Cell Division; 4.6-4.7, 5.1-5.2

Describe the roles/purposes of cell division

1. Reproduction

2. Growth and development

3. Tissue repair and renewal

Differ between chromatin and chromosomes

Chromatin → uncondensed, DNA

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Chromosomes → tightly coiled and condensed DNA

Sister Chromatids

The two identical halves of a replicated chromosome

• connected at the centromere by proteins (cohesin)

• tightly condensed coils of DNA

Centromere

The central region of the replicated chromosome

• where sister chromatids connect (Cohesin)

• where spindle fibers attach (Kinetochore)

Cohesin

The protein that connects **sister chromatids** at the **centromere**

Kinetochore

proteins at the **centromere** where spindle fibers attach

Centrosomes

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

• Acts as a microtubule-organizing center (MTOC)

• Produces spindle fibers

Centrioles

The organelle where **spindle fibers** are created/branch off from

Spindle Fibers

a network of microtubule fibers + proteins

• Aka “Mitotic Spindle”

• Extend from the centrioles

• attach to the kinetochores at the centromere of replicated chromosomes

Describe the two major periods of time in the cell cycle

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

Summarize the specific events that occur during the sub-phases of interphase

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)

Summarize the specific events that occur during the phases of mitosis

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

Describe the purpose of cytokinesis

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

How does cytokinesis differ between animal and plant cells

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

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

* Cell Plate = early/beginning cell wall

Describe how prokaryotic cell division is similar and different from mitosis in eukaryotic cells

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)

Describe what a dividing cell “checks” at the 3 checkpoints in the cell cycle

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

What is apoptosis? Why is it important

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

Explain the role of cyclin and CDKs in the regulation of the cell 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

Describe how growth factors and density-dependent inhibition regulate the cell cycle

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

Describe what occurs when regulation of the cell cycle is lost

Cells become cancerous

• Accumulate genetic mutations

• Avoid cell regulation

• Grown and divide uncontrollably

• Invade other tissue

Describe some of the characteristics of cancer cells

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

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

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__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

Homologous Chromosomes

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

* Also known as “homologs”

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Sex Chromosomes

Chromosomes that determine the sex of an individual

* Humans, 1 pair (X, Y)

Autosomes

Chromosomes that do not determine the sex of an individual

* Human, 22 pairs

Diploid Cells

Cells that contain both homologs from each pair

• Somatic (body) cells

• 2n

example cell → ( II ii )

Haploid Cells

Cells that contain one homolog from each pair

• Gametes (sex cells)

• n

example cell → ( I i )

Summarize the role of mitosis and meiosis in the life cycle of a sexually reproducing organism

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)

Why is meiosis sometimes referred to as reduction division

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

Describe synapsis and crossing over. When do these events occur in 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

Differ between: Prophase I & II, Metaphase I & II, Anaphase I & II, and Telophase I & II

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

What is the __first__ point in the process when the cells are considered haploid? Explain

After telophase I and cytokinesis

* Each cell at this point has 1/2 __the original__ number of chromosomes

Why does meiosis II occur?

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

Describe the differences and similarities between mitosis and meiosis

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)

(On study guide canvas) Practice identifying what phase and what type of division is occurring in cells

Describe the ways in which genetic diversity is introduced via meiosis and sexual reproduction

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