2. The Eukaryotic Cell Cycle, Mitosis and Meiosis

How many cells does the adult human body contain?

10-50 trillion cells

What is the exception for not having 10-50 trillion cells?

Rare mutations (DNA sequences of all chromosomes are the same in all cells)

What does the careful reproduction of cells ensure?

The integrity of the genetic material through mitosis and meiosis

What is a cell cycle?

A highly regulated series of events that leads to cell division

What happens when cells get ready to divide?

The chromosomes become compact enough to be seen with a light microscope

Chromosomes

Big, long, thready strands of DNA that coil and fold upon themselves

What’s the disease where cells divide uncontrollably?

Cancer

Do chromosomes form daughter cells?

Yes

What is cytogenetics?

Filed of genetics involving microscopic examination of chromosomes

How many sister chromatids are in a chromosome?

Each chromosome has a pair of sister chromatid. 2 altogether

Do Eukaryotic chromosomes occur in sets?

Yes

How many sets of chromosomes does ONE set of human chromosomes have? What do they include?

23

• 22 autosomes (non-sex chromosomes)

• Sex chromosomes - X and Y

How many sets of chromosomes do most human cells have?

46 chromosomes in total

• Two chromosome sets = diplous or 2n

How many sets of chromosomes does a gamete have? What is a gamete?

A gamete (sperm and egg) has 1 set of chromosomes = haploid or n

What are members of a pair of chromosomes called in a diploid species?

Homologs or homologous chromosomes

Homologous chormosomes

• Pairs are nearly identical in size and genetic composition

• Contain some sequence differences that provide genetic variation

What % do homologs differ by?

Less than 1%

Which sex chromosomes is bigger?

X

• Sex chromosomes are very different from each other in size and composition

The cell cycle in order

G₁: First gap

S: Synthesis of DNA

G₂: Second gap

M: Mitosis and cytokinesis 

• A cell may also exit the cell cycle and enter a nondividing phase called G0

What happens during the G₁ phase?

A cell grows and becomes committed to divide and accumulates molecular changes that promote progression through the cell cycle

What happens in the S phase

Each chromosomes is replicated and forms a pair of sister chromatids

What happens during the G₂ phase?

A cell synthesizes proteins needed for chromosome sorting and cell division; some growth may occur

What happens during the M phase?

Cell undergoes:

• Mitosis

• Cytokinesis, which divides the cytoplasm into two daughter cells

What occurs during mitosis?

Divides one cell nucleus into two and distributes the duplicated chromosomes so that each daughter cell receives the same complement of chromosomes

Length of cycle for quickly growing embryos

Several minutes

Length of cycle for slow growing adult cells

Several months

Length of cycle for fast dividing mammalian cells in adults (skin cells)

10 to 24 hrs

How long does each phase last in a cell that divides in 24 hrs?

G1 – 11 hours

S phase – 8 hours

G2 phase – 4 hours

M phase – 1 hour

Why is the cell cycle highly regulated?

To ensure that cells only divide at the appropriate time

What is advancement through the cell cycle controlled by?

Two proteins

• Cyclins: levels rise and fall during the cell cycle

• Cyclin-dependent kinases (cdks)

What do cyclins and cdks form when combined?

Form an activated cyclin/cdk complex

What does the activated cyclin/cdk complex do?

It phosphorylates and activates other proteins needed to advance the cell cycle

• If the right signals and nutrients are present, the cell will divide

What is needed for the cell to divide in an activated cyclin/cdk complex?

The right signals and nutrients are present, the cell will divide

Cyclins rise and cyclins fall. What does this mean?

• Made → rise

• Degraded → down

Eukaryotic cell check points

• G1 checkpoint

• G2 checkpoint

• Metaphase checkpoint

G1 checkpoint?

This is the restriction point and the proteins determine if conditions are favourable for cell division and also can sense DNA damage

G2 checkpoint?

This checks for damaged DNA and enures that all DNA has been replicated. This also monitors the levels of proteins needed to advance through M

Metaphase checkpoint

This monitors the integrity of the spindle apparatus. It also checks that all chromosomes are correctly attached to the spindle apparatus.

What do checkpoint proteins act as?

They act as sensors to determine if the cell is in proper condition to divide. It also acts as an inhibitor to stop the cell cycle using cyclin-dependent kinases.

Checkpoint steps in order?

• G1 checkpoint

  • Determined favourable conditions, damaged DNA

  • G1 cyclin degraded after cell enters S phase

• G2 checkpoint

• Metaphase checkpoint

What happens to the mitotic cyclin as it progresses through mitosis?

it is degraded

What organism did Masui and Markert study to investigate oocyte maturation?

Frog oocytes.

In what phase are frog oocytes normally dormant?

The G2 phase of the cell cycle

What happens to the oocyte when it is dormant in G2 phase?

It does not progress through the cell cycle; it is paused before mitosis.

What hormone triggers progression of frog oocytes from G2 to M phase?

Progesterone.

What happens when the oocyte advances to the beginning of M phase?

Chromosomes condense, signaling maturation.

What was the main question Masui and Markert sought to answer?

How progesterone causes dormant frog oocytes to enter M phase.

Why is understanding oocyte maturation important?

It reveals mechanisms controlling cell cycle progression in development.

What factor did Masui and Markert identify as required to advance the oocyte cell cycle?

Maturation-promoting factor (MPF).

What is MPF composed of?

A complex of a mitotic cyclin and a cyclin-dependent kinase (cdk).

What is the function of MPF in oocyte maturation?

It drives the transition from G2 phase to M phase, triggering chromosome condensation and maturation.

What happens in mitotic cell division?

Cell divides to produce two new cells (daughter cells) genetically identical to the original (mother cell)

What does mitotic cell division involve?

Mitosis followed by cytokinesis

What is mitosis?

Division of one nucleus into two nuclei

What is cytokinesis?

Division of one cell into two

What is mitotic cell division used for?

• Asexual reproduction (e.g. in single-celled yeast or amoeba)

• Development and growth of multicellular organisms

When does each chromosome replicate?

Prior to mitosis, during the S phase.

What happens to chromosomes at the start of mitosis?

They become compact.

What is the “first squiggle” in terms of DNA replication?

The initial appearance of DNA before S phase.

How identical are sister chromatids?

100%

How are sister chromatids positioned before division?

One goes to one side of the cell, the other to the opposite side.

What cellular structures help separate sister chromatids?

Microtubules.

What is the spindle apparatus composed of?

Microtubules 

What is the spindle apparatus responsible for?

Responsible for the organizing and sorting of chromosomes

What are the two major types of microtubules that extend into the middle of the cell?

Polar microtubules and kinetochore microtubules.

Where does microtubule growth and organization begin?

At two centrosomes (microtubule-organizing centers, or MTOCs).

When does a centrosome duplicate produce?

Two centrosomes that define the two poles of the spindle apparatus.

What do centrosomes do in mitosis?

They build microtubules and allow the cell to rapidly extend or retract them.

What role do spindle poles play?

They pull sister chromatids apart toward opposite sides.

What do kinetochore microtubules do?

They attach to chromosomes at the kinetochore and tug them toward a pole.

How do microtubules behave during spindle formation?

They grow and retract; some bump into a chromosome, attach, and hold on tightly.

What are the phases of interphase?

G1, S, and G2.

What is the state of chromosomes during interphase?

They are decondensed in the nucleus.

When does DNA replication occur?

During S phase of interphase.

What are the phases of mitosis in order?

Prophase, Prometaphase, Metaphase, Anaphase, Telophase (PPMAT)

What happens during cytokinesis?

The cell divides into two daughter cells.

What is the mnemonic to remember the order of mitotic phases?

PPMAT

What happens to chromatids during prophase?

They condense into highly compact, visible structures.

What happens to the nuclear membrane in prophase?

It begins to dissociate into small vesicles.

What happens to the nucleolus in prophase?

It is no longer visible.

What happens to the nuclear envelope in prometaphase?

It completely fragments.

What forms fully during prometaphase?

The spindle apparatus.

What attaches sister chromatids to the spindle in prometaphase?

Kinetochore microtubules from opposite poles

What proteins hold sister chromatids together during prometaphase?

Cohesin proteins

What characterizes metaphase?

Sister chromatids align in a single row on the metaphase plate.

What is the metaphase plate?

A plane halfway between the two spindle poles.

What happens mechanically during metaphase?

Microtubules tug chromatids back and forth until they line up.

What happens to the connection between sister chromatids in anaphase?

It is broken when cohesin proteins are cleaved.

What do kinetochore microtubules do in anaphase?

hey shorten, pulling chromatids to opposite poles.

How do the poles move during anaphase?

The two spindle poles move farther apart.

Mechanistically, how does each chromatid move?

Each microtubule “takes” one chromatid to its pole.

What happens to chromosomes during telophase?

hey reach the poles and begin to decondense.

What happens to the nuclear membrane in telophase?

It reforms around each set of chromosomes, producing two nuclei.

Prophase — Order of Events

• Chromosomes begin to condense.

• Chromatids become visible under the microscope.

• Nucleolus disappears.

• Nuclear membrane starts breaking down into vesicles.

• Early spindle fibres begin to form.

Prometaphase — Order of Events

• Nuclear envelope completely fragments.

• Centrosomes move farther apart, establishing poles.

• Spindle apparatus is fully formed.

• Microtubules extend and retract toward chromosomes.

• Kinetochore microtubules attach to kinetochores on sister chromatids.

• Cohesin proteins still hold sister chromatids together.

Metaphase — Order of Events

1. Kinetochore microtubules tug back and forth on chromatids.

2. Chromosomes migrate to the center of the cell.

3. Sister chromatids line up at the metaphase plate (middle).

4. Cell pauses at metaphase checkpoint to ensure proper attachment.

Anaphase — Order of Events

• Cohesin proteins are cleaved (released).

• Sister chromatids separate.

• Kinetochore microtubules shorten, pulling chromatids to poles.

• Polar microtubules lengthen, pushing poles apart.

• Poles move farther apart as chromatids arrive.

Telophase — Order of Events

• Chromosomes arrive at the poles.

• Chromosomes begin to decondense.

• Nuclear membranes reform around each set.

• Nucleoli reappear.

• Spindle apparatus disassembles.

Cytokinesis — Order of Events

(Overlaps with telophase)

1. Contractile ring (actin + myosin) forms.

2. Cleavage furrow develops.

3. Cell membrane pinches inward.

4. Cell fully splits into two daughter cells.

When does cytokinesis occur relative to mitosis?

It usually follows mitosis quickly.