Cellular Division

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Cell Division Occurs in all Organisms

• In unicellular organisms the function of cell division is reproduction.

• In multicellular organisms functions of cell division include GROWTH, DEVELOPMENT, and REPAIR.

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GrowthMulticellular organisms grow because cell division increases

the number of cells in it.

• We already learned that one of the characteristics of living things is that they GROW!

• One cell becomes two, two cells become four, four cells become eight, etc

Growth

• In general, a large organism does not have larger cells than a small organism; it simply has many more cells than the small organism.

• In multicellular organisms growth happens mostly because cells grow in number rather than in size. (cells do grow in size, but there are limits to the size that cells can reach)

• By the time you reach adulthood, your body will be made up of about 100 trillion cells.

• Individual cells grow in size, but there are limits to the size that cells can reach.

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DevelopmentDevelopment occurs as cells specialize to perform particular functions.

• If cell division were the only process occurring in cells, all multicellular organisms would end up as spheres of identical cells.

• During development, cells become specialized to perform particular functions.

• These cells may take on shapes or structures that help them to perform their functions.

• These cells still have the same set of genetic material as all the other cells in an organism's body, but as the organism develops they specialize (they only use what they need from the DNA).

Development

RepairCells need to be replaced and repaired over the lifetime of an organism.

• The body repairs injuries, such as cuts, scrapes, and broken bones, by means of cell division.

• As cells age and die, they need to be replaced; they are replaced at different rates.

• Every minute or so, your skin loses about 40,000 cells, which are replaced with new ones.

• Most of the cells in your brain live a long time and do not divide very often.

REPAIR

break in skin

dividing cells

new cells pushed up to fill cut

new cell layers

wound is healed

cells stop dividing

DNA

• When a cell divides into 2 new cells, each new cell receives a full set of genetic material. This material is contained in the DNA.

• DNA is made of 2 strands of molecules joined in a structure that looks like a twisted ladder-it is called a double helix.

• So….DNA is the genetic material in the cell and it is shaped like a double helix!

Chromosomes

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http://www.youtube.com/watch?v=9kQpYdCnU14

What is a Chromosome?• Chromatin: loose, uncoiled form.

• Chromosome: tightly wound, condensed form of DNA that contains the information about an organism’s traits.

• Gene: one section of a DNA strand that codes for one trait.

Eukaryotic ChromosomesEach chromosome is composed of a single, tightly coiled DNA molecule

Chromosomes can’t be seen when cells are not dividing and are called chromatin

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Eukaryotic ChromosomesAll eukaryotic cells store genetic

information in chromosomesMost eukaryotes have between

10 and 50 chromosomes in their body cells

Human body cells have 46 chromosomes or 23 identical pairs

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Compacting DNA into Chromosomes

DNA is tightly coiled around proteins called histones

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Chromosomes in Dividing Cells

Duplicated chromosomes are called chromatids & are held together by the centromere

15Called Sister Chromatids

A picture of the chromosomes from a human cell arranged in pairs by size

Last pair are the sex chromosomesXX female or XY male

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Boy or Girl?

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Y - Chromosome

X - Chromosome

The Y Chromosome Decides