Ch 12.

Gene Expression and Regulation

Chapter 12 At a Glance

12.1 How Is the Information in DNA Used in a Cell?

12.2 How is the Information in a Gene Transcribed into RNA?

12.3 How is the Base Sequence of mRNA Translated into Protein?

12.4 How Do Mutations Affect Protein Structure and Function?

Case Study: Cystic Fibrosis

If all you knew was her music, you’d think Alice Martineau had it made – a young, pretty singer-songwriter under contract with a major recording label. However, like about 70,000 other people worldwide, Martineau had cystic fibrosis, a recessive genetic disorder caused by defective alleles of a gene that encodes a crucially important protein called CFTR. CF occurs when a person is homozygous for these defective alleles. Before modern medical care, most people with cystic fibrosis died by age 4 or 5; even now, the average life span is only 35 to 40 years. Martineau died when she was 30. The CFTR protein forms channels that allow chloride to move across plasma membranes down its concentration gradients. CFTR also helps sodium movement in many parts of the body, including the sweat glands, CFTR helps to reclaim sodium chloride form the sweat and transport it back into the blood, so that the body doesn’t lose too much salt. Probably the most crucial role of CFTR is in the cells lining the airways are covered with a film, of mucus, which traps bacteria and debris. The bacteria-laden mucus is then swept out of the lungs by cilia on the cells of the airways.

Case Study: Cystic Fibrosis Normally, chloride moves through CFTR channels out of the airway cells into the mucus, and sodium follows. The resulting high concentration of sodium chloride causes water to move into the mucus by osmosis, resulting in a thin liquid that can be removed easily by the cilia. However, mutations in the CFTR gene produce a defective chloride channel proteins. As a result, chloride and sodium do not move from the cells into the mucus, so water doesn’t move into the mucus, either. The mucus becomes so thick that the cilia can’t move it out of the lungs, leaving the airways partially clogged. Bacteria multiply in the mucus causing the chronic lung infections. In this chapter, we examine the processesby which the instructions in genes are translated into proteins. As you will learn, changes in those instructions – mutations – alter the structure and function of proteins as CFTR.

12. 1 How Is the Information in DNA Used in a Cell?

• Information must be translated into action in order for a particular process to work

• DNA contains the “molecular blueprint” of every cell

• Proteins = construction workers of the cell

• Proteins control cell shape, function, reproduction & synthesis of biomolecules

• Must be a flow of information from DNA to protein

• DNA provides instructions for protein synthesis via RNA intermediaries

• DNA info must be carried by ribonucleic acid (RNA) from the nucleus to the cytoplasm

• RNA is usually single- stranded unlike DNA’s double helix

• RNA has the sugar ribose rather than deoxyribose in its backbone

• RNA contains the nitrogenous base uracil (U) instead of thymine (T)

There are 3 types of RNA involved in protein synthesis:1. Messenger RNA (mRNA) carries DNA gene information to the ribosome• contains codons (groups of 3 bases) which specify which amino acids will be incorporated

into a protein

2. Ribosomal RNA (rRNA) is part of the structure of ribosomes• Consist of 2 subunits (made up of rRNA and other proteins) that contain various binding and

catalytic sites needed for protein synthesis; carry out translation

3. Transfer RNA (tRNA) brings amino acids to the ribosome• contain anticodons (groups of 3 bases) which deliver specific amino acids to the ribosome

where they are incorporated into a protein.

Genetic information is transcribed into RNA and then translated into protein.

• DNA directs protein synthesis in a 2-step process1. Information in a DNA gene is copied into RNA in the process of transcription (occurs in nucleus

of eukaryotic cells)

2. mRNA, together with tRNA, amino acids, and a ribosome synthesize a protein in the process of translation of the genetic information contained in mRNA (occurs in cytoplasm of eukaryotic cells)

12.2 How Is the Information in a Gene Transcribed into RNA?

1. Initiation – RNA polymerase binds to

promoter region at the beginning of a gene so

RNA can be synthesized

2. Elongation – RNA polymerase travels along

DNA template strand (3’ to 5’) adding RNA

bases that are complementary to DNA (as the

RNA strand forms; DNA helix re-forms).

3. Termination – transcription stops when RNA polymerase reaches a termination sequence; completed RNA strand is released and detaches from DNA; RNA polymerase free to bind to promoter region on a different gene

https://www.youtube.com/watch?v=ztPkv7wc3yU

12.3 How Is the Base Sequence of Messenger RNA Translated into Protein? mRNA synthesis differs btwn prokaryotes and eukaryotes

Prokaryotes- No nuclear membrane- Transcription/translation not separated in space/time- ribosomes immediately begin translating the mRNA into protein while still attached to DNA

Eukaryotes- DNA contained in nucleus and ribosomes reside in cytoplasm- Genes not clustered by many disperse among several chromosomes- mRNA molecule formed is not a functional mRNA that can be immediately translated into

protein

• In eukaryotes, a precursor RNA is processed to form mature mRNA that is translated into protein• Pre-mRNA contains exons (segments of DNA that encode for protein)

interrupted by introns (segments of DNA that are not translated)

5’ Cap & Poly (A) tail help 1) move the RNAthrough the nuclear envelope, 2) bind mRNA toribosome, 3) prevent cellular enzymes from breaking down the mRNA

RNA Splicing – introns cut out and exons spliced together

• During translation, mRNA, tRNA, and ribosomes help to synthesize proteins• 3 steps (initiation, elongation, termination)

1. Initiation – begins when tRNA and mRNA bind to a ribosome

• Pre-initiation complex forms

• UAC anticodon of methionine tRNA binds to mRNA by base paring

with AUG start codon at (5’ end) of mRNA

• large ribosomal unit attaches to small subunit, holding mRNA btwn 2 subunits and holding methionine tRNA in its first tRNA binding site

2) Elongation – amino acids are added one at a time to growing protein chain

• 2nd tRNA anticodon base-pairs with 2nd codon on mRNA

• bond holding methionine to its tRNA is broken and forms peptide bond btwn amino acid on 1st tRNA and amino acid on 2nd tRNA

• Empty tRNA is released and ribosome moves down the mRNA one codon

https://www.youtube.com/watch?v=5bLEDd-PSTQ

3) Termination: stop codon (UAG, UAA, UGA) signals the end of translation

https://www.youtube.com/watch?v=itsb2SqR-R0

12.4 How do Mutations Affect Protein Structure and Function?• Mutations – changes in base sequence of DNA caused by mistakes made during

replication or environmental factors• 1-2. Inversions & translocations: when pieces of DNA are broken apart and

reattached within a single chromosome or to a different chromosome- These mutations relatively benign if entire gene is moved- If gene is split in 2, it won’t code for a complete functional protein

• Depending on how many nucleotides are involved, mutations can cause a misreading of a gene’s codons

• Deletions – occurs when 1 or more nucleotides are removed from the sequence• Insertion – occurs when 1 or more nucleotides are added to thesequence• Substitution (point mutation) – an incorrect nucleotidetakes the pace of a correct one.

• There are about 1,500 different defective alleles of the CFTR gene, all of which can causecystic fibrosis. The most common defective allele is missing a single codon. The resulting lack of one crucial amino acid causes the CFTR protein to be misshapen. A normal CFTR protein is synthesized by ribosomes on rough endoplasmic reticulum, enters the ER, and then is transported to the plasma membrane. The misshapen CFTR protein, however, is broken down within the ER and never reaches the plasma membrane. Four other common mutant CFTR alleles code for a stop codon in the middle of the protein, so translation terminates partway through. Still other mutant alleles produce proteins that are completely synthesized and inserted intothe plasma membrane, but do not form functional chloride channels.

Androgen InsensitivitySometime between 7-14 yrs of age, a girl usually goes through puberty: breasts swell, hips widen, and menstruation begins. In rare instances, however, a girl may develop all of the outward signs of womanhood, but without menstruation. If her physician performs a chromosome test, in some cases the results seem to be impossible: the girls’ sex chromosomes are XY. The reason she has not begun to menstruate is that she lacks ovaries and a uterus but instead has testes inside her abdominal cavity. She has about the same concentrations of androgens (male sex hormones, i.e. testosterone) circulating in her blood as would be found in a boy her age.In fact, androgens have been present since early in her development. However, her cells cannot respond to them – a condition called androgen insensitivity. The affected gene codes for a protein known as an androgen receptor. In normal males, androgens bind to the receptor proteins, stimulating the transcription of genesthat help to produce many male features, including formation of a penis and descent oftestes. Androgen insensitivity is caused by defective androgen receptors.

There are more than 200 mutant alleles of the androgen receptor gene. The most serious are mutations that create a premature sstop codon. Bc the androgen receptor gene is on the X chromosome, a person who is genetically male (XY) inherits a single allele for the androgen receptor. If this allele is seriously defective, the person will not synthesize functional androgen receptor proteins. The person’s cells will be unable to respond to testosterone, and male characteristics will not develop. In many

respects, female development is the “default” option in humans, and without functional androgen receptors, the affected person’s

body will develop female characteristics. Thus a mutation that changes the nucleotide sequence of a single gene, causing a single type of defective protein to be produced, can cause a person who is genetically male to look like and perceive herself to be female.

Why Bruises Turn ColorsBruises typically progress from purple to green to yellow. This sequence is visual evidence of the control of gene expression. If you bang your shin on a chair, blood vessels break and release red blood cells, which burst and spill their hemoglobin. Hemoglobin and its iron-containing heme group are dark bluish-purple in the deoxygenated state, so fresh bruises are purple. Heme, which is toxic to the liver, kidneys, brain, and blood vessels, stimulates transcription of the heme oxygenase gene. Heme oxygenase is an enzyme that converts heme to biliverdin, which is green. A second enzyme, which is always present because its gene. A second enzyme, which is always present because its gene is always expressed, converts biliverdin to bilirubin, which is yellow. The bruise finally disappears as bilirubin moves to the liver, which secretes it into the bile. You can follow the detoxification of heme by watching your bruise change color.