Chapter Chapter #9#9

Gene Gene ExpressionExpression

Section 9-Section 9-1:1:

From From Genes to Genes to ProteinsProteins

A. The Path of Genetic Information1) Proteins are not built directly

from genes. Your cells preserve hereditary information by transferring the information in genes into sets of working instructions for use in building proteins.

2) The working instructions of the genes are made of molecules of RNA. RNA differs from DNA in three ways:

a) RNA is single-stranded, while DNA is double-stranded.

b) The sugar in RNA is ribose, while DNA has deoxyribose.

c) RNA has the nitrogen base uracil instead of the base thymine found in DNA.

3) RNA is present in cells in three different forms:a) Messenger RNA (mRNA): Carries

hereditary information from DNA to the ribosomes.

b) Transfer RNA (tRNA): Acts as an interpreter molecule, translating mRNA sequences into amino acid sequences.

c) Ribosomal RNA (rRNA): Plays a structural role in the ribosomes.

B. Gene expression occurs in two stages:

1) Transcription: The information in DNA is rewritten into mRNA.

2) Translation: The mRNA is deciphered in the ribosomes and made into proteins.

C. Transcription: Making RNA1) The first step in using DNA to

direct the making of a protein is transcription, the process that “rewrites” the information in a gene in DNA into a molecule of mRNA.

2) In eukaryotic organisms, transcription occurs inside the nucleus. In prokaryotic organisms, it takes place in the cytoplasm.

3) Transcription begins when an enzyme called RNA polymerase binds to the beginning of a gene on a region of DNA called a promoter.a) A promoter is a specific

sequence of DNA that acts as a “start” signal for transcription.

b) After RNA polymerase binds to the promoter, the enzyme starts to unwind and separate the double helix’s two strands, exposing the DNA’s bases.

4) In transcription, only one of the two strands of DNA serves as a template.

5) Once the DNA has separated, the RNA polymerase moves along the bases of the template strand -- always in the same direction.

6) The enzyme reads each nucleotide and pairs it with a complementary RNA nucleotide (see Fig 9-2). Transcription follows the same base-pairing rules as DNA except that uracil, rather than thymine, pairs with adenine.

7) As RNA polymerase works its way down the strand, a single strand of RNA grows and dangles off the enzyme like a tail.

8) Behind RNA polymerase the two strands of DNA close up and re-form the double helix.

9) Transcription proceeds at a rate of about 60 nucleotides per second until the RNA polymerase reaches a stop signal called a terminator. Then the enzyme detaches from the DNA and releases the RNA molecule for the next stage of gene expression.

D. In eukaryotic cells, both exons and introns are transcribed into mRNA. Before mRNA leaves the nucleus, the introns are cut out. The exons are joined to form a single molecule of mRNA, which leaves the nucleus through a pore and enters the cytoplasm.

E. The Genetic Code1) After transcription, the

genetic message is ready to be translated from the language of RNA to the language of proteins.

2) The instructions for building a protein are written as a series of three-nucleotide sequences called codons.

3) Each codon along the mRNA strand either corrresponds to an amino acid or signifies a stop signal.

4) From trial-and-error experiments, biologists worked out which codons correspond to which amino acids.

5) Table 9-1 presents the entire genetic code, the amino acids and stop signals that are coded for by each of the possible mRNA codons.

6) The genetic code is virtually the same for all organisms. The only exceptions are in the way cell organelles and a few microscopic protists read stop codons.

F. Translation: Making Proteins1) The equipment for

translation is located in the cytoplasm, where a cell keeps its supply of tRNA.

2) A tRNA molecule is a single strand of RNA folded into a compact shape with three loops (See Fig. 9-4).

3) One of the loops has a three-nucleotide sequence called an anticodon.a) It is called an anticodon because the

3-nucleotide sequence is complementary to one of the 64 codons of the genetic code.

b) In most organisms, there is no tRNA molecule with an anticodon complementary to the codons UAG, UAA, or UGA. This is why these three codons are called stop codons.

c) Opposite the anticodon is a site at which the molecule carries an amino acid which corresponds to the codon the tRNA anticodon matches.

4) The ribosome is the cell organelle where proteins are made. (See Fig 9-5) Each ribosome has three binding sites that play important roles in translation.a) One binding site holds the mRNA so

that its codons are accessible to tRNA molecules.

b) The A site holds a tRNA molecule that is carrying its specific amino acid.

c) The P site holds a tRNA molecule that is carrying its specific amino acid attached to the growing protein chain.

5) When translation begins, mRNA is bound to a complete ribosome so that the start codon (usually AUG) is positioned in the P site, ready for the first amino acid of the protein chain. (Fig 9-6)

6) When both sites on the ribosome are filled, a peptide bond can form and link the amino acids. (Fig 9-7)

7) After a peptide bond is formed, mRNA shifts on the ribosome so that a new codon is present in the A site. (Fig 9-8)

8) This process repeats until a stop codon reaches the A site. Since there is no anticodon to match the stop codon, the ribosome complex falls apart and the newly made protein is released into the cell. (Fig 9-9)

Section 9-Section 9-2:2:

Regulating Regulating Gene Gene

ExpressionExpression

A. Being able to translate a gene into a protein is only part of gene expression. Every cell must also be able to regulate when particular genes are used. Every function that a living organism carries out is the controlled expression of genes, each used at the proper moment to achieve precise effects.

B. Gene Regulation in Prokaryotes1) In order to survive, bacteria must

be able to adjust to changes in their environment, such as fluctuations in available nutrients. This allows a bacterium to save energy and resources on producing a substance when it is either not needed or readily available in the environment.

2) Bacteria utilize a mechanism called an operon to regulate gene expression. An operon is a cluster of genes that codes for proteins with related functions.

3) The lac operon is a well understood mechanism of gene regulation in E. coli bacteria. It is a cluster of genes that enables a bacterium to build the proteins needed for lactose metabolism only when lactose is present.

a) The lac operon has three structural genes that code for proteins used to metabolize lactose.

b) When lactose is absent from a bacterium’s environment, the repressor protein is bound to the operator and the lac operon is switched off. This prevents transcription from taking place.

c) When lactose is present, the repressor protein detaches from the operator and the lac operon is switched on. This permits transcription to occur.

C. Gene Regulation in Eukaryotes1) Like prokaryotic cells, eukaryotic

cells must continually turn certain genes on and off in response to signals from their internal and external environments.

2) Operons, however, have not been found in eukaryotic cells. Instead, genes with related functions are often scattered among different chromosomes.

3) Gene expression in eukaryotes is affected by the way the DNA in a chromosome is physically arranged.

4) Transcription takes place in the regions of a chromosome where DNA has uncoiled.

5) This is visible as a chromosome puff, which is a region of intense transcription that forms when DNA loops out from the chromosome, perhaps making the genes in that region more accessible to the RNA polymerase.

D. Enhancer Control1) Enhancers are regions of DNA

that stimulate transcription of certain genes in eukaryotic organisms.

2) To understand how an enhancer stimulates transcription, look at the action of estrogen.

a) When estrogen passes through the cell membrane of specific cells it binds to a receptor protein in the nuclear envelope, forming a hormone-receptor complex.

b) The hormone-receptor complex has the right shape to bind to a specific protein called an acceptor protein, which in turn binds to enhancer regions of the DNA.

c) Binding of the acceptor protein to the enhancer region stimulates RNA polymerase to begin transcription.

Section 9-Section 9-3:3:

Genes, Genes, Mutation, and Mutation, and

CancerCancer

A. Mutations are changes in DNA1) A mutation is a change in the

DNA of a gene.2) The effects of a mutation vary,

depending on whether it occurs in a gamete or in a body cell.

a) Mutations in gametes can be passed on to offspring.

b) Mutations in body cells affect only the individual in which they occur.

3) Mutations are an important basis of evolution, because they produce the variations that exist in species.

4) Some mutations alter the structure of the chromosome itself, while others, called point mutations, change one nucleotide or just a few nucleotides in a gene.

a) Substitutions occur when one nucleotide in a gene is replaced with a different nucleotide.

b) Insertions occur when one nucleotide is added to a sequence.

c) Deletions occur when one nucleotide is removed from a sequence.

5) Insertions or deletions can cause frameshift mutations because the upset the three-nucleotide sequence of the gene. This could have a drastic effect on the protein’s function.

B. What causes mutations?1) Some mutations are chemical

mistakes that happen spontaneously during DNA replication.

2) Other mutations are induced by exposure to environmental agents called mutagens.

Examples include:a) Ionizing radiation (x-rays & gamma rays)

b) Ultraviolet lightc) Carcinogens: Cancer-causing agents such as asbestos, benzene, and other pollutants.

C. Mutations can cause cancer1) Cancer is a term used to indicate

a disease characterized by abnormal cell growth.

2) A cell that is cancerous does not respond to the signals that stop cell division. As a result, cancerous cells divide without stopping.

3) Health problems begin when a cancerous cell invades the body’s immune system, which normally destroys cancerous cells. The cancerous cell then proliferates, forming a mass of cells called a tumor.

a) A benign tumor does not invade surrounding tissues.

b) A malignant tumor spreads into other tissues and interferes with organ functions. The cells of malignant tumors have the ability to break free of the tumor and enter blood and lymph vessels, which spreads the malignant cells beyond their original site. This is called metastasis.

D. Cancer Genes1) Researchers have learned that a

cell becomes cancerous when mutations occur in genes that regulate cell growth.

2) An example of growth-regulating genes are called ras genes (because they were first discovered in rats -- genus Ras).

3) The ras genes code for proteins that help prevent uncontrolled cell division.

4) When there is a mutation in a ras gene, a faulty ras protein in built.

5) As a result, the cell divides more rapidly than normal, a sign that it has become cancerous.

6) The ras genes are examples of oncogenes -- genes that, when mutated, can cause a cell to become cancerous.

7) Researchers have found that a mutated ras gene usually contains a point mutation. For example, in a form of human bladder cancer caused by a mutated ras gene, a single G nucleotide has been replaced with a T.

8) The ras gene is only one of several controls that the cell normally exercises over unwanted growth. All of these controls must be inactivated before cancer results. This is why most cancers occur in people over 40 years old -- it takes time for an individual cell to accumulate the necessary mutations.