AP Bio Unit 3: Cell Cycle

Mitotic phase

• Also known as M phase

• Mitosis and cytokinesis

• Shorter than interphase

Interphase

• G1, S, and G2 phases

• Cells spend around 90% of their time here

Mitosis

• Nucleus divides and DNA is distributed between two daughter cells

• The process of duplicated chromosomes separating into two daughter cells

• Has five phases: prophase, prometaphase, metaphase, anaphase, and telophase

Cytokinesis

• Cytoplasm is divided between the two daughter cells after mitosis

• In animal cells, happens by cleavage

• In plant cells, happens with a cell plate

G1 phase

• “Gap 1”

• Cell grows and makes proteins needed for DNA replication

S phase

• “Synthesis phase”

• Chromosomes duplicate via DNA replication

G2 phase

• “Gap 2”

• Cell continues to grow and proteins needed for mitosis are synthesized

Timing of Cell Division

• Can vary between cell types and species

• E.g. skin and esophageal cells divide frequently, liver cells only do so to repair damaged tissue, and nerve cells don’t divide at all

G0 phase

• Cells that never divide (or are temporarily not dividing) are in this phase

• E.g. nerve cells

• Cells will enter this phase if they do not make it through the G1 checkpoint

Genome

• All of a cell’s genetic material contained in its nucleus

Genes

• Units within the genome that code for certain traits

Chromatin

• DNA and proteins (not necessarily condensed)

Chromosomes

• Condensed DNA and proteins

• Made up of two identical sister chromatids held together by the centromere

Chromatid

• One of two identical halves of a chromosome that has been replicated in preparation for cell division

Gamete

• Reproductive cells

• Have only half as many chromosomes as somatic cells and only one copy of each chromosome

Somatic cells

• Non-reproductive cells

• Have two copies of each chromosome

Prophase

• Chromatin condenses into chromosomes

• Nucleolus disappears

• Mitotic spindle begins to form from microtubules extending from the centrosome

• In animal cells, this involves the centrioles, but not in plant cells

Aster

• Short microtubules on the non-spindle side of the centrosome

Prometaphase

• Nuclear envelope breaks apart

• Spindle microtubules attach to proteins on the centromeres of the chromosomes called kinetochores

• Each chromosome has two kinetochores, and they each attach to a microtubule from a different pole

Metaphase

• Mitotic spindle is complete

• Chromosomes align along the metaphase plate (an imaginary line in the center of the cell)

• Asters extend the cell membrane

Anaphase

• The proteins holding the sister chromatids together deactivate

• Kinetochore microtubules pull the sister chromatids apart toward opposite poles of the cell

• Nonkinetochore microtubules elongate the cell

Telophase

• Daughter nuclei form

• Chromosomes begin to uncoil

• Cytokinesis occurs:

  • Animal cells separate at the cleavage furrow

  • Plant cells begin forming the cell plate which fuses with the cell membrane and will become a new cell wall

Binary fission

• How prokaryotes and single-celled eukaryotes (e.g. amoeba) divide

• Chromosomes replicate and move to opposite ends of the cell, which then separates into two identical daughter cells

G1 checkpoint

• Most important checkpoint in mammals

• Occurs at the G1/S transition

• This is the primary point where cells decide whether or not to divide

• Checks for:

  • Size (is it big enough to divide?)

  • Nutrients (does it have enough nutrients or energy reserves?)

  • Molecular signals (is the cell receiving positive cues (e.g. growth factors) from neighboring cells?)

  • DNA integrity (is any of the DNA damaged?)

• If a cell doesn’t pass through this checkpoint, it will enter G0 phase

• A transcription repressor protein is degraded, allowing genes to be expressed that allow the cell to progress to DNA replication

G2 checkpoint

• Occurs at the G2/M transition

• Checks for:

  • DNA integrity (is any of the DNA damaged?)

  • DNA replication (was the DNA completely copied in the S phase?)

• If errors or damage are detected, the cell will pause the cycle here and try to repair the damage or complete replication

• If the damage is irreparable, then the cell may undergo apoptosis (programmed cell death) to prevent the damaged DNA from being passed on.

• Triggered by MPF

M checkpoint

• At the transition from metaphase to anaphase

• Also known as the spindle checkpoint

• Makes sure that all sister chromatids are correctly attached to spindle microtubules and lined up at the metaphase plate

• The cell will look for “straggler” chromosomes and pause mitosis until they are captured

• Results in daughter cells with the right number of chromosomes

Why are cell cycle control systems necessary?

To ensure that the cell is replicating and dividing properly.

For example, if the proteins regulating cell division aren’t functioning properly, then this may lead to cancer.

Where are cell cycle controlling signaling molecules located?

The cytoplasm

Cyclin-dependent protein kinases (CDKs)

• Proteins that are present throughout the cell cycle

• Usually, they’re inactive. They are activated when a cyclin protein binds to them

Maturation-promoting factor (MPF)

• A CDK-cyclin complex that forms during the S and G2 phases when cyclin levels rise

• It decreases during mitosis

• This complex is primarily responsible for allowing the cell to progress past the G2 checkpoint and begin mitosis

Cyclins

• Proteins that are present at fluctuating levels at different times throughout the cell

• Bind to CDKs to help the cell progress through cell cycle checkpoints

Cancer

• Uncontrolled cell growth

• Cancer cells often exhibit traits like:

  • excessive cell division

  • unusual numbers of chromosomes

  • abnormal cell surface

  • detachment from neighboring cells and the ECM

  • ability to secrete molecular signals that attract extra bloodflow

• Detachment and extra bloodflow for nutrients allow the cells to spread to other area

Growth factors

• Proteins that stimulate cell division

• Cancer cells that divide without these

Density-dependent inhibition

• Normal cells stop growing when they become too crowded

• Cancer cells do not display this and will overlap and clump

Radiation therapy

• Damages the DNA in cancer cells

• Cancer cells lack the ability to repair this damage

• Halts the cell cycle at the G2 or M checkpoints

Chemotherapy

• These treatments are toxic to actively dividing cells

• E.g. it might freeze mitotic spindles and prevent mitosis from progressing beyond metaphase

• E.g. it might prevent DNA from replicating

Zygote

• A diploid fertilized egg cell

• Through mitosis, a zygote can divide and grow into a multicellular organism

First thing a zygote does?

• Divides until there are lot of cells

Cell differentiation

• During embryonic development, cells become specialized in structure and function

• They are organized into tissues and organs

Morphogenesis

• Development of the shape and form of tissues and organisms

Cytoplasmic determinants

• Maternal substances in the egg cell’s cytoplasm (e.g. proteins, mRNA, organelles)

• These are distributed to daughter cells during early mitotic divisions

• The cytoplasm divides without making more cytoplasm, so these cytoplasmic determinants are different from cell to cell

• Different cytoplasmic determinants regulate gene expression during cell differentiation

Induction

• Gene expression is controlled by environmental signals between cells

• E.g. contact between cell surfaces and secretion and reception of growth factors

• Creates regions of differentiated cells that can then become tissues and organs (morphogenesis)