Question

Explain cell division in detail. You may use images or figures if you wish. 25 scores
2. Integrate the knowledge provided you about the regulation of blood pressure in different topics including the physiology of Endocrine system, Renal & Fluid, Nervous system, and Cardiovascular system. Try to integrate all of this data and draw a scheme/trace (like the figure below) to explain the mechanism involved in regulation of blood pressure. 25 scores

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Answer 1

1)   Cell division is the process by which a parent cell divides into two or more daughter cells. It usually occurs as part of a larger cell cycle.

A) In eukaryotes, there are two distinct types of cell division:

a) a vegetative division, whereby each daughter cell is genetically identical to the parent cell called mitosis

b) a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes called meiosis.

• Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.
• Homologous chromosomes are separated in the first division, and sister chromatids are separated in the second division.
• Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle.
• Both are believed to be present in the last eukaryotic common ancestor.

B) In Prokaryotes (bacteria and archaea)

• Cell division usually undergo a vegetative cell division known as binary fission, where their genetic material is segregated equally into two daughter cells.
• While binary fission may be the means of division by most prokaryotes, there are alternative manners of division, such as budding, that have been observed.
• All cell divisions, regardless of organism, are preceded by a single round of DNA replication.

C) For simple unicellular microorganisms such as the amoeba.

• one cell division is equivalent to reproduction – an entire new organism is created.
• On a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings.
• Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by meiotic cell division from gametes.
• After growth, cell division by mitosis allows for continual construction and repair of the organism.
• The human body experiences about 10 quadrillion cell divisions in a lifetime

[Note:  The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information that is stored in chromosomes must be replicated, and the duplicated genome must be separated cleanly between cells.A great deal of cellular infrastructure is involved in keeping genomic information consistent between generations.]

  

Cell Division in Eukaryote

Cell division in eukaryote is much more complicated than prokaryote.

Depending upon chromosomal number reduced or not, eukaryotic cell divisions can be classified as:

• Mitosis (equational division)
• Meiosis (reductional division).
• A premitive form of cell division is also found which is called amitosis( The amitotic or mitotic cell division is more atypical and diverse in the various groups of organisms such as protists namely diatoms, dinoflagellates etc. and fungi.)

a) In mitotic metaphase, typically the chromosomes (each with 2 sister chromatid that they developed due to replication in the S phase of interphase) arranged and sister chromatids split and distributed towards daughter cells.

b) In meiosis, typically in Meiosis-I the homologous chromosomes are paired and then separated and distributed into daughter cells. Meiosis-II is like mitosis where the chromatids are separated.

[Note: In human and other higher animals and many other organisms, the meiosis is called gametic meiosis, that is the meiosis gives rise to gametes. Whereas in many group pf organisms especially in plants, the meiosis gives rise to kind of spores that germinates into haploid vegitative phase or gametophyte. Such kind of meiosis is called sporic meiosis].

Phases of eukaryotic cell division

Interphase

• Interphase is the process through which a cell must go before mitosis, meiosis, and cytokinesis.
• Interphase consists of three main phases: G₁, S, and G₂.
• G₁ is a time of growth for the cell where specialized cellular functions occur in order to prepare the cell for DNA Replication.
• In S phase, the chromosomes are replicated in order for the genetic content to be maintained.
• During G₂, the cell undergoes the final stages of growth before it enters the M phase, where spindles are synthesized.
• The M phase, can be either mitosis or meiosis depending on the type of cell.
• Germ cells or gametes, undergo meiosis, while somatic cells will undergo mitosis.
• After the cell proceeds successfully through the M phase, it may then undergo cell division through cytokinesis.

Prophase

• Prophase is the first stage of division.
• The nuclear envelope is broken down in this stage, long strands of chromatin condense to form shorter more visible strands called chromosome the nucleolus disappears, and microtubules attach to the chromosomes at the disc-shaped kinetochores present in the centromere.
• Microtubules associated with the alignment and separation of chromosomes are referred to as the spindle and spindle fibers.
• Chromosomes will also be visible under a microscope and will be connected at the centromere.
• During this condensation and alignment period in meiosis, the homologous chromosomes undergo a break in their double-stranded DNA at the same locations, followed by a recombination of the now fragmented parental DNA strands into non-parental combinations, known as crossing over.
• This process is evidenced to be caused in a large part by the highly conserved Spo11 protein through a mechanism similar to that seen with toposomerase in DNA replication and transcription.

Metaphase

• In metaphase, the centromeres of the chromosomes convene themselves on the metaphase plate or equatorial plate(an imaginary line that is at equal distances from the two centrosome poles and held together by complex complexes known as cohesins).
• Chromosomes line up in the middle of the cell by microtubule organizing centers (MTOCs) pushing and pulling on centromeres of both chromatids thereby causing the chromosome to move to the center.
• At this point the chromosomes are still condensing and are currently one step away from being the most coiled and condensed they will be, and the spindle fibers have already connected to the kinetochores.
• During this phase all the microtubules, with the exception of the kinetochores, are in a state of instability promoting their progression towards anaphase.
• At this point, the chromosomes are ready to split into opposite poles of the cell towards the spindle to which they are connected.

Anaphase

• Anaphase is a very short stage of the cell cycle and it occurs after the chromosomes align at the mitotic plate.
• Kinetochores emit anaphase-inhibition signals until their attachment to the mitotic spindle.
• Once the final chromosome is properly aligned and attached the final signal dissipates and triggers the abrupt shift to anaphase.
• This abrupt shift is caused by the activation of the anaphase-promoting complex and its function of tagging degradation of proteins important towards the metaphase-anaphase transition.
• One of these proteins that is broken down is securin which through its breakdown releases the enzyme separase that cleaves the cohesin rings holding together the sister chromatids thereby leading to the chromosomes separating.
• After the chromosomes line up in the middle of the cell, the spindle fibers will pull them apart.
• The chromosomes are split apart while the sister chromatids move to opposite sides of the cell.
• As the sister chromatids are being pulled apart, the cell and plasma are elongated by non-kinetochore microtubules.

Telophase

• Telophase is the last stage of the cell cycle in which a cleavage furrow splits the cells cytoplasm (cytokinesis) and chromatin.
• This occurs through the synthesis of a new nuclear envelopes that forms around the chromatin which is gathered at each pole and the reformation of the nucleolus as the chromosomes decondense their chromatin back to the loose state it possessed during interphase.
• The division of the cellular contents is not always equal and can vary by cell type as seen with oocyte formation where one of the four daughter cells possess the majority of the cytoplasm.

Cytokinesis

• The last stage of the cell division process is cytokinesis. In this stage there is a cytoplasmic division that occurs at the end of either mitosis or meiosis.
• At this stage there is a resulting irreversible separation leading to two daughter cells.
• Cell division plays an important role in determining the fate of the cell.
• This is due to there being the possibility of an asymmetric division.
• This as a result leads to cytokinesis producing unequal daughter cells containing completely different amounts or concentrations of fate-determining molecules.

2) Regulation of Blood Pressure

Changes in blood pressure are routinely made in order to direct appropriate amounts of oxygen and nutrients to specific parts of the body.

Blood pressure can be adjusted by producing changes in the following variables:

• Cardiac output can be altered by changing stroke volume or heart rate.

• Resistance to blood flow in the blood vessels is most often altered by changing the diameter of the vessels (vasodilation or vasoconstriction). Changes in blood viscosity (its ability to flow) or in the length of the blood vessels (which increases with weight gain) can also alter resistance to blood flow.

The following mechanisms help regulate blood pressure:

A) The cardiovascular center provides a rapid, neural mechanism for the regulation of blood pressure by managing cardiac output or by adjusting blood vessel diameter. Located in the medulla oblongata of the brain stem, it consists of three distinct regions:

• The cardiac center stimulates cardiac output by increasing heart rate and contractility. These nerve impulses are transmitted over sympathetic cardiac nerves.

• The cardiac center inhibits cardiac output by decreasing heart rate. These nerve impulses are transmitted over parasympathetic vagus nerves.

• The vasomotor center regulates blood vessel diameter. Nerve impulses transmitted over sympathetic motor neurons called vasomotor nerves innervate smooth muscles in arterioles throughout the body to maintain vasomotor tone, a steady state of vasoconstriction appropriate to the region.

• The cardiovascular center receives information about the state of the body through the following sources:

  • Baroreceptors are sensory neurons that monitor arterial blood pressure. Major baroreceptors are located in the carotid sinus (an enlarged area of the carotid artery just above its separation from the aorta), the aortic arch, and the right atrium.

  • Chemoreceptors are sensory neurons that monitor levels of CO ₂ and O ₂. These neurons alert the cardiovascular center when levels of O ₂ drop or levels of CO ₂ rise (which result in a drop in pH). Chemoreceptors are found in carotid bodies and aortic bodies located near the carotid sinus and aortic arch.

  • Higher brain regions, such as the cerebral cortex, hypothalamus, and limbic system, signal the cardiovascular center when conditions (stress, fight‐or‐flight response, hot or cold temperature) require adjustments to the blood pressure.

B) The kidneys provide a hormonal mechanism for the regulation of blood pressure by managing blood volume.

• The renin‐angiotensin‐aldosterone system of the kidneys regulates blood volume.

• In response to rising blood pressure, the juxtaglomerular cells in the kidneys secrete renin into the blood. Renin converts the plasma protein angiotensinogen to angiotensin I, which in turn is converted to angiotensin II by enzymes from the lungs.

• Angiotensin II activates two mechanisms that raise blood pressure:

  Various substances influence blood pressure. Some important examples follow:

  • Angiotensin II constricts blood vessels throughout the body (raising blood pressure by increasing resistance to blood flow). Constricted blood vessels reduce the amount of blood delivered to the kidneys, which decreases the kidneys' potential to excrete water (raising blood pressure by increasing blood volume).

  • Angiotensin II stimulates the adrenal cortex to secrete aldosterone, a hormone that reduces urine output by increasing retention of H ₂O and Na ⁺ by the kidneys (raising blood pressure by increasing blood volume).

  • Epinephrine and norepinephrine, hormones secreted by the adrenal medulla, raise blood pressure by increasing heart rate and the contractility of the heart muscles and by causing vasoconstriction of arteries and veins. These hormones are secreted as part of the fight‐or‐flight response.
  • Antidiuretic hormone (ADH), a hormone produced by the hypothalamus and released by the posterior pituitary, raises blood pressure by stimulating the kidneys to retain H ₂O (raising blood pressure by increasing blood volume).
  • Atrial natriuretic peptide (ANP), a hormone secreted by the atria of the heart, lowers blood pressure by causing vasodilation and by stimulating the kidneys to excrete more water and Na ⁺(lowering blood pressure by reducing blood volume).