Urinary System

Urinary System

Gross Anatomy

Consist of six organs

–Kidneys (2)

–Ureters (2)

–Urinary bladder

–Urethra

•Right slightly lower than left due to space occupied by liver

Renal parenchyma is divided into two zones

–Renal cortex- about 1cm thick

–Inner medulla

Micro Anatomy

•Each kidney contains 1.2 million functional units called Nephrons.

•A nephron consist of two principal parts

–Renal corpuscle- (glomerulus) where the blood plasma is filtered

–Renal tubule- processes the filtrate into urine

•Consist of a ball of capillaries called a glomerulus enclosed in a two-layered glomerular (Bowman's) capsule

Structure of the Bowman's capsule

In the glomerulus the filtrate of plasma has to pass through three layers:

-          The fenestrated epithelium of the capillary- the filtering membrane

-          The basement membrane of the bowman's capsule- It contains connective tissue and Mesangial cells which are both phagocytic and contractile.

-          The epithelial cells of the Bowman's capsule called podocytes which are wrapped around the capillaries.

• Renal Tubule
  

•Is a duct that leads away form the glomerular capsule and ends at the tip of a medullary pyramid

•Divided into four major regions

–Proximal convoluted tubule

–loop of Henle

–Distal convoluted tubule

–Collecting duct -(not really a part of the nephron- receives urine from many Nephrons

• Proximal Convoluted Tubule

–Longest and most coiled

–Simple cuboidal epithelium with microvilli (brush border for absorption capacity) and numerous mitochondria

• Loop of Henle

•Descending limb- first portion of the loop, passes from the cortex into the medulla

•At its deep end it turns 180^o and forms an ascending limb that returns to the cortex. 

•It is divided into thick and thin segments

–Thin segments- has a simple squamous epithelium

•Cells have low metabolic rate but are very permeable to water

•Forms the lower part of the descending limb, the bend, and partway up the ascending limb

–Thick segments- have a simple cuboidal epithelium with lots of mitochondria in the cells due to the high metabolic activity of active transport.

•Form initial parts of the descending limb and part or all of the ascending limb

• Distal Convoluted Tubule

•Shorter and less coiled than the PCT.

•Has a cuboidal epithelium with smooth-surface cells nearly devoid of microvilli

• Collecting Duct

•The DCTs of several Nephrons drain into a this straight tube, which passes into the medulla

•Are both lined with simple cuboidal epithelium

There are two populations of Nephrons:

•Cortical Nephrons- Nephrons close to the kidney surface

–Have shorter nephron loops that dip only slightly into the outer medulla before turning back

•Juxtamedullary nephron- Nephrons close to the medulla

–Have very long loops that extend to the apex of the renal pyramid

–Responsible for maintaining the salinity gradient

Blood Supply

•Receives 21% of the cardiac output

•Each kidney is supplied by a renal artery (sometimes 2 or more)

•The interlobular artery branches off afferent arterioles to the glomerulus of each nephron

•The glomerulus is drained by an efferent arteriole

Nerve Supply

•Renal nerves arise from the superior mesenteric ganglion and enter the kidney at the hilum.

•They follow the branches of the renal artery to innervate each nephron

•Consist mostly of sympathetic fibers that regulate blood flow into and out of each nephron thus controlling the filtration rate and urine formation

Functions of the kidney

-The main function is to regulate the volume and composition of the ECF.

-Excrete metabolic wastes e.g. Urea

-Excrete foreign substances and their derivatives .g. Drugs and food additives

-synthesize Prostaglandins and kinins which act within the kidney

-Function as an endocrine organ; produces EPO, renin, Calcitrol (The active form of Vit D)

Blood and Blood Clotting

BLOOD & clotting

Specific Objectives

1.       describe the mechanisms that contribute to blood clotting  

2.       identify the stages involved in blood clotting

3.       describe how chemicals called serotonin and Thromboxane A2 (released by platelets) play a role in Haemostasis

4.       describe the terms used to describe the difference between stationary versus moving blood clots

5.       describe the similarities and differences between the extrinsic and the intrinsic pathways of blood clotting

6.       describe the role of vitamin K in blood clotting

7.       list the 4 common blood types of the ABO blood group system

8.       describe which antibodies are found circulating in each of the respective ABO blood types (if any)

9.       Describe the Rh antigen.

10.   distinguish between the terms Rh negative and Ph positive  

11.   describe the complications that can occur when there is a mismatch between a mother who is Rh positive and a baby who is Rh negative , and what if anything that can be done to prevent problems.

Definition: The sequence (cascade) of reactions that prevent blood loss from a broken vessel.

 It's a fast, localized, and carefully controlled process. It occurs within seconds to minutes of the injury. Has three phases:

1. Vascular spasms – constriction of damaged blood vessels. Triggers include:

-          Direct injury to vascular smooth muscle

-          Presence of chemicals released by endothelial cells and platelets especially serotonin.

-          Reflexes initiated by local pain receptors

2. Platelet plug formation – sealing the vessel walls .Platelets adhere to exposed collagen fibers, become sticky, and form spiked processes. Attachment to collagen fibers activates the release of several chemicals including:

-          Serotonin – enhances the vascular response

-          Adenosine diphosphate (ADP) – attract platelets to site of injury

-          Thromboxane A2 – stimulates positive feedback cycle

-          PGI₂ limits the platelet plug formation to the site of injury

3. Coagulation or blood clotting – Blood clotting (coagulation) is normally complete within 3-6 minutes after blood vessel damage. In summary blood is transformed from a liquid to a gel. Consists of three crucial steps:

-          Formation of prothrombin activator. Step 1 can be divided into Two Separate mechanisms, called the Intrinsic Pathway and the Extrinsic Pathway. Intrinsic Pathway is initiated from damage from within the blood vessel (e.g., damage to endothelial lining exposes Collagen Fibers that, in turn, activate clotting factors within or intrinsic to the blood).  Platelets participate. Extrinsic Pathway-activated by chemicals released from damaged tissue, that is by factors Outside (Extrinsic to) the blood.

 Note:  Extrinsic Pathway has Fewer Steps than the intrinsic pathway, and, thus, is more rapid than the intrinsic pathway (e.g., in cases of Severe Tissue Trauma, the Extrinsic mechanism may clot blood within 15 seconds).
       :  a typical cut on the skin will activate both the Extrinsic and the intrinsic pathways.  How?  Well, damaged tissues in the area of the cut will activate the extrinsic pathway.  Also blood vessel damage inside the skin will exposes collagen fibers that activate clotting factors Intrinsic to the blood (the intrinsic pathway).
Disease Atherosclerosis might put one at risk for activation of the intrinsic pathway.  Narrowing of blood vessels----> increased Blood Pressure ----> increased risk of damaging blood vessels from within.  Thus, there is increased risk of blood clot formation of person with "hardening of arteries."
-The liver synthesizes many of the clotting factors present within the blood.  Almost all of these clotting factors exist in an inactive form until they are acted upon by something else.  Clotting Factor X is common to both intrinsic and extrinsic pathways.
 --exists in an inactive form until acted upon by intrinsic or extrinsic pathway.
Extrinsic pathway:  tissue factor TF, clotting factor VII activate---> Activated Factor X.
Intrinsic pathway:  platelet Factors XII, XI, IX, and VIII act on Factor X (inactive) ---> Activated Factor X.

-          Conversion of prothrombin to the active enzyme called thrombin

-          Activation of Fibrinogen to Fibrin by thrombin. Which forms a mesh that traps blood cells and seals the hole until the blood vessel can be permanently repaired.

Anticoagulants inhibit clot formation (e.g. Heparin, warfarin)

-unneeded clots are removed when healing has occurred (within 2 days of clot formation) by Fibrinolysis to prevent vessel blockage. Plasmin is the active digesting enzyme responsible for Fibrinolysis and is confined to the clot

Vitamin K is necessary for the synthesis of 4 clotting factors prothrombin (II), VII, IX, and X)

Calcium is also necessary for proper blood clotting.  Chelators like EDTA and Citrate Phosphate Dextrose will bind up the calcium in a test tube, and prevent the blood from clotting.  Useful in "blood banking" clinics.

Factors Limiting Clot Growth or Formation:

-          Size of clot is limited by the rapid removal of clotting factors and inhibition of activated clotting factors

-          Heparin is released in small amounts to inhibit thrombin activity

-          Unwanted clotting is prevented by the secretion of antithrombic substances (PGI₂ and heparin) secreted by endothelial cells; vitamin E is also a potent anticoagulant

Fibrinolysis – removal of unneeded clots when healing has occurred (within 2 days of clot formation)

• Prevents vessel blockage
• Plasmin is the active digesting enzyme responsible for Fibrinolysis
• Plasmin action is confined to the clot

Mechanism - plasminogen (a plasma protein) is activated by many factors & becomes Plasmin. Which then breaks down fibrin meshwork & phagocytic WBCs remove products of clot dissolution

        Factors Limiting Clot Growth or Formation

o   Size of clot is limited by the rapid removal of clotting factors and inhibition of activated clotting factors

o   Heparin is released in small amounts to inhibit thrombin activity

o   Unwanted clotting is prevented by the secretion of antithrombic substances (PGI₂ and heparin) secreted by endothelial cells; vitamin E is also a potent anticoagulant

Clinical Disorders of Haemostasis – Can be due to improper clot formation or the inhibition of natural clot formation

1.      Thromboembolytic Conditions – result from undesired clot formation:

• Thrombus – a stationary blood clot.  a clot that develops in an unbroken vessel. can block circulation and kill viable tissues
• Embolus – a mobile "moving" blood clot in the bloodstream. Dangerous if it encounters narrow vessels. if it lodges in lungs, called a Pulmonary Embolism.
• Embolism – clot obstructing a vessel
• Disseminated Intravascular Coagulation (DIC) – bacterial toxins cause clotting in healthy tissue; the remaining blood in the system is unable to clot and patient experiences heavy bleeding.

 Treatment: aspirin, heparin, and dicumarol are used to prevent unwanted clotting

2.      Bleeding Conditions – normal clot formation is inhibited

• Thrombocytopenia – due to a platelet deficiency

§  Seen in patients with bone marrow disorders; people exposed to ionizing radiation (ex. X-rays); and certain drugs

§  Spontaneous bleeding occurs in these individuals and Haemostasis cannot occur because of platelet deficiency

§  As a consequence of this, the patient may experience bruising and the formation of purplish spots on the skin.

• Liver Impairment – causes are reduction in the production of procoagulants needed for the clotting response.

• Vitamin K deficiency – due to lack of vitamin K in the diet (rare) or because antibiotic use has disrupted the intestinal flora responsible for making vitamin K. Prothrombin Synthesis in the liver requires the presence of Vitamin K.

-          Disruption in fat absorption, especially in the intestines – Vitamin K is fat-soluble and any disruption involving lipids or fat will impair vitamin K absorption

-          Disruption in bile production or a bile duct obstruction - The salts present in bile are critical for vitamin K absorption in the intestines; imbalances or disruptions will also impair vitamin K absorption

-          prolonged antibiotic therapy (decreases bacteria that make Vitamin K)

• Hemophilias - genetic 'defect' that leads to inability to produce certain clotting factor(s). Temporary treatment is by infusion with whole blood.
• Anticoagulant therapy (e.g., heparin--acts as an anti-thrombin; coumadin--interferes with liver's utilization of Vitamin K; that is, coumadin is a vitamin K antagonist).  Note:  another name for Coumadin is Warfarin or Dicumarol.

Transfusions and Blood Typing

·         The word "antigen" means Antibody Generating. A person will produce antibodies against blood group antigens that are foreign (not on the surface of one's own red blood cells). For some 80% of the population, the ABO blood group antigens are also present in salivary secretions (saliva).

·         Antigens (agglutinogens) present on the surface of red blood cells can cause adverse reactions in transfusion recipients. They are glycolipids or glycoproteins.

·         Prior to receiving transfusions a patient's blood type must be determined and the blood they receive must match to avoid complications

·         The following agglutinogens are screened in all cases: ABO and Rh

ABO Blood Typing

determined by the presence or absence of AB agglutinogens (Antigens) on RBC cell membranes. An individual with type AB:

• Both the A and B antigens are present on RBC cell membrane
• Antibodies against A and B antigens are not produced in the individual
• AB individuals can receive A, B, AB, or O blood transfusions (Universal Recipient)
• AB individuals can only donate to AB individuals

• An individual with type A:
• Only the A antigens are present on the RBC cell membrane
• Antibodies against B antigens are produced in the individual
• A individuals can receive A or O blood transfusions
• A individuals can only donate to A or AB individuals

• An individual with type B:
• Only the B antigens are present on the RBC cell membranes
• Antibodies against A antigens are produced in the individual
• B individuals can receive B or O blood transfusions
• B individuals can only donate to B or AB individuals

• An individual with type O:
• Neither A nor B antigens are present on the RBC cell membranes
• Antibodies against A and B antigens are produced in the individual
• O individuals can only receive O blood transfusions
• O individuals can donate to A, B, AB, and O individuals (Universal Donor)

Rh factor Blood Typing

determined by the presence or absence of at least 8 different types if Rh agglutinogens.

• An Rh⁺ individual:
• Rh agglutinogens are present on the RBC cell membrane
• No antibodies are formed against the Rh antigens in the individual
• Individuals can receive blood from Rh⁺ and Rh^- individuals, provided they have the correct ABO blood group

• An Rh^- individual:
• Rh agglutinogens are absent on the RBC cell membrane
• No antibodies are formed against Rh antigens unless the individual has prior exposure to them (ex. Transfusion Rh⁺ blood or delivery of an Rh⁺ baby to an Rh^-mother)
• Rh^- individuals can only receive Rh^- blood transfusions
• Rh^- can donate to Rh^- and Rh⁺ individuals (because of the absence of the Rh antigen on the RBC cell membrane), provided they have the correct ABO blood group

Pregnancy and Rh^- Individuals

• If the fetus of an Rh^- mother is Rh⁺, the mother will produce large amounts of antibodies to the Rh antigen when the placenta detaches and the fetal blood is mixed with the maternal blood.  This first pregnancy will not be affected. 

Following delivery (live or dead), the mother must receive a shot containing antibodies against the Rh antigen. This will inhibit an immune response against an Rh⁺ fetus in future pregnancies.  This will prevent severe birth defects in the fetus and improve chances of a successful pregnancy; Hemolytic Disease of the Newborn (HDN).

Plasma and Blood Volume Expanders

• Clinical use:  colloidal or isotonic salt solutions are used in emergency situations in which the patient is experiencing severe blood loss and immediate blood transfusions are not available.  Plasma and blood volume expanders will only bring the blood volume back to normal, not increase the levels of the formed elements.
• Colloid solutions – e.g. Dextran and purified human serum albumin – used to replenish plasma volume
• Isotonic salt solutions – e.g. Normal saline, Ringer's solution – used to increase blood volume by increasing water volume

1. Define and classify the various forms of Anemia
2. Explain the roles of Vitamin K, B12, EPO

Created by Musango 28/03/11

Essential Terminologies and Concepts of CVS

Essential Terminology and Concepts of CVS

Systole- each period of contraction

Diastole – each period of relaxation

Heart rate/Pulse rate (HR) - the number of times the heart contracts/beats per minute (Av.-75 at rest)

Stroke volume (SV) - the amount of volume ejected by each ventricle (equal for both ventricles- 70ml at rest)

Cardiac output (CO) - total volume each ventricle ejects per minute (= HR*SV) i.e. at rest CO= 70/min * 70ml= 4900ml/min or approximately 5l/min

Venous returns- amount of blood that returns from the veins to the heart per minute. Under normal circumstances = CO.

Hydrostatic pressure – the pressure exerted by the blood onto the walls of blood vessels. We have systolic and diastolic pressure.

Vascular resistance- the resistance of the blood vessels to blood flow.

–        Total Peripheral resistance (TPR) –The resistance of all blood vessels down stream to the Aorta.

–        Pulmonary vascular resistance (PVR) - The resistance of all blood vessels down stream to the Pulmonary artery.

        TPR is 7 times more than PVR. This is because the pulmonary blood vessels are shorter and wider than the systemic, and this is why the walls of the left ventricle are thicker and more muscular than the right. to over come the resistance.

Blood pressure- in the arteries average- 90-120mmHg, in the veins 1-2mmHg.

Þ    describe CCF, Aneurysm

Anatomical distribution of the CO- The distribution depends on the resistance of the arterioles in the region. Tissues with low vascular resistance receive a higher proportion of the CO. The resistance depends mainly on the diameter of the vessels. In certain circumstances these can be changed e.g. during exercise, the arterioles in the skeletal muscle dilate while those to the GIT constrict, thus allowing more blood to go to the skeletal muscle at the expense of the supply the GIT.

Þ    List the distribution of the CO in the body systems and the blood volume in the vascular system. (The veins keep 60% of the blood)

Þ    Why are the veins referred to as blood reservoirs? Why do women have less blood than males?

Þ    Define the term Haematocrit and state its significance to the blood flow.

Þ    Discuss the Mechanical events of the cardiac cycle and relate them with the electrical events in the following format:

1. Ventricular filling
2. Isovolumic ventricular contraction
3. Ejection
4. Isovolumic ventricular relaxation

Þ    Explain the origin of the heart sounds viz lubb, dup and murmurs

Þ    Differentiate the parasympathetic innervation of the heart from the sympathetic, in terms of their location and effects.

Þ    Define inotropic and chronotropic properties, tachycardia and bradycardia.

Þ    Explain why the heart rate will increase to about 100 beats/min, if you block both the sympathetic and parasympathetic flow to the heart.

Þ    Describe the structure of the various blood and lymph vessels.

The Blood

It is a tissue composed of:

-          a solid phase of formed elements- Cells and part of cells and

-          Liquid phase-plasma.

Lymph is formed from blood by filtration at the capillaries due to hydrostatic pressure. It's low in protein content as compared to the blood. It is returned to the venous circulation via the lymphatic vessels at the thoracic duct. The lymphatic vessels have one way valves. Along the way the fluid passes through Lymph nodes where it is inspected for foreign substances e.g. bacteria and they are filtered.

Blood constituents

1. RBC- carries and protects Haemoglobin.

Describe the structure of RBCs

Define the terms; red cell count, mean corpuscular volume, erythrocyte sedimentation rate and their significance.

2. WBCs (Leucocytes)

List the various types and their functions

3. Megakaryocytes and Platelets

-Platelets have no nucleus

-contain Golgi apparatus, ER, Enzymes, Microtubules and filaments.

-Functions- Haemostasis, Phagocytosis, storage and transport of 5-HT and Histamine.

4. List the constituents and functions of Plasma.

Functions of Blood

1.       Ionic and Osmotic balance- the blood proteins i.e. Albumins and Globulins are the ones that maintain osmotic pressure in the capillaries. (They do not cross the capillary walls due their size) this is called Oncotic pressure.

2.       Nutrition and excretion- transports nutrients, hormones, waste products, drugs and their metabolites.

3.       Blood gases

4.       Buffering

5.       Protection

6.       temperature control

7.       Turgor- controls tone of the skin, turgidity of the organs and provides the hydraulic pressure for the erection of the penis.

Created by: Musango 26/3/11

Meiosis Explained

It is a type of cell division that occurs only in the reproductive cells and hence it is known as germ cell division. The daughter cells resulting from meiosis will carry half the number of chromosomes and half the amount of genetic material compared to the parent cell. Hence, meiosis is also known as reductional division.

fig. 17.5 - Stages of Meiosis

Meiosis occurs during the formation of gametes (spermatozoa and ova) in animals and spores in higher plants. It is mainly meant for bringing about a decrease in the chromosome number from the diploid (2n) condition to the haploid (n) condition.

The most characteristic feature in meiosis is that nucleus divides twice successively. Hence, meiosis is conventionally divided into first meiotic division (or meiosis - I) and second meiotic division (or meiosis - II).

In both the meiotic divisions, karyokinesis can be distinguished into four stages - prophase, metaphase, anaphase and telophase.

First Meiotic Division

The first meiotic division or meiosisI is commonly described as reductional division since at the end of this division the two resulting daughter cells will have half the number of chromosomes as that of the parent cell.

Interphase

It is the preparatory phase. Cell organells replicate and size of the cell increases. DNA molecule undergoes replication. Each chromosome exists as a pair of chromatids joined together by a centromere.

fig. 17.6 Stages of Meiosis-I

Prophase I

It is the phase of longest duration and involves a series of significant changes in the chromosomes. These changes are often described in five substages namely leptontene, zygotene, pachytene, diplotene and diakinesis.

Leptotene

• : Chromosomes shorten and become visible as single structures. In some cases they have a beaded appearance showing densely staining material called chromomeres alternating with nonstaining regions.

Zygotene

• : Paternal and maternal chromosomes come together and pair up. This pairing of homologous chromosmes is called synapsis. The paired chromosomes are described as bivalents. The bivalents shorten and thicken (spiralisation).

Pachytene

• : Each chromosome splits into two chromatids and thus each pair will have four chromatids two paternal and two maternal. They are now called tetrads. The non-sister chromatids of the paternal and maternal chromosomes overlap each other. They appear to be joined at several regions along their length. These points are called Chiasmata. Each chiasma is the site of an exchange of genetic material between the two chromatids. It occurs due to breakage and reunion between the two non-sister chromatids. This process is called genetic recombination.

Diplotene

• : The synaptic forces holding the two chromosomes in the pair come to an end. The chromosomes start separating. This separation is called as disjunction.

Diakinesis

• : Separation of the chromosomes is now complete with paternal and maternal chromosomes having exchanged portions of chromatids. The chromosomes condense again. The chiasmata disappear by sliding towards the tips of the chromatids. This process is called terminalisation.

By the time these changes are completed in the chromosomes, the nuclear membrane and nucleolus disappear. Asters and spindle fibres make their appearance.

fig. 17.7 - Behaviour of Chromosomes during Meiosis-I

Metaphase-I

In this change of a very brief duration, the chromosomes move towards the equator of the cell and come to lie in two parallel metaphase plates. These two parallel plates are formed by one set each of the homologous chromosomes. Each homologous chromosome has two kinetochores, one for each of its two chromatids.

Anaphase-I

There is no splitting of the centromere. As a result homologous chromosomes of each pair rather than the chromatids of a chromosome separate and move to the opposite poles. As a result, half the number of chromosomes that appear in the early prophase, move to each opposite pole. It is here that an actual reduction in the chromosome number (from (2n) to (n)) occurs. However, each chromosome found at the poles consists of two chromatids.

This is in contrast to the single stranded chromosomes in the anaphase of mitosis.

Telophase-I

The chromosomes at each pole uncoil and elongate to form the chromatin. A nucleolus reappears at each pole. Spindle fibres and asters disappear and centrioles split. A nuclear membrane is formed at each pole resulting in the formation of two daughter nuclei.

Cytokinesis - I

Simultaneously with the formation of two daughter nuclei, a cleavage furrow appears in the middle of the cell. The furrows gradually deepen and divide the cell into two daughter cells. Each of the resulting daughter cell prepares itself to undergo the second meiotic division.

Interkinesis

There is no interphase preceding second meiotic division. There is a brief intervening period called interkinesis. During this period there may be synthesis of some reserve food and proteins. However, there is no replication of DNA prior to meiosis II.

Second Meiotic Division

The second meiotic division or meiosis II almost always follows the first meiotic division. This division is primarily meant for separating the two chromatids of each chromosomes. Meiosis-II also has a Karyokinesis and a cytokinesis.

Karyokinesis of meiosis-II (Karyokinesis-II) can be distinguished into four stages namely prophase-II, metaphase-II, anaphase-II and telophase-II.

Prophase-II

It is of a short duration compared to prophase-I. No significant changes take place in the chromosomes. Nuclear membrane and nucleolus disappear. Asters and spindle fibres are formed.

Metaphase-II

The chromosomes line up at the equator of the cell forming a single metaphase plate (as in mitosis).

fig. 17.8 - Stages of Meiosis-II

Anaphase-II

The centromere splits and two chromatids in each chromosome start moving away from each other. Finally, they reach the poles of the cell. Each pole now has haploid number of chromosomes and half the amount of DNA.

Telophase-II

Chromosomes at each pole uncoil and elongate to form chromatin. Nucleolus and nuclear membrane are formed surrounding each chromatin network. Asters and spindle fibres disappear and centrioles divide. Daughter nuclei are formed.

Cytokinesis-II

A cleavage furrow appears in the middle of the cell, deepens gradually and divides the cell into two.

Thus, at the end of second meiotic division, four daughter cells are formed. Each daughter cell has not only half the number of chromosomes but also half the amount of DNA, as that of the parent cell. Thus, the resulting cells are truly haploid cells.

Significance of Meiosis

Meiosis becomes significant for the following reasons.

• It brings about a reduction in the chromosome number from a diploid (2n) condition to a haploid (n) condition. Such a reduction becomes necessary for maintaining the chromosome number.

• It provides chance for the appearance of new gene combinations as a result of crossing over. This situation brings about variations.

• It is a division necessary for the formation of gametes in animals and spores in plants.

Both mitosis and meiosis essentially follow the same sequence in all living organisms, which is an evidence of the basic relationship between diverse groups of living organisms

Mitosis Explained

It is a common type of cell division that occurs in all the cells of an organism. Hence, it is commonly called as somatic cell division. In mitosis, the resulting daughter cells will have the same number of chromosomes and contain the same amount of DNA, as that of the parent cell. Hence, mitosis is commonly described as equational division.

Mitosis occurs in two stages namely karyokinesis, the division of nucleus and cytokinesis, the division of cytoplasm. Just prior to karyokinesis, the cell will be in interphase.

fig. 17.2 - Stages of Mitosis

Interphase

It is the preparing phase. It is of varying duration depending on the cell type function. It is the period in which the cell carries out synthesis of organelles and increases in size. The nucleoli are prominent and actively synthesising ribosomes. Just prior to division, the DNA undergoes replication. Each chromosome exists as a pair of chromatids joined together by a centromere.

Karyokinesis

It is the division of nuclear material, represented by a sequence of events in the cell. It can be distinguished into four phases namely prophase, metaphase, anaphase and telophase.

Prophase

It is the longest stage of the division cycle. It is characterised by significant changes.

• Chromatids shorten (to about 4% of their original length) and thicken by spiralisation and condensation of DNA

• Centrioles move to the opposite poles of the cell

• Short microtubules develop, radiating from the centrioles. These are called asters

• Nucleolus gradually decreases in size and disappears

• Nuclear membrane disintegrates

• Spindle fibres appear in the cytoplasm

Metaphase

In this phase, chromosomes move to the equator of the cell.

• Pairs of chromatids become attached to the spindle fibres at their centromeres

fig. 17.3 - Stages of Mitosis

Anaphase

It is a rapid stage.

• Each centromere splits into two

• Spindle fibres pull the daughter centromeres to the opposite poles

• The separated chromatids, now called chromosomes, are pulled along with centromeres to the opposite poles

Telophase

It is the last phase of Karyokinesis.

• Chromosomes reach the poles of the cell, uncoil and lengthen to form chromatin

• Spindle fibres disintegrate and centrioles replicate

• A nuclear membrane is formed around chromosomes in each pole

• Two daughter nuclei are formed

As telophase is in progress, cytokinesis begins in the cell.

Cytokinesis

It is the division of cytoplasm. It occurs in animal cells by the appearance of a furrow in the middle of the cell. The furrow deepens and divides the cell into two. Two daughter cells are formed.

fig. 17.4 - Differences between Mitosis in Plant and Animal cells

Significance of Mitosis

Mitosis becomes significant for the following reasons.

• Mitosis forms two daughter cells which will have the same chromosome number and same genetic material as the parent cell.

• Daughter cells formed from mitosis are genetically identical to their parent cell and no variation would be introduced during mitosis. This results in genetic stability within the populations of cells derived from parental cells, as in a clone.

• The number of cells within an organism increases by mitosis and this process is called hyperplasia. It forms the basis for growth.

If mitotic division goes uncontrolled in any part of the body, it results in the formation of malignant cells. These cells continue to divide resulting in the formation of malignant tumours. This condition is called cancer.

• Mitosis is the basis of asexual reproduction in both plants and animals. This becomes the basis for vegetative propagation.

• Mitosis is also responsible for repair and regeneration of the injured and lost parts of the body.

Intro to the Cell Cycle

Every cell that is capable of undergoing division passes through a cyclic sequence of events involving growth and division. It is called Cell Cycle. It encompasses the entire sequence of events that occur in a cell from the time it is formed from its parent cell till the time of its own division into daughter cells.

Cell cycle has three main stages namely:

Interphase

This is a period of intense synthesis and growth in the cell. The cell produces many materials required for its own growth and activities. The genetic material DNA replicates during interphase.

Karyokinesis

It is the process of nuclear division, which involves separation of chromatids and their redistribution as chromosomes into daughter cells.

Cytokinesis

It is the process of division of the cytoplasm to result in the formation of daughter cells.

fig. 17.1 - The Cell Cycle

Phase	Events within cell
G1	Intensive cellular synthesis, mitochondria, chloroplasts (in plants), ER, lysosomes, golgi complex, vacuoles and vesicles produced. Nucleus produces rRNA, mRNA and tRNA and ribosomes are synthesised. Cell produces structural and functional proteins. Cell metabolic rate high and controlled by enzymes. Cell growth occurs. Substances produced to inhibit or stimulate onset of next phase.
S	DNA replication occurs. Protein molecules called histones are synthesised and cover each DNA strand, Each chromosome has become two chromatids.
G2	Intensive cellular synthesis. Mitochondria and chloroplasts divide. Energy stores increase. Mitotic spindle begins to form.
Mitosis	Nuclear division occurs in four phases
C	Equal distribution of organelles and cytoplasm into each daughter cells

The length of the cycle depends on the nature of cell and various external factors like temperature food and oxygen availability. Bacterial cells may divide every 20 minutes, epithelial cells living the small intestine divide once in 8 to 10 hours, onion root tip cells take about 20 hours to divide. Some specialised cells like the nerve cells never divide.

Tissue and Cell Types

Overview of Meiosis

Overview of Meiosis

Meiosis is a two-part cell division process in organisms that sexually reproduce. Meiosis produces gametes with one half the number of chromosomes as the parent cell.

In some respects, meiosis is very similar to the process of mitosis, yet it is also fundamentally different.

The two stages of meiosis are meiosis I and meiosis II. At the end of the meiotic process, four daughter cells are produced. Each of the resulting daughter cells has one half of the number of chromosomes as the parent cell.

Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.

During interphase the cell increases in mass, synthesizes DNA and proteins, and duplicates its chromosomes in preparation for cell division.

Meiosis I

Meiosis I encompasses four stages:

• Prophase I
  
• Metaphase I
  
• Anaphase I
  
• Telophase I
  

In most cases, at the end of meiosis I, two daughter cells are produced.

Meiosis II

Meiosis II encompasses four stages:

• Prophase II
  
• Metaphase II
  
• Anaphase II
  
• Telophase II
  

At the end of meiosis II, four daughter cells are produced. Each of these resulting daughter cells is haploid 

Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.

Interphase:

• G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
• In the latter part of interphase, the cell still has nucleoli present.
• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
• In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.

Overview of Mitosis

Overview of Mitosis

Cell division is an elegant process that enables organisms to grow and reproduce. Through a sequence of steps, the replicated genetic material in a parent cell is equally distributed to two daughter cells. While there are some subtle differences, mitosis is remarkably similar across organisms.

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Interphase is the "holding" stage or the stage between two successive cell divisions. In this stage, the cell replicates its genetic material and organelles in preparation for division.

Mitosis is composed of several stages:

• Prophase
  
• Metaphase
  
• Anaphase
  
• Telophase
  

Let's briefly look at some important events in each step in the process.

Prophase

In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and spindles form at opposite "poles" of the cell.

Metaphase

In metaphase, the chromosomes are aligned at the metaphase plate (a plane that is equally distant from the two spindle poles).

Anaphase

In anaphase, the paired chromosomes (sister chromatids) move to opposite ends of the cell.

Telophase

In this last stage, the chromosomes are cordoned off in distinct new nuclei in the emerging daughter cells. Cytokinesis is also occurring at this time.

At the end of mitosis, two distinct cells with identical genetic material are produced.

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Some 90 percent of a cell's time in the normal cellular cycle may be spent in interphase.

View image of a cell in interphase.

Stages of Interphase

• G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
• S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
• G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
• In the latter part of interphase, the cell still has nucleoli present.
• The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
• In animal cells, two pair of centrioles formed from the replication of one pair are located outside of the nucleus.