 You could ask what is the force that this motor exerts during a single step and the force for kinesin is roughly of the order of 10 piconewton. So, these are not small forces ah piconewton ordered forces which in the context of these nanometer sized objects are pretty large forces that these motors exert. Another another characterization another quantity that you can use to characterize motors is their processivity and what processivity means is that how long does a motor walk before it unbinds from the filament and falls off ok. So, these motors are walking on the filament these motors are walking on the filament, but before they walk they need to come and bind to this they need to come and bind to this filament right. So, it binds with some rates let us say k on, but similarly once a motor is bound and it is carrying a cargo it can also unbind with some rates right some k off ok. So, the typical sort of distance that a motor covers before it unbinds from this filament is what is called its processivity and different motors have different processivities. For example, RNA polymerase is a highly possessive motor it needs to walk thousands of bases basically it needs to walk the entire length of the gene ah before it needs it can unbind because if it unbound in the middle then you would get partial transcripts which are of no use to you. So, an RNA polymerase once it binds stays bound until typically the gene is completely red. So, it is a highly possessive motor. On the other hand if you in the other extreme if you consider muscle myosin some myosin that are found in muscles the actin fibers in muscles they are not very processive at all they just take two three steps before they unbind from the actin filaments ok and you can have a anywhere in between for example, dyneins are more processive than kinesins and so on. So, that is another measure that you can use to characterize this motor properties. So, the force exerted is sort of a measure of how much force would you typically need to bring this motor to a stop ok. So, what this means is that for kinesins typically you would need 10 piconewton force to completely stop this motor and I will I will show a slightly different context for this force measured sorry this force exerted has a more direct sort of meaning. Now, let us focus a little bit on let us say this kinesins and dyneins. So, here I have a microtubule right it has a minus end and a plus end. So, this is my microtubule and I have some cargo right which needs to be transported. I know that a kinesin if it attaches to this cargo will tend to walk towards the plus end. On the other hand if I had a dynein bound to this cargo let me draw if I had a dynein bound to this cargo it would try to move towards the minus end ok. Typically a cargo will have multiple motors bound to it and not necessarily of the same type. So, it could have multiple kinesins bound as well as multiple dyneins bound ok. If you had multiple kinesins as well as dyneins bound to this cargo what sort of motion would I see if I observe this cargo as a whole would it move towards the plus end or would it move towards the minus end. So, what these cargoes typically do is what is called as bidirectional motion. So, if you see the these dots these fluorescent dots these are cargos. If you see there are cargos that are moving sometimes sometimes moving this way and then sometimes moving back in the other way right. So, there is a single cargo depend and then depending on something we do not really understand completely what it depends on. It will sometimes move in this direction it will sometimes move in the other direction. But yet so and most cargos are going to be bound by both kinesins and dyneins. So, it is not that generally only one or the other is bound and what these movies show is that these cargos will typically do some sort of a bidirectional sometimes it will move this way sometimes it will move back that way. And yet somehow the cell sort of manages to deliver cargo very reliably to where it needs to go right and in the time scales that it needs to go in. So, even though there are both types of motors that are bound let us say your cargo has been produced here and it needs to go here in some amount of time. The cell will generally every time get you there even though the underlying motion might look very noisy bidirectional sort of motion. The macroscopic results are fairly robust and the mechanism of that also is not very clear how the cell regulates this sort of bidirectional transport in order to reliably achieve directed motion in the presence of these sort of opposing motors like kinesins and dyneins is not very clear. But at least at the microscopic level this is what the cargo does it executes bidirectional motion where it moves sometimes this and sometimes that. You can see it also over here this is I think mouse experiments on a mouse neuron where it again shows this sort of bidirectional motion. Ok. Now, you can think of this motors in a slightly different context. For example, instead of binding a cargo like this what it could do in principle is that if this motor one end is bound to the microtubule the other end is bound to another microtubule let us say. But let us say in the reverse direction let us say plus 1. So, these would generally be sort of complexes. So, this leg wants these are kinesins let us say. So, this leg wants to walk in this direction this leg wants to walk in this direction. They these because these are kinesins they both want to move walk towards the plus end of the microtubule. So, in a setup like this what would you observe it would observe sort of sliding of these microtubules related to each other right. So, if you have these sort of doublet microtubules you can have sliding of these microtubules related to each other which way this slide depends on the relative orientation of these microtubules whether they are parallel or anti-parallel. You could also imagine that this sort of walking causes bending of these microtubule structures oops. So, if you have these sort two microtubules on which these motors are walking on which this motors are walking, but you also introduce some sort of chemical cross links between them you also introduce some sort of chemical cross links at certain positions. So, that these motors so that these filaments cannot slide against each other then what this sort of walking will do is that it will produce local bending it will produce local bending of these structures. So, these are contexts in which directly these sort of motors can apply force and this force can result either in sliding or in sort of this bending of these structures. So, these are not typical cargo binding scenarios of these motors, but scenarios where these motors bind two filaments either parallel or anti-parallel or indeed a bunch of filaments. So, it can cause sliding it can cause bending. Another one of the most common examples of the roles of this sort of motor motion comes in this context of muscles and in muscles the relevant fiber is actually actin and the motor is myosin. So, instead of kinesins or dinanes it is actin it is myosin is walking on actin. So, here is my muscle where somebody is muscle and as you flex it you know the muscle fibers move. So, if you zoom inside this muscle what you find is that the basic unit is what is called the basics muscle cell in some sense is what is called the sarcomere. So, here is one sarcomere and then this sort of sarcomere structure repeats to form your muscle. Inside this sarcomere you will have these two sorts of filaments. So, thick red one this is called the thick myofilament and this thinner orange ones which are called the thin myofilaments. So, this orange ones are called the thin myofilaments. What happens when you contract or expand the muscle is that these two the thick and the thin myofilaments slide against each other. So, here is if you zoom in even further the thick myofilaments are nothing but thousands and thousands of myosin motors all bunched up into a filament with only the myosin head domain which binds to this actin that being free. So, I have this let me watch the over here the thin myofilaments are what are your actins and this myosin heads these go and bind to the actins. They walk along the actin, but because the myosins are formed into this thick bundle the bundle this myosins themselves do not move, but these actins actually slide relative to this myosin bundle. So, when this actin head sorry when this myosin head executes what is called as a power stroke. So, it takes in ATP it converts that ATP into ADP plus energy and it takes a stroke. So, let us see I think this is what will show. So, it gets activated by absorbing ATP and it forms a cross bridge with this actin fiber that is the release of the inorganic phosphate. Once it is formed a cross bridge it will execute a power stroke which means it will so, it releases the ADP as well it executes a power stroke like this. So, it slides this actin filament relative to this myosin filament and then it sort of release this bond releases which is called the cross bridge it releases and then it gets reactivated again to walk once more. So, ATP binds again and it releases this bond and then the cycle will be bond repeating. So, this thick myofilament is so, I have these actin heads and let me say this actin bodies. So, sorry myosin ATP. So, these are all my myosin motors ok. The bodies of these actin myosin motors all bunch up together to form this thick myofilament to form this thick myofilament. What is what is visible are these heads of these motors of these myosin motors and these heads walk along this actin filament over here this helical actin filament over here. As it walks on this actin filament there is a related sliding of the actin relative to this thick myofilament and that causes this movement of the this sort of relative sliding that causes the contraction and expansion of the muscles. This is sort of governed by the release of calcium ions. So, why does it not always walk that is because the myosin binding domain on this actin that is not always visible to the myosin. It gets visible only when calcium ions come and bind and it causes a sort of structural change in this actin filaments. So, this sort of motion of this these molecular motors like myosin on these actin filaments is what underlies this muscle movement any sort of muscle movement. So, this is just that same thing, but in terms of pictures. So, these are my thick filaments over here and you can see the myosin heads and then there are these thin filaments on which these myosins walk. As they walk they produce a related sliding of the thin relative to the thick filaments and that is how your muscles contract. So, this whole thing is one sarcomere and then this structure is repeated n number of times to form a complete muscle. One more context where these actins and myosins exert forces comes in context of cell division. So, this was the spindle formation by Michael T. Views, but once the once this the chromosomes have been separated out for example, over here there is an actin ring that forms in the center of the cell along with myosin filaments which walk on these actin rings on these actin filaments. This actin actomyosin ring rather this actomyosin ring is contractile. So, it slowly pinches off the middle of the cell until it you form two daughter cells basically from a single cell. So, this contractility is again a function of these myosin motors walking on the actin filaments. So, the same sort of underlying biology motors walking on filaments. So, this sort of translational motors underlie a lot of very different phenomenon cargo transport which is straightforward, but also this sort of cell division or muscle movement muscle contraction and so on. So, very different very different macroscopic phenomenon, but the underlying biology is very similar. So, you have these motors walking on these filaments. A nice way to visualize these sort of forces that these motors exert on the filaments comes in the form of these gliding assay experiments. So, where you take a glass slide and you fix your motors onto the glass slides and then you throw in a bunch of filaments let us say actin filaments. So, if these motors were myosins then you throw in a bunch of actin filaments because these motors are fixed to the glass slide the motors themselves cannot move, but what you will see is motion of these actin filaments as a whole. So, these are called gliding assays and so, here for example, is a typical gliding assay, what you are you are seeing are these actin filaments that are moving. It seems they are moving on their own, but what is happening is that underneath which we cannot see are these myosin motors they are walking along these actin filaments causing a sort of sliding of these actin filaments relative to the cover slip. This is a somewhat dilute system. So, you have very few sort of actin filaments and so on. You can make this very concentrated for example, and you get very interesting phenomena. So, for example, this is an experiment done with again actin and myosin similar sort of experiment, but extremely high density now ok. So, what you are seeing again you are not seeing the motors what you are seeing are simply the filaments, but now at a much higher density than the previous experiment and you see this very beautiful patterns that form and merge and dissolve and you have different vortices that arise. So, there is a collective sort of motion of these actin filaments because of the coupling with this underlying motors walking on these filaments. It can give rise to a variety of phenomenon and different scales depending on the density and so on. So, that is one sort of way to visualize the effect of the interactions of the motors with these filaments this gliding assays. You could also do sort of single motor assays in which you observe a single bead being moving along a track whether it be microtubule or actin. So, for example, if you can see these are this is a bead for example, this is a kinasein experiment if I remember correctly. So, it is moving on microtubules and you will see that the bead sort of attaches and then it moves along the microtubule after some time it will detach and go off. So, one sort of thing that you can ask from these experiments for example, is how do these motors walk along the how do these motors walk along the underlying microtubule or actin.