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CELLREPRODUCTION

Kimberly LozanoBiology 490Spring 2010

CELL CYCLE• Interphase G1: Growth (1)

New organelles formwithin the cell.

S: Synthesis Cell duplicates its DNA. G2: Growth (2) Cell prepares for

division to take placeduring the mitoticphase.

• Mitotic M phase Mitosis. Chromosomes

divide. Cytokinesis.

Cytoplasm and cellmembrane divide.

Chromosomes as PackagedGenes

• DNA double helix bound tohistones

• DNA wrapped around histonesform nucleosome

• Further coiling of nucleosomeproduce 30nm wide chromatinfiber

• Coiling of chromatin fiberproduce “x” shapedchromosome

• Chromosome consists of twoidentical sister chromatidsjoined at the centromere

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Prokaryotic Chromosome

• Single circular chromosome• Plasmids

• Mitosis produces two daughter cells that are identical to the parent cell.

• If the parent cell is haploid (N), then the daughter cells will be haploid.

• If the parent cell is diploid, the daughter cells will also be diploid.

• N N 2N 2N

• This type of cell division allows multicellular organisms to grow and repair damagedtissue.

What is Mitosis?

Interphase

•Chromatin is condensing into chromosomes and centrioles have duplicated in preparation for mitosis.• Chromosomes are not visible because they are uncoiled

Prophase

•Duplicated chromosomes are visible.•Centrioles begin moving apart.•Nuclear envelope is fragmenting and will disappear.

Chromatin

centrioles

nuclear envelope

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Metaphase

• The spindle fully develops and the chromosomes align at the metaphase plate

Anaphase

• Chromosomes are pulled along the microtubules toward opposite poles of the cell

Telophase

•The cell divides into two.•The chromosomes uncoil.•The nucleus reforms.•The spindle apparatus disassembles.

Cytokinesis

•Not a phase of mitosis.•Division of cytoplasm.•Begins as the cell lengthens in anaphase and ends with separation of daughter cells being complete at end of telophase.

• Meiosis produces daughter cells that have one half the number ofchromosomes as the parent cell.

• 2N N

• Meiosis enables organisms to reproduce sexually. Gametes (sperm andeggs) are haploid.

• Meiosis involves two divisions producing a total of four daughter cells.

What is Meiosis?

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Prophase I

Early•Homologous pairs within cell become visible. Replication of genetic material has already occurred.

Middle•Homologous chromosomes synapse, become shorter, thicker.

Late•Chromosomes become double stranded..

Crossing-over andRecombination DuringMeiosis

•Variation

•Survival of organism

Crossing over

Metaphase I

•Homologous pairs become aligned inthe center of the cell.

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Anaphase I

Centromeres on the chromosomes do not divide. Double stranded chromosomes move apart to opposite poles.

Telophase I

This stage is absent in some species. Two haploid cell start to form (Half the number).

Interkinesis

Interkinesis is similar to interphase except DNA synthesis does not occur. Chromosomes have faded from view.

Prophase II

• Nuclear envelope breaksdown and spindle apparatusappears.

Metaphase II

• Chromosomes becomearranged on the metaphaseplate and are attached tothe now fully formedspindle.

Anaphase II

• Centromeres separate and two chromatids of each chromosome move toopposite poles on the spindle. Separated chromatids are now calledchromosomes.

Telophase II

• A nuclear envelope forms around eachset of chromosomes and cytokinesisoccurs, producing four daughter cells, eachwith a haploid set of chromosomes

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Meiosis results

• Chromosomes fade from view.• Total of four haploid cells are produced.• Next phase would be fertilization.

Daughter Cells

• Sexual Reproduction andGenetic Variability

• Enhances potential for survival ofspecies

• Genetic variability byrecombination of wholechromosomes

Cited Literature Fried, George and Hademos, George Schaum’s Outline of

Biology. New York: McGraw Hill, 2009.

Sadava, David, C. Heller, G. Orians, W. Purves, D. Hillis, Life.The Science of Biology, Eighth Edition,Volume 1, The Cell andHeredity. Virginia: W.H.Freeman, 2006.