Mitosis

Mitosis Overview

• Mitosis is the division of the nucleus.
• The nucleus undergoes distinct changes during mitosis.

Stages of Mitosis

• Prophase:
  • The nucleus starts to change; chromatin begins to cluster up into visible chromosomes.
  • Early prophase: Initial changes in the nucleus.
  • Late prophase (sometimes called prometaphase): Chromosomes are almost lined up, nuclear envelope dissolves away, mitotic spindle forms.
• Metaphase:
  • Chromosomes line up along the middle (the metaphase plate).
  • Each chromosome is a duplicate of itself, connected in the middle.
• Anaphase:
  • The two halves (chromatids) of each chromosome break apart and move to opposite sides of the cell.
  • Defined by chromatids separating and moving to opposite poles.
• Telophase:
  • New nuclei form around the separated chromosomes.
  • The cell itself starts to divide (cytokinesis) simultaneously.

Cytokinesis

• The division of the cytoplasm, resulting in two new cells.
• Happens simultaneously with telophase.

Mitotic Phase

• Encompasses mitosis and cytokinesis; essentially, cell division.
• A single cell divides into two daughter cells during this phase.

Interphase

• The nondivision portion of the cell cycle that daughter cells enter.
• G1 (Gap 1):
  • A growth phase; cells grow and mature.
  • Many cells remain in G1 indefinitely.
• G0 (Gap 0):
  • Cells that stay in G1 for a very long time are said to be in G0.
• S (Synthesis):
  • DNA is copied in preparation for cell division.
  • The cell still appears normal.
• G2 (Gap 2):
  • Another growth phase after DNA synthesis.
  • The cell prepares all the necessary materials for division.

Prophase Details

• Chromatin clusters up into visible chromosomes.
• The nuclear envelope disappears.
• Centrosomes (which duplicated during interphase) migrate to opposite sides of the cell.
• Microtubules form between the centrosomes, creating the mitotic spindle.

Metaphase Plate

• The region where chromosomes line up during metaphase.

Chromosome Structure During Metaphase

• Each chromosome consists of two identical halves (sister chromatids) connected in the middle.
• During anaphase, these chromatids separate, becoming individual chromosomes.

Kinetochore

• Attachment point on the chromosome for microtubules.
• Microtubules attach to the kinetochores on both sides of the chromosome, ensuring proper alignment at the metaphase plate. These "arms" pull the chromosomes apart.
• (\text{Attachment of kinetochores} \Rightarrow \text{Metaphase Line Up})

Cytokinesis in Animal vs. Plant Cells

• Animal Cells:
  • The cell membrane pinches in the middle, forming a cleavage furrow until the cell divides into two.
• Plant Cells:
  • A cell plate forms in the middle of the cell. Vesicles containing cell wall material fuse together to create a new cell wall and membrane, dividing the cell.

Identifying Cytokinesis

• Animal Cells:
  • Cleavage furrow: A pinching in of the cell membrane indicates cytokinesis.
• Plant Cells:
  • Cell plate: Formation of a new dividing wall indicates cytokinesis.

Cell Cycle Checkpoints

• Controls regulate cell division and its pacing.
• These are stopping points at critical phases of the cell cycle.
• If checkpoints are not passed, a cell can be stuck indefinitely at that stage.

G1 Checkpoint

• Determines if the cell will divide or enter G0.
• If a cell stops here, it enters G0 and may stay there permanently (e.g., muscle cells).
• To pass this checkpoint, enzymes must be activated.

G2 Checkpoint

• The barrier between interphase and mitosis.
• Controlled by cyclin-dependent kinases.
• Once this checkpoint is passed, prophase starts.

M Checkpoint (Metaphase Checkpoint)

• Occurs during metaphase to ensure microtubules are correctly attached to chromosomes.
• Prevents progression to anaphase until everything is in order.
• MPF (Mitotic Phase Promoting Factor): Cyclin dependent kinase at G2 checkpoint that triggers Mitosis.
• APC (Anaphase Promoting Complex): Checkpoint in the middle of mitosis that makes Anaphase happen.

Cyclin-Dependent Kinases (CDKs)

• Enzymes that regulate the cell cycle.
• Activated by cyclin proteins.
• Once a checkpoint is passed, cyclin is removed, and the enzyme turns off.

Internal vs. External Signals

• Internal Signals: Enzymes inside the cell that regulate its progression through the cell cycle.
• External Signals: Chemicals, such as growth factors, that come from outside the cell and stimulate cell division.

Uncontrolled Cell Division: Cancer

• Cancer cells constantly release growth factors, leading to uncontrolled division and tumor formation.
• HeLa Cells: Famous cell line from Henrietta Lacks, whose cancer cells divided uncontrollably.

Binary Fission

• Cell division in prokaryotic cells (bacteria, archaea).
• DNA loops are copied as the cell grows, eventually growing into two separate cells.
• Also found in mitochondria and chloroplasts.
• Different than mitosis as growing and cell division are simultaneous.

Eukaryotic Cells

• Gradual transistions/changes.
• There are intermediate cells that lead to intermediate steps.

Karyotypes

• A picture of a complete set of chromosomes from a cell.
• Humans have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes).
• Typical Male Karyotype: 22 pairs of chromosomes + XY.
• Typical Female Karyotype: 22 pairs of chromosomes + XX.
• Somatic Cell: Typical cell that you have in you.
  • Germ cells and gametes are not somatic cells.

Genetic Abnormalities

• Atypical karyotypes (e.g., XXY, X, or triplicate chromosomes) can lead to genetic disorders.
• Down syndrome (trisomy 21) is a more common example.

Genome

• The complete set of genetic material of an individual, including DNA in chromosomes and mitochondria.
• Tracing ancestory by looking at mitochondria.

Chromosome Composition

• Chromosomes are made of chromatin.
• Chromatin is composed of DNA wrapped around proteins.

Gene Definition

• A segment of DNA that codes for a specific protein, which determines a trait.
• A gene is the piece of DNA that codes for the Protein (recipe).

Genetic Relationships

• Sharing a gene with another individual indicates a shared ancestor.
• We are all related and cousins of each other.

Sister Chromatids

• When a chromosome copies itself, the two identical copies are called sister chromatids, connected in the middle.
• During anaphase, these chromatids separate and become daughter chromosomes.

DNA Data Storage

• DNA is highly efficient at storing information in a compact form.
• 21 gigabytes of text can be held in the nucleus.
• DNA is compacted within the nucleus.
• Enough DNA as big as a big family sized Snickers bar is enough to store every bit of information ever recorded by humans throughout human history.