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DNA

DNA. DNA is the molecule that carries all of the inherited information in the cell. DNA was discovered as “nucleic acid”—an acidic material in the nucleus in the later 1800’s.

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DNA

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  1. DNA • DNA is the molecule that carries all of the inherited information in the cell. • DNA was discovered as “nucleic acid”—an acidic material in the nucleus in the later 1800’s. • Its importance was discovered until later. For a long time, DNA was considered too simple to carry genetic information.

  2. Structure of DNA • DNA is a macromolecule, a large molecule composed of many subunits. The subunits of DNA are nucleotides. • Each nucleotide is composed of 3 parts: a nitrogenous base, a sugar (called deoxyribose), and a phosphate group (which is a phosphorus atom bonded to 4 oxygen atoms). • There are 4 kinds of nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).

  3. More DNA Structure • The nucleotides are joined into long chains that connect the phosphate of one nucleotide to the sugar of the next nucleotide. • The nitrogenous bases of 2 different chains pair with each other, giving a DNA molecule that has 2 sugar-phosphate chains on the outside, with bases paired in the center. • Base pairing occurs by hydrogen bonds: partial positive and negative charges attract each other. Hydrogen bonds are weak, but there are lots of them in a DNA molecule. • The 2 chains are “anti-parallel”—they run in opposite directions. They are twisted together into a corkscrew shape: a double helix. • Base pairing is very specific: A pairs with T, and G pairs with C.

  4. Gene Expression • Each gene is a short section of a chromosome’s DNA that codes for a polypeptide. • Recall that polypeptides are linear chains of amino acids, and that proteins are composed of one or more polypeptides, sometimes with additional small molecules attached. The proteins then act as enzymes or structures to do the work of the cell. • All cells have the same genes. What makes one type of cell different from another is which genes are expressed or not expressed in the cell. For example, the genes for hemoglobin are on in red blood cells, but off in muscle and nerve cells. “Expressed” = making the protein product. • Genes are expressed by first making an RNA copy of the gene (transcription) and then using the information on the the RNA copy to make a protein (translation). • This process: DNA transcribed into RNA, then RNA translated into protein, is called the “Central Dogma of Molecular Biology”.

  5. RNA • RNA is a nucleic acid, like DNA, with a few small differences: • RNA is single stranded, not double stranded like DNA • RNA is short, only 1 gene long, where DNA is very long and contains many genes • RNA uses the sugar ribose instead of deoxyribose in DNA • RNA uses the base uracil (U) instead of thymine (T) in DNA. • There are 3 main types of RNA in the cell: • 1. messenger RNA: copies of the individual genes • 2. ribosomal RNA: part of the ribosome, the machine that translates messenger RNA into protein. • 3. transfer RNA, which is an adapter between the messenger RNA and the amino acids it codes for.

  6. More Transcription

  7. Genetic Code • There are only 4 bases in DNA and RNA, but there are 20 different amino acids that go into proteins. How can DNA code for the amino acid sequence of a protein? • Each amino acid is coded for by a group of 3 bases, a codon. 3 bases of DNA or RNA = 1 codon. • Since there are 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons. • This is far more than is necessary, so most amino acids use more than 1 codon. • 3 of the 64 codons are used as STOP signals; they are found at the end of every gene and mark the end of the protein. • One codon is used as a START signal: it is at the start of every protein.

  8. Transfer RNA • Transfer RNA molecules act as adapters between the codons on messenger RNA and the amino acids. Transfer RNA is the physical manifestation of the genetic code. • Each transfer RNA molecule is twisted into a knot that has 2 ends. • At one end is the “anticodon”, 3 RNA bases that matches the 3 bases of the codon. This is the end that attaches to messenger RNA. • At the other end is an attachment site for the proper amino acid. • A special group of enzymes pairs up the proper transfer RNA molecules with their corresponding amino acids. • Transfer RNA brings the amino acids to the ribosomes, which are RNA/protein hybrids that move along the messenger RNA, translating the codons into the amino acid sequence of the polypeptide.

  9. Translation • Three main players here: messenger RNA, the ribosome, and the transfer RNAs with attached amino acids. • First step: initiation. The messenger RNA binds to a ribosome, and the transfer RNA corresponding to the START codon binds to this complex. Ribosomes are composed of 2 subunits (large and small), which come together when the messenger RNA attaches during the initiation process. http://www.youtube.com/watch?v=NJxobgkPEAo&feature=related

  10. More Translation • Step 2 is elongation: the ribosome moves down the messenger RNA, adding new amino acids to the growing polypeptide chain. • The ribosome has 2 sites for binding transfer RNA. The first RNA with its attached amino acid binds to the first site, and then the transfer RNA corresponding to the second codon bind to the second site. • The ribosome then removes the amino acid from the first transfer RNA and attaches it to the second amino acid. • At this point, the first transfer RNA is empty: no attached amino acid, and the second transfer RNA has a chain of 2 amino acids attached to it.

  11. Translation, part 3 • The ribosome then slides down the messenger RNA 1 codon (3 bases). • The first transfer RNA is pushed off, and the second transfer RNA, with 2 attached amino acids, moves to the first position on the ribosome.

  12. Translation, part 4 • The elongation cycle repeats as the ribosome moves down the messenger RNA, translating it one codon and one amino acid at a time. • Repeat until a STOP codon is reached.

  13. Translation, end • The final step in translation is termination. When the ribosome reaches a STOP codon, there is no corresponding transfer RNA. • Instead, a small protein called a “release factor” attaches to the stop codon. • The release factor causes the whole complex to fall apart: messenger RNA, the two ribosome subunits, the new polypeptide. • The messenger RNA can be translated many times, to produce many protein copies.

  14. Summary of translation

  15. Post-translation • The new polypeptide is now floating loose in the cytoplasm. It might also be inserted into a membrane, if the ribosome it was translated on was attached to the rough endoplasmic reticulum. • Polypeptides fold spontaneously into their active configuration, and they spontaneously join with other polypeptides to form the final proteins. • Sometimes other molecules are also attached to the polypeptides: sugars, lipids, phosphates, etc. All of these have special purposes for protein function.

  16. Genetically Modified Plants • It is fairly easy to insert genes into plants, using a special plasmid vector derived from crown gall tumors. This vector grows in bacteria, but transfers its DNA into the plant genome after infection. • One use of this techniques is to insert nutrient genes into plants. For instance, rice is the staple food for a large part of the world’s population. It contains no carotene, the orange pigment that is the precursor for the main visual pigment retinol. People who live on rice alone often develop blindness because they don’t eat enough carotene. • Golden rice was developed to solve this problem: genes fro producing carotene were put into rice. This pigment gives the rice its color. • Problems: will people eat this oddly-colored food? Will it work well in cooking? Will they accept “Frankenfood”?

  17. Nuclear Cloning • Why not just take the nucleus from any cell and put it into an egg, producing a new person genetically identical to the original? This is the idea behind nuclear cloning, and it does work on occasion. • Dolly the Sheep was the first example: a nucleus from her parent’s mammary gland was extracted and put into an egg whose own nucleus was removed. The egg was them implanted into the uterus of another sheep, and Dolly was born. She is genetically identical to the donor sheep.

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