BIOLOGY - CLUTCH CH.17 - GENE EXPRESSION.

Transcription

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2 CONCEPT: GENES Beadle and Tatum develop the one gene one enzyme hypothesis through their work with Neurospora (bread mold). This idea was later revised as the one gene one polypeptide hypothesis. The central dogma of molecular biology: 1. Transcription the code of DNA is read, and transcribed into an RNA code 2. RNA processing in eukaryotes, hnrna must be processed into an mrna transcript 3. Translation ribosomes read the mrna transcript and build proteins with the assistance of trna molecules 4. Post-translational modification proteins undergo folding, cutting, and chemical modification Page 2

3 CONCEPT: GENES The two strands of DNA are known as the sense and antisense strands, or coding and template strands. Ribosomes read the code in triplets called codons. Reading frame the consecutive, non-overlapping triplets of a protein code The code of codons is nearly universal across all life on the planet. Page 3

4 CONCEPT: TRANSCRIPTION Transcription has three stages: initiation, elongation, and termination. Bacterial transcription: Initiation RNA polymerase and sigma subunit holoenzyme binds to the promoter region The sigma subunit is responsible for detecting the promoter Holoenzyme an enzyme activated by the binding of a coenzyme Elongation RNA polymerase catalyzes the addition of nucleotides on the growing 3 end of the RNA transcript Page 4

5 CONCEPT: TRANSCRIPTION Termination RNA polymerase transcribes a termination signal that causes the formation of a hairpin Eukaryotic transcription: Eukaryotes have three RNA polymerases; RNA polymerase II transcribes protein-coding genes. Most eukaryotic promoters have a TATA box (~30 base pairs) upstream of the transcription start site. Transcription factors proteins that help RNA polymerase bind to the promoter Poly A signal a sequence that is transcribed, leading an enzyme to cut the RNA transcript downstream of the signal Page 5

6 CONCEPT: RNA PROCESSING The primary transcript, or pre-mrna, must be processed into mature mrna before it can be translated. 5 cap a protective cap of modified guanine Poly-A tail a long string of adenines ( ) not encoded by the DNA that are added to the transcript RNA splicing the process that removes introns, and joins exons snrps (small nuclear ribonucleoproteins) RNA that helps form the spliceosome Page 6

7 CONCEPT: TRANSLATION Translation is carried out by ribosomes and trna, and occurs in three stages: initiation, elongation, and termination. trna (transfer RNA) binds to codon of RNA and matches appropriate amino acid Anticodon complementary code to codon on mrna Aminoacetyl-tRNA synthetases catalyze the transfer of an amino acid to the trna. Wobble hypothesis there are some unconventional base pairing in the third base of the codon/anticodon There are 61 potential anticodons, but most cells only contain about 40 different types of trna. Page 7

8 CONCEPT: TRANSLATION Ribosome a structure of protein and rrna that synthesizes polypeptides from mrna with the help of trna A site the acceptor site where the charged trna enters P site the polypeptide site where the trna that holds the growing polypeptide chain is held E site the exit site, where the uncharged trna exits Initiation: 1. The small subunit binds the mrna by the 5 cap aided by proteins called initiation factors. 2. The initiator trna carrying methionine binds to the start codon with the help of initiation factors. 3. The large subunit binds to the initiation complex. Page 8

9 CONCEPT: TRANSLATION Elongation: 1. A charged trna with the appropriate anticodon enters the A site. 2. A peptide bond forms between the two amino acids. 3. Translocation occurs, shifting the ribosome down one codon, and shifting the trnas to new sites within the ribosome. Termination: 1. A protein with a similar shape to trna, called a release factor, recognizes the stop codon and enters the A site. 2. The release factor hydrolyzes the bond connecting the polypeptide to the trna in the P site. 3. The polypeptide and trnas are released, and the ribosomal subunits separate. Page 9

10 CONCEPT: TRANSLATION The polypeptides released from the ribosome are rarely fully-formed, and must go through post-translational modifications. Most proteins need to be folded into their proper shape, and are often assisted by chaperones. Some proteins are chemically modified with the addition of various chemical groups. Some proteins contain a signal peptide that targets the to the endoplasmic reticulum. The signal peptide is recognized by a protein-rna complex called a signal-recognition particle. Page 10

11 CONCEPT: MUTATIONS There are three main types of mutations: beneficial, neutral, and deleterious. Point mutation a change in the DNA sequence at a single base pair Substitution the replacement of one nucleotide for another Transitions when a purine is replaced with a purine, or a pyrimidine with a pyrimidine Transversions when a purine is replaced with a pyrimidine, or vice versa Silent mutations a mutation with no observable effect on phenotype Missense mutations a mutation that changes the codon for one amino acid, to that for another Nonsense mutations a mutation that changes a codon for an amino acid to a stop codon Page 11

12 CONCEPT: MUTATIONS Insertion the addition of nucleotides into the sequence Deletion the deletion of nucleotides from the sequence Frameshift mutation a mutation that shifts the reading frame of the codons Deletion loss of a chromosome fragment Duplication can be cause by attachment of extra chromosome fragment Inversion chromosome fragment attaches in reverse orientation Translocation chromosome fragment attaches to a nonhomologous chromosome Page 12