GENERAL BIOLOGY LESSON 3: Cell Cycle and Cell

Rudolf Virchow

In 1858, he coined the cell doctrine which states that "When a cell arises, there must have been a previous cell, just as animals can only arise from animals and plants from plants."

The Cell Cycle

Most eukaryotic cells will proceed through an ordered series of events in which the cell duplicates its contents and then divides into two cells. This cycle of duplication and division is called __________.

G1

S

G2

M

The cell cycle is a series of events that lead to cell division. It consists of four phases namely:

Gap

In the cell cycle, G means _______

Synthesis

In the cell cycle, S means ______.

Mitosis

In the cell cycle, M means _____.

Interphase

Mitotic phase

G0 phase

A eukaryotic cell cycle consists of two main parts: 1) _______ and 2) _________, phase, and an alternative part called the ________.

G1

S

G2

G0

Parts of the Interphase

G1 phase

During this phase, the cell reviews the cellular environment and the cell size to ensure that the conditions are appropriate to support DNA replication.

G0

If all is not ready to undergo DNA replication, cells can pause during G1 and enter a phase called ____.

G0

Depending on a cell's preparedness to continue in the cell cycle, ___ can last days, weeks, or even years

S phase

When the cell has reached an appropriate size and is in a supportive environment for DNA replication, it will exit either G1 or G0 and enter the next phase of interphase called _________.

S phase

Cell growth continues through this phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis.

G2 phase

Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called ______.

G2 phase

Occurring between the end of DNA replication in S phase and the beginning of cell division in mitosis, this phase can be thought of as a safety gap during which a cell can check to make sure that the entirety of its DNA and other intracellular components have been properly duplicated.

G2 phase

In addition to acting as a checkpoint along the cell cycle, ______ also represents the cell's final chance to grow before it is split into two independent cells during mitosis.

quiescent stage

Cells in G0 phase are not actively preparing to divide. The cell is in a _______ _______ that occurs when cells exit the cell cycle.

Cardiac Muscles

Nerve Cells

Examples of cells that remain permanently in G0 phase.

Checkpoint

A stage in the eukaryotic cell cycle at which the cell examines internal and external cues and "decides" whether or not to move forward with division.

G1 Checkpoint

G2 Checkpoint

M Checkpoint

Three checkpoints of the cell cycle.

G1

The integrity of the DNA is assessed at the ___ checkpoint.

Proper chromosome duplication

________________ is assessed at the G2 checkpoint.

Attachment of each kinetochore to a spindle fiber

What is assessed at the M checkpoint?

G1 Checkpoint

Present just before the entry into S-phase, a cell checks whether internal and external conditions are right for division.

Size

Nutrients

Molecular signals

DNA integrity

Factors a cell might assess in the G1 Checkpoint

DNA integrity

DNA replication

What is checked in the G2 Checkpoint?

The Spindle Checkpoint

The M checkpoint is also known as _______________.

M Checkpoint

The cell examines whether all the sister chromatids are correctly attached to the spindle microtubules.

M Checkpoint

This checkpoint looks for "straggler" chromosomes that are in the wrong place (e.g., floating around in the cytoplasm) If a chromosome is misplaced, the cell will pause mitosis, allowing time for the spindle to capture the stray chromosome.

Regulator molecules

These may act individually, or they can influence the activity or production of other regulatory proteins.

Positive Regulators

Promote progress of the cell to the next phase

Cyclins

Cyclin-dependent kinases

Groups of proteins that are responsible for the progress of the cell through the various checkpoints.

Concentration of cyclin proteins

Increases in the ____________ ___________ are triggered by both external and internal signals.

Cyclin-dependent protein kinase enzymes

DNA replication and mitosis are dependent on the activity of ______________, which are heterodimers of a catalytic subunit with a cyclin subunit.

Heterodimers of a catalytic subunit with a cyclin subunit

Cyclin-dependent protein kinase (CDK) enzymes are _______________________.

Cyclin-dependent protein kinase

An enzyme that adds negatively charged phosphate groups to other molecules in a process called phosphorylation

Phosphorylation

Through this process, Cdks signal the cell that it is ready to pass into the next stage of the cell cycle.

Cyclin-dependent protein kinases

As their name suggests, they are dependent on cyclins, another class of regulatory proteins.

Cyclins

Named as such because they undergo a constant cycle of synthesis and degradation during cell division.

Cyclin-Cdk complex

This complex then acts as a signal to the cell to pass to the next cell cycle phase.

Mitotic Cyclins

G1 Cyclins

Two classes of cyclins

Phosphorylation

Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through ________.

Phosphorylation

When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via ________.

Negative Regulation

Halts the cell cycle

Retinoblastoma proteins

A group of tumor-suppressor proteins common in many cells.

53 and 21 designations

Refers to the functional molecular masses of the proteins (p) in kilodaltons.

Damaged or non-functional

All three of these regulatory proteins were discovered to be ___________ in cells that had begun to replicate uncontrollably (became cancerous).

G1 Checkpoint

Rb, p53, and p21 act primarily at the ______________.

p53

A multi-functional protein that has a major impact on the commitment of a cell to division because it acts when there is damaged DNA in cells that are undergoing the preparatory processes during G1.

p53

It halts the cell cycle and recruits enzymes to repair the DNA if damaged DNA is detected.

p53

Can trigger apoptosis, or cell suicide, to prevent the duplication of damaged chromosomes if the DNA cannot be repaired.

p21

As p53 levels rise, the production of ______ is triggered. It enforces the halt in the cycle dictated by p53 by binding to and inhibiting the activity of the Cdk/cyclin complexes.

p53 and p21

As a cell is exposed to more stress, higher levels of __________ accumulate, making it less likely that the cell will move into the S phase

“The guardian of the genome”

a famous tumor suppressor often described as ______________.

Mutations

p53 prevents _________ from being passed on to daughter cells.

If When p53 is defective or missing, it can potentially lead to ___________.

24 hours

A typical human cell might takes how many hours to divive?

Mitosis

A fundamental process for life. During this, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells.

Prophase

Metaphase

Anaphase

Telophase

Four Stages of Mitosis

Prophase

Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.

Mitotic spindle, the microtubules, the proteinaceous components

In the Prophase, initiation of the assembly of _______, _________, and __________ of the cell cytoplasm help in the process.

Kinetochores

In the Metaphase, Spindle fibres attach to _________ of chromosomes.

Metaphase

Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles

Metaphase plate

The imaginary center/equator of the cell where they move and align to be attached to spindle fibers

Anaphase

Centromeres split and chromatids separate. Chromatids move to opposite poles.

Telophase

Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.

Nuclear envelope

In the Telophase, __________ assembles around the chromosome clusters as the nucleolus, golgi complex and ER reform.

Cytokinesis

The process by which the cell divides into two daughter cells.

Contractile ring of actin and myosin filaments

In animal cells, a _________________ ________ _________ forms around the cell, contracting and pinching the cell membrane until the cell is divided into two separate cells.

Cell Plate

In plant cells, a structure called the ____ ____ forms along the equator of the cell, eventually dividing the cell into two separate daughter cells.

Meiosis I

Reduces the number of chromosomes from diploid to haploid

Meiosis II

Produces four haploid daughter cells

Interkinesis

The period of time between meiosis I and meiosis II is called __________.

Interkinesis

No replication of DNA occurs during _________ because the DNA is already duplicated.

Homologous chromosomes

Paired chromosomes with genes for the same trait arranged in the same order

Allele

Gene form for each variation of trait of an organism

Alleles

Homologous chromosomes may have different ________ on them.

Synapsis

Each pair of homologous chromosomes come together to form a tetrad

Tetrad

Two homologous chromosomes come together and the four chromosomes overlap.

Crossing Over

The exchange of genetic material by non-sister chromatids during late prophase I of meiosis. which result in a new combination of alleles.

Chiasmata

X-shaped regions where crossing over occurred.

Metaphase I

Tetrads line up at the metaphase plate, with one chromosome facing each pole

Microtubules

___________ from one pole are attached to the kinetochore of one chromosome of each tetrad.

Anaphase 1

Pairs of homologous chromosomes separate.

Centromere

Sister chromatids remain attached at the ________ and move as one unit toward the pole.

Spindle Apparatus

In Anaphase I, chromosome moves toward each pole guided by the ________ ________.

Telophase 1

The cell now undergoes cytokinesis that divides the cytoplasm of the original cell into two daughter cells.

Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell.

Prophase II

A spindle apparatus forms as chromosomes (each still composed of two chromatids) move toward the metaphase plate

Metaphase II

The sister chromatids are at the metaphase plate. The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.

Anaphase II

The sister chromatids separate. The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles. (sister chromatids are not identical)

Telophase II

The chromosomes arrive at opposite poles and cytokinesis separates the cytoplasm.

There are four daughter cells, each with a haploid set of unreplicated chromosomes. Each daughter cell is genetically distinct from the others and from the parent cell.

Crossing Over

Random Assortment of Chromosomes

Random fusion of gametes from different parents

The three main sources of genetic variation arising from sexual reproduction

Chiasmata

The exchange of genetic material occurs between non-sister chromatids at points.

Recombinants

Are chromatids that consist of a combination of DNA derived from both homologous chromosomes.

recombinant chromosomes

Offspring with ________ ________ will have unique gene combinations that are not present in either parent.

Random

When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is _______.

Independently

The orientation of each bivalent occurs ___________, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I.

8,388,608

Human have 46 chromosomes, and thus can produce ________ different gametes by random orientation.

46 chromosomes

Number of chromosomes in humans

Diploid zygote

The fusion of two haploid gametes results in the formation of a ________ _________.