosome dyad, not one on each chromatid
what happens in metaphase I in meiosis?
- spindle attached bivalents align along metaphase plate
what happens in anaphase I in meiosis?
- chromosome dyads are pulled apart by spindle fibres to opposite poles of cell
what happens in telophase I in meiosis?
- nuclear membrane reforms and cytokinesis commences
- each cell contains only one maternal or one paternal version of each chromosome - haploid
what happens in meiosis II?
- only 23 chromosomes are replicated into each separate cell rather than diploid complement of 46 replicated in meiosis
- at end of meiosis, each gamete mother cell therefore produces 4 gametes
what dies replication of a single chromosome produce?
- two identical chromatids
- each carry exactly the same versions of each allele ( alternative versions of same gene) in exactly the same loci
where do two copies of each chromosome come from?
- one copy from the mother
- one copy from the father
these are not identical
will a gamete that receives the maternal version of a chromosome be the same as a gamete that receives a paternal version?
no
what is a source of variation that happens during prophase I?
- homologous chromosome pair together, chiasmata form
- points at which some of the genetic material from maternal and paternal versions of same chromosome become entangled
what happens to the homologous pairs are separated at anaphase I when they formed chiasmata?
- chiasmata leads to exchange of genes between one chromatid of maternal and paternal chromosomes
what will happen to 2 out of the 4 gametes if cross over occurs at the chiasmata?
- each gamete mother cell will not contain pure maternal or paternal chromosomes but rather a mixture of the two
what is cancer a result of?
the degradation of the regulator mechanisms of the cell cycle
which regulatory genes must be affected for cancer to develop?
- oncogenes, cell division promoters
- tumour suppression genes, inhibit cell division
describe what happens if p53 is mutated or damaged?
- DNA damage will persist
- cells will continue to divide
what is an example of a tumour suppressor gene?
p53
what is an example of an oncogene?
Ras
describe what happens if Ras is mutated or damaged?
- once activated cannot be de-activated
- constantly stimulates cell division
what are other checkpoints that cancerous cells ignore?
- density dependent inhibition (stop cells diving if crowded)
- anchorage dependence (cells require attachment to solid base before division)
