CH 15 - GENES & HOW THEY WORK

I.  PROTEIN SYNTHESIS = (Gene expression) (p. 280)

                  A. CENTRAL DOGMA: DNAè RNAè PROTEIN

                  B. 1 gene è 1 enzyme (protein)

                  C. 2 STEPS: TRANSCRIPTION & TRANSLATION

            II. TRANSCRIPTION - synthesis of RNA along a DNA template

                 A. INTRODUCTION  DNA  è RNA

                  1. In nucleus.

                  2. U replaces T in RNA.

                  3. Only gene is transcribed, not entire DNA.

                  4. RNA polymerase is enzyme.

                  5. Only 1 strand of DNA transcribed = ANTISENSE/TEMPLATE - ACTIVE

                  6. Non-transcribed strand is = CODING/SENSE - INACTIVE

                 B. STEPS OF TRANSCRIPTION

                  1.  INITIATION (p. 285)

                        a. DNA is cut open and unwound

                        b.  Promoter (start of gene) is read

                  2.  ELONGATION

                        a.  RNA polymerase reads the DNA 3'è5'

                        b.  RNA polymerase assembles the RNA 5'è 3'     

                        c.  Many RNA molecules can be transcribed simultaneously.

                        d.  Only the needed gene is transcribed.

                  3.  TERMINATION (p. 286)

                        a.  Transcription stops at a STOP site after gene transcribed.

                  4.  Example:

                        DNA   3' ATGCATGCA 5'  

                        RNA               5' UACGUACGU 3'

                 C. 3 MAJOR TYPES OF RNA TRANSCRIBED (p. 281)

                  1. mRNA - messenger RNA

                        a. copy of gene, "blueprint" of protein

                        b. dictates sequence of amino acids to be translated

                  2. rRNA - ribosomal RNA.

                        a. combines w/proteins to form ribosomes

                        b. "workbench" for AA assembly

                        c. transcribed in nucleolus

                        d. 1 ribosome = 3 rRNA + 50 proteins! (p. 293)

                  3. . tRNA - transfer RNA. (p. 291)

                        a. "clover leaf" shaped

                        b. transports correct A.A. to ribosome for assembly

            III TRANSLATION - synthesis of a protein from mRNA

                 A. INTRODUCTION:

                  1. Only 1 strand (mRNA) - is read (only one strand exists).

                  2. In cytoplasm.

                  3. Almost the entire mRNA transcript is translated.

                  4. Product of translation is a protein/polypeptide.

                  5. Assembly of AA into correct sequence (polypeptide)

                 B. GENETIC CODE: - not discovered until early 1960's (p. 283)

                  1. 20 Amino Acids – make up all proteins

                  2. Each AA is coded for by a triplet of bases (p. 282)

                  3. One triplet = codon - 3 bases long; found on mRNA

                  4. Anticodon - complementary to codon, found on tRNA (p. 291)

                  5. Only 4 bases in RNA - A, C, G & U

                  6. 3 characteristics of Genetic code:

                        a. TRIPLET - 3 bases - codon

                              i.    4¹ = only 4 combos - not nearly enough

                              ii.   4² = 16 combos, not quite enough

                              iii.  4³ = 64 combos, more than enough

                              iv.   61 code for the 20 AA

                              v.    3 are "Stop" sequences    

                        b. DEGENERATE - most AA coded for by more than 1 codon

                              i. "Wobble" hypothesis - @ 3rd base site in codon

                        c. UNIVERSAL. . . practically! - works for all living things

                              (except mitochondria, chloroplasts & some protists) (read p. 284)

                 C. STEPS OF TRANSLATION:

                  1.  Charging  - tRNA chemically bonded to each AA. (p. 292 bottom)

      2.  Initiation - Needs mRNA, tRNA, rRNA, a ribosome, GTP, Mg⁺²,        

                  AA & protein factors. (p. 294 top)

                        a. AUG = "start sequence" is read by the ribosome

                        b. MET is brought in by the tRNA

                        c. Gets cut out after about 30 AA added

                        d. Anticodon on tRNA pairs with codon on the mRNA

                  3.  Elongation & Translocation (p. 294- 295)

                        a.  Next codon gets read

                        b.  tRNA (carrying the AA) attaches

                        c.  Peptide bond forms between AAs

                        d.  Old tRNA is kicked out as ribosome reads next codon

                        e.  Old tRNA gets "recharged" with another AA in cytoplasm

f.     a-e repeats until. . .

                  4.  Termination (p. 296 top)

                        a.  One of 3 stop codons is read.

                        b.  Completed polypeptide released

                        c.  Ribosome splits into large & small subunits, mRNA released

                  5. Tape player analogy

                              DNA = original full length tape (genotype)

                              mRNA = a copy of one song

                              protein = music from playing the song (phenotype)

                              rRNA + proteins (ribosome) = tape player

                              tRNA = listens to the tape, brings in correct AA

            IV. COMPARISON

            REPLICATION                                 TRANSCRIPTION              TRANSLATION

            Nucleus                                             Nucleus                                 Cytoplasm

            DNAèDNA                                      DNAèRNA                          RNAèPolypeptide

            Read 3'è5'                                       Read 3'è5'                           Read 5'è3'

            Assemble 5'è3'                               Assemble 5'è3'                   AA joined by pep. bond

            DoubleèDouble                             DoubleèSingle                   SingleèSingle

            A,C,G,T                                              A,C,G,U                                 20 AA

            Entire DNA copied                           1 Gene copied                      1 Gene expressed

            GenotypeèGenotype                     GenotypeèGenotype         GenotypeèPhenotype

            V. EUKARYOTIC RNA PROCESSING  (p. 288-290)

                 A. INTRONS - intervening sequences - non-coding interruptions

     B. EXONS - coding sequences of a gene

     C. RNA PROCESSING

           1. All EUK.RNA processed/edited before use

           2. Gene transcribed into mRNA

           3. 5’ cap and 3’ Poly-A tail added to transcript

           4. Introns removed & degraded

           5. Exons spliced together è functional mRNA = mature/final transcript

           6. ALTERNATIVE PROCESSING/SPLICING

                 a. 30,000 genes - yet up to 120,000 mRNA transcripts ! ! !

                 b. THYROID vs PITUITARY

                 c. Same gene results in different protein product

            VI. OVERVIEW (p. 298)

            VII. PROKARYOTIC VS. EUKARYOTIC GENE EXPRESSION (p. 298 bottom)

            VIII. MUTATIONS (p. 299)

                A. DEFINITION

                  1. Abrupt, abnormal hereditary changes

                  2. Most harmful, many lethal

                  3. Somatic - not passed on to offspring

                  4. Germinal - passed on to offspring