Cell Cycle, Mitosis, Meiosis, and Recombination Flashcards
Outline the basic phases of the cell cycle and the processes that take place during each phase.
Cell division is the process by which a parent cell divides into two or more daughter cells by progression through the cell cycle.
G0 - A resting phase where the cell has left the cycle and has stopped dividing (quiescent / senescent).
G1 - Growth phase during which proteins and RNA are synthesised. Each chromosome exists as a single double stranded helix - at no point is DNA synthesised in this phase. At the G1 checkpoint - the restriction point - the cell is committed to division and moves into the S phase. 9-12h.
S - DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids. 6-8h.
G2 - Cell continues to grow. The G2 checkpoint ensures enough cytoplasmic materials necessary for mitosis and cytokinesis. 2-5h.
Note: G1, S, and G2 collectively form interphase.
M - The cell stops growing. Nuclear division (mitosis) followed by a cell division (cytokinesis). The Metaphase checkpoint in the middle of mitosis ensures that the cell is ready to complete cell division. 1h.
*Average duration for lymphocytes in culture is ~24 hours; this varies for different cell types.
What is the average duration of the cell cycle for lymphocytes in culture?
The average duration for lymphocytes in culture is ~24 hours; this varies for different cell types.
What is mitosis?
Mitosis is part of the cell cycle in which chromosomes in the nucleus are separated into two identical sets of chromosomes contained within their own nucleus.
What are cell cycle checkpoints?
Cell cycle checkpoints are regulatory pathways that control the order and timing of cell cycle transitions ensuring critical events are completed with high fidelity.
How is the cell cycle regulated?
The cell cycle is regulated by heterodimeric protein kinases composed of:
Cyclins
Cyclin-dependent kinases (CDKs)
What are cyclins?
Cyclins - Form the regulatory subunit and have no catalytic activity
What are Cyclin-dependent kinases (CDKs)?
Inactive in the absence of a partner cyclin. Becomes the catalytic subunit (serine/threonine protein kinases) of an activated heterodimer which phosphorylates target proteins to orchestrate coordinated entry into the next phase of the cell cycle
When are CDKs and cyclins expressed?
CDKs are constitutively expressed whereas cyclins are synthesised at specific stages in response to various external stimuli / molecular signals.
Why are checkpoints important?
Checkpoints are essential to ensure that the cell cycle halts if chromosomal DNA is damaged or critical processes such as DNA replication or alignment have not been completed properly.
What are the three main checkpoints?
G2/M Checkpoint
G1/S (restriction) Checkpoint
Metaphase / Spindle Checkpoint
G2/M Checkpoint
CDK1 is activated by phosphorylation and de-phosphorylation of specific amino-acid residues by Cyclin-Activating Kinase (CAK, as well as the inhibition of the wee1 protein (which inhibits CDK1)
Enables CDK1-cyclin B formation (aka MPF)
Allows G2-M phase transition
G1/S (restriction) Checkpoint
Cell growth enables formation of the CDK4/6-cyclin D
Phosphotylates retinoblastoma protein
Relieves inhibition of E2F transcription factor
Cyclin E now expressed, binds to CDK2
Metaphase / Spindle Checkpoint
Chromosomes assemble on metaphase plate
Anaphase-promoting complex (APC) activated
Degrades cyclin B = MPF disassembly
Relieves inhibition of ‘separase’ which cuts cohesion
Sister chromatid separation = anaphase entry
How do Cytogeneticists manipulate the cell cycle?
Mitogens - used to induce division of resting cells:(PHA, pokeweed, concanavilin A)
Synchronisation - Inhibitors block cell cycle during S phase by slowing/stopping DNA synthesis (FudR/uridine, Thymidine).
Block released after 16-22h - cells continue through G2 together
Mitotic arrestants - stop division during mitosis (colchicine/Colcemid®). Prevents spindle fibre apparatus formation. Stops cell at metaphase.
What does p53 do to checkpoints?
p53 (“the guardian of the genome”) plays an important role in controlling progression through G1/S and G2/M checkpoints. P53 is a critical component of DNA damage checkpoints
DNA damage -> Activated p53-> Inhibits progression through checkpoint
What is meiosis?
Meiosis is a specialised type of cell division that reduces the chromosome number by half.
What are the broad stages of meiosis?
Interphase (G1, S and G2 )- this is followed by two rounds of cell division (meiosis I and meiosis II) to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell. The daughter cells are not genetically identical to the parent cells (unlike mitosis).
Describe Prophase I of meiosis
Prophase I
Leptotene
Nuclear chromatin begins to condense/become visible. Chromosomes are unpaired fine threads consisting of 2 tightly bound sister chromatids (AKA “A string with beads.”)
Zygotene
Maternal and paternal homologues pair to form bivalents- held together by formation of synaptonemal complex. In males, X and Y condense to form sex vesicle. The 2 sets of sister chromatids is termed a bivalent and the connecting points are chiasmata.
Pachytene (early)
All homologues have paired (synapsis). Bivalent at this stage is called a tetrad.
Pachytene (late)
Chromosomes thicken, cross over and recombination of genetic material occurs. Two non-sister chromatids cross over, the other 2 remain unaltered. Approx 60 (sperm) to 90 (ova) crossover events occur per cell with at least one per chromosome arm.
Diplotene
Homologues start to separate (desynapsis) but are held together by chiasmata- may resemble cross-like appearance. Sex vesicle disappears and X and Y appear associated end to end. The small chiasma formed is enough to keep X/Y chromosomes paired on spindle during metaphase- resulting sperm cells will have either X or Y chromosome.
Diakinesis
Bivalents are more contracted. Nuclear envelope breaks down. Oocytes reach this stage at ovulation
Describe Metaphase I of meiosis.
Spindle formed, bivalents align along metaphase plate. Organisation critical to ensure one copy of each chromosome is received in each new nucleus. In females, spindle is off centre and one resultant cell will contain more cytoplasm. X and Y may separate to form univalent
Describe Anaphase I of meiosis.
Homologous chromosomes drawn apart. Chromatids remain together
Describe Telophase I of meiosis.
Chromosomes at poles. Haploid daughter cells formed. In females, the cell with larger amount of cytoplasm is called secondary oocyte and the smaller one is first polar body.
Describe Prophase/ Metaphase II of meiosis.
Cells pass directly from meiosis I to metaphase II with no real prophase II. Occurs immediately in males. In females it is co-ordinated with ovulation and fertilisation. Nuclear envelope breaks down, new spindle formed, chromosomes (consisting of 2 chromatids) align.
Describe Anaphase II of meiosis.
Separation of centromeres, migration of sister chromatids to opposite poles
Describe Telophase II of meiosis.
Further cell division occurs forming 2 haploid cells (4 in total). In secondary oocyte, spindle is again off centre resulting in a large cell (ovum) and second polar body. Two more polar bodies may arise from the first but the ovum is the only viable gamete.
What is Gametogenesis ?
Gametogenesis occurs by meiotic division of diploid gametocytes into various gametes
Begins in utero for females (Oogenesis) and puberty in males (Spermatogenesis)
What is MI (meiosis 1) ?
MI (meiosis 1) – random independent assortment of chromosome pairs and crossover enables genetic recombination prior to separation
What is MII (meiosis 2)?
MII (meiosis 2) – separation of chromatids (to haploid state)
How diverse can human gametes be?
Independent assortment of maternal and paternal homologues in human cells creates the possibility of 2^23 (8.4 million) genetically different gametes
Recombination of genetic material via crossing over forms daughter cells that contain composites of both parental genes; therefore, the actual number of genetically different gametes is infinite.
Both of the above increase genetic diversity between daughter cells
What does genetic recombination involve?
Recombination takes place during prophase I stage of meiosis. Homologous chromosomes line up in pairs and swap segments of DNA, i.e., “crossing over”.
Genetic recombination involves :
Alignment of two homologous DNA strands
Precise breakage of each strand
Equal exchange of DNA segments between the two strands
Sealing of the resultant recombined DNA molecules through the action of enzymes called ligases.
What are chaisma?
Recombined homologs are physically connected at specific points which mark the location of the crossover. These points are known as chaisma. There are on average 55 chiasmata per cell in male meiosis and possibly 50% more in female meiosis. Chiasmata are also thought to be essential for correct chromosome segregation in meiosis I.
Give an overview of meiotic recombination
Recombination is initiated by DNA double-strand breaks
Ends are rapidly processed to form long single-stranded tails with 3’-termini
Inter-homolog single-end invasions (SEI) by the single-stranded tails takes place
SEI results in double-Holliday junctions (DHJ) formation
DHJs are resolved into cross-over products
Potential consequences of recombination
Non–allelic homologous recombination (NAHR)
Single gene disorders (Mendelian disorder) - where recombination results in the deletion or duplication of a single gene (autosomal dominant/recessive and X-linked) such as Rubinstein–Taybi
Contiguous gene disorders – caused by chromosome abnormalities where several genes are deleted/duplicated resulting in alteration to normal gene dosage such as Wilms’ tumor Aniridia Genitourinary abnormalities and mental Retardation (WAGR)(11p13)
Segmental aneuploidy (aneusomy) syndromes – refers to the relative excess or deficiency of specific chromosome regions; contiguous gene syndromes with well-defined phenotypes such as Di George/VeloCardioFacial Syndrome (VCFS)/22q11.2 deletion or Prader-Willi Syndrome (PWS) (paternal 70%/maternal 25%/genomic imprinting defects), these recur due to non-allele homologous recombination between low copy repeats
