Thursday, January 22, 2009

Chapter 17 Mutations Start Chapter 18


Mutations are alternations in the genetic material of the cell; point mutations are alternations of just one base pair of a gene. Point mutations are the most common. They come in 2 basic types:

1. Base pair substitutions refer to the replacement of one nucleotide and its complementary base pair in DNA with another pair of nucleotides.

  • Missense Mutations are those substitutions that enable the codon to still code for an amino acid, although it might not be the correct one

  • Nonsense Mutations are those substitutions that change a regular amino acid codon into a stop codon, ceasing translation

  • Silent Mutations (another example of base pair substitution from the book) are substitutions that have no effect on the new proten due to the redundancy of the genetic code.

2. Insertions and Deletions refer to the additions and losses of nucleotide pairs in a gene. They can cause a frameshift mutation which causes the mRNA to be read incorrectly.

Mutagens are substances or forces that interact with DNA to cause mutations. X-rays and some other forms of radiations are known mutagens, as are certain chemicals.

Chapter 18: Regulation of Gene Expression

The control of gene expression enables individual bacteria to adjust their metabolism to enviornmental change. Cells control metabolism by regulating enzyme activity or by regulating enzyme synthesis through activating or inactivating genes.

Repressible vs. Inducible Operons

The Repressible Operon : The trp operon is said to be a repressible operon because its transcription is usually ON but can be inhibited (repressed) when a specific molecule (ex. tryptophon) binds allosterically to a regulatory protein.

This "binding" makes the repressor active not the operon.

The enzymes for tryptophan synthesis are said to be repressible. Repressible enzymes generally function in anabolic pathways (building up), which synthesize essential end products from raw materials (precursors). By suspending production of an end product when it is already present in sufficient amounts, the cell can allocate its organic precursors and energy for other uses.

Here is a little preview of Chapter 18!

Review Questions!
1. This kind of mutation will occur whenever the number of nucleotides inserted or deleted is not a multiple of three. This causes improperly grouped codons and a nonfunctional protein.
A. Frameshift mutation
B. Nonsense Mutation
C. Missense Mutation
2. What is an example of a mutagen?
A. Visible light
B. UV light
C. Radio waves
3. The trp operon is what kind of operon?
A. Repressible
B. Corepressor
C. Inducible
4. Repressible Operons are usually turned off
A. True
B. False
5. Repressible enzymes function in catabolic pathways
A. True
B. False
Answers: A, B, A, B, B
So don't forget tomorrow is Chapter 18 vocab quiz and you Chapter 17 essay is due!

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