CELL CYCLE

What is the purpose of the cell cycle?

The series of stages a cell goes through as it grows, copies its DNA, and divides. It allows organisms to grow, repair damage, and reproduce.

Why must the cell cycle be regulated?

To ensure cells divide at the correct time and prevent uncontrolled growth and tumors.

What are the main phases of the cell cycle?

Interphase (G1, S, G2), mitosis (M phase), and cytokinesis.

What is interphase?

The longest part of the cell cycle. The cell grows, performs normal functions, and copies its DNA to prepare for division.

What happens during G1?

The cell grows, prepares to copy its DNA, and makes more cell structures (organelles), such as centrioles.

What happens during S phase?

DNA is copied (replicated). Each chromosome now has two identical copies called sister chromatids, which are attached at a centromere.

What are chromosomes, chromatids, sister chromatids, and centromeres?

Chromosome: A structure made of DNA that carries genetic information.

Chromatid: One copy of a duplicated chromosome.

Sister chromatids: Two identical copies of one chromosome.

Centromere: The region holding sister chromatids together.

What happens during G2?

The cell continues growing and prepares for mitosis by making materials needed for division, including proteins that form spindle fibers.

How much DNA and how many chromosomes are present after S phase?

The DNA amount doubles because each chromosome has two sister chromatids. However, the chromosome number stays the same because the copies are still attached.

What is mitosis?

The division of the nucleus. It makes sure each of the two new nuclei receives a complete set of the parent's genetic information (genome).

What are the four stages of mitosis?

Prophase

Metaphase

Anaphase

Telophase

(PMAT)

What happens during prophase?

Chromosomes condense (become shorter, thicker, and visible).

The nuclear membrane (the covering around the nucleus) dissolves.

Spindle fibers begin forming.

Centrosomes move to opposite sides of the cell.

What are spindle fibers and centrosomes?

Spindle fibers: Structures that attach to chromosomes and help move them during cell division.

Centrosomes: Structures that organize spindle fibers and move toward opposite ends of the cell.

What happens during metaphase?

Spindle fibers attach to the centromeres of chromosomes. The chromosomes line up across the middle of the cell, called the metaphase plate (equator).

What happens during anaphase?

The centromeres split, separating the sister chromatids. Spindle fibers pull them toward opposite ends (poles) of the cell. Each separated chromatid is now considered its own chromosome.

What happens to the chromosome number during anaphase?

The chromosome number temporarily doubles inside the dividing cell because the separated sister chromatids are now individual chromosomes.

What happens during telophase?

Two new nuclear membranes form around the separated chromosomes. Each new nucleus has the same number of chromosomes and the same genetic information as the original parent cell.

What is cytokinesis?

The division of the cytoplasm (the material inside the cell outside the nucleus) and its contents between the two new daughter cells. It happens after mitosis.

How does cytokinesis differ in animal and plant cells?

Animal cells: A cleavage furrow forms, pinching the cell into two.

Plant cells: A cell plate forms in the middle and develops into new cell wall material that separates the daughter cells.

What is G0?

A nondividing stage that cells enter when they temporarily or permanently stop dividing. These cells have exited the cell cycle.

Why do cells enter G0, and can they leave it?

Cells may enter G0 because they are fully mature (differentiated) or because conditions aren't favorable for growth. Some cells can reenter the cycle when stimulated by molecular signals.

What are cell-cycle checkpoints?

Control points that check whether a cell is ready to move to the next stage. They help make sure cell division happens correctly.

What are cyclins and cyclin-dependent kinases (CDKs)?

Cyclins: Proteins whose levels rise and fall during the cell cycle.

CDKs: Proteins called kinases that stay at fairly constant levels and add phosphate groups to other molecules, activating them.

How are cyclin-dependent kinases (CDKs) activated?

CDKs are inactive by themselves. They become active when they bind to cyclin proteins.

What is MPF, and what does it do?

MPF (mitosis-promoting factor) is a complex formed when cyclin binds to a CDK. It triggers the beginning of mitosis.

How do cyclin, CDK, and MPF levels change during the cell cycle?

Cyclin levels rise and fall.

CDK levels stay constant.

When cyclin binds to CDK, MPF forms and triggers mitosis.

What are somatic cells?

Body cells that are NOT involved in sexual reproduction. Their division can be controlled by density-dependent inhibition.

What is density-dependent inhibition?

When cells become too crowded, they stop dividing. This prevents too many cells from growing in the same area.

What is anchorage dependence?

Cells must be attached to a surface in order to divide.

How do cancer cells differ from normal cells in density-dependent inhibition and anchorage dependence?

Cancer cells ignore both controls. They continue dividing even when crowded or not properly attached, allowing uncontrolled growth

What are proto-oncogenes?

Normal genes that help promote and regulate controlled cell growth and division. Think of them as the accelerator of a car.

What are oncogenes, and how do they form?

Oncogenes are mutated proto-oncogenes that cause cells to divide too quickly and can lead to tumors.

How can one mutated proto-oncogene allele affect cell growth?

One mutated allele can be enough to cause uncontrolled growth. Oncogenes act dominantly because one abnormal copy can affect the cell.

What are tumor suppressor genes, and how do they work?

Genes that detect mutations and prevent abnormally fast cell division.

They act like the brakes of a car. Both alleles generally must become nonfunctional for the cell to lose this control, so they act recessively.

What are BRCA mutations and apoptosis? Why are they important?

BRCA: A tumor suppressor gene (sometimes called the breast cancer gene). A mutation in one allele increases the risk of certain cancers, but cancer is not guaranteed because one working allele remains.

Apoptosis: Programmed cell death. It can remove cells with cancer-causing mutations and help shape developing organs.

Example: During embryonic development, apoptosis removes the webbing between fingers to create separate fingers.