Bryan Bishop October, November, 02005 DNA Preview 1. What are the main functions of DNA? DNA is a nucleic acid which contains genetic instructions (code) for the creation of all other forms of life and some viruses, not to mention proteins. DNA is responsible for "inherited traits". 2. Identify the types and locations of covalent bonds and hydrogen bonds in a DNA molecule. Hydrogen bonds hold the two strands of the double-helix DNA molecule. Carbon bonds are most commonly seen in the deoxyribose sugar molecule, a 5-carbon structure. Since carbon atoms always have 4 bonds, there is noticeably three atoms which are bonded to one carbon in the structure, however one of those atoms is bonded through a double bond. 3. List the base-pairing rules. The base-pairing rules are: Adenine connects with Guanine, and Thymine connects with Cytosine. 4. What roles do enzymes play in DNA replication? Enzymes work with the strands of DNA to specifically control what's going on. 5. How would the deoxyribose sugar-phosphate backbone of nucleotide chains look if purines paired only with purines and pyrimidines paired only with pyrimidines? 6. Define transcription. List the main steps involved in this process. Transcription is the process where DNA is copied by an RNA polymerase to produce an extra RNA. In prokaryotic transcription, RNAP specifically binds with the promoter region on the double-helix strands of DNA. A promoter is a region of DNA that allows a gene to be transcribed (read/write). The DNA is then unwound. The RNAP tries to transcribe the DNA, but doesn't do so correctly about 10 times because an exit channel is blocked which normally isn't. The enzymes work on that problem to remove the block, and continue on. In eukaryotic transcription, there is an initiation stage, an enlongation stage, and a termination stage. The enlongation stage is common to both prokaryotic and eukaryotic transcription. 7. In what ways does the structure of RNA differ from that of DNA? Usually, RNA structurally differs from DNA, except in cases such as transfer RNA (tRNA). RNA differs by the extra hydroxyl group attached to each pentose ring, a ring of the 5-carbon monosaccharide. 8. Describe the structure and function of each of the three types of RNA. 1. Messenger RNA (mRNA) 1. Structure: 5' cap, 5' UTR, Coding Region (composed of codons), 3' UTR, Poly(A) tail 2. Function: The purpose of mRNA is to carry information from DNA to ribosomal sites of protein synthesis. Once mRNA has finished being transcribed from DNA, it is `exported` and bound to ribosomes and translated into protein. The message the RNA was carrying starts to degrade. 2. RNA genes (Non-Coding RNA) (rRNA, tRNA, snRNA, snoRNA, miRNA, gRNA, eRNA, SRP, pRNA, tmRNA, fRNA) 1. Structure: There is no general structure. 2. Function: These are RNA genes that encode RNA which is not translated into any protein. Beyond this, there are many specialized functions/purposes of "RNA genes". 3. Double Stranded RNA (dsRNA) 1. Structure: RNA with two extra strands like DNA. 2. Function: Forms the genetic material of some viruses 9. List three roles of RNA polymerase in transcription. RNAP binds to DNA at a specific region. The DNA is "unwound" and creates the "open omplex" with a structure resembling RNAP. RNAP transcribes the DNA, however sometimes the exit channels are blocked. 10. What basic principle ensures that the transcribed RNA molecule is carrying the right genetic message? There is a second RNA polymerase following the first, like trucks in a convoy. 11. Compare transcription with translation. Transcription is where mRNA is created from the 5-prime (leading strand) of the DNA double-helix molecule. Translation requires the mRNA from transcription, and then proceeds to match the mRNA with tRNA. (In translation, the specific amino acids coded for by the codons are connected to the tRNA). 12. Distinguish a codon from an anticodon, and explain the significance of each. A codon is a set of coding for an amino acid, e.g., it's the make up of the genetic code stored in DNA, a gene sequence. Anti-codons contain the complementary bases of codons. Codons and anti-codons first "meet" when tRNA is using the recognition sites for the codons. 13. How does the structure of tRNA relate to its function in translation? Every tRNA molecule bonds only one type of amino acid because of the sites for amino-acid attachment and the codon (three-base-sequence) recognition, which is different for each tRNA molecule (although one tRNA molecule sometimes recognizes more than one type of codon). 14. Using the information in Table 10-1, list the amino acids that are coded for by the codons AGU, GGG, CCU, and GUG. AGU - Serine GGG - Glycine CCU - Proline GUG - Valine (start) 15. Explain the significance of the start codon and the stop codon. Do all polypeptides begin with the amino acid coded for by the start codon? The start and stop codon signal where transcription is to begin and end. Without these codons, it would be very hard - if not impossible - to determine where to start reading the DNA and where to stop. No, not all polypeptides begin with the amino acid coded for by the start codon. 16. How does the inheritance of sex chromosomes result in approximately equal numbers of males and females among the offspring of fruit flies? There's a 50% chance of X, and a 50% chance of Y. The XY chromosome is "decided by" the father. 17. Offer an explanation for why Morgan did not find white-eyed female Drosophilia in the second generation of the first of the parent generation when he crossed white-eyed males with red-eyed females. The white-eyed female characteristic would have appeared in the first generation and "skipped" the second generation. This probably has something to do with dominant and recessive phenotypes, and known/unknown genotypes. 18. How does crossing-over show that genes are found on chromosomes? Unequal crossover shows that the genes which are in certain chromosomes are duplicated, transferred, or removed in certain instances when the chromosomes break and reconnect at different ends. 19. How can crossing-over between two alleles be used to map their locations on chromosomes? A genetic map based on recombination frequencies (the crossing over of genes) can be viewed to see how the genes end up in the final chromosome. The genes will always end up in the same relative position. 20. What are point mutations? A point mutation is a permanent change in the genetic material of a single base pair with another pair. This often includes "insertion" of base-pairs and the deletion of base pairs in the coding sequence. 21. A husband and wife are heterozygous for cystic fibrosis. Their son has cystic fibrosis. Their second child, a daughter, does not. Prepare a pedigree for this family. c c -------------------- C | Cc | Cc | -------------------- c | cc | cc | -------------------- C = cystic fibrosis dominant c = cystic fibrosis recessive (not appearing) It seems that there is a 50% chance of an offspring having cystic fibrosis. 22. A husband and wife have the ABO blood group genotypes 1A1B and ii. What ABO blood types can their children have? A or B. 23. Compare the inheritance of Huntington's disease with the inheritance of sickle cell anemia. Huntington's disease is a dominant trait, whereas Sickle cell disease is recessive. A person who receives the defective gnee from both father and mother develops sickle cell disease; a person who receives one defective, yet one helathy allele, remains healthy, but can pass on the disease as a carrier. Sickle cell disease is known to be caused by a defect in a single nucleotide (A to T) of a globin gene. 24. Is pattern baldness a sex-linked trait or a sex-influenced trait? Explain your answer. Pattern baldness is a sex-influenced trait because the allele in both men and women happens to be recessive in women. My explanation is a link to a website: http://facweb.bhc.edu/academics/science/robertsk/biol101/geneticsnotes.htm 25. How can nondisjunction change chromosome number? Nondisjunction is when the chromosome fails to split correctly during a basic form of reproduction.