Cell Cycle & Regulation, Mitosis, Cyclins, RB, P53 & Tumor Suppressors (USMLE Esssentials)

hello everybody and welcome to the med a

USM step one Essentials video series

where we talk about very high yield step

one topics in this video we will be

talking about a topic which shows up

very frequently on the exam that topic

is the cell cycle and the proteins

involved in regulating it namely cyclin

retinol blastoma protein or RB

mp53 as we will soon learn the genes

which code the these proteins are known

as tumor suppressor genes and they are

extremely important because they are

associated with the development of

Cancer all right so let's get started

the cell cycle can be broken down to two

major phases inase and mitosis interface

and mitosis can be further broken down

to various subphases as well an easy way

to conceptualize and memorize the cell

cycle is to picture a new cell which has

just freshly divided from a parent cell

this cell will start off here right

after mitosis because mitosis is

essentially the part of the cell cycle

in which the cell physically divides if

the cell starts right after mitosis then

it must start off in the G1 phase which

is the first phase of

interphase as the cell matures it will

eventually go through the S phase and

then the G2 phase the S phase always

occurs before the G2 phase we will talk

about why shortly lastly mitosis occurs

after the G2 phase during mitosis the

cell essentially physically splits to

form two new identical

cells it is not as worthwhile to

memorize the sequence of the phases of

mitosis as it is to know what is exactly

happening in each

phase another worthwhile thing to

understand is that before a cell

progresses to a new phase in the cell

cycle certain conditions must be met

these conditions can be thought as

quality control requirements the process

by which this happens is often referred

to as cell cycle checkpoints and we will

talk about them in just a

minute for now let's talk about mitosis

as we have already mentioned mitosis is

the part of the cell cycle in which the

nucleus and cytoplasm of a parent cell

splits into two genetically identical

cells known as daughter

cells the Reon reason it is important to

know that the daughter's cells are

genetically identical to each other and

the parent cell is because there is

another type of cell division in which

this is not the

case this type of cell division is known

as meosis and it is the type of cell

division which occurs in germ cells

which is another name for sperm and US

sites so remember in mitosis the

daughter cells are always genetically

identical and in meiosis they are not

errors in mitosis can lead to one of

three consequences it can lead to

inhibition of cell division and

therefore the cell cycle this is the

principle behind the anti-neoplastic

drugs such as pxel and Vin blastin these

drugs create errors and mitosis which

cause the entire cell cycle to stop and

therefore the growth of the tumor errors

and mitosis can also lead to cell death

which explains why these drugs can also

kill cancer

cells lastly errors and mitosis can lead

to genetic mutations such as anlo which

is when the cell has an incorrect number

of

chromosomes okay so let's briefly go

over the individual steps of

mitosis the first phase of mitosis is

known as prophase the principal thing

that happens during prophase is that the

nuclear envelope breaks down and

chromatine begins to condense in order

to form

chromosomes the next phase is called a

metaphase several things occur in

metaphase first the chromatine condenses

even more to form full chromosomes next

the chromosomes align at the center of

the cell and microtubules attach from

the chromosomes to the centrosome which

are located in the poles of the cell the

next phase is called the anaphase during

anaphase the chromosomes are pulled to

the opposite poles of the cell by the

action of

microtubules it is important to

recognize that metaphase and anaphase

are the phases of mitosis which the

antineoplastics such as pxel and Vin

blastin exert their effect these drugs

work worked by inhibiting the action of

microtubules which are essential for

mitosis

progression the last phase of mitosis is

called telophase in this phase the

chromosomes begin to decondense back to

chromatine and the nuclear envelope

reforms while these are all important

things to know regarding mitosis perhaps

the most important thing to know about

mitosis in regards to the cell cycle is

that once mitosis has occurred any DNA

changes or mutations which occurred

during replication cannot be corrected

in other words the DNA mutations become

permanent for this reason mitosis can be

thought as an end point in the steps of

the cell cycle because once mitosis is

complete there is nothing else that can

be done to ensure that DNA mutations can

be

repaired throughout the cell cycle there

are numerous complex quality control

checkpoints which ensure that mitosis

occurs only when the cell is free of DNA

mutations the culmination of all of

these checkpoints and complex molecular

mechanisms is to ensure that no DNA

mutations have occurred during the

process of DNA

replication okay so now let's talk about

the other major components of the cell

cycle

interphase interphase is the part of the

cell cycle in which the cell grows

replicates its DNA and synthesizes new

proteins in preparation for

mitosis cells which are constantly

dividing such as bone marrow cells gut

epithelium skin and hair follicles spend

a short period of time in interphase

while cells which do not divide such as

neurons and muscle cells are permanently

stuck in

interphase neurons and muscle cells

which do not divide are said to be in a

stage of interphase known as

g0 so just like mitosis interface is

composed of several phases as we said

before it is useful to think as the G1

phase as the first phase of the cell

cycle since this is the phase that

occurs right after a pair of new cells

have formed Following

mitosis during the G1 or growth Phase 1

the cell grows makes new organel and

synthesizes proteins required for DNA

replication during this phase the cell

is preparing for DNA

replication during the S phase or

synthesis phase the cell's DNA is

replicated during the G2 or growth Phase

2 the cell begins to synthesize the

proteins required for mitosis in

preparation for cell

division lastly cells in the g0 phase

are not preparing for application but

instead are in a resting state this is

also known as the resting phase cells

which enter the g0 phase may stay

permanently in the g0 phase or may enter

the cell cycle and undergo

mitosis now that we understand the steps

of interface and mitosis let's talk

about the cell cycle checkpoints these

checkpoints are extremely important and

relevant because dysfunction of these

checkpoints is believed to be one of the

major mechanisms by which cancer occurs

throughout the cell cycle there are

various quality control checkpoints

which function to ensure that various

cellular conditions are met before

proceeding with the cell cycle and

ultimately cell

division one of the conditions which

these checkpoints checks is that no DNA

errors are present which can potentially

be passed down to the Future daughter

cells remember once mitosis has occurred