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'4 4GENIE 111-1 ovs, The knowledge of gene activity arose from the experiments of several investigators. Garrod reasoned the cause for inborn errors of metabolism. Beadle and Tatum, working with red bread mold, suggested the one gene— one enzyme hypothesis. Pauling and Itano refined this to the one gene—one polypeptide hypothesis. RNA differs from DNA in several ways: (1) The pentose sugar is ribose, not deoxyribose; (2) the base uracil replaces thymine; and (3) RNA is single stranded. According to the central dogma of molecular biology, DNA is the template for its own replication and also for RNA formation. The sequence of bases in DNA specifies the proper sequence of amino acids in a polypeptide. The genetic code is a triplet code, and each codon (code word) carried by mRNA consists of three bases. The code is almost universal among organisms. Polypeptide (protein) synthesis requires transcription and translation. Transcription, the synthesis of an EIVES WORK_ RNA molecule off a DNA template, begins when RNA polymerase attaches to a promoter. Elongation of an RNA molecule occurs through the process of complementary base pairing until there is a DNA stop sequence. Messenger RNA (mRNA), which now carries codons, is processed before it leaves the nucleus; in particular, introns are removed by RNA splicing. In humans, 95% of proteincoding genes are introns, which increases the proportion of non-protein-coding DNA sequences, a finding that is of interest to investigators. Translation, the making of a polypeptide in the cytoplasm, requires several types of RNA. Ribosomal RNA (rRNA) and various proteins make up a ribosome where a polypeptide is formed. As a ribosome moves down an mRNA strand, the codons pair with the anticodons of transfer RNA (tRNA), which bring amino acids to the ribosomes. Because of this process, the amino acids are joined according to the sequence of bases in DNA. Study the text section by section as you answer the questions that follow. I 4 71.1119r4 E S (Pp_ 238 - 2;19 1r)' • Each gene specifies the amino acid sequence of one polypeptide of a protein, a molecule that is essential to the structure and function of a cell. Indicate whether these statements are true (T) or false (F). a. Garrod was the first to suggest an association between genes and proteins. b. Beadle and Tatum induced mutations in asexual haploid mold spores. c. Beadle and Tatum proposed the one gene–one enzyme hypothesis. Pauling and Itano showed that a mutation leads to a change in the structure of protein in hemoglobin. d. Pauling and Itano proposed the one gene–one enzyme hypothesis. 2. The work of these investigators made it possible to conclude that F RC> ri DF.IArc) Ft 1■1 A -rc> 240) P Ft COT E I ( P. • The expression of genes leading to a protein product involves two steps, called transcription and translation. • Three different types of RNA molecules are involved in transcription and translation. 3. Because genes (DNA) reside in the a. the b- , of the cell and polypeptide (protein) synthesis occurs in they must have a go-between. The most likely molecule to fill this role is C. 4. Indicate whether these statements about differences between DNA and RNA are true (T) or false (F). a. DNA is double stranded; RNA is single stranded. b. DNA is a polymer; RNA is a building block of that polymer. c. DNA occurs in three forms; RNA occurs in only one form. d. The sugar of DNA is ribose, which is absent in RNA. e. Uracil, in RNA, replaces the base thymine, found in DNA. 5. Complete this table to describe the function of the various types of RNA involved in protein synthesis. RNA Function messenger RNA (mRNA) a. ribosomal RNA (rRNA) b. transfer RNA (tRNA) C. 6. Label the following diagram, which pertains to the central concept of genetics. 1, mi ,r 0 cc, xc. . ; 1 a. J cc J J GI . ii mRNA i K I i I II I I I codon 1 codon 2 codon 3 0 0 0 I I I polypeptide —N—C—C—N—C—C—N—C—C- I , R, . I I i arginine 1 R, " tyrosine i R, " tryptophan Ill I 4.2 - r 1-1 E ■COCIE EPIEII (P. 24 I ) • The genetic code is a triplet code; each code word, called a codon, consists of three nucleotide bases, and stands for a particular amino acid of a polypeptide. 7. Study the following diagram, which lists the mRNA codons. First Base u C A G Second Base Third Base U C A G UUU phenylalanine UUC phenylalanine UUA leucine UUG leucine CUU leucine CUC leucine CUA leucine CUG leucine AUU isoleucine AUC isoleucine AUA isoleucine AUG (start) methionine GUU valine GUC valine GUA saline GUG valine UCU serine UCC serine UCA serine UCG serine CCU proline CCC proline CCA proline CCG proline ACU threonine ACC threonine ACA threonine ACG threonine GCU alanine GCC alanine GCA alanine GCG alanine UAU tyrosine UAC tyrosine UAA stop UAG stop CAU histidine CAC histidine CAA glutamine CAG glutamine AAU asparaqine AAC asparagine AAA lysine AAG lysine GAU aspartic acid GAC aspartic acid GAA glutamic acid GAG qlutamic acid _ UGU cysteine UGC cysteine UGA stop UGG tryptophan CGU arginine CGC arginine CGA arginine CGG arginine AGU serine AGC swine AGA arginine AGG arginine GGU glycine GGC glycine GGA glycine GGG qlycine u c A G u c A G u C A G U c A G a. What does it mean to say that the genetic code is a triplet code? b. What are the mRNA codons for leucine? c. What does it mean to say that the genetic code is degenerate? _ d. What does it mean to say that the genetic code is unambiguous? • During transcription, a segment of the DNA strand serves as a template for the production of an RNA molecule. 8. During transcription, an mRNA molecule is formed that has a sequence of bases a of one DNA strand. The bases pair in this manner: A in DNA pairs with b c. ' to a portion ' , and G pairs with (and vice versa) in the mRNA being formed. If the sequence of bases in DNA is CGA AGC TCT, then the sequence in mRNA is d Why is there a space between every three bases? e 118 ' 9. a. Which are spliced out when primary RNA is processed—exons or introns? b. What tells a ribosome where to attach—the cap at the 5' end or the poly-A tail at the 3' end? c. How are introns removed from mRNA? d. Which one, a spliceosome or a ribozyme, is an intron and also an enzyme? e. What are two possible functions of introns? • During translation, the amino acids of a specific polypeptide are joined in the sequence directed by a type of RNA called messenger RNA (m RNA). • Transfer RNA (tRNA) molecules transfer amino acids to the ribosomes. • Ribosomal RNA (rRNA) is produced from a DNA template in the nucleolus. 10. Features of tRNA structure include: At one end, an a. attaches, and at the other end there is a(n) complementary to a codon in mRNA. 11. Features of rRNA structure include: Each ribosome is composed of a a. subunit and a b. subunit. Ribosomes have a binding site for C. and three d ' at a time. Several ribosomes moving down the same mRNA plus the mRNA make up a e. 12. Label the following diagram using the alphabetized list of terms. amino acid A site codon E site large ribosomal subunit mRNA P site small ribosomal subunit h. 9. f. e. 13. Use the diagram in question 12 to help you answer questions with regard to initiation. During initiation, two ribosomal subunits come together to form a a. . An initiator tRNA occupies the P site of the ribosome. A b. will soon occupy the A site. b. c. 14. What are the three steps of translation? a. 119 15. Use the following diagram to help you answer the questions with regard to elongation. During elongation, the tRNA at the a- b. site passes its peptide to the tRNA-amino acid at the site. Then C. occurs, and the spent tRNA leaves from the d. site. The tRNA bearing the peptide moves to the P site, and the A site is now empty. The next tRNA-amino acid arrives, and this tRNA pairs with its e. . This sequence of events reoccurs until the polypeptide is complete. 16. Use the following diagram to help you answer questions with regard to termination. .to 3' *- 4411414r4--441t, During termination, the ribosomal a last tRNA before the stop codon UGA. 120 separate, and the b. is released from the 17. Complete this diagram by identifying each of the components depicted. TRANSCRIPTION -c 1. DNA' in nucleus1 ..„7,—,..t r-r-, ,,,, t,_ b. ir /.."._ -. . ,„-. serves as a template ,----'' '\ introns d. „ ...."'"' e. , ---' ---- mature ,...- -„, ---::.a&-z--- 1 ,....................--_..,::„ .-- ...--' A L a. , f. ...."4 ' — I. . - 3. mRNA moves into cytoplasm and 5' becomes associated 4.. with ribosomes. -.. . 9. , /-* 4. tRNAs with anticodons carry amino acids to mRNA. a h 6. Polypeptide synthesis takes place one amino acid at a time. cccUGGUUU 5' . 0 GG ACC 1 4t k. 1 i rn L-T---/ ""rairl441,..__ j. 5. Anticodon—codon complementary base pairing occurs. If you identified all correctly, you have found your way out. 0A 3' -ow. Now 010 K E W4C1 Ft CO .114c■ ss,vv4colzt Review key terms by completing this crossword puzzle, using the following alphabetized list of terms: anticodon codon exon intron messenger RNA mutagen ribosomal RNA ribozyme RNA polymerase transcription transfer RNA translation triplet code Across 2 Type of RNA found in ribosomes that coordinates the coupling of anticodons with codons during polypeptide synthesis (two words) Genetic code (mRNA, tRNA) in which sets of three bases call for specific amino acids in the formation of polypeptides (two words) Noncoding segment of DNA that is transcribed but removed before mRNA leaves the nucleus Type of RNA formed from a DNA template and bearing coded information that directs the amino acid sequence of a polypeptide (two words) 11 Enzyme that speeds the formation of RNA from a DNA template (two words) 12 Process whereby the sequence of codons in mRNA determines the sequence of amino acids in a polypeptide I Down 1 Type of RNA that transfers a particular amino acid to a ribosome during protein synthesis; at one end, it binds to the amino acid, and at the other end, it has an anticodon that binds to an mRNA codon (two words) 3 Three nucleotides on a tRNA molecule that are attracted to a complementary codon on mRNA 5 In a gene, the portion of DNA base sequence that is expressed resulting in polypepetide formation 6 Three nucleotides of DNA or mRNA; it stands for a particular amino acid 8 Process whereby a DNA strand is a template for the formation of mRNA 9 Agent, such as radiation or a chemical, that brings about a mutation in DNA 10 Enzyme that carries out mRNA processing