Cell cycle phases | Cells | MCAT | Khan Academy

Summary

This script explores the fascinating world of cell biology, comparing the vast scale of a human to the microscopic realm of a cell. It delves into the cell cycle, akin to seasons, with interphase being the 'growing' season and mitosis the 'active division' phase. The script outlines the stages of interphase, including G1 for growth and protein synthesis, the S phase for DNA replication, and G2 for preparing for mitosis. It also touches on the G0 phase, where cells like neurons cease division. The cycle's continuous nature is highlighted, emphasizing the cell's life stages, from growth to division, unless they enter the non-dividing G0 phase.

Takeaways

• 🧬 The average human height is more than 5 feet or about 2 meters, while a cell's diameter is approximately 100 micrometers, making it one one-millionth the size of a human.
• 🌿 Cells, like humans, grow and reproduce through a process known as cell division.
• 🔁 The cell cycle is a series of phases that a cell goes through, similar to the changing seasons in a year.
• 🌱 Interphase is the longest phase of the cell cycle where cells primarily grow but do not divide, and most cells spend their time in this phase.
• 🔍 Cancer cells are an exception to normal cell behavior, as they tend to divide more frequently than they grow.
• 📈 The cell cycle includes phases named G1, S, and G2, each with specific roles in cell growth and preparation for division.
• 🧬 G1 phase is the longest phase where cells produce extra organelles and proteins necessary for cell division.
• 🔬 S phase stands for synthesis, specifically DNA synthesis, where cells replicate their DNA from 23 pairs to 46 pairs of chromosomes.
• 🛑 Some cells may enter the G0 phase, where they cease to divide, like neurons in the brain which do not typically divide after brain development.
• 🔄 Mitosis is the phase where active cell division occurs, resulting in two new cells that can then enter the G1 phase to grow and prepare for future division.

Q & A

What is the average height of a person in meters?

The average person is a little less than two meters tall.

How does the size of a cell compare to the size of a human being?

A cell has a diameter of about 100 micrometers, which is about one one-millionth of the size of a human being.

What is the smallest unit of life?

The smallest unit of life is a cell.

How does cell division in cells compare to human reproduction?

Just as humans grow and have babies, cells also grow and undergo division, which is a form of reproduction.

What is the term used to describe the life span of a cell?

The life span of a cell is described by the cell cycle.

What are the two main phases of the cell cycle?

The two main phases of the cell cycle are interphase, where the cell grows but does not divide, and mitosis, where the cell actively divides.

What is interphase and why is it significant?

Interphase is the period in the cell cycle where the cell primarily grows but does not divide. It is significant because this is where cells spend most of their time.

What exception to the typical cell cycle is mentioned in the script?

Cancer cells are an exception to the typical cell cycle because they have a defect that causes them to divide more than they grow.

What is the first phase of interphase called and what happens during this phase?

The first phase of interphase is called G1, during which the cell grows and produces extra organelles and proteins.

What is the S phase in the cell cycle, and what occurs during this phase?

The S phase stands for synthesis, specifically DNA synthesis, where DNA replication occurs and chromosomes are duplicated.

What is the G2 phase, and what is its purpose in the cell cycle?

G2 is another growth phase where the cell prepares for mitosis by making microtubules and other structures necessary for cell division.

What happens to a cell after it completes the cell cycle?

After completing the cell cycle, each of the resulting cells enters the G1 phase again, where they grow and produce organelles and proteins for future division.

Outlines

00:00

🌱 Understanding the Cell Cycle and Its Phases

This paragraph introduces the concept of the cell cycle, comparing it to the changing seasons in a year. It explains that cells, like humans, grow and divide, but on a much smaller scale. The cell cycle is divided into two main phases: interphase, where cells grow but do not divide, and mitosis, where active cell division occurs. Interphase itself consists of three sub-phases: G1, where cells grow and produce organelles and proteins; S phase, where DNA replication takes place; and G2, where cells prepare for mitosis. The paragraph also touches on the exception of cancer cells, which have a defect causing them to divide more frequently, and the G0 phase, where cells cease to divide, as seen in neurons.

05:01

🔄 The Continuous Nature of the Cell Cycle

This paragraph delves deeper into the cell cycle, emphasizing its cyclical nature. It describes the final phase of mitosis, where the cell divides into two, and each new cell enters the G1 phase to grow and prepare for future division. The comparison to seasons continues, illustrating the repetitive cycle of growth and division unless a cell enters the G0 phase, indicating a permanent halt in cell division. The paragraph concludes by reinforcing the idea of the cell cycle as a continuous process of cellular renewal and growth, with the exception of specialized cells that do not require further division.

Keywords

💡Cell

A cell is the smallest unit of life that can function independently. In the context of the video, cells are compared to humans to illustrate the vast difference in size and complexity. Cells, like humans, grow and reproduce through a process called cell division. The video uses the analogy of seasons to describe the cell cycle, emphasizing that cells, too, have periods of growth and division, akin to the changing seasons in nature.

💡Cell Cycle

The cell cycle refers to the series of events that take place in a cell, leading to its division and duplication. It is likened to the changing seasons in a year, with periods of growth and division. The video script explains that the cell cycle consists of interphase, where cells grow, and mitosis, where cells divide. Understanding the cell cycle is crucial for grasping how cells function and reproduce.

💡Interphase

Interphase is the phase of the cell cycle where the cell grows and carries out its normal functions but does not divide. It is the longest phase of the cell cycle, and most cells spend the majority of their existence in interphase. The script mentions that interphase is akin to the fall or winter season, where growth occurs but not division, which is a critical concept for understanding the cell's life cycle.

💡Mitosis

Mitosis is the phase of the cell cycle during which a cell divides to produce two daughter cells, each with the same number of chromosomes as the parent cell. The video script describes mitosis as the 'active cell division' phase, where cells replicate their genetic material and then physically split into two. This process is essential for growth, repair, and maintenance of an organism's tissues.

💡G1 Phase

The G1 phase, or the first gap phase, is the initial phase of interphase where the cell grows and produces organelles and proteins necessary for cell division. The video script uses the analogy of spring, where growth occurs in preparation for division. The G1 phase is the longest phase of the cell cycle, indicating its importance in cellular growth and preparation for division.

💡S Phase

The S phase stands for the synthesis phase, during which DNA replication occurs. The video script explains that in this phase, the cell duplicates its 23 pairs of chromosomes, resulting in 46 pairs. This is a critical step in ensuring that each daughter cell receives a complete set of genetic information during cell division.

💡G2 Phase

The G2 phase, or the second gap phase, follows the S phase and is another growth phase where the cell prepares for mitosis. The video script mentions that during G2, cells produce microtubules, which are essential for the separation of chromosomes during cell division. This phase is crucial for ensuring that the cell is ready for the physical process of division.

💡G0 Phase

The G0 phase is a resting phase where cells exit the cell cycle and do not divide. The video script uses the example of neurons in the brain, which, once formed, do not typically divide again. Understanding G0 is important for grasping that not all cells in an organism are actively dividing and that some cells have specialized roles that do not require continuous division.

💡Cancer

Cancer is mentioned in the video script as an exception to the normal cell cycle, where cells have defects that cause them to divide more than they grow. Cancer cells often bypass the normal regulatory mechanisms of the cell cycle, leading to uncontrolled division and growth. This concept is crucial for understanding the difference between normal cellular growth and the abnormal growth seen in cancer.

💡Chromosomes

Chromosomes are structures within the cell that contain genetic information. The video script explains that during the S phase of the cell cycle, chromosomes are replicated, ensuring that each daughter cell receives a complete set of genetic information. Chromosomes play a critical role in the process of cell division and the transmission of genetic traits.

💡DNA Replication

DNA replication is the process by which a cell duplicates its genetic material. The video script describes this process as occurring during the S phase of the cell cycle, where the cell's 23 pairs of chromosomes are copied. This is essential for ensuring that each new cell formed during cell division has the same genetic information as the parent cell.

Highlights

An average person is a little less than two meters tall.

A cell has a diameter of about 100 micrometers, which is about one one-millionth of the size of a human.

Cells, like humans, grow and undergo cell division to 'have babies'.

The cell cycle is introduced as a concept similar to seasons in a year.

Interphase is the period where cells primarily grow but do not divide, and it's where cells spend most of their time.

Cancer cells are an exception, as they tend to divide more than they grow.

Mitosis is the phase of active cell division, abbreviated as 'M'.

The G1 phase is the longest phase of the cell cycle, where cells produce extra organelles and proteins.

The S phase stands for synthesis, specifically DNA synthesis, where DNA replication occurs.

Some cells may enter the G0 phase, where they cease to divide, like neurons in the brain.

The G2 phase is another growth phase where cells prepare for mitosis by making microtubules.

Mitosis is the final phase of the cell cycle where the cell divides into two.

After mitosis, cells enter the G1 phase again to grow and produce organelles and proteins for future division.

The cell cycle is cyclical, much like the changing of seasons, with cells continuously dividing unless they enter the G0 phase.

Transcripts

So here I've got a picture of an average person.

They're a little more than 5 feet tall.

So if we were to convert that into meters,

the average person is a little less than

about two meters tall.

Now, a person is the largest unit of life we consider.

When we talk about the smallest unit of life,

we're talking about a cell

that relatively looks like that.

And while a human being is about two meters tall,

a cell has a diameter of about 100 micrometers,

which, to put into perspective relative to a human being

is about one one-millionth of the size.

And yet there's so much that goes on here.

Just as human beings grow and maybe will have babies,

cells do the exact same thing.

They grow and have babies, as well,

or they undergo cell division.

So, in the next couple of videos,

we're going to talk about how a cell grows and divides.

So let's zoom in on the cell right here

and spend the next couple of videos talking about

how a cell grows and divides.

The life span of a cell can be described by what's called

the cell cycle.

The cell cycle can be thought of as seasons in a year.

Just as we have seasons such as the spring or the summer

where things grow, versus fall and winter where they don't,

the cell has times when it grows and divides

and other times when it doesn't divide.

There are two main overarching seasons,

or types of seasons,

that we can talk about here.

There's this period here that's more like

the fall or the winter, where you don't have

as much cell division, but you have

more growth of the cell.

This period is where the cell spends most of its time,

and it's called interphase. Interphase: where we

primarily have cell growth occur,

but not cell division.

Interphase is where cells spend most of the time,

so most cells live here.

But there is one key exception.

What do you think that one exception might be?

I think I heard you correctly if you said, "Cancer."

You are absolutely right.

Cancer cells have some defect in them that causes

them to want to divide more so than grow,

and we'll talk more in detail

about how that occurs in a minute.

The other main phase of the cell cycle here

is where you have active cell division,

and it's called mitosis.

Mitosis, or sometimes it's abbreviated with just an "M."

Mitosis is the time where you have active cell division.

Now, there are a few other phases that occur within interphase.

The first part of interphase is a growth phase.

It's usually abbreviated, "G1."

As you can see here, G1 is the

longest phase of the cell cycle, so

most of a cell's life is spent here,

and it's in this phase that we produce

extra organelles, such as ribosomes and proteins.

So we make proteins that will be useful when

we get to the point of cell division.

>From here, the cell has a choice.

If it wants to continue growing

and move towards the direction of cell division,

it will move forward this way

to the next phase that's called the, "S phase."

The S phase just stands for synthesis,

more specifically DNA synthesis,

because here we're going to have DNA replication.

That's where we take 23 pairs of chromosomes,

so 23 pairs. We call them pairs because half of them

are from your mom, half of them are from your dad,

and we duplicate them, we replicate them,

and we end up with 46 pairs.

and most cells go in this direction

as I mentioned over here.

Some cells however, instead of going forward,

from G1 they'll go in another direction here

to a phase that's called, "G not," or G 0,"

where you have no more division.

No more cell division.

Because there are certain cells in the body that

don't like to divide or don't tend to divide,

and you can think of a quick example like neurons in the brain.

Once your brain is formed

it doesn't necessarily need to divide any more.

You just have cells grow.

So that means it's sort of

the end point for these types of cells.

They won't usually come back and enter

the cell cycle in this way.

But let's continue as if it had.

The next step, or the next phase of the cell cycle,

is called, "G 2," which is another growth phase

where we are more directly preparing for mitosis.

So, we prepare for mitosis in a couple of ways.

Perhaps a good example is we make microtubules,

which will be used to pull our chromatids apart

when it comes time for anaphase,

and we'll talk about what that means in a separate video.

And finally, to be complete, we have our last phase

right here, which is just mitosis,

which is our final season of the cell cycle

where our cell will divide.

And once it's divided and turned into two cells,

each of the cells will next enter the G1 phase,

where they will grow and produce

extra organelles and proteins that will eventually

allow them to divide again. So, as you can see,

just like seasons in the year,

the cell cycle goes around and around as we

divide cells further and further,

unless it's a cell that goes off to this G not or G0 phase

where we don't need any more cell division.