Cell

• The smallest structural and functional unit of an organism.

• Somatic cells are any cells other than germ cells.

  • Found throughout the body.

  • 46 chromosomes in humans.

• Germ cells contain half the number (23) of chromosomes.

  • Only found in the reproduction organs. (ex: Ovaries)

Cell Division

There are two types of cell division

1. Mitosis in somatic cells results in offspring exactly the same as the parents. Involves one “set” of stages.

2. Meiosis in germ cells results in a variety of genetically different offspring. Involves two “sets” of stages. (Meiosis 1 and Meiosis 2)

Vocabulary

• Chromatid - each of 2 thread-like strands into which a chromosome divides during mitosis.

• Sister chromatids - 2 identical copies of a chromatid.

• Centromere - a structure in a chromosome that holds the two chromatids together.

• Spindle fibers - control the movement and separation of chromosomes during mitosis.

• Centriole - helps in the formation of spindle fibers.

• Nuclear envelope - a membrane that separates the nucleus from the cytoplasm in eukaryotic cells.

Mitosis

• Used for cell division.

• Asexual reproduction

  • Binary fission

  • Budding

  • Regeneration

  • Vegetative Reproduction

  • Fragmentation

• Examples: Some types of jellyfish, worms, and plants.

4 basic phases.

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase/cytokinesis

Occurs in a strict sequential order called the cell cycle.

Produces diploid cells (2) - same genetic makeup as the parent cell.

Prophase

• The first phase of Mitosis.

• Chromosomes become visible through a microscope

• spindles begin to form

• Nuclear membrane breaks down

• The chromosomes replicate and are seen as a pair of chromatids.

Metaphase

• Second phase of mitosis

• The chromosomes, guided by the spindle fibers, line up in the middle of the dividing cell.

• The centrosomes are now at opposite ends (spindle poles) of the cell.

Anaphase

• Third phase of mitosis

• the two sister chromatids of each chromosome are pulled apart by the spindle fibers.

• Chromosomes pull away from each other toward the poles.

• The cell elongates so that the poles are farther apart.

Telophase

• The last stage of mitosis

• The chromosomes have reached the poles.

• Two new nuclear envelopes form around each of the two separated sets of unreplicated chromosomes.

• The cell has divided into two daughter cells exactly like the parent cell.

Interphase

• Mitosis is now over.

• Chromatin is unwound

• Here the cell replicates its chromosomes in preparation for another round of cell division.

Meiosis

• Cell division occurs in 2 sets of stages. Meiosis 1 and Meiosis 2.

• Reduces the number of chromosomes in the parent cell by half.

• Produces 4 gamete cells.

Vocabulary

homozygous chromosomes - During meiosis there are matching paternal and matching maternal

Crossing over - the process where homologous chromosomes pair up and exchange genetic material to form new chromosomes.

haploid cells - four cells as a result of meiosis, which are genetically different from each other and their parent cells.

Meiosis 1 and Prophase 1

• The first phase of Meiosis 1

• Chromosomes become visible under a microscope.

• Duplication of homologous chromosome pair and cross-over occurs.

• The nuclear envelope disappears.

• Spindles enter nucleus

Meiosis 1 and Metaphase 1

• The crossover is complete.

• Chromosomes move toward the center and line up.

• Spindle fibers attach to the centromere of each chromosome.

Meiosis 1 - Anaphase 1

• Cell starts to lengthen.

• Two chromatids of each chromosome pair separate and move toward opposite poles by the spindle fibers.

• In meiosis the chromatids remain together.

Contrast with mitosis, where sister chromatids seperate.

Meiosis 1 - Telophase 1

• Complete haploid (4) sets of chromosmes

• A clevage of furrow appears

• By the end of the stage the parent cell has divided into two daughter cells.

• This separation of cytoplasm is called cytokinesis.

Interkinesis in Meiosis

• a period of rest, called interkinesis

• No replication of DNA occurs during this phase.

Meiosis 2 - Prophase 2

• first step in Meiosis 2

• Begins with 2 daughter cells from meosis 1.

• Chromosomes are condensed.

• Nuclear envelope begins to break down.

• Centrosomes have replicated and are moving toward the poles.

Meiosis 2 - Metaphase 2

• The second step of Meiosis 2

• The spindles draws the chromomes to the center plate.

The centromeres are bound to the spindle fibers from opposite sides.

Meiosis 2 - Anaphase 2

• the third step of Meiosis 2

• Very similar to Meiosis 1 anaphase.

• The two sister chromatids of each chromosome are pulled apart by the spindle fibers.

• Chromosomes move away from each other toward the poles.

• The cell elongates so that the poles are farther apart.

Meiosis 2 - Telophase 2

• Chromosomes reach opposite poles.

• Cytokinesis occurs (seperation of cytoplasm) and nuclear envelopes form.

• Meiosis is complete with 4 daugher cells (haploid) each different from each other and different from the parent cell.

Summary of meiosis

From of cell division that results in half the number of chromosomes in gametes.

Maintains the same number of chromosomes from generation to generation.

Results in an assortment of genetic material passed on to offspring.