Cell Cycle Overview

Summary

This podcast script offers an insightful overview of the cell cycle, mitosis, and meiosis, emphasizing the production of genetically identical cells in mitosis and the vast genetic diversity in meiosis. It delves into the stages of the cell cycle, including interphase and the M phase, and explains the regulatory role of cyclin-dependent kinases. The script also discusses cell cycle checkpoints, crucial for preventing genetic damage, using an analogy of a washing machine to illustrate the sequential and progressive nature of cell cycle events.

Takeaways

• 🔬 The podcast discusses the cell cycle, focusing on the differences between mitosis and meiosis, using the mathematical examples of 1 to the 46th power and 2 to the 23rd power to illustrate the genetic diversity in cells produced by each process.
• 🌟 The cell cycle's main objectives include understanding the events in each phase, the stages of mitosis and meiosis, and the regulatory mechanisms involving cyclins and cyclin-dependent kinases (CDKs).
• 📚 The cell cycle is divided into discrete, non-overlapping stages: interphase (G1, S, G2) and M phase (mitosis and cytokinesis), with mitosis involving one round of DNA synthesis and chromosome separation, and meiosis involving one round of DNA synthesis and two rounds of chromosome separation.
• 🧬 The cell cycle is controlled by heterodimeric proteins consisting of a regulatory cyclin subunit and a catalytic CDK subunit, which play a crucial role in regulating the cell cycle's progression.
• 📈 Interphase has three components (G1, S, G2), and M phase has two components (karyokinesis and cytokinesis), with cells spending most of their time in interphase, especially in G1 and G2 phases.
• 🔄 Cyclins and CDKs are crucial for cell cycle regulation, with specific complexes activating or inhibiting different stages of the cell cycle, such as G1 cyclin-CDK for cell cycle initiation, S phase cyclin-CDK for DNA replication, and M phase cyclin-CDK for mitosis progression.
• 🌱 The cell cycle's progression is sequential and unidirectional, with checkpoints ensuring that each stage is completed correctly before moving on to the next, preventing catastrophic genetic damage.
• 🔍 The podcast highlights the importance of the cytoskeleton in cell division, particularly spindle fibers for chromosome movement and the contractile ring for cell separation.
• 📊 The DNA content in human somatic cells varies depending on the cell cycle phase, with cells in G1 having 2C DNA content, cells in S phase having 2-4C, and cells in G2 having 4C DNA content.
• 🛠 Cell cycle control is essential for preventing genetic damage, with checkpoints acting as brakes to halt or slow the cell cycle if necessary, such as in response to DNA damage or improper chromosome attachment to the spindle.

Q & A

What is the main purpose of mitosis in the cell cycle?

The main purpose of mitosis is to produce genetically identical daughter cells from one parent cell.

How does the concept of 2 to the 23rd power relate to meiosis?

2 to the 23rd power represents the potential number of genetically distinct cells that can be formed during meiosis, which is over 8 million, highlighting the diversity of gametes produced.

What are the three components of interphase in the cell cycle?

The three components of interphase are G1 phase, S phase (where DNA synthesis occurs), and G2 phase.

What are the two main components of the M phase in the cell cycle?

The two main components of the M phase are karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).

What role do heterodimeric proteins play in controlling the cell cycle?

Heterodimeric proteins, consisting of a regulatory cyclin subunit and a catalytic cyclin-dependent kinase subunit, control the progression of the cell cycle.

What are the functions of cyclin and cyclin-dependent kinases (CDKs) in the cell cycle?

Cyclin and CDKs regulate the passage through the cell cycle by forming complexes that trigger events at specific stages, such as DNA replication and mitosis.

What is the significance of the G1 checkpoint in the cell cycle?

The G1 checkpoint ensures that DNA is replicated before the cell proceeds to the S phase, preventing errors and maintaining genomic integrity.

How does the cell cycle differ between somatic cells and cells in early embryos?

In early embryos, the cell cycle progresses rapidly with minimal gap phases, whereas in somatic cells, the G1 and G2 phases can be more extended.

What is the concept of 'N' and 'C' in the context of the cell cycle and human chromosomes?

'N' refers to the haploid number of chromosomes, which is unique to each species and is 23 for humans. 'C' represents the amount of DNA in a haploid cell, approximately 3.5 picograms for humans.

How do differentiated cells, like neurons or cardiac myocytes, relate to the cell cycle?

Differentiated cells, such as neurons or cardiac myocytes, are typically in the G0 phase, meaning they have exited the cell cycle and no longer divide.

What is the importance of cell cycle checkpoints in preventing genetic damage?

Cell cycle checkpoints act as safeguards to ensure that critical events, such as DNA replication and chromosome segregation, occur correctly, thus preventing catastrophic genetic damage to daughter cells.

Outlines

00:00

🔬 Introduction to the Cell Cycle and Meiosis

The script begins with an introduction to the cell cycle, emphasizing the distinction between mitosis and meiosis through mathematical examples. It sets the learning objectives for understanding the phases of the cell cycle, stages of mitosis and meiosis, and the role of cyclin and cyclin-dependent kinase in cell cycle regulation. The cell cycle's central events—chromosome duplication, separation, and cell division—are described as occurring in a unidirectional and progressive manner. The script also introduces the concept of heterodimeric proteins controlling the cell cycle and outlines the stages of interphase and M phase.

05:02

🌀 Cyclin-Dependent Kinase Regulation in the Cell Cycle

This paragraph delves into the regulatory mechanisms of the cell cycle, focusing on cyclin and cyclin-dependent kinase complexes. It explains the role of G1, S, and G2 phases within interphase and the activation of specific cyclin-CDK complexes that drive the cell cycle forward. The paragraph also provides a simplified view of the cell cycle with a table and figure, highlighting the activity of cyclins and CDKs and their impact on various cellular processes, including the regulation of tumor suppressor proteins and transcription factors.

10:04

📈 Understanding Cell Cycle Dynamics and Cytoskeleton's Role

The script discusses the dynamics of the cell cycle, emphasizing the importance of the cytoskeleton in processes like chromosome movement and cell division. It describes the role of microtubules in spindle fiber formation and actin in the contractile ring. The paragraph also classifies somatic cells based on their mitotic activity, distinguishing between static, stable, and renewing cells, and explains the concept of differentiated cells that exit the cell cycle. Additionally, it provides a simplified overview of the cell cycle stages and the DNA content changes during the cycle.

15:05

🧬 DNA Content Analysis and Cell Cycle Control

This section presents an analysis of DNA content in human somatic cells using fluorescence-activated cell sorting, illustrating the distribution of cells in different phases of the cell cycle. It explains the DNA content for cells in G1, S, and G2/M phases and introduces the concepts of 'n' for the haploid number of chromosomes and 'C' for the amount of DNA in a haploid cell. The script also discusses the importance of cell cycle control, using an analogy of a washing machine to explain the sequential and progressive nature of the cell cycle and the role of checkpoints in ensuring proper cell cycle progression.

20:07

🛑 The Importance of Cell Cycle Checkpoints

The final paragraph highlights the critical role of cell cycle checkpoints in preventing genetic damage. It explains the necessity of checkpoints for ensuring that events like DNA replication and chromosome attachment to the spindle occur correctly. The script uses a cartoon to illustrate the potential consequences of checkpoint failure, such as nondisjunction, which could lead to daughter cells with abnormal chromosome numbers. The importance of these checkpoints in maintaining genomic stability and preventing catastrophic genetic outcomes is emphasized.

Keywords

💡Cell Cycle

The cell cycle is the series of events that take place in a cell, leading to its division and duplication. It is central to the video's theme as it provides the framework for understanding how cells grow and divide. The script discusses the cell cycle's phases, including interphase (G1, S, G2) and the M phase, which includes mitosis and, in the case of meiosis, two rounds of chromosome separation.

💡Mitosis

Mitosis is a type of cell division that results in two genetically identical daughter cells, each having the same number of chromosomes as the parent cell. It is a key concept in the script, used to illustrate the process of cell duplication where the main objective is to maintain genetic identity among cells.

💡Meiosis

Meiosis is another form of cell division that results in four non-identical daughter cells, each with half the number of chromosomes of the parent cell. The script uses the concept of meiosis to contrast with mitosis, highlighting the difference in genetic diversity produced through the potential of 2^23 combinations during meiosis.

💡Chromosomes

Chromosomes are thread-like structures made of DNA and protein found in the nucleus of a cell. They carry genetic information crucial for the functioning, traits, and reproduction of all living organisms. The script discusses how chromosomes duplicate and segregate during the cell cycle, which is fundamental to both mitosis and meiosis.

💡Cyclin-Dependent Kinase (CDK)

Cyclin-Dependent Kinases are a group of protein kinases that regulate cell cycle progression by forming complexes with cyclins. The script explains that these heterodimeric proteins, composed of a regulatory cyclin subunit and a catalytic CDK subunit, control the transitions between different phases of the cell cycle.

💡Cyclin

Cyclins are proteins that regulate the cell cycle by binding to and activating cyclin-dependent kinases. The script mentions different types of cyclins, such as cyclin D in the G1 phase, cyclin E in the S phase, and cyclin B in the M phase, each playing a role in moving the cell cycle forward.

💡Interphase

Interphase is the phase of the cell cycle during which the cell prepares for division by carrying out metabolic activities and duplicating its DNA. The script describes interphase as consisting of G1, S, and G2 phases, emphasizing the importance of DNA synthesis (S phase) and the preparatory gap phases (G1 and G2).

💡M Phase

The M phase of the cell cycle encompasses the processes of mitosis and cytokinesis, leading to the division of the cell into two daughter cells. The script details the components of the M phase, including karyokinesis (nuclear division) and cytokinesis (cytoplasmic division), as the culmination of the cell cycle.

💡Checkpoints

Checkpoints in the cell cycle are control mechanisms that ensure the accuracy of cell division by monitoring and coordinating the sequence of events. The script discusses the importance of checkpoints for preventing errors such as unreplicated or damaged DNA and improper chromosome attachment to the spindle.

💡DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. The script mentions that DNA replication occurs during the S phase of interphase and is a prerequisite for the cell to proceed to the M phase, ensuring that each daughter cell receives a complete set of genetic information.

💡Differentiation

Differentiation refers to the process by which a cell becomes specialized in structure and function. The script touches on the G0 phase, where cells that have differentiated and are no longer dividing reside, such as neurons or cardiac myocytes, indicating a state of cellular maturity and specialization.

💡Cytoskeleton

The cytoskeleton is a network of protein fibers that provide structural support and facilitate cell movement and intracellular transport. The script highlights the role of the cytoskeleton in the cell cycle, particularly in the formation of spindle fibers for chromosome movement during mitosis and the contractile ring for cell division.

Highlights

The podcast provides an overview of the cell cycle, mitosis, and meiosis, emphasizing the difference between producing genetically identical cells in mitosis and the vast genetic diversity in meiosis.

The cell cycle's purpose is to ensure the orderly sequence of events for cell reproduction, including chromosome duplication, separation, and cell division.

The cell cycle progresses uni-directionally through discrete, non-overlapping stages, with DNA synthesis in the S phase and chromosome separation in the M phase.

Heterodimeric proteins, consisting of a regulatory cyclin subunit and a catalytic cyclin-dependent kinase subunit, control the cell cycle.

Interphase has three components: G1, S, and G2 phases, while M phase includes karyokinesis and cytokinesis.

Cyclin-cyclin-dependent kinase complexes regulate the cell cycle's progression, including G1, S, and M phase transitions.

The G1 cyclin-CDK complex with cyclin D allows cells to pass the restriction point and start the cell cycle.

S phase cyclin-dependent kinases, primarily involving cyclin E, initiate DNA replication.

M phase cyclin-CDK complexes, mainly cyclin B or MPF, drive cells through mitosis or meiosis.

Activation of cyclin-dependent kinases requires cyclin binding, and inactivation involves cyclin degradation, essential for cell cycle progression.

The Nobel Prize in Physiology or Medicine was awarded for the fundamental understanding of the cell cycle.

Differentiated cells, such as neurons or cardiac myocytes, enter a G0 phase, exiting the cell cycle.

Somatic cells can be classified by their mitotic activity into static, stable, and renewing cells, each with distinct division characteristics.

The cell cycle's sequential and progressive nature is likened to a washing machine's operation, emphasizing the importance of checkpoints.

Checkpoints act as brakes, preventing or slowing cell cycle progression to ensure proper DNA replication and chromosome segregation.

Failure of cell cycle checkpoints can lead to nondisjunction and catastrophic genetic consequences for daughter cells.

The podcast concludes by underscoring the importance of cell cycle control in preventing genetic damage and maintaining cellular integrity.

Transcripts

[Music]

this podcast will be an overview of the

cell cycle and then we'll do a brief

podcasts on mitosis and another brief

podcast on mitosis

whenever I talk about the cell cycle I

like to start it with this very simple

slide and just ask question what is 1 to

the 46th power and what is 2 to the 23rd

power I ask this question because it's a

good simple way to understand the

difference between mitosis and meiosis

clearly 1 to the 46th power is 1 and

when you think about mitosis the object

of mitosis is to produce genetically

identical daughter cells from one parent

cell on the other hand if you look at 2

to the 23rd that's a really big number

it's more than 8 million and that's a

good way to describe meiosis because in

meiosis the chromosomes behave as pairs

and they segregate independently of each

other at the first meiotic division we

have 23 pairs of homologous chromosomes

so if the first meiotic division there's

a minimal potential of more than 8

million genetically distinct types of

cells that will be going into forming

the gametes your learning objectives for

these 3 podcasts you should be able to

describe the events that occur in each

phase of the cell cycle you should be

able to describe stages of mitosis and

meiosis it's useful to be able to do

this by comparing and contrasting you

should be able to describe the concepts

of C and N and you should be able to

describe how cyclin and cyclin dependent

kinase is regulate passage through the

cell cycle the central events of the

cell cycle would be duplication of

chromosomes separation of chromosomes

and cell division or formation of

daughter cells we like to say that these

stages occur in discrete non overlapping

stages the cell cycle is progressive and

it's uni-directional that means it only

goes in one

rection you have to have DNA synthesis

that occurs in the S phase of the cell

cycle and then chromosome separation

occurs in the M phase of the cell cycle

and for mitosis there's one round of DNA

synthesis one chromosome separation

meiosis has one round of DNA synthesis

and two chromosome separations we've

come to understand over recent years

that heterodimeric proteins control the

cell cycle there's a regulatory cyclin

subunit anise catalytic cyclin-dependent

kinase subunit and this cartoon is just

meant to give you that view so there's

the regulatory cyclin subunit and the

catalytic cyclin dependent kinase

subunit and as we've said the cell cycle

is an orderly sequence of events in

which a cell is going to reproduce by

duplicating its contents that's going to

occur an interphase and then it's gonna

divide and that's going to occur in the

M phase of the cell cycle we understand

there are three components to interphase

there's a g1 component an S component

and a g2 component and then there are

two components to M phase there's

karyokinesis and cytokinesis now let me

just quickly review these s is the

synthesis phase g1 stands for a gap

phase before DNA begins to synthesize

itself so it's a gap from the end of

cell division to the start of DNA

synthesis g2 is a gap from the end of

DNA synthesis to the start of cell

division and then we talked of course

about a g0 phase where cells have

differentiated and have taken themselves

out of the cell cycle cells in very

early embryos don't have a significant

of protracted gap phases so the cell

cycles in early embryos pretty well

progress rapidly through DNA synthesis

and

cell division come to understand that

there are several cyclin

cyclin-dependent kinase is that regulate

passage through the cell cycle in all

cells and this would include human cells

in this slide is a little bit of an

oversimplification but it has the main

points that I would want you to worry

about for our course we can talk about a

g1 cyclin-cdk neighs complex the cyclin

would be cyclin D and that causes cells

to pass a restriction point and you

might think of this if you want as the

start of the cell cycle it's a little

bit artificial but it helps to sometimes

think about things in a linear manner so

we could say the g1 cyclin-cdk neighs

complex

allow cells to start the cell cycle then

we have an S phase cycling cyclin

dependent kinase conflicts the main

cyclin here would be cyclin e and that's

involved in initiating DNA replication

and then there's an M phase or a mitotic

cyclin-cdk ace the main cyclin here

would be cyclin B or for historical

reasons MPF for maturation phase

promoting factor and the M phase cyclin

cyclin dependent kinase complex allows

the cells to progress through mitosis or

there are mitotic cyclins that allow the

cell cycle to progress through meiosis

this is a little bit more complicated

view of the cell cycle this table and

figure come from the textbook that we

use for the course it's beyond the scope

of what I want you to know for our

course but it makes a couple of

interesting points first of all the

figure shows you in general where the

cyclin and cyclin-dependent kinases are

active in the cell cycle tracing through

from g1 through SG to nm and then the

table shows you some of the targeted

phases of the cell cycle where these

cyclones and cyclin dependent

NASA's operate and gives you an idea in

broad scope of some of the effective

proteins that are targeted and noticed

that some of them are tumor suppressor

proteins others proteins that regulate

passage through the cell cycle various

transcription factors things like

nuclear lamins might be affected etc

this is way beyond the scope of what you

need to know for our course but I put it

here just to give you a general

perspective of what might be going on in

regulating the cell cycle going back to

a simplified view of things we can say

that activation of cyclin dependent

kinase --is would require cyclin binding

so here's a mitotic cyclins

binding on to the mitotic cyclin-cdk

neighs that would for example trigger

machinery for mitosis likewise to

inactivate the cyclin cyclin dependent

kinases you have to degrade the cyclin

and the only way you can go from one

stage to the next in the cell cycle is

to degrade the appropriate cyclin to

progress forward so in this simplified

figure it shows the activation and

degradation of the mitotic cyclin-cdk

NACE and then its subsequent degradation

to progress from g1 beyond ass into g2

this basic understanding of the cell

cycle led to the Nobel Prize in

Physiology and medicine

this shows a simplified human somatic

cell cycle across the top one can see

the stages of mitosis

so this prophase prometaphase metaphase

anaphase a anaphase B and telophase and

then the simplified diagram shows the

major stages of the cell cycle

interphase consisting of g1 s g2 and

then mitosis and then of course it shows

differentiated cells cells will go into

G sub 0 and for our course we might

think of differentiated cells

cells like neurons or cardiac myocytes

any cells essentially that no longer

divide some cells remain in a prolonged

g1 stage so for example adult stem cells

for various organs or tissues within

organs might be in a prolonged g1 stage

memory cells of the immune system would

be in a prolonged g1 stage and we've

already told you that in embryos g1 and

g2 a very brief in a typical cell cycle

g1 is usually the longest phase g1 and

g2 can be variable but g1 is usually the

longest phase in a typical somatic cell

cycle this again is a simplified

overview of cell cycle time and we

sometimes in our minds I think of the

cell cycle is a 24 hour time frame that

may or may not be accurate but we can

just kind of make a point and rapidly

dividing adult cells most time is spent

in interphase so for example through g1

s and g2 and rapidly dividing cells

cells like the bone marrow cells in the

GI epithelium or tumor cells and most of

their time in interphase the bottom

piece of the cartoon is just to remind

you of the importance of the

cytoskeleton in relation to the cell

cycle clearly the spindle fibers for

moving the chromosomes in the

contractile ring for separating the cell

during cell division the spindle fibers

of course are mostly microtubules the

contractile ring mostly act and

interacting with myosin now somatic

cells can be classified by their mitotic

activity and we talked about static

cells those would be cells that are no

longer dividing that'd be post mitotic

cells they usually long-lived cells so

for example cells of the central nervous

system skeletal muscle cells cardiac

muscles

we usually consider those as static

cells we can also talk about stable

cells these are cells that might divide

episodically

to maintain tissue or organ structure

they can be stimulated to divide more

rapidly if there's an injury so for

example we might think of periosteal or

perichondrium cells as stable cells

smooth muscle cells would be stable

cells endothelial cells fibroblasts

stable cells these would divide

episodically but if there's an injury

they can divide much more rapidly to

heal the injury and we can talk about

renewing cells these would be cells of

display regular mitotic activity and the

daughter cells will either differentiate

or they'll remain as stem cells

differentiated cells may be lost from

the body we can talk about a slowly

renewing population of cells they may

increase in size so for example smooth

muscle of hollow organs fibroblasts in

the uterine wall lands epithelial cells

or even slow growing tumor cells will be

slowly renewing cells we might also talk

about a rapidly renewing population of

cells like the progresses of the blood

cells in bone marrow epithelial cells

and dermal fibroblasts of skin the

mucosa lining of the GI tract or tumor

cells in rapidly growing tumors these

would be rapidly renewing cell

populations here is a simplified way to

look at the cell cycle we talk about g1

we talk about s we talk about g2

remember that an S DNA replicates so in

the simple-minded diagram we could say

this cell would be 2 n + 2 C this is in

g1 as the cell passes through s the DNA

is replicating and as the cell is in g2

we would say that that cell is 2 n 4 C

and then as the cell goes through my

for example two daughter cells are

produced that about two to n and to see

so here I want to define for you the

concept of N and C first in general

terms and then specifically for humans

when we talk about n we're talking about

the haploid number of chromosomes for a

given species n is unique to that

species for humans the haploid number of

chromosomes is 23 C is the amount of DNA

in a haploid cell usually we take that

as the amount of DNA in a mature sperm

so in humans

C is three and a half Piko grams of DNA

here's an interesting diagram that looks

at DNA content in human somatic cells

looking at the DNA content by a

fluorescence activated cell sorter or a

flow cytometer on the y-axis one plots

the number of cells on the x-axis one

plots the relative amount of DNA per

cell and you can immediately see that

there are three populations of cells and

these types of experiments there's a

population of cells that have not

replicated that DNA that's what's shown

by the cells that are in peak a these

would be cells in g1 and then there are

cells that have replicated the DNA needs

to be the cells represented in peak B

and those could be cells that are either

in g2 or in the early M phase of the

cell cycle and then there are cells that

are intermediate between these two peaks

these would be cells in the S phase of

the cell cycle and you can notice on the

x-axis that there's a relative increase

in DNA amount as you move along the x

axis another point to make from this

type of experiment if you look at the

distribution of cells there are more

cells in g1 and that's how we've come to

understand

g1 is the longest phase in the cell

cycle now I told you that the DNA

content for human sperm equals three and

a half Piko grams with that information

what would the DNA content be for the

cells that are represented in Peaks a

and Peaks B I hope you understand that

peak a would represent cells that are to

see before DNA replication they would

have seven picograms of DNA cells at

peak B would be in g2 they would have a

four C amount of DNA or fourteen Piko

grams of DNA here I'd like to spend just

a few minutes and talk about cell cycle

control and the first point I want to

make is that cell cycle control relates

to the fact that the stages of the cell

cycle occur in a sequential or

progressive manner this is a cartoon

that comes out of the Albert's textbook

and I really like it because it's so

oversimplified that it makes good sense

this is a washing machine dial and if

you think about the way a washing

machine works to wash your clothes for

example before you close wash this

machine has to fill with water once the

machine fills with water the Machine

washes the clothes once the machine

washes the clothes the machine empties

the water and then rinses the clothes

you can't go backwards you can't rinse

the clothes before you wash the clothes

so it's a simple-minded analogy but it's

a good way to explain how the cell cycle

works

it occurs in stages in sequential stages

and it occurs in a progressive manner

and to move from one stage to the next

requires that certain events must occur

just like in the washing machine so with

respect to the cell cycle you enter the

cell cycle you trigger DNA replication

machinery only when that machinery has

been triggered can

be replicated you cannot trigger the

mitosis machinery until DNA is fully

replicated once DNA is fully replicated

the machinery to trigger mitosis is

activated and then once that happens the

mitotic spindle is assembled once all of

that occurs you can put complete cell

division so again sequential progressive

manner this allows the cell to go

through various checkpoints so for

example you can say is DNA replicated

the g1 checkpoint would say is DNA

replicated if it's not then you stop you

replicate DNA or you repair DNA if it's

damaged and then replicate DNA other

chromosomes on the spindle if they're

not on the spindle you stop and attach

the chromosomes to the spindle that's

necessary before the cell can divide so

the checkpoint signals act as brakes on

negative controls to halt or slow

progression through the cell cycle

checkpoints allow input of extracellular

signals from neighboring cells as well

so we can say that control of the cell

cycle is all about triggers and

checkpoints this is a very much more

complicated slide than we need to worry

about for our costs but you can see some

major triggers in the cell cycle there's

an S phase entry point there's an M

phase entry point and then there's an

anaphase and telophase trigger point and

then you can see a variety of

checkpoints around the cell cycle again

more complicated than we need to worry

about but there's a unreplicated DNA

checkpoint a spindle assembly checkpoint

a chromosome segregation checkpoint and

then look at the DNA damage checkpoints

that can operate all the way around all

the triggers

in the cell cycle so again a much more

complex slide than we need to worry

about for our costs but you can ask the

simple question why do you have to have

cell cycle checkpoints that would be an

absolute requirement to prevent

catastrophic genetic damage and this

simple-minded cartoon shows what happens

if there's a failure of the chromosome

segregation checkpoint you would get

nondisjunction somehow the chromosome

segregation checkpoint fails not all the

chromosomes are attached to the spindle

so when the cell divides one cell is

going to miss a chromosome whereas

another cell is going to get an extra

chromosome you can imagine that could

lead to very catastrophic genetic

consequences to the daughter cells

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you

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