The Cell Cycle Flashcards
What are cells that continue to divide regularly in definite intervals described as?
Cycling cells.
What are cyclins?
A family of proteins whose concentration increases and decreases throughout the cell cycle.
What are the cell cycle proteins molecules?
Cyclins A,B, and E.
What do cyclins activate?
Cyclin dependent kinases (CDKs) which act in conjunction with cyclins.
Upon completion of cell cycle, what happens to cyclins and CDKs?
They are degraded: residues of used molecules are taken up by cytoplasmic care taker protein; ubiquitin.
What is interphase?
The interval between the end of mitosis and beginning of next. Cell is either resting or performing its specialised work not in mitosis.
What happens in S phase?
Relation of DNA to genes is established/ new DNA is synthesized. DNA would have to be duplicated (replication) sometime in the cell cycle to have enough genes for two daughter cells. DNA is duplicated in interphase long before the chromosomes are detected as thread like bodies.
What is S phase promoted by?
Cyclin A and CDKs.
When does S phase begin and how long does it take?
8 hours after the end of mitosis and takes about 7-8 hours to complete.
What happens when crucial steps in the cell cycle, such as DNA replication is not completed?
The cell cycle will be arrested.
What is the common axis where the two chains of DNA are coiled around called?
Helical axis.
How are the DNA chains paired?
In an antiparallel manner.
Which side is the hydrophilic deoxyribose-phosphate backbone of each chain located?
The outside.
Which side are the hydrophobic bases stacked?
The inside.
What does the spatial relationship between the two strands in the helix create?
A major and minor groove.
What does anticancer drugs such as dactinomycin (actinomycin D) do?
Exert their cytotoxic effect by intercalating into the narrow groove of the DNA double helix, thereby interfering with DNA synthesis.
What does DNA damage during DNA replication cause?
Deoxynucleotide incorporation error during replication by spontaneous deamination of bases during normal genetic functions. From X-radiation that cause ‘nicks’ in the DNA. From UV irradiation. From various chemicals that interact with DNA.
What is Xeroderma Pigmentosum?
DNA repair mechanisms are defective. DNA damage produced by UV irradiation. Results from inborn deficiency of the enzyme “nicking endonuclease”.
What are examples of Xeroderma Pigmentosum?
Dry keratosis, hyperpigmentation, and skin atrophy. Increased cutaneous sensitivity to UV light. Ulcers in the cornea could form.
What is developed from Xeroderma Pigmentosum?
Squamous cell carcinoma of the skin.
What is homologous recombination (HR)?
A major pathway for the repair of DNA double-strand breaks. The defining step of which is homologous directed by the RAD51 protein.
What is required for HR?
Hereditary breast cancer suppressors BRCA1 and BRCA2.
What is linked to tumour predisposition?
Mutations in a number of HR genes.
What is Hereditary Nonpolyposis Colon Cancer (HNPCC)?
Mismatch repair. Chromosome 2 has been shown to contain a genetic error; the affected gene is known as hMSH 2.
What does mutations of hMSH 2 account for?
50-60% of HNPCC cases.
What is G1 phase?
The period when cells respond to growth factors directing the cell to initiate another cycle; once made, this decision is irreversible. It is also the phase in which most of the molecular machinery required to complete another cell cycle is generated.
What is the G1 phase controlled by?
Cyclin E and CDKs.
How long does G1 phase last?
Considerable variation, sometimes ranging from less than 2 hrs in rapidly dividing cells, to more than 100 hrs within the same tissue.
What is G0 phase?
Cells that retain the capacity for proliferation and in the process of differentiation, but which are no longer dividing, enter this phase. They are described as quiescent and do not enter S phase.
What is the role of growth factors in G0 phase?
They can stimulate quiescent cells to leave G0 and re-enter the cell cycle.
What is the role of proteins encoded by certain tumour suppressor (eg the gene mutated in retinoblastoma, Rb) genes in G1 phase?
Block the cycle in G1.
What is G2 phase?
The cell prepares for division; this period ends with the breakdown of the nuclear membrane and the onset of chromosome condensation.
What is the G2 phase controlled by?
Cyclin B and CDKs.
How long does G2 phase last?
2-4 hrs to complete.
State 4 facts about interphase.
Individual chromosome are not visible. They are uncoiled into very long narrow structures.
The nucleus of the cell possess a nuclear envelope and contains a network of chromatin threads or granules and a nucleolus.
Replication of DNA takes place in chromosome.
Outside the nuclear envelope two paired centrioles are located in cytoplasm.
What happens in prophase?
The individual chromosome are first visualized as thin threads and are tightly coiled along the entire length. Each chromosome splits longitudinally into two chromatids except at the centromere. The two paired centriole are separated from each other by elongation of continuous microtubules of the achromatic spindle and occupy the opposite pole of nucleus.
What happens in prometaphase?
Subsequently, the nuclear membrane and nucleolus disappear. The chromosomes are now entangled in a meshwork of continuous microtubules.
How long does prophase take to complete?
About 1.5hrs.
What happens in metaphase?
Centromere region from each chromosome presents a bilateral disc Kinetochores. From kinetochores, a set of chromosomal microtubules are organized and extend bilaterally towards opposite centrioles. The growth of the chromosomal microtubules allows centromere of individual chromosomes with their paired chromatids to occupy equatorial plane of the spindle.
How long does metaphase last?
Persists for 20mins.
What is the role of colchicine?
Colchicine arrest the cell division at metaphase - prevents the formation of microtubules of the spindle. In colchicine treated metaphase the chromosomes are found to assemble around centriole. Application of colchicine in cell culture is a important tool in karyotyping - this underpins the rationale for many types of cytotoxic drugs used in cancer therapy.
What happens in anaphase?
The centromeres split longitudinally and chromatids separate to form two new chromosome. The spindle fibres contract, each pair of newly formed chromosomes separates and migrates to the opposite pole of the spindle. Two identical and complete chromosomes are formed.
The separation of the chromatids to form new chromosomes occurs by what?
Contraction of chromosomal microtubules which exert a centrifugal force in centromere region.
What is non-disjunction?
Abnormal function of spindle apparatus one or more chromosomes fail to migrate properly in anaphase. One daughter cell receives extra chromosomes and other daughter cell is deficient in this chromosome.
What is isochromosome?
The centromere splits transversely instead of longitudinal division.
This leads to formation of two daughter chromosome of unequal length.
What happens in telophase (in progress)?
A reorganization period - the daughter chromosomes are enveloped by new nuclear membrane and nucleolus appears. The chromosomes become uncoiled and their individual identity is lost.
The cytoplasm divides and two complete cells are formed.
What happens in telophase (complete)?
In the process of cytokinesis, the cleavage furrow develops around the equator region of achromatic spindle. The contraction of fibrillar component of cytoplasm help in hour glass constriction of cleavage.
