Chapter 7Gene Expression and Control Part 1

Ribosomes are THAT importanto Ricin is a deadly toxin found in castor-oil seeds. A

dose as small as a few grains of salt can kill an adult.

o Even though ricin use as a weapon has been banned in most countries as a result of the Geneva Protocol, this poison is easily enough obtained that its use as a poison has been documented many times throughout history.Eating only 8 of these deceivingly beautiful seeds

would be enough to kill an adult.

Ribosomes are THAT importanto During the Cold War, a Bulgarian writer, Georgi

Markov, who had defected to England was killed when an assassin used the tip of a modified umbrella to jam a small, ricin-laced ball into his leg.

o Traces of ricin were found in a United States Senate mailroom and State Department building, as well as in an envelope addressed to the White House in 2004.

o In 2005, jars of banana baby food laced with ground castor-oil beans made the news.

Ribosomes are THAT importanto So how is it that a poison that is so easily obtained

could be so deadly?o Ricin is so toxic because it inactivates ribosomes,

the organelles upon which amino acids are assembled into proteins in all living cells.

o Because proteins are critical to all life processes, cells that cannot make them die very quickly.

o Someone who inhales ricin typically dies from low blood pressure and respiratory failure within a few days of exposure.

The Nature of Genetic Informationo You already know that DNA contains four letters

(which are the nitrogen bases): A, G, T, and C.o The sequence of these four letters, one after the

other, carries the information encoded by the DNA, called the genetic code.

o The subsets of the information contained within this code are called genes.

o A cell uses the base sequence of a gene to build and RNA molecule and eventually a protein product.

The Nature of Genetic Information

o The first step in creating a protein product from a gene (DNA) is transcription.

o During transcription, enzymes use the DNA sequence of a gene to assemble a strand of RNA.

The Nature of Genetic Informationo RNA is similar to DNA in that both are chains

containing four kinds of nucleotides.o However, while DNA is a double-stranded

molecule, RNA consists of only a single strand.o In addition, the nucleotides that make up RNA

differ slightly from those that make up DNA. While DNA contains A, G, T, and C, RNA contains A, G, U, and C.

o Finally, while the sugar in DNA is deoxyribose, the sugar in RNA is ribose.

DNA vs. RNA

The Nature of Genetic Informationo Even though the structural differences in RNA and DNA seem

small, the function of these two molecules varies greatly.o DNA’s only role is to store inheritable genetic information.o In contrast, cells make three different kinds of RNA molecules,

each of which has a different function.o Even though all three RNA molecules are encoded by information

in the DNA, only one of these RNA’s, messenger RNA (mRNA), carries the protein-building message from the DNA.

o That message is encoded with the mRNA sequence by sets of three bases. These three letter “genetic words” are called codons.

Types of RNA

The Nature of Genetic Information

o Like the words of a sentence, a series of the se “genetic words” can form a meaningful package of information, resulting in a specific series of amino acids in a protein after the next, and last, step in the creation of a protein, called translation.

o Translation occurs when the protein-building message in the mRNA is translated into a different language, the amino acid language of proteins.

o The result is a polypeptide chain that will twist and fold, forming the secondary, tertiary, and quaternary structures of functional proteins.

Translation

The Nature of Genetic Informationo The processes of transcription and translation

together result in gene expression, the process by which genetic information encoded in the DNA is converted to RNA and then to a protein product.

o This is called the Central Dogma of biology.