Module 10

Gap 1 phase

first of 4 phases of interphase of the cell cycle in this part the cell synthesizes mRNA and proteins in preparation for subsequent steps leading to mitosis.

meiosis

involves one round of DNA replication and two rounds of cell division. sexual reproduction where gametes from two organisms combine (fertilization) resulting in a 2n zygote. 2 cycles of mitosis are performed after interphase (meiosis I and meiosis II) results in 4 haploid (n) genetically distinct daughter cells

Provide information about the following: the roles of kinases in cell cycles (as activators of other proteins) and why they are called cyclin-dependent

kinases (Cdk) are what signal that the cell is ready to pass on to the next step of the cell cycle. For only with activation of Cdks will the cell be propelled into the next phase. Cdks are often associated with activation of other proteins as well. For example for the cell to enter into the S phase (replication phase) of the cell cycle a transcription regulator must be activated, as it is bound to an Rb protein. While bound to the active Rb protein the transcription regulator cannot bind to the DNA strand and begin transcription. It is only due to the activation of the G1-Cdk and G1/S-Cdk as a result to cyclin binding onto the Cdk. Cyclin production is promoted by the cell signalling done by the binding mitogen. Only now can the Rb protein be phosphorylated (causing a conformational change) by the G1-Cdk and G1/S-Cdk. In turn releasing the transcription regulator and allowing for transcription/translation to proceed. Thus driving the cell cycle forward Kinases are called cyclin dependent because only when kinases have been bound with cyclin will they become activated. Thus the cell cycle seems to be cyclin dependent as the cell cycle can not proceed without activation of proteins by kinases, and kinases cannot become activated without being bound to cyclin.

cyclin-dependent kinases (Cdk)

known for their role in regulating the cell cycle a family of sugar kinases by definition Cdk binds a regulatory proton called cyclin. Without cyclin, Cdk has little kinase activity. Only the cyclin-Cdk complex is an active kinase. Cdk concentrations change over the course of the cell cycle

interpolar microtubule

microtubules produced by centrosomes for use in the separation of chromatids during anaphase and generated in prophase/prometaphase. a sliding force is generated between interpolar microtubules from opposite poles to push the poles apart via microtubule growth at the plus end of interpolar microtubules.

meiosis II

no DNA replication consists of Prophase II, metaphase II, anaphase II and cytokinesis II results in four cells with one set (n) of single-copy chromosomes

fertilization

occurs during meiosis. In which two haploid gametes from two organisms combine (fertilization) resulting in diploid (2n) zygote

crossing over of chromosomes

occurs during metaphase I of meiosis during which chromosomes are aligned along the equatorial plate. Crossing over causes genetic variation in sexual reproduction. Which means that daughter cells have different ploidy to that of parents as well.

Gap 2 phase

the second subphase of interphase in the cell cycle directly preceding mitosis, and following S phase. During G2 the cell has already replicated its DNA in S phase and thus will continue to grow and produce new proteins ensuring the cell is completely prepared for mitosis.

homologous recombination of chromosomes

type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical DNA molecules.

zygote

a 2n result from a fertilization event between that of two parental haploid (n) gametes. The zygotes genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual.

mitogen

a chemical substance found in G1 phase, that encourages the production of cyclins and stimulates cell division, thus triggering mitosis. A mitogen is usually some form of a protein. mitogenesis is the induction (triggering) of mitosis, typically via a mitogen mitogens will bind to a mitogen receptor on a cell, activating it in the process. This activation causes the use of an intracellular signalling pathway into the cell thus causing the activation of G1-Cdk and G1/S-Cdk, which in turn causes the steps of cell division to begin.

kinetochore

a complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach. another word for centromere

cyclin

a family of proteins that control the progression of cell through the cell cycle by activating Cdk enzymes

gamete

a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to from a zygote

cyclin-Cdk inhibitor protein

a protein that inhibits Cdk activation which would halt the cell cycle. the cell cycle is controlled through the use of Cdks (cyclin-dependent kinases) they are cyclically activated activate key proteins in cell cycle processes cyclin protein concentrations change over cell cycle different cyclins regulate different phases of cell cycle (e.g. M-cyclin control mitosis)

germ cell

a reproductive cell

mitotic spindle

a structure made of microtubules that controls chromosome movement during mitosis

centrosome

an organelle that is the main place where cell microtubules are organized. Also it regulates the cell division cycle, the stage which lead up to cell division.

haploid

and organism having (n) number of chromosomes an organism or cell having only one complete set of chromosomes

anaphase

at anaphase the sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached. The kinetochore microtubules get shorter, and the spindle poles also move apart. A sliding force is generated between interpolar microtubules from opposite poles to push the poles apart. A pulling force acts to pull the poles toward the cell cortex, thereby moving the two poles apart. With microtubule growth at plus end of interpolar microtubules. All contributing to chromosome segregation.

centromere (of chromosome)

attachement point for kinetochore microtubules in chromosomes. Seen as the center point of the X in a chromosome.

diploid

being 2n, where n is the number of chromosomes in a particular organism

kinetochore microtubule

bind to the centrosomes of chromatids during prophase/prometaphase and pull chromatids apart during anaphase. Via shortening of kinetochore microtubules forces are generated at kinetochores to move chromosomes towards spindle poles.

mitosis

cell cloning interphase (G1, S, G2, M) must be completed properly before the cell will enter mitosis phases: prophase metaphase anaphase cytokinesis a 2n zygote is cloned into two 2n daughter cells (2n->4n->2n0

cell cycle checkpoint

cell cycle can be paused at checkpoints. G1 check point (integrity of DNA is assessed and determined whether to enter into G0 or S phase) G2 checkpoint (ensures proper chromosomes duplication has been performed) M checkpoint (attachment of each kinetochore to a spindle fiber is assessed)

How are chromosomes packaged for cell division (compare this to normal G0 DNA structure)?

chromosomes are condensed during the prophase of mitosis. This is accomplished in conjunction with cohesin and condensin. For it is with the use of condensin that it loops and condenses the strands of DNA into the arms of the stereotypical chromosome. In addition to this cohesin is used to bind the two sister chromatids together at their centromeres. When you compare with with the normal G0 DNA structure is is noted that the DNA structure present in G0 is that of uncondensed DNA strands like spaghetti in the nucleus because the cell is lying dormant before entering into the cell cycle.

metaphase

chromosomes line up along the metaphase plate, where kinetochore microtubules from opposite centromere poles attach to the centrosome of the aligned chromosomes in preparation for separation of sister chromatids in anaphase. Interpolar microtubules also connect at the metaphase plate in order to generate a sliding force via microtubule growth at the plus end aiding is pushing the sister chromatids apart. Aster microtubules generate a pulling force on the kinetochore in turn pulling the chromosomes apart. The enzyme separase then hydrolyzes cohesin, releasing the sister chromatids from one another, thus propelling the cell into anaphase.

The roles of cyclin proteins in regulating cell cycle - example: what happens to non proliferating chicken oocyte if it is injected with cytoplasmic material from an M-phase oocyte?

cyclins are cell cycle signals different cyclin-Cdk complexes trigger different steps in the cell cycle and are found at different concentrations throughout the cell cycle. For example the two primary cyclins present during the cell cycle are S-Cdk and M-Cdk. As S-Cdk is required to drive the cell cycle out of G1 into/through S phase and into G2 phase it is found in higher concentrations midway through G1 -> the beginning of M phase. In contrast since M-Cdk is required to drive the cell cycle from G2 into M phase it is not found in concentration until the beginning of G2 phase and increases in concentration through to the middle of M phase. Should you inject cytoplasm from an M-phase cell in that of an oocyte the oocyte will be driven into M phase. Where as if you injected cytoplasm from a cell in interphase there would be no effect on the oocyte.

S phase

during S phase replication occurs Cdc6 bound to the ORC (origin recognition complex sitting on origin) is dissociated with the association of helicase to binding slightly up stream of the ORC. Resulting in the generation of the prereplicative complex (pre-RC). S-Cdk activates helicase and recruits the appropriate replication machinery. Helicase unwinds the DNA allowing for a replication fork to be generated and for DNA polymerase to begin making the leading a lagging stands of the replicating DNA. Once the replication forks reach the end of the DNA strands replication has completed and the cell is ready to enter the G2 phase.

contractile ring

during telophase the division of the cytoplasm begins with the assembly of the contractile ring. During cytokinesis, the cytoplasm is divided in two by the contractile ring. Composed of actin an myosin filaments the contractile ring pinch the cell into two daughter cells each with their own nucleus.

Explain two kinds of regulation of Cdk activity: - regulatory kinases and phosphates - cyclin-Cdk-complex inhibitor proteins

- Kinases add phosphate groups to molecules, and phosphatase removes phosphate groups from molecules. Phosphorylation and dephosphorylation is a common mechanism for regulation of enzymes. This will in turn in many molecules cause a conformational change, resulting in activation or deactivation of a particular process. For example transcription is inhibited by an active Rb protein which deactivates a transcription regulator. Only upon G1-Cdk and G1/S-Cdk being activated by cyclin can a phosphate group be added to the Rb protein. This in turn causes a conformational change in the Rb protein which releases its hold on the transcription regulator. With its release it is able to facilitate transcription. - these are a class of proteins that can block cyclin-Cdk complexes and inactivate them causing the cell cycle to pause. p27 is an example of a cyclin-Cdk-complex inhibitor which generates an inactive p27-cyclin-Cdk- complex when bound around a cyclin-Cdk.

Why is cell signalling important for cell division? - describe why cell cycle might be influenced by the external environment or by signals from other cells? - what is a mitogen

- control of the cell cycle is very important and dependent on signals from within cells and from outside. Cell signalling is a cellular communication action between cells, which senses the external and internal environment/conditions and responding appropriately. It is only when the appropriate signals are present that the cell will begin the cell cycle. If all checkpoints have been passed then the cell will carry out the entire process; one parent cell-> two daughter cells. But should something happen to disrupt the checkpoints cell signalling will prevent the cell from further processing until deeper appropriate. For example should DNA become damaged due to x-rays or external environmental conditions can temporarily halt progression of G1 phase. Cells can delay division for prolonged periods by entering specialized non dividing states (G0). It is because of the damaging to DNA that the cell signals for the activation of kinases for phosphorylation of p53 allowing it to bind to the regulatory region of the p21 gene and halt replication until signalled to do otherwise. - mitogens promote the production of cyclins that stimulate cell division. Through the use of cell signalling. For mitogen binds to a mitogen receptor which in turn activates it, generating an intracellular signalling pathway which activates G1-Cdk and G1/S-Cdk and causes the phosphorylation of the Rb protein allowing for the activation of a transcription regulator.

nondisjunction

Error in meiosis in which homologous chromosomes fail to separate.

M-Phase: Explain the entire process, including mitosis and cytokinesis

Interphase is the first stage of the cell cycle, broken into G1/G0, S, and G2 phase. In G1 the cell will grow and synthesize mRNA for later use in subsequent stages. Once the cell has grown enough it will enter into S phase where DNA is replicated. Following S phase the cell enters into G2 phase where the cell will continue to grow further as well as ensure that all DNA has been properly replicated before entering into M phase. Once the cell has completed G1, S and G2 phase the cell will enter into M phase. The mitotic phase of the cell cycle. Where the 2n parental cell will be genetically copied to create two genetically identical 2n daughter cells. M phase is broken into multiple steps; prophase, metaphase, anaphase, and telophase/cytokinesis. During prophase the nuclear envelope beings to degrade and the chromosomes condense (via condensin and cohesin) as the centrosomes are moved to opposite poles and spindle fibers are developed. The cell then moves into metaphase where the chromosomes become aligned along the metaphase plate and kinetochore microtubules attach to the centromere of the chromosomes. During anaphase the kinetochore microtubules pull the sister chromatids to opposite poles of the cell. Once the sister chromatids are in position on opposite poles the cell enters into telophase/cytokinesis. Where a contractile ring composed of actin and myosin filaments surrounds the interpolar microtubules still connected between the two kinetochores and the nuclear envelope reassembles. As the contractile ring begins to pinch off between the two cells they undergo cytokinesis in which the two cells are cleaved from one another resulting in two genetically identical 2n daughter cells to that of the parent cell.

Describe the location, activities and movement of the key cytoskeleton and chromosome structures during each of the phases of mitosis: 1) prophase 2) metaphase 3) anaphase 4) telophase

1) nucelar envelope degrades, chromosomes condense, kinetochores move to opposite poles, spindle fibers are formed 2) condensed chromosomes line up along metaphase plate, kinetochore microtubules attach to centromere of chromosomes, interpolar microtubules attach and grow to produce side force which helps push sister chromatids apart, aster microtubules anchor kinetochore and pull to aid in sister chromatid separation, separase dissociates cohesins 3) sister chromatids are pulled to opposite poles, interpolar microtubules remain connected between opposing kinetochores but push sister chromatids apart via plus end growth, aster microtubules pull in kinetochores to help in separation of sister chromatids, actin and myosin filaments create contractile ring around interpolar microtubules 4) nuclear envelope is reassembled, contractile ring tightens around interpolar microtubules causing cell to pinch, kinetochores attached to sister chromatids are on opposite poles, cytokinesis occurs at site of contractile ring, resulting in two genetically identical daughter cells

somatic cell

Any of the cells of a plant or animal except the reproductive cells.

What different structure are present that take part in plant cytokinesis?

At the end of telophase instead of having a contractile ring generated golgi derived vesicles arrive at the metaphase plate. These golgi derived vesicles and associated microtubules are known as a phragmoplasts. These are used to generate a new cell wall between that of the two new daughter cells. Thus completing cytokinesis in plant cells.

regulatory phosphatase

Cdk is activity regulated phosphatase results in the dephosphorylation of an inactive M-Cdk thus activating it and proceeding the cell cycle further

regulatory kinase

Cdk is activity regulated regulatory kinases result in the phosphorylation of mitotic Cdk which in turn generates an inactivated Cdk thus halting the cell cycle.

condensin and cohesin

Condensin is a 5 subunit complex, conserved among eukaryotes and is the key molecular machine of chromosome condensation. Promotes chromosomes compaction by linking two distant segments of a single chromatid. Cohesin maintains sister chromatid cohesion mainly at centromeres. during M phase cohesions and condensin organize duplicated chromosomes for separation. proteolysis of condensin and cohesion triggers separation of sister chromatids in anaphase.

meiosis I

Consists of prophase I, metaphase I, anaphase I and cytokinesis I. crossing over occurs during prophase I/metaphase I chromosome pairing and crossing over ensure the proper segregation of homologs in anaphase I results in two cells with one set (n) of duplicated chromosomes (sister chromatids)

DNA repair

DNA damage can temporarily halt progression through G1. DNA damage can be caused by a plethora of things. But should it occur it needs to be repaired quickly. DNA damage causes the activation of kinases that phosphorylate p53 which stabilizes and activates it which allows it to to bind to the regulatory region of the p21 gene (CDK-inhibitor protein) and halts cell division due to the damaged DNA. The cell does not want extra copies of chromsomes. Transcription of the p21 gene into p21 mRNA and translation of the p21 mRNA results in production of p21 CDK inhibitor protein. S-Cdk initiates DNA replication and block re-replication, as it activates DNA helicase for unwinding and repair of the damaged DNA. S-Cdk targets initiator protein (Cdc6) for destruction and allows helicase to bind. The p21 inhibitor protein translated from the p21 gene is used to control the activation of the S-Cdk. As incomplete replication can arrest the cell cycle in G2. allowing for the cell to finish DNA replication and complete the DNA repairs.

Give examples of important checkpoints in the cell cycle. - what might halt cell cycle progress at G1 and prevent entry into S-phase - what might halt cell cycle progress at S-phase and prevent entry into G2?

DNA damage may cause the cell cycle to halt in G1 as the DNA is not properly prepared to undergo replication, for if it did it would result in incorrect replication. Errors in the replication of the DNA during S phase could cause the cell cycle to halt. Similarly with above if the cell cycle was allowed to continue without DNA editing of the replication errors the cell would undergo M phase and create genetically identical daughter cells, both of which would be defective and in turn would cause great genetic disruption for the organism as this effect is compounded over multiple trips through the cell cycle.

DNA replication

DNA replication occurs in S phase of the cell cycle. G1 phase must be completed correctly first. at the beginning of G1, the ORC (origin recognition complex sitting on origin of replication) is bound with Cdc6. As helicase binds to the DNA Cdc6 dissociates, allowing for the prereplicative complex (pre-RC) to be formed. At the beginning of S phase integration of active S-Cdk activates replication machine. Generating a replication fork and recruiting the designated DNA polymerase for creation of growing and lagging strands of replicated DNA. Eventually resulting in completion of DNA replication.

prophase

Degradation of the nuclear envelope begins and condensation of chromosomes into typical X shape occurs, via the use of condensin to produce to the cylindrical arms/bodies of the sister chromatids and cohesin to bind the two sister chromatids together at their centromere. Outside the nucleus the mitotic spindle assembles between the two centrosomes, which have begun to move apart.

M phase

Mitosis and cytokinesis. controlled by M-Cdk cohesions and condensing organize duplicated chromosomes for separation M phase occurs in stages: prophase metaphase anaphase telophase followed by cytokinesis results in conservation of 2n diploid cells (2n->4n->2n) and 2 genetically identical daughter cells to that of the parent cells.

What are the phases of the eukaryotic cell cycle? Identify the main activates related to cell division that occur in: - G0 phase (hint: this is not really part of cell division and reproduction) - interphase: -- G1 phase -- S phase -- G2 phase - M phase: -- mitosis (nuclear divison) -- cytokinesis

Phases: interphase(G1, G0, S, G2) M phase broken down into: prophase metaphase anaphase telophase cytokinesis G0 Phase: This a a non-dividing stage of the cell cycle that occurs just before the cell would enter G1 phase. This allows for the cell can lie dormant without having to undergo cellular replication in preparation to be sent through G1. Interphase: - G1 phase during this phase of the cell cycle Cdks are stably inactivated and the cell grows and synthesizes mRNAs in preparation for subsequent phases. inactivated helicase binds upstream of the ORC and generates the inactivated pre-RC - S phase it is during S phase that replication takes place. S-Cdk activates helicase which causes recruitment of replication machinery and DNA replication is performed. Generating new genetically identical DNA strands. - G2 phase follows after S phase has completed and directly precedes mitosis. During this phase the cell will continue to grow/produce new proteins and ensure that no mistakes have been made the gene sequences during DNA replication. If the cell is up to standards then it will commence in mitosis. M phase: - mitosis is nuclear division resulting in the genetic cloning of a 2n diploid parent cell with conservation of chromosome numbers resulting in a 2n daughter cell (2n->4n->2n). Mitosis is broken down into parts; prophase, metaphase, anaphase and telophase/cytokinesis. The process of mitosis in a sparks notes version goes as such. Spindle poles and fibers are formed and move to opposite poles with the initial degradation of the nuclear envelope as well as the condensation of chromosomes via cohesins and condensins. Chromosomes are then organized along the metaphase plate where kinetochore microtubules from opposite poles bind to the centromeres of the condensed chromosomes. interpolar microtubules help to push the sister chromatids apart via using a slide force generated via the addition of tubulin to the plus end of the growing microtubules. Once the chromosomes and fibers are oriented appropriately the sister chromatids are pulled to opposite poles of the cell. Upon arrival at opposite poles the nuclear envelope begins to reassemble all the while actin an myosin form a contractile ring. The contractile ring is the primary force in the final step of mitosis, cytokinesis, which separates the two genetically identical cells. Each as 2n diploid cells. - Cytokinesis: the final stage of the cell cycle is cellular division. As the contractile rings generated during telophase encircle the mitotic spindle the cells get pinched into two daughter cells, each with one nucleus. The mitotic spindle determines the plane of cytoplasmic cleavage. The contractile ring is only present in animal cells. Cytokinesis in plant cell involves the formation of a new cell wall along the metaphase plate. Called a phragmoplast.

Why do different cyclins exist (provide examples of different cyclins), and how do their levels relate to progress through the cell cycle.

different cyclins exist because different cyclins are associated with different aspects of activation in the cell cycle. As cyclins activate kinases which become the appropriate Cdk necessary for their particular step in the cell cycle. Hence why particular cyclin concentrations are regulated throughout the cell cycle. For example M-cyclin is needed to propel the cell from G2 into M phase. Another cyclin known as S-cylin is required for driving the cell cycle from G1 through to G2. There is also G1-Cdk which is needed in conjunction with G1/S-Cdk for the cell cycle to pass out of G1 phase. These cyclins will not be produced though without the appropriate signalling being received. Since each type of cyclin has a particular job they are not found at the same concentrations through the cell cycle. Since S-cyclin is only need to drive the cell out if G1 and through S/G2 phase the concentration of that cyclin will peak at the beginning of s phase and remain in high concentration till the end of G2. In contrast M-cyclin is only needed to drive the cell out of G2 and into M phase and thus is only found in concentration from the end of S phase and through into M phase.

cytokinesis

separation of daughter cells in mitosis or meiosis

Briefly describe the cell cycle control system in general?

the cell cycle control system is based cellular signalling. For if the proper signal is not received the cell cycle will not proceed. Primarily the cell cycle control system is dependent on certain protein kinases. As through phosphorylation Cdks signal that the cell is ready to pass into the next stage of the cell cycle, thus overall controlling the rate of the cell cycle. For when they are activated Cdks will drive the cell cycle forward, and when they are inhibited they will halt the cell cycle. Cdks can become activated via mitogen molecules who cause intracellular signalling and result in the production of cyclin which in turn facilitates the activation of Cdk.

how does the cytoskeleton facilitate cytokinesis and cell division in animals?

the cytoskeleton uses actin and myosin filaments to generate a contractile ring around the interpolar microtubules during telophase/cytokinesis. The contractile ring generated by the cytoskeleton acts as the site of cleavage between the two new genetically identical daughter cells.

telophase

the final phase of mitosis in which the kinetochores have pulled their respective sister chromatids to their respective poles. A new nuclear envelope reassembles around each set of chromosomes, completing the formation of the two nuclei and marking the end of mitosis. From here actin and myosin filament begin forming the contractile ring.

independent assortment of chromosomes

the formation of random combinations of chromosomes in meiosis and of genes on different pairs of homologous chromosomes by the passage according to the laws of probability of one of each diploid pair of homologous chromosomes into each gamete independently of each other pair.