Mitosis and meiosis + experimental evidence Flashcards
Cell cycle definition
Repeated cycles of macromolecular synthesis (growth) and division (mitosis)- the regular sequence of events that produces new cells.
Two main phases of the cell cycle
Interphase and mitosis
What periods are within interphase?
G1, G2 and S phase
G1 length
Longest of the phases
G1 explained
Period of active RNA and protein synthesis, including proteins controlling progress through the cell cycle.
Cell volume, that was halved in mitosis, is restored
S phase explained
Characterised by DNA replication, histone synthesis and centriole duplication
G2 length and explained
Relatively short, proteins required for mitosis accumulate
What is the overall function of the gap phases?
Allow time delays for cell growth
What does the length of G1 depend upon?
How favourable the extracellular conditions are. If conditions are unfavourable then the cells delay progress through G1.
What happens if conditions are unfavourable?
If conditions are unfavourable then the cells delay progress through G1.
May enter a specialised resting state known as G0, in which the cells can remain for days, weeks or months before resuming proliferation.
What activates postmitotic G0 cells + process?
Growth factors or signals from the extracellular environment called mitogens bind and to the cell surface receptors and trigger a cascade of kinase signalling in the cell.
Cells are maintained at the restriction point at the G1/S boundary until sufficient enzymes and nutrients are required for DNA synthesis have accumulated.
Checkpoint function and locations
Entry or progression into the next phase of the cell cycle is monitored at specific checkpoints before the beginning of each phase. Certain conditions must be met to continue
What regulates the checkpoints +function?
Cyclins. Different cyclins present during different cell cycle phases each activating one or more specific cyclin-dependent kinases.
How do CDK work?
Phosphorylate specific proteins including enzymes, transcription factors and cytoskeletal subunits triggering activities in the phase.
What happens after the phase is completed?
CDKs are ubiquitinated and broken down by proteasomes and a new cyclin for the next phase takes over.
Gene that encodes for an important CDK?
CDC2
Why is CDC2 important?
Codes for a CDK that appears both at G1 and G2 phase
How was CDC2 proven to checkpoint two phases?
Lee and Nurse
Known already that CDC2 mutants
6 stages of mitosis
Prophase, pro metaphase, metaphase, anaphase, telophase and cytokinesis
Explain each stage of mitosis and their appearance
prophase- nucleolus disappears, replicated chromatin condenses, into threadlike chromosomes, with the sister chromatids attached to one another at the centromere. Two centrosomes with their duplicated centrioles move to opposite poles. Late in prophase lamina and inner nuclear membrane are phosphorylated causing the nuclear envelope to disappear into vesicles
Prometaphase- Late in prophase lamina and inner nuclear membrane are phosphorylated causing the nuclear envelope to disappear into vesicles
metaphase- chromosomes condense further into large protein complexes called kinetochores. Microtubules produced by centrioles move the chromosomes to the equatorial plate
anaphase- sister chromatids (bound by cohesin protein) separate and move to opposite poles- by a combination of microtubule motor proteins and dynamic changes in length of the spindle.
Telophase- two sets of chromosomes begin reverting to their uncondensed state. Microtubules depolymerise, two nuclear envelopes begin to reform.
Cytokinesis- belt like contractile ring of actin filaments forms at the cell’s equator. Constriction of the ring produces a cleave furrow and progresses until organelles and cytoplasm are divided, with each daughter cell containing a nucleus.
Meiosis definition
Specialised process involving two closely associated cell divisions that occurs only in the cells that will form sperm and egg cells
What are sperm and egg cells called?
Gametes
What occurs during sexual reproduction?
Two gametes fuse to form offspring with a new gene combination, derived from both parents.
Two features that characterise meiosis
Early in meiosis, homologous chromosomes of each pair come together in a process known as synapsis where double stranded breaks and repairs may result in DNA exchange known as crossovers.
Cells produced are haploid, meaning they have half the genetic material of a diploid cell (23 chromosomes in humans)
What is the benefit of crossovers?
Produce new combinations of genes in the chromosomes increasing genetic diversity.
Two stages of meiosis
Meiosis 1 and meiosis 2
Meiosis 1 explained
Division where homologous pairs are pulled to separate poles.-producing two identical diploid cells
What is a tetrad?
Term used to describe synaptic chromosomes, as there are 4 copies of each genetic sequence present in each homologous pair (due to the sister chromatid formation)
Meiosis 2 explained
Each of the cells divide again- chromatids separate- form 4 haploid cells in total. Resembles mitosis
S phases in both meiosis 1 and 2
meiosis 1- normal S phase
meiosis 2- no intervening s phase
How is cell proliferation measured?
Radioactively tag DNA using 3H-labeled thymidine. Able to see the formation of new DNA molecules, which occurred in S phase. Increase in S phase is synonymous with increase cell proliferation.
What did Walther Flemming do?
Used aniline dyes to see structures that absorbed basophilic dyes, which he named chromatin. Studied cell division in vivo and in stained preparation using material from the fins and gills of salamanders. Drew out the different stages of mitosis.
