Cell Cycle And Its Regulation

 

A cell cycle is a series of events that takes place in a cell as it grows and divides. A cell spends most of its time in what is called interphase, and during this time it grows, replicates its chromosomes, and prepares for cell division.

The two main events occurs in each cell division. The 1^st is the doubling of genome in synthesis phase also called s phase and the 2^nd is halving of genome during mitosis phase.

The eukaryotic cell cycle complete in two phases –the interphase and M phase.

Interphase is the preparatory phase of cell cycle which leads to doubling the genome.

The interphase is further divided into G1 or post mitotic gap phase, s phase or synthesis phase and G2 or pre-mitotic gap phase.

 

M phase which is a real phase of division is itself composed of two tightly coupled processes: karyokinesis, in which the cell's nucleus divides, and cytokinesis, in which the cell's cytoplasm divides forming two daughter cells.

The mitotic phase is divided into prophase, metaphase, anaphase and telophase. In M phase the genome becomes half.

Now let’s see in detail the different phase of cell cycle

In G1 phase the different enzymes which is needed for the DNA replication are synthesized as well as ATP and RNA molecule also synthesize for transcription and translation process.

So many thing are synthesized in G1 phase so the size of the cell increases.

In G₁ phase, a cell has three options.

Either

• To continue cell cycle and enter S phase
• Stop cell cycle and enter G₀ phase for undergoing differentiation.
• Become arrested in G₁ phase hence it may enter G₀ phase or re-enter cell cycle.

Cells of liver kidney and neuron after remaining in G1 phase for some time, come out of the cell cycle and enter to G0 phase also known as quiescent phase.

In quiescent phase the cell division stop but the other activity within the cells remain continue.

 

After the completion of G1 phase the cell cycle transits into S phase.

DNA replication is the main events that occur during S phase, some kind of histone protein is also synthesize in S phase.  Usually cell takes 5-6 hrs. to complete s phase.

After s phase the cell inter into G2 phase which is shorter lasting only 3-4 hrs.

In G2 phase the following events occur-

1.     The tubulin protein synthesize which is required for spindle formation.

2.     For plasma membrane formation certain protein are synthesized.

3.     ATP synthesis and RNA synthesis also occur during G2 phase.

The next stage is M phase. M stands for mitosis. This is where the cell actually partitions the two copies of the genetic material into the two daughter cells.

In prophase of mitosis chromatid coiling, spindle formation and disintegration of nuclear membrane occurs.

In metaphase the chromosome oriented at equatorial plane.

In anaphase the sister chromatid separate from each other and moves towards the opposite poles.

And In telophase which is the last phase of mitosis, the chromosome reach the poles and nuclear membrane reforms.

Telophase is followed by cytokinesis or the division of the cytoplasm into two daughter cells.

Check Point in Cell cycle

Activation of each phase of cell cycle is dependent on the proper progression and completion of the previous one.

The deciding point is called check point . The check point ensure the proper progression of cell cycle.

The whole cell cycle is controlled by the three check point which are-

The G1 check point at G1/S transition.

The G2 checkpoint at G2/M transition

And the M checkpoint or spindle checkpoint at the transition from metaphase to anaphase

The G1 checkpoint is the main decision point for a cell where it decide whether the cell will divide or remain suspended.

The G1 check point check all the preparation needed for a cell to enter into s phase.

The next check point is G2 check point before M phase which ensure the Proper DNA replication, and check whether M phase cyclin and cdk complex is activated to initiate mitosis.

The third check point is at the transition of Metaphase and anaphase, here the check point examine whether all the sister chromatids are correctly attached to the spindle microtubule or not.

Regulation of cell cycle

In addition to the check point there are two groups of intracellular molecules that regulates cell cycle. The two groups of proteins, called cyclins and cyclin dependent kinase are responsible for the progress of cell through the various checkpoints.

 

 

Hs of testis and ovary

 

Histological structure of ovary and testis

 

Anatomy and counter current mechanism of Kidney

 

Kidney Structure

 

 

 

 Hello viewers welcome to scientech biology. Today’s I am going to explain the structure and counter current mechanism of human kidney

 

 

 

 

 

The structure of urinary system function to filter blood and remove waste from the body. The kidneys are the blood filtering organ.

Blood enters the kidneys through the renal arteries. Within the kidneys, the substances are filtered out of the blood into urinary structure.

Some substances are then reabsorbed back into the blood, and others are secreted into the urine.

This three step process cleans blood and create the waste products urine. Urine exit the kidneys and moves down the ureter to the urinary bladder.

As the bladder fills, interaction with the brain, and nerves contract and relax the muscular structures and urine is pushed out through the urethra in the process called micturition.

 

The major excretory organs of mammals is the kidney. Human have two kidneys located in the upper rear region of the abdominal cavity.

The urine they produce is conducted to the urinary bladder through the ureters. The urethra drains the bladder.

Externally, the kidneys are surrounded by three layers. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perineal fat capsule, which helps anchor the kidneys in place. The third and innermost layer is the renal capsule

The internal structure of kidney includes an outer cortex and inner medulla. The ureter divides into branches, the end of which envelop medullary tissue called renal pyramids.

The actual work of kidney is carried out by functional unit called nephrons.  Each human kidney contains about millions nephrons.

Each nephron consist of vascular and tubular component. An afferent arteriole carries blood to the knot of capillaries called the glomerulus. Draining each glomerulus is an efferent arteriole that give rise to the peritubular capillaries most of which surround the cortical portions of the nephron tubules.

Blood pressure forces water and small molecules to be filtered from the glomerulus and collected in Bowman’s capsule. The initial segment of the renal tubule is called the proximal convoluted tubule. The glomerulus, Bowman’s capsule and proximal convoluted tubule of each nephron are located in the cortex.

From the proximal convoluted tubule, the nephron tubule turns down into the medulla. The portion of the tubule in the medulla is called the Loop of Henle., where the ascending limbs of the loop of Henle reaches the cortex, it becomes the distal convoluted tubule.

The distal convoluted tubule of many nephrons joins a common collecting duct in the cortex. The collecting ducts then run in parallel with the loop of Henle down through the medulla and empty into the ureter at the tips of the renal pyramids.

A few peritubular capillaries run into the medulla in parallel with the loop of Henle and the collecting ducts and forms the vasa recta. . These capillaries carry away the molecule that are reabsorbed from the tubules. All the peritubular capillaries join back together into a venule that eventually leads to the renal veins.

Nephron regulates the composition of blood and urine by the combination of filtration, secretion and reabsorption. Viewed schematically we will see how these processes are facilitated by the regular arrangement of the segments of the nephron.

The proximal convoluted tubule is responsible for most of the reabsorption of water and solutes form the glomerular filtrate. The cells of this section of the nephron actively transport Na+ and other solutes such as glucose and amino acids, out of the tubule fluids.

The active transport of solute out of the proximal convoluted tubule into the tissue fluid causes water to flow by diffusion. The water and solutes moved into the tissue fluids are taken up by the peritubular capillaries and returned to the venous blood leaving the kidney.

 

Despite the large volume of water and solute reabsorbed out of the proximal convoluted tubule, the overall concentration or osmolality, of the fluid that enters the loop of the Henle is similar to that of the blood plasma, although its composition is quite different. The ability of the kidney to produce urine that is hypertonic to the blood plasma is due to the loop of Henle. The loop of Henle does not concentrate the urine directly rather, it function as a countercurrent multiplier creating a concentration gradient in the surrounding medulla.

To understand the countercurrent multiplier mechanism it is easiest to move backward through the tubule, starting with thick ascending limbs. The thick ascending limb actively transport Na+ from the tubule fluid and move it into the surrounding tissue fluid. Chloride ions follow passively.

The thick ascending limbs is not permeable to water, so the reabsorption of Na+ and Cl- out of this part of the tubule is not followed by the outward diffusion of water. This reabsorption of Na+ and cl- raises the concentration of solutes in the surrounding tissue fluids.

The descending limbs, in contrast is permeable to water, but not very permeable to Na+ and Cl- . Since the surrounding fluid has been made more concentrated, water leaves the tubule by osmosis. As a result the fluid in the descending limb become more concentrated as it flow towards the bottom of the medulla.

The thin ascending limb is not permeable to water. It is however permeable to Na+ and Cl-. Since the tubule fluid is more concentrated than the surrounding tissue, Na+ and Cl- diffuses out. The thick ascending limbs continues to move Na+ and Cl- to the medulla by active transport.

As a result of this process, the tubule fluid reaching the distal convoluted tubule is less concentrated than the blood plasma and the solute that have been left behind in the renal medulla have created a concentration gradient in the surrounding tissue fluid.

Since the fluid entering the distal convoluted tubule is less concentrated than the surrounding cortex the tubule loses water osmotically as it flow towards the collecting ducts.

The tubule fluid entering the collecting ducts is at the same concentration as the blood plasma. However since Na+ and Cl- have been moved out of the tubule fluid, urea and other waste products make up a greater proportion of its total solute content. As the collecting ducts descend from the cortex to the tip of the renal pyramids, the concentration gradient established by the loop of Henle increases. This increasing solute concentration causes more and more water to be absorbed from the fluid, thus concentrating the urine in the collecting ducts.