- Watson and Crick, along with the structure, proposed a theory for the replication of this DNA.
- This is the semi-conservative model.
- This was later proven by Meselsohn and Stahl:
-Growing bacteria on heavy N15, which when centrifuged caused a band at the bottom.
-Then transfering this bacteria to material containing only N14.
-With every generation taking the bacteria out and centrifuging a sample.
-The first generation provided one band between that of N15 and N14, wiping out the possibility for the theory of conservative replication.
-Then with the second generation there were two equal bands at N14 and the midpoint.
-And the third provides a thicker N14 band a thin midpoint, removing possibility of other theories.
- The accepted method is as such:
-H bonds break and the two strands seperate.
-DNA unwinds and DNA polymerase catalyses the addition of free nucleotides.
-Each chain is a template for a new strand to be attached.
-The result is two DNA molecules, both consisting of one original and one new strand.
- THE SEMI-CONSERVATIVE HYPOTHESIS.
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The Genetic Code
- DNA acts as a store for genetic information, divided up into genes.
- The code is triplet because there are 20 amino acids and 3 corresponding bases as a code provides enough possibilites for all amino acids, whereas 2 does not.
-Contains 3 bases: triplet code.
-The code is degenerate, as most amino acids have more than one code.
-Three codes are STOP CODES, and are the end point of a code, like a full stop almost; the terminating signal.
-The portion of DNA that corresponds to a whole polypeptide is called a gene.
-Each 3 bases is a codon.
-The codons are universal, in all living organisms.
-The code is non-overlapping.
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- Basic outline:
-DNA acts as a template for the instructions to form long sequences of nucleotides.
-A complementory section is made into mRNA by transcription.
-The mRNA acts as a template for complementory tRNA to attach amino acids, translation.
- DNA does not leave the nucleus, acting as a template for the formation of mRNA; which carries the genetic information into the cytoplasm.
-DNA helicase acts on a specific region called the cistron on the DNA and breaks the H bonds, causing the two strands to seperate.
-RNA polymerase links the template strand to the begining of the sequence, unwinding and then unzipping in the relavent region.
-Free RNA nucleotides attach themselves opposite correpsonding bases.
-So G-C, C-G, T-A, A-U.
-The DNA moves along attaching these bases, synthesising an mRNA strand.
-The DNA rezips behind the forming bonds, and then this all ends at a stop code.
-The mRNA then moves this information to a ribosome outside the nucleus.
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Protein Synthesis (Cont.)
- Involves translating codons on the mRNA into a sequence of amino acids, on a ribosome.
- Two tRNA are associated with a ribosome at one time, the ribosome acts as framework.
-Ribosome becomes attached to the starting codon on the mRNA.
-The first tRNA with the complementory anticodon attaches to the ribosome, the second tRNA with the complentory anticodon for the next code also attaches; these are close enough for a peptide bond to form between them.
-The tRNA leaves and the ribosome moves one along.
-One site binds tRNA to the growing polypeptie, the other site carries tRNA carrying the next amino acid in sequence.
-Translation by ribosomes allows the assembly of amino acids in accordance with the original DNA, the ribosome moving along the mRNA one code at a time.
-Each time the ribosome moves, a new molecule of polypeptide is produced.
-Usually more than one ribosome is associated with the mRNA at one time, this is called the polysome system and so many molecules can be formed at the same time.
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- The anticodon determines which amino acid attaches to the binding site.
-ie. CCC = glycine will attach to the tRNA
- This anticodon will combine with codon GGG on the mRNA molecule.
- This codon therefore translates into glycine.
- Once the tRNA is released from the mRNA it is free to collect another amino acid from the amino acid pool in the cell, this requires ATP.
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