Unit 2 - Chapter 12

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What is cell division required for?

- Reproduction

- Embryonic and fetal development

- Call replacement following death or damage


What are the two main stages of the cell cycle?

- Interphase, cell growth and DNA replication

- Mitotic phase, redistribution of duplicated chromosomes and division of cell.


What are the stages in interphase?

Has several substages:

G1 phase, cells may leave G1 to G0 and may re-enter G1 at any point. S phase (DNA synthesis) and then G2.


What happens in the G1 stage of interphase?

- Cell growth in preparation for cell division

- RNA synthesis

- Important site of control for cell cycle progression.


What happens in the S phase of interphase?

- Chromosomes duplicate


What happens in the G2 stage of interphase?

- Prepares proteins for mitosis

- Checks the duplicated Chromosomes for nay errors.


What are the steps in mitosis?

Prophase, prometaphase, metaphase, anaphase, and telophase


What happens in prophase?

Chromosomes condense, centromere forms and mitotic spindles form.


What happens in prometaphase?

The nuclear envelope fragments and spindles make contact with chromosomes. Kinetochore forms.


What is the chromosomes structure after replication?

- Two sister chromatins attached along their length by cohesins.


What are the mitotic spindles?

- Made of microtubules that control chromosome movement.

- The assembly of spindle microtubules begins in the centrosome.

- The centrosome replicates during interphase and each copy migrate to opposite ends of the cell.


What happens in Metaphase?

-Centrosomes at opposite poles of cell and the chromosomes are positioned at the metaphase plate.


What happens in Anaphase?

Sister chromatins separate and move towards the opposite spindle poles, each pole has an equal number fo chromosomes.


What happens in telophase and cytokinesis?

- Daughter chromosomes arrive at poles and daughter nuclei form, the chromosomes uncoil and the spindle disassembles.

- Cytokinesis divides cytoplasm in two and cell division is complete.


What is asexual reproduction?

Reproduction by uni-cellular organisms, some animals and many plants.

Offspring are genetically identical to parents.


What are the different types of asexual reproduction?

- Budding

- Sporulation


How does asexual budding occur?

Genetic material is split equally between parent and offspring but less cytoplasm in offspring.


What is asexual sporulation?

Spores containing genetic material form within the parent and the spores are released to germinate to form a mature organism.


What are gametes?

- Haploid gametes are produced by meiosis which results in one set of chromosomes in each gamete.


What happens in Meiosis?

Chromosomes replicate, resulting in four daughter cells. Each daughter cell has half the number of chromosomes as the parent cell.


What are the stages of meiosis?

- Chromosomes duplicate during interphase

- Sister chromatids are associated along their lengths 'sister chromatid cohesion'

- Chromatids sorted into four haploid daughter cell in two steps meiosis 1 and meiosis 2.


What are the four stages in meiosis 1?

- Prophase 1 - Chromosome crossing over occurs, X-shaped regions called chiasmata are sites of crossover.

- Metaphase 1 - Homologous chromosomes line up at metaphase plate. Microtubules attach to the kinetochore.

- Anaphase 1 - Chromosomes separate, moving towards opposite poles, guided by spindles.

- Telophase 1 and cytokinesis - Each half of the cell has haploid chromosomes, two haploid daughter cells form.


How does crossing over occur?

  1. Sister chromatids held together by cohesions
  2. The chromosomes cross over
  3. Nonsister chromatids broken precisely corresponding positions.
  4. DNA breaks are repaired, DNA from one non sister chromatid joined together to corresponding segment of other.

What does a diagram of crossing over look like

card image

What are the four stages of Meiosis 2?

- Prophase 2 - Spindles form and chromosomes move towards metaphase plate.

- Metaphase 2 - Sister chromatids are arranged at the plate and attach to microtubules from opposite poles.

- Anaphase 2 - Chromatids separate and move as two new individual chromosomes to opposite poles.

- Telophase 2 and cytokinesis - Chromosomes arrive eat opposite poles and nuclei form, chromosomes decondense.


What are the differences between mitosis and meiosis?

Mitosis - Conserves chromosome sets, producing genetically identical cells to parent.

Meiosis - Reduces the number of chromosomes sets from diploid, to haploid. Producing cells which are genetically different from each other.


What causes variation in humans and animals?

- Crossing over of chromosomes

- Random fertilisation as any sperm can fuse with any ovum.


What is the Cell cycle control system?

A cyclically aerating set of molecules in the cell which triggers and coordinates key events in the cell cycle.


What are the 3 main checkpoints in the cell cycle?

G1, G2 and M phase.


What are the main regulatory proteins in the cell cycle?

Protein kinases and cyclins. Kinase activate cyclins, so are called cyclin-dependant kinases or Cdks.


What is MPF?

Maturation-promoting factor which is a complex of cyclin B and Cdk1, which triggers a cell passage from G2 to M phase.


What is the molecular mechanism which helps regulate the cell cycle?

  1. Synthesis of cyclin begins in the late S phase and continues through G2. Because cyclin in protected from degradation during this cell, so it accumulates.
  2. Cyclin combines with Cdk, producing MPF, when enough has accumulated the cell passes through the G2 checkpoint and begins mitosis.
  3. MPF promotes mitosis by phosphorylating various proteins. MPF's activity peaks in metaphase.
  4. In anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters G1.
  5. During G1 the degradation of cyclin continues and the Cdk component of MPF is recycled.

How is passaged through G1 regulated and mediated?

Regulated by - growth factors, nutrients and cell size.

Mediated by - Cyclin D and E.


How is passaged through G2/M regulated and mediated?

Regulated by - Cell size, DNA damage and DNA replication.

Mediated by - Cyclin B and A.


What is a growth factor?

A protein released by certain cells that stimulate other cells to divide.


What is platelet-derived growth factor and how is it used within the body?

PDGF is made by the blood platelets and is required for the cell division of fibroblasts. Fibroblasts have receptors on there plasma membrane which triggers a transduction pathway that allows cells to pass G1.


What is density-dependent inhibition?

Crowded cells stop dividing when a receptor from one cell links to its counterpart on another cell, forming a single layer.


What is anchorage dependance?

For cells to divide they must be attached to a substratum, such as the extracellular matrix.


What is the basis of an abnormality in a cancer cell?

A change in one or more genes that alters the function of their protein products, resulting in a faulty cell cycle control.


What is a benign tumor?

Abnormal cells which that remain at original site as they have two few genetic changes to move site and survive.


What is a malignant tumor?

Abnormal cells which can move to new tissues due to large genetic and cellular changes, can also be called transformed cells.


What is metastasis?

When a tumor separates from a main tumor and travels through the blood or lymph vessels to other parts of the body.


How are localised tumors treated?

High-energy radiation which damages DNA in cancer cells more than DNA in normal cells.


How are metastasis tumors treated?

Chemotherapy - drugs which are toxic to dividing cells pass through the circulatory system, it prevents cell division. E.g. preventing spindle fibre formation.