Meiosis and mitosis Flashcards
What are the stages of the cell cycle
G1 first gap
S synthesis
G2 second gap These 3 stages are collectively
known as interphase.
In interphase the cell grows and copies its chromosomes in preparation for cell division. Alternatively the cells may exist the cell cycle and remain for long periods of time in the phase called G0 a non-divind phase. During interphase the chromosomes are decondensed and found in the nucleus.
G1: cell growth normally occurs in this phase due to the environmental conditions and signalling molecules.
S- phase: where the chromosomes are copied, leading to twice as many chromosomes. After this phase the chromosomes are still attached to one another they are called sister chromatids.
G2-phase: The cell synthesis proteins that are needed for chromosome sorting and cell division.
M phase mitosis and cytokinesis
Mitosis:
prophase, prometaphase, metaphase, anaphase, telophase
Mitosis results in the division of one cell into two nuclei distributing the duplicated chromosomes so that each daughter cell will receive the same complement of chromosomes. Produces two genetically identical daughter cells from the mother cell. The purpose of mitosis is for asexual reproduction and multicellularity.
Cytokinesis: is the division of the cytoplasm to produce two distinct daughter cells.
G1 phase is relatively long around 11 hours and M phase short 1 hour. The most time the cell spends is in interphase.
Discuss how checkpoint proteins control cell division
The cell cycle is highly control so that it minimises the occurrence of mutations that can lead to cancer. Cyclins and cyclin-dependent kinases are the molecules that control the cell through the cell cycle.
The cell makes different types of cyclin proteins which are typically degraded once the cell has entered the next stage of the cell cycle. The formation of activated cyclin/cdk complexes is regulated by checkpoint proteins. The cyclin/cdk phosphorylate other proteins.
At G1 checkpoint (restriction point)- determines if conditions are favourable for cell division and if the DNA is damaged. G1 cyclin is made in response to sufficient nutrients and growth factors. Forms a activated G1 cyclin/cdk complex. It is degraded when the cell enters S phase. It acts by phosphorylating other proteins that are needed to advance the cell cycle to the next phase in the cell cycle. E.g. those to do with DNA synthesis needed for S phase. If damage is found the checkpoint proteins will prevent G1 cyclin/cdk complex forming stopping the progression of the cell into the S-phase, so stopping the cell cycle.
At the end of G2 the G2 checkpoint checks for any DNA damage and checks if all the DNA has been replicated and monitors the levels of proteins needed for M phase. Forms a mitotic cyclin/cdk complex. It is degraded as the cell progress through mitosis.
Metaphase checkpoint determines if all the chromosomes are attached to the spindle apparatus. If a chromosome is not attached properly to the spindles the metaphase checkpoint protein will stop the cell cycle. the. Prevents incorrect sorting of the chromosomes during division.
Checkpoint proteins can pause the cycle cell while it is being fixed or they can stop it completely if they cannot be fixed.
Describe the process of meiosis
Meiosis is used for sexual reproduction, in which two haploid gametes unite to form a diploid cell called a zygote. This zygote can then go onto undergo mitotic division to develop into a multicellular organism, with many diploid cells.
Meiosis is the process by which haploid cells are produced from a cell that was originally diploid. The chromosomes must be sorted and half in a specific way to ensure that each gamete receives one chromosome from each of the 23 pairs. Meiosis has two rounds of cell division.
Like mitosis the cell firstly goes through interphase including G1, S, G2. However it has additional steps.
The synaptonemal complex connects homologous chromosomes sister chromatids to form a bivalent called a tetrad. Crossing over also occurs which involves a physical exchange between chromosome segments of the bivalent. The arms then tend to separate but the chromosomes stay attached at the crossover site, this is called the chiasma. Normally crossing over occurs aroun 2-5 times.
Meiosis I: separates homologous chromosomes
Prophase I: Homologous chromosomes synapse to form bivalents, and crossing over occurs. Chromosomes condense and the nuclear envelope begins to dissociate into vesicles.
Prometaphase I: The nuclear envelope completely dissociates into vesicles, and bivalents become attached to kinetochore microtubules. In mitosis a pair of sister chromatids are attached to both poles. In meiosis a pair of sister chromatids is attached to just one pole.
Metaphase I: Bivalent randomly align along the metaphase plate in a double row compared to a single row in mitosis. The random alignment results in a huge genetic variation between resulting gametes.
Anaphase I: Homologous chromosomes separate and move towards opposite poles. The connections between bivalents break but not the connections that hold sister chromatids.
Telophase I: the chromosomes decondense and the nuclear envelope re-forms. the 2 daughter cells are separated by a cleavage furrow. They are considered haploid as they do not contain homologous chromosomes.
Meiosis II: separate sister chromatids. S-phase does not occur between these two phases.
Exactly the same happens but with two cells. Sister chromatids align in metaphase instead of bilavents. At anaphase individual chromosomes move towards the poles instead of homologous chromosomes. Cleavage furrows separate the two cells into four cells. That are haploid.
Describe each stage of mitosis in detail
When cells divide the sister chromatids are tightly compacted and readily visible under the microscope. The sister chromatids are tightly associated at the centromere. The protein called cohesion is needed to hold the sister chromatids together. The centromere act as a site of attachment for a group of proteins that form the kinetochore which is needed to sort each protein.
The mitotic spindles that are composed of microtublues-protein fibers that are components of the cytoskeleton are responsible for organising and sorting the chromosomes during cell division.
Each centrosome organises the construction of the microtubules by rapidly polymerising tublin proteins. A cell inherits a single centrosome at cell division, which is duplicated by the cell before a new round of mitosis begins, giving a pair of centrosomes. The three types of spindle microtubules are termed astral, polar and kinetochore. The astral microtubules emanate away from the region between the poles. the polar microtubules project intot he region between the the two poles. The kinechore microtubules are attached to the kinetochore of sister chromatids.
Prophase: The chromosomes have already been replicated and are in sister chromatids. The nuclear envelope begins to dissociate into vessicles. Chromosomes condense into tightly compact structures that are visible under the microscope. Mitotic spindles start to form.
Prometaphase: At the beginning of prometaphase in animal cells, phosphorylation of nuclear lamins causes the nuclear envelope to disintegrate into small membrane vesicles. As this happens, microtubules invade the nuclear space. This is called open mitosis. The nuclear envelope has completely dissociated into vesicles, and the mitotic spindle fibers are completely formed. Sister chromatids attach to the spindle via kinetochore microtubules from opposite poles. Kinetochore microtubules begin to search for and attach to chromosomal kinetochores. A kinetochore is a proteinaceous microtubule-binding structure that forms on the chromosomal centromere during late prophase
Metaphase: Sister chromatids align along the metaphase plate in a single row. When they get to this position metaphase has started. After the microtubules have located and attached to the kinetochores in prometaphase, the two centrosomes begin pulling the chromosomes towards opposite ends of the cell. The resulting tension causes the chromosomes to align along the metaphase plate.
Anaphase: At this stage the connections between the sister chromatids are broken the cohesins that bind sister chromatids together are cleaved, now individual chromosomes is linked to only one of the two poles by one or more kinetochore microtubules. These tubules shorten, pulling the chromosome towards the pole. The two poles also move further away from each other. This occurs because the overlapping polar microtubles lengthen and push against each other, thereby pushing the poles farther apart. In late anaphase, chromosomes also reach their overall maximal condensation level, to help chromosome segregation and the re-formation of the nucleus.
Telophase: Chromosomes have reached their respective poles and decondenses. The nuclear envelopes now re-forms to produce two separate nuclei.
Cytokinesis: involves the formation of a cleavage furrow, which constricts like a drawstring to separate the cells. They use 20 proteins to form a contractile ring, it encircles a region of the mitotic spindles called the central spindle, is a network of myosin and actin. The motor activity of the the myosin moves the actin filaments in a way that causes the contractile ring to constrict. Membrane vesicles are inserted into the contractile site to achieve division.
