the cell cycle

why is cell division needed

• growth and development from single fertilised eggs

• tissue renewal and repair (hair follicles, skin, wound healing)

• maintain homeostasis - new blood cells, immune cells

• reproduction - produce gametes

what’s the consequence of mutations

• inactivates tumour supressor genes and/or activate genes that promote cell proliferation

• accumulation of mutations - uncontrolled cell growth

• formation of tumours

cells that cannot be replaced

• cardiomyocytes

• photoreceptors

• neurons

• hair cells inside cochlear

the cell cycle stages

• interphase - normally takes around 23hours, contains G1, S phase and G2

• M phase - takes around an hour, consists of prophase, prometaphase, metaphase, anaphase, telophase and cytokinethesis

what happens during interphase

• the cell grows, doubling its protein content

• the organelle doubles in size

• during the S phase, DNA is synthesised

• the centrosome replicates

what happens during prophase

• chromatin condenses into visible chromosomes and two sister chromatids join at the centromere

• mitotic spindle forms from centrosomes

• centrosomes move to opposite poles of the cell

• microtubules attach to the kinetochores (protein complex at centromere of the chromosome which connect chromosomes to microtubules)

what happens in prometaphase

• nuclear envelope completely breaks down so spindle microtubules can access chromosomes

• spindle microtubules attach to kinetochores, connect chromosomes to opposite spindle poles ready to pull the chromosomes apart

what happens in metaphase

• chromosomes align at the equator

• sister chromatids attach to the opposite poles by kinetochore microtubules

• cell at this stage is ready for chromatid separation, spindle checkpoint - all chromosomes are properly attached before proceeding to the next phase

what happens during anaphase

• cohesive link between sister chromatids is released, allowing them to separate

• spindle fibres shorten, pulling the chromatids

• centromeres are separated, sister chromatids are pulled apart

• cell elongates, ready for separation into two daughter cells

what happens during telophase

• chromosomes reach opposite poles of the cells and de-condense to chromatin

• reformation of the nuclear envelope to give two genetically identical nuclei

• contractile ring begins to form around the equator to transition to cytokinethesis

what happens in cytokinethesis

• contractile ring tightens to partition cytoplasm into two daughter cells with shared organelles

• DNA de-condense and the cells return to interphase

how is the cell cycle controlled through cyclin

• cyclin are regulatory proteins that control progression of cell cycle and they rise and fall in a cyclical pattern

how do cyclin dependent kinases work

• inactive on their own

• cyclin binds to CDKs which causes a conformational change which activates the kinase activity

• active cyclin - CDK complex phosphorylate target proteins to drive specific phase of cell cycle

• different cyclin are synthesised and degraded, cell cycle progress in order

what do mitotic spindles do in each phase of the cell cycle (importance of the cytoskeleton)

• microtubules main constituents of mitotic spindles for pulling chromosome apart

• prometaphase - attach to chromosomes via kinetochores

• metaphase - pull the chromosome around until they are aligned at the equator

• anaphase - pull the sister chromatids apart by shortening of kinetochore microtubules

importance of meiosis to produce gametes

• maintain the same number of chromosomes across generations, prevent chromosomes doubling

• genetic stability, all start at the same time diploid number after fertilisation

• genetic diversity, crossing over/independent assortment for natural selection

key stages of meiosis

• interphase I - DNA replication, each chromosome is duplicated into two sister chromatids

• prophase I - homologous chromosomes pair up via synapsis synaptonemal complex, crossing over between non sister chromatids, exchange of DNA segments at chiasmata

• metaphase - homologous chromosome pairs align randomly, independent assortment

• anaphase I - homologous chromosomes are pulled to opposite poles

• telophase I and cytokinesis - two haploid cells form - reduction

• meiosis II - seperation of sister chromatids, no DNA replications and produces 4 haploid gametes

end products of meiosis

• chromosome number halved

• 4 haploid gametes, genetically unique

meiosis vs mitosis

• in meiosis cells divide twice, it mitosis cells divide once

• meiosis produces 4 cells with half the number of chromosomes of the parent cell, cells are unique and mitosis produces two cells with the same chromosome number as the parent, cells are identical

• role of meiosis is to produce gametes (sperm & eggs), required for sexual reproduction, in mitosis it increases the number of cells in a multicellular organism