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DNA & the Cell Cycle
Transcript of DNA & the Cell Cycle
The generalized model of DNA is of a twisted ladder - the DNA double helix.
The sides of the ladder are alternating sugars (deoxyribose) and phosphate groups.
The "rungs" of the ladder are the base pairs - adenine with thymine, and guanine with cytosine. Base pairs are held together by hydrogen bonds. http://commons.wikimedia.org/wiki/File:DNA_Double_Helix.png http://commons.wikimedia.org/wiki/File:Base_pair_GC.svg http://commons.wikimedia.org/wiki/File:Base_pair_AT.svg Base Pairs Guanine always pairs with Cytosine: Adenine always pairs with Thymine: Three hydrogen bonds between G & C Two hydrogen bonds between A & T The order of the four bases (A, T, G, C) in DNA determines the information stored within the molecule. This information (genes) determines an organism's characteristics, and it is because of the base pairing rules that DNA can be used to store information and pass it on accurately to the next generation. Eukaryotic chromosomes are made of chromatin.
chromatin is approximately 50% DNA & 50% protein.
the protein is called histone, and acts like a spool that keeps DNA from tangling
chromatin resembles beads on a string - the beads are called nucleosomes Chromosomes http://commons.wikimedia.org/wiki/File:Chromatin_chromosome.png (1) Single DNA strand.
(2) Chromatin strand (DNA with histones).
(3) Chromatin during interphase with centromere.
(4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present)
(5) Chromosome during metaphase. During cell reproduction, a cell's chromosomes are condensed (coiled) so that they are easy to move through the cell.
For most of a cell's life cycle, the chromosomes are uncoiled, so that the information they contain can be accessed. Every hour, about a billion cells die in your body, replaced by another billion cells. Reproduction of these cells is crucial for the functioning and survival of any multicellular organism, and must be done in a precise manner that ensures that each cell is an exact copy of its 'parent' cell. The Cell Cycle http://upload.wikimedia.org/wikipedia/commons/8/8e/Cell_Cycle_3.png Scientists have identified a series of events that we now call the cell cycle. Most of a cell's lifetime is spent in interphase, where the cell is actively carrying out the processes of life. The rest of the time cells are preparing for cell division, or cytokinesis. The four phases of the cell cycle are:
G1 - 1st growth phase
S - DNA synthesis phase
G2 - 2nd growth phase
M - mitosis
Mitosis is followed immediately by cytokinesis. Interphase makes up 90% of a cell's lifetime. During this time it is growing - but cells cannot grow indefinitely. As a cell grows, its need for oxygen and food increases, and its waste production also increases. These are related to the volume of cytoplasm which increases at a greater rate than the area of the cell membrane, across which all materials must pass. If a cell gets too big, it becomes inefficient and dies. G1 is a period of growth for the cell. The cell will also be producing new proteins and organelles at this time. S is a synthesis phase - namely, synthesis of new DNA. All the DNA in a cell is copied, so that each cell formed by cytokinesis contains the same genetic information. G2 is a second growth period. During this time, structures and organelles needed for cell division are produced. G2 is the shortest period of interphase. Mitosis refers specifically to the duplication of a cell's nucleus in preparation for cell division. There are four distinct phases of mitosis, though the transition from one to the next may be indistinct. Mitosis http://upload.wikimedia.org/wikipedia/commons/3/3a/Mitosis.png In prophase, the chromatin fibres in the nucleus start to coil up, making each strand shorter and thicker. The nuclear envelope and nucleoli disappear, to make the sorting of chromosomes easier. In addition, spindle fibres form a network that reaches from the middle of the cell (its equator) to each pole. This is formed by centrioles in animal cells. In metaphase, the coiled-up chromosomes align themselves along the cell's equator. They connect to the mitotic spindle at the centromere, the point of connection between sister chromatids in each chromosome. The sister chromatids separate in anaphase, when the spindle fibres contract, breaking the centromere. The spindle fibres pull each sister chromatid (now called a chromosome) to opposite poles of the cell. Telophase is the reverse of prophase. The nuclear envelope and nucleolus reform, and the chromosomes uncoil, returning to chroma7n fibres. Telophase is followed immediately by cytokinesis, or cell division. In animal cells (1, top), the cell membrane begins to pinch in along the cell's equator. Eventually, two cells are formed as the membrane separates - think of cutting a soap bubble into two.
In plant cells (2, bottom), a new cell wall begins to form along the cell's equator, dividing the cell into two. Cytokinesis http://upload.wikimedia.org/wikipedia/commons/5/5f/Cytokinesis.png In multicellular organisms, growth occurs because of cell division. Division of the meristem cells at the top of a plant's stem makes it taller. Root meristem cells divide to make roots grow deeper into the soil. The stem gets thicker due to lateral meristem growth.
Repair is necessary sometimes, too. Skin cells are constantly being regenerated, as are the cells that line the digestive tract. A broken bone is repaired when new cells are produced to join the break.
And don't forget... a cell must divide if it gets too big, otherwise its surface area to volume ratio will be inefficient and it will die. Why is cell division necessary? The environment can have an effect on the rate of cell division:
Ever wonder why indoor plant stems bend towards the light coming in from a window? This is because of an increase in cell division on the opposite side of the stem, causing the stem to bend towards the light.
Your body will produce more red blood cells in response to a change in altitude (e.g. a vacation in the Himalayas).
Antibiotics are drugs that halt cell division in bacteria, by interfering with DNA replication or production of new bacterial cell walls.