DNA, RNA, and Protein Synthesis

Notes

1. General Information

1. Archilbard Garrod believed that the function of a gene is to dictate the production of a specific enzyme
2. Auxotroph - nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild type strains of the same species

1. Beadle and Tatum - each mutant lacks a different enzyme
1. One gene - one enzyme hypothesis
2. Revised into the one gene - one polypeptide hypothesis
1. Common to refer to proteins rather than polypeptides as the gene products

2. Transcription and Translation

1. Genes provide instructions for making specific proteins

1. Must go through RNA to get to DNA

1. Transcription - synthesis of RNA under the direction of DNA

1. Messenger RNA - carries a genetic message from the DNA to the protein-synthesizing machinery of the cell

2. Translation - actual synthesis of a polypeptide, under the direction of mRNA

1. Translates sequence of mRNA molecule into the amino acid sequence of a polypeptide
2. In a Prokaryotic cell, transcription and translation occur in the same time and place

2. RNA processing - modification of RNA before it leaves the nucleus, a process unique to eukaryotes

1. DNA à RNA à protein

3. Nucleotide triplets specify amino acids
4. Triplet code - a set of three-nucleotide-long words that specify the amino acids for poleypeptide chains
5. Template strand - the DNA strand transcribed that serves as a template for the sequence of nucleotides in an RNA transcript

1. mRNA molecule is complimentary to its DNA template

1. Pairs are similar, except that U is used for T

2. Codon - mRNA base triplet

1. Because they are base triplets, the number of nucleotides must be three times the number of amino acids making up the protein product
2. Marshall Nirenberg decoded the first codon
3. AUG signals translation to begin

3. Transcription

1. An RNA poleymerase separates the two strands of DNA and connects RNA nucleotides as they base pair
2. Transcription unit - stretch of DNA that is transcribed into an RNA molecule
3. Promoter - region of DNA where RNA polymerase attaches and initiates transcription
4. Transcription factors - collection of proteins that mediate the binding of RNA polymerase and the initiation of transcription

1. Completed assembly is called the transcription initiation complex

5. Terminator - signals the transcription to stop

4. After Transcription

1. The 5’ end of a pre-mRNA molecule is capped off with a guanine nucleotide

1. The cap protects mRNA from degradation by hydrolytic enzymes
2. It also tells ribosomes to attach there

2. The 3’ end receives a poly(A) tail of 30-200 adenine nucleotides

1. Also prevents degradation and helps ribosomes attach
2. Seems to facilitate the export of mRNA from the nucleus

3. RNA splicing - removal of noncoding portions of the RNA molecule after the initial synthesis
4. Introns - non coding, intervening sequence within a eukaryotic gene
5. Exons - coding region of a eukaryotic gene that is eventually expressed (translated into amino acids)

1. These together form an oversized RNA molecule, but it doesn’t leave the nucleus

1. Introns are excised and exons join together to form an 'abridged' mRNA molecule with a continuous coding sequence

6. Small nuclear ribonucleoproteins (snRNPs) are short nucleotide sequences that are the signals for RNA splicing

1. Found on the ends of introns

5. Synthesis of Proteins

1. Transfer RNA (tRNA) - RNA molecule that functions as an interpreter between nucleic acid and protein language by picking up specific amino acids and recognizing the appropriate codons in the mRNA

1. Transfers amino acids from the cytoplasm’s amino acid pool to a ribosome

1. Bears a specific amino acid at one end and an anticodon
2. The anticodon binds to a complementary codon

2. A transfer RNA molecule consists of a single RNA strand about 80 nucleotides long

1. Used repeatedly
2. Folds back upon itself to form a 3-D structure
3. There are 45 tRNAs, which is okay because some tRNAs can recognize two or more different codons

3. Aminoacyl-tRNA synthetase - family of enzymes, at least 1 for each amino acid, that catalyze the attachment of an amino acid to its specific tRNA molecule
4. Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis

1. Ribosomal RNA (rRNA) - ribosomal subunits constructed of proteins and RNA molecules

5. Three stages in building a polypeptide

1. Initiation - brings together mRNA, a tRNA bearing the first amino acid of the polypeptide, and the two subunits of a ribosome
2. Elongation - amino acids are added one by one to the first amino acid. It is a three step cycle:

1. codon recognition - mRNA codon in A site bonds with the anticodon of a tRNA. Require hydrolysis of GTP
2. Peptide bond formation - rRNA unit catalyzes formation of a peptide bond to the amino acid in the A site
3. Translocation - tRNA in the A site is translocated to the P site. MRNA brings next codon to be translated into the A site. The tRNA that was in the P site leaves the ribosome. Requires hydrolysis of GTP.

3. Termination - UAA, UAG, UGA act as signals to stop translation

6. Polyribosomes - strings of ribosomes
7. Signal peptide - stretch of amino acids on polypeptides that targets proteins to specific destinations in eukaryotic cells

1. Recognized as it emerges from the ribosome by a signal recognition particle

6. Point Mutations

1. Mutations - changes in the genetic material of a cell

1. Point mutations - chemical changes in 1 or a few base pairs in a single gene
2. If a mutation has an adverse effect on an organism, it is referred to as a disorder

2. Mutations types

1. Substitutions

1. Base pair substitution - replacement of a nucleotide and its complement with another pair of nucleotides
2. Missense mutations - includes all substitution mutations; altered codon still codes for an amino acid, though it might not make sense
3. Nonsense mutations - changes a codon to a stop signal

1. Insertions and deletions

1. Insertion - addition of one of more nucleotide pairs in a gene
2. Deletion - loss of one or more nucleotide pairs in a gene
3. Frameshift mutation - mutation occurring when the number of nucleotides inserted or deleted is not in multiples of 3

3. Mutagens - physical and/or chemical agents that interact with DNA

1. X-rays and other high energy radiation are harmful to genetic material