Cell Cycle + Cell Cycle Regulation

Why do unicellular organisms divide?

To reproduce

Why do multicellular organisms divide?

1. Growth and development

2. To replace damaged/dead cells

Binary fission

A method of cellular division that only requires replicating and splitting genetic material to create an identical daughter cell; used by prokaryotes

Interphase

The part of the cell cycle in which the cell grows, carries out normal functions, and replicates its DNA (consists of G₁, S, and G₂)

G₁ phase

The cell goes through normal growth and processes

S phase

The cell duplicates its DNA

G₂ phase

The cell prepares for nuclear division

M phase

Division of the nucleus (eukaryotic cells) and the rest of the cell

• Mitosis - somatic cells

• Meiosis - gametes

Chromatin

“Unwound” form of genetic material - consists of DNA/histone protein complex

Chromosomes

Densely wound and packed chromatin - each species has a different number of chromosomes per cell, represented by n

How many chromosomes do humans have?

n = 23; this means gametes (haploids - n) have 23 chromosomes and somatic cells (diploids - 2n) have 46

Homologous chromosomes

Chromosomes that encode for the same gene, but different alleles; one from mom, one from dad

Sister chromatids

Two copies of the same chromosome stuck together by a centromere

Kinetochore

The protein complex in a chromosome’s centromere where the spindle fiber attaches

G₀ phase

A phase in which the cell is ‘arrested’ in interphase. May happen because

1. The cell has differentiated 

2. There are not enough nutrients to go further with the cell cycle

Phases of Mitosis

Prophase

• Chromosomes condense

• Nuclear membrane dissolves

• Spindle fibers begin to form

Metaphase

• Chromosomes line up on the metaphase plate

• Spindles attach to sister chromatids

Anaphase

• Spindles pull sister chromatids toward opposite poles

Telophase

• Chromosomes begin to unravel

• The nuclear membrane begins to rebuild around the separated chromatin

Cytokinesis

The splitting of the cytoplasm between the daughter cells

In animal cells: the cell membrane pinches off at a place called the cleavage furrow

In plant cells: vesicles of cellulose form a cell plate and eventually a new cell wall in between the new cells

What is the Chi-Squared test used for?

To tell whether the difference in between two groups of data is statistically significant or not

How to tell how many degrees of freedom you should use in the Chi-Squared test

Use one less than the number of possible outcomes (ex. in the cell cycle lab, we used 3 df because there were four possible outcomes: interphase, prophase, metaphase, and ana/telophase)

Null hypothesis (H₀)

A statistical assumption that states there is no statistically significant difference between the two data sets

Alternative hypothesis (H_A)

A claim that states there is a statistically significant difference between the two data sets

Cell cycle checkpoints

Control points where ‘stop’ and ‘go ahead’ signals from inside and outside the cell can regulate the cell

G₁ checkpoint

The cell checks for

• Regular growth

• Sufficient resources

• Normal DNA

If all these are ‘yes,’ the cell enters S phase

If any of these are ‘no,’ the cell goes to G₀

G₂ checkpoint

The cell checks for

• Correctly copied DNA

If this is ‘yes,’ the cell goes to mitosis

If this is ‘no,’ the cell goes through apoptosis

M checkpoint

During metaphase, the cell checks for

• Chromosomes correctly lined up on the metaphase plate

If this is ‘yes,’ the cell goes to anaphase

If this is ‘no,’ the cell goes through apoptosis

Growth factors

Molecular signals released by certain cells that stimulate surrounding cells to divide

Cyclin-Dependent Kinase (CDK)

Enzymes that are always present in the cell; they help the cell proceed to the next step of the cell cycle (in checkpoints) when activated by a cyclin. Each different checkpoint had a different type of CDK and cyclin. 

Cyclin

Proteins that activate CDKS in checkpoints to help the cell proceed to the next step of the cell cycle. Each different checkpoint had a different type of CDK and cyclin. 

APC/C

A protein that facilitates the degradation of the centromere during the M checkpoint

Mutation

A change in DNA sequence resulting from exposure to chemicals/radiation/UV or incomplete repairs

Cancer

The uncontrolled proliferation of cells

Tumor

A cluster of cancerous cells

Metastasis

When cancerous tumors spread to other parts of the body

Proto-oncogenes

Genes that make the cycle go forward when necessary

Tumor suppressor genes

Genes that slow the cycle down when necessary

What happens when a proto-oncogene is mutated?

When a proto-oncogene mutates, it is called an oncogene. Oncogenes make the cycle move forward even when it’s not supposed to, like the accelerator of a car being much too strong. 

What happens when a tumor suppressor gene is inactivated?

If a tumor suppressor gene is inactivated, it can no longer make the cycle slow down when it has to; this is like the brake petal of a car not working

Contact inhibition

When cell division is inhibited by contact with another cell, preventing an excess of cells

Density-dependent inhibition

When cells get too dense in a certain area, they stop dividing - cancer cells do not have this ability, which is why they form tumors.

p53

“Guardian of the genome:” tumor-suppressor gene that either stops the cell cycle or initiates apoptosis in cases of DNA damage