Cell Division; 4.6-4.7, 5.1-5.2

1. Reproduction

2. Growth and development

3. Tissue repair and renewal

Chromatin → uncondensed, DNA

Chromosomes → tightly coiled and condensed DNA

The two identical halves of a replicated chromosome

• connected at the centromere by proteins (cohesin)

• tightly condensed coils of DNA

The central region of the replicated chromosome

• where sister chromatids connect (Cohesin)

• where spindle fibers attach (Kinetochore)

The protein that connects sister chromatids at the centromere

proteins at the centromere where spindle fibers attach

a region near the side of an animal cell containing 2 centrioles

• Acts as a microtubule-organizing center (MTOC)

• Produces spindle fibers

The organelle where spindle fibers are created/branch off from

a network of microtubule fibers + proteins

• Aka “Mitotic Spindle”

• Extend from the centrioles

• attach to the kinetochores at the centromere of replicated chromosomes

1. Interphase

   1. Majority of a cell’s life cycle (~90%)

   2. where cell growth, DNA replication, and overall prep for cell division occur

   3. normal function

   4. NOT dividing

2. Mitotic

   1. Very Brief (~10% of cell life cycle)

   2. Divided into Mitosis and Cytokinesis

   3. Pause in normal function

   4. Focus on active cellular division

G1 Phase (First Gap)

• Normal Function

• Growth

• Acquiring Nutrients

• Organelles Replicated

S Phase (“Synthesis”)

• DNA Replication

  • makes sister chromatids (not condensed yet)

G2 Phase (Second Gap)

• Growth

• Continued prep for division (making enzymes)

Prophase

• DNA starts condensing (chromatin wrapping around histone proteins in tight coils)

• Spindle Fibers begin to form

• Centrosomes move apart

Prometaphase

• Nuclear envelope starts breaking down

• DNA finishes condensing (replicated chromosome “X”)

• Centrosomes reach opposite ends of the cell

• Spindle Fibers attach to kinetochores

Metaphase

• Spindle Fibers pull the chromosomes back and forth until they are at the center/middle

Anaphase

• Spindle Fibers pull the sister chromatids apart to opposite poles of the cell

  • Cohesins are broken down by enzymes beforehand

Telophase

• Nuclei begin to reform

• DNA uncondenses into chromatin

• Spindle Fibers break down

• Cytoplasm starts to divide

To finish divide the cytoplasm and separate the dividing cell into two

Animal Cells → cleavage furrow; a ring of microfilaments that gets tighter until the cell divides

Plant Cells → build a new cell wall on the inside (cell plate) using vesicles from the Golgi

• Cell Plate = early/beginning cell wall

Similar:

• DNA replication occurs

• Experiences growth before dividing

• Cytokinesis occurs

• Results in 2 identical daughter cells

Different:

- Prokaryotes

• Called “Binary Fission” (Division in Half)

• Cell elongates (growth)

• Experiences both a cleavage furrow and cell wall building

- Eukaryotes

• Called “Mitosis”

• Goes through interphase (duplication of organelles and nutrition prep) and mitotic phase

• The nuclear envelope has to break down (prophase) and redevelop (telophase)

• Can experience either a cleavage furrow (animal) or cell wall building (typically not both)

G1 checkpoint

• Cell growth/size

• Nutrients

• Growth factors

• DNA damage

G2 checkpoint

• DNA is accurately and completely replicated

M checkpoint

• @ transition through prometaphase

• checking that spindle fibers are attached to each chromosome

Apoptosis → programmed cell death

• Ensure that cells don’t randomly die and damage other cells in the process

• Make sure that cells that aren’t fit for cell division (and cannot be helped) don’t continue on with the cycle

Cyclin is the “go” signal for the cell cycle and isn’t created until the S/G2 phase. It’s required to activate the CDKs which remain at a constant concentration throughout the cell cycle. When cyclin and CDKs come together, MPF is created and phosphorylates proteins that regulate/activate the M phase.

• (side note) Eventually, cyclin is degraded by enzymes, and CDK becomes inactive, the cell exits M phase, repeat

Growth Factors

• the “go” signals and bind to receptor tyrosine kinase + triggers signal trans. pathway

• Lead to the production of cyclin

• Needed in order for the cell cycle to proceed

Density-dependent Inhibition

• A “stop” signal that stops cells from dividing when they touch

• Prevent the overproduction of cells, ensuring the cell cycle doesn’t keep going after max capacity is reached

Cells become cancerous

• Accumulate genetic mutations

• Avoid cell regulation

• Grown and divide uncontrollably

• Invade other tissue

mutated Proto-oncogenes → become oncogenes and produce too many “go” signals (aka cannot stop, and therefore surpass cell regulation

mutated Tumor-suppressor genes → produce too few “stop” signals (aka not receiving the sign to stop and continues surpassing cell regulation)

Production of telomerase → rebuilds telomeres and therefore cannot go through apoptosis

• Telomeres: a repetitive sequence of DNA at the ends of chromosomes that get shorter as regular cells divide → when finished, apoptosis for the cell

Pair of chromosomes that are the same size and contain the same DNA (1 maternal, 1 paternal)

• Also known as “homologs”

Chromosomes that determine the sex of an individual

• Humans, 1 pair (X, Y)

Chromosomes that do not determine the sex of an individual

• Human, 22 pairs

Cells that contain both homologs from each pair

• Somatic (body) cells

• 2n

example cell → ( II ii )

Cells that contain one homolog from each pair

• Gametes (sex cells)

• n

example cell → ( I i )

Meiosis

• The creation of the gametes for a sexually reproducing organism

  • 2 rounds of division

Mitosis

• After 2 gametes come together to form a diploid zygote (the first diploid cells of an embryo), the embryo undergoes mitosis to make more cells

Meiosis (creates the gametes) → Mitosis (continues the growth of the embryo) → Meiosis (to make more gametes once the organism is sexually mature)

Because the number of chromosomes is reduced in each cell as the parent cell goes through meiosis

Synapsis

• Homologs pair up in tetrads ( XX )

• Held together by synaptonemal complex (proteins)

Crossing over

• Exchange of DNA between non-sister chromatids of homologs

• Occurs at the chiasmata (an area at the ends of the sister chromatids)

• Mixes up maternal and paternal DNA

Both occur during Prophase I

Prophase I vs. II

• Prophase II has 2 cells to start off while Prophase I has 1 cell

• Prophase II does not experience synapsis or crossing over like Prophase I

Metaphase I vs. II

• Metaphase I → homologous pairs lining up in the middle

• Metaphase II → Single, replicated, chromosomes lining up in the middle in both cells

Anaphase I & II

• Anaphase I is the pulling apart of the homologous pairs

• Anaphase II is the pulling apart of sister chromatids in both cells

Telophase I & II

• Not much of a difference

• Telophase I → still condensed chromosomes

• Telophase II → becomes chromatin

After telophase I and cytokinesis

• Each cell at this point has 1/2 the original number of chromosomes

At the end of meiosis I there are still 2 copies of DNA, so to make sure that each gamete ends with 1 copy of DNA the cell must divide again during meiosis II

Similar:

• Cell division

• DNA Replication (during interphase, s phase)

• Start with Diploid Parent Cells

Different:

Mitosis

• Purpose: Cell Growth, replication, and division

• Somatic Cells made

• 1 round of division

• 2 daughter cells created (diploid cells)

  • Genetically similar

Meiosis

• Purpose: cells for reproduction

• Gametes

• 2 rounds of division

• 4 daughter cells created (haploid cells)

  • Genetically different

• Experience Crossing over and synapsis (prophase I)

1. Crossing Over

   1. The Exchange of DNA creates recombinant chromosomes (chromosomes that carry both parent’s DNA)

2. Independent Assortment

   1. Chromosomes line up and separate randomly during metaphase and anaphase

   2. leads to diff. combos of chromosomes in sex cells

3. Random Fertilization

   1. Nothing influences which sperm arrives first

   2. Endless possible combos for the zygotes