Ch. 17: From Gene to Protein Mendel’s work revealed that proteins are the link between genotype and phenotype Tall vs. dwarf height in pea plants was due to a growth hormone synthesized or not; due to a presence of an enzyme!! DNA directs synthesis of proteins: Transcription Translation

Beadle and Tatum Worked with breadmold; x-rayed and examined mutant growth Discovered the function of a gene is to dictate the production of a specific enzyme Restated hypothesis as one gene – one polypeptide However, keep in mind…some genes code for RNA that have important functions but are not translated into protein

Genes The DNA provides the instructions to make the protein RNA is the link between gene and protein DNA codes for RNA and RNA codes for the protein….known as the “central dogma” of biology

Transcription/Translation The DNA and RNA molecules are composed of nucleotide monomers. When converting from DNA to RNA you are simply transcribing the code from the language of DNA nucleotides to RNA nucleotides Proteins are “written” in the language of amino acids. When converting from RNA to protein we are translating from the nucleotide language to amino acid language

RNA In what ways are RNA molecules different from DNA? RNA is single stranded In RNA Uracil replaces Thymine Nucleotides have ribose instead of deoxyribose. In eukaryotes RNA leaves the nucleus

What are the functions of these 4 different types of RNA?

4 types of RNA mRNA  takes DNA’s message out to the ribosome for protein synthesis tRNA  brings amino acids to the ribosome for protein synthesis rRNA  structural component of ribosomes snRNA  involved in RNA splicing

The Genetic Code mRNA strand is complementary and antiparallel to DNA template RNA consists of four “letters”  A, U, G, and C Proteins consist of 20 “letters”  the amino acids If 1 RNA base codes for 1 amino acid, then only 4 amino acids can be coded for.

How many different amino acids can be coded for if 2 RNA’s code for 1 amino acid? 4 2 = 16 : Not enough! How many different amino acids can be coded for if 3 RNA’s code for 1 amino acid? 4 3 = 64: More than enough for the 20 different amino acids….

Codons mRNA base triplets are called codons Codons are read in the 5’  3’ direction # of nucleotides making up the genetic message is 3x the # of amino acids 64 codons deciphered by mid 1960’s Stop codons: UAA, UGA, UAG Start signal and methionine: AUG

There is redundancy …2 codons for one a.a., but not ambiguity …1 codon doesn’t code for 2+a.a.!

The Code is Universal The code is shared by almost all organisms CCG codes for what amino acid? Proline. This holds true for all species of living organisms. Bacteria, therefore can be programmed to synthesize human proteins by inserting human DNA

Eukaryotic Transcription 3 steps: Initiation Elongation Termination RNA polymerases are used RNA pol. II used for mRNA synthesis RNA pol. I and III used for all other RNA (not coded into protein) Direction of transcription  downstream (5’  3”)

Initiation Signaled by a promoter DNA sequence is TATAAAA, called a “TATA” box

Elongation RNA pol. moves along DNA and untwists it bases at a time RNA nucleotides are added to 3’ end (about 60/sec in eukaryotes) DNA double helix reforms as new RNA peels away

Termination Prokaryotes: terminator sequence on DNA causing RNA pol. to detach and mRNA to be released Eukaryotes: premRNA is cleaved due to a particular DNA sequence but needs to be processed into mRNA! 1. 5’end cap is added 2. 3’ tail called a poly-A tail is added

In prokaryotes, RNA is directly translated into the polypeptide

RNA in eukaryotes is processed before translation

The function of the cap is: prevent mRNA degradation by hydrolytic enzymes helps attach to the ribosome Function of the 3’ tail: same functions as the 5’cap also helps facilitate export of mRNA from nucleus

RNA splicing Removes non-coding regions (introns) snRNP (short nuclear ribonucleoproteins) recognize the splicing signals that are at the ends of introns The RNA in the snRNP is called snRNA (small nuclear RNA) snRNP + protein = spliceosome The spliceosome cuts and releases the introns, and then joins exons together

Evolutionary role of introns Introns may play regulatory role Different intron removal may lead to different proteins Introns may enhance crossing over between homologous regions by increasing the distance between exons

Translation mRNA delivers the message in the “nucleotide language” tRNA translates the message into the “amino acid language” End of tRNA molecule is an anticodon…triplet, complementary to mRNA Ex. mRNA  UUU; tRNA  AAA + phenylalanine

Structure and function of tRNA Transcribed from template DNA strand in nucleus Used repeatedly About 80 nucleotides long, single stranded with H- bonds causing a 3D structure

Recognition steps to translation 1. Amino acid joined to correct tRNA by aminoacyl-tRNA synthetase…20 of those (each specific to an individual amino acid) This step is catalyzed ATP The tRNA with the amino acid is known as aminoacyl tRNA 2. Correct match between tRNA anticodon and mRNA codon Wobble  relaxation in the base pairing rules with 3 rd base at the 3’ end of mRNA

tRNA tRNA Assembly

Messenger RNA consists of leader, reading frame, and trailer sequences.

Ribosomes 2 subunits (large and small) Constructed of protein and rRNA Only functional when attached to mRNA 2/3 of ribosomal mass is rRNA (most abundant type of RNA)

Ribosomal binding sites P site  peptidyl tRNA site; holds the tRNA carrying the growing polypeptide chain A site  aminoacyl tRNA site; holds the tRNA carrying the next amino acid E site  exit site; site where tRNAs leave the ribosome

Ribosomes, consist of two subunits, each of which contains rRNA and ribosomal proteins…rRNA serves as the catalyst (called a ribozyme)of peptide bond formation!

Building a polypeptide 3 stages of translation Initiation Elongation Termination

Initiation Small ribosomal subunit binds to mRNA and initiator tRNA carrying methionine Small subunit scans downstream along mRNA until it reaches start codon … AUG, establishing the “reading frame”. Initiator tRNA H-bonds to start codon mRNA + initiator tRNA + small ribosomal subunit + large subunit = translation initiation complex … requires proteins called initiation factors and energy in the form of GTP Proteins synthesized from N-terminus  C-terminus

Elongation Proteins called elongation factors are required to add new amino acids to preceding ones GTP required Ribosomes moves along mRNA in the 5’  3’ direction 3 steps to elongation 1. Codon recognition 2. Peptide bond formation 3. Translocation (moving along A, P, E sites)

Termination Protein called release factor binds to stop codon in the A site bringing in a water molecule instead of an amino acid Polypeptide is released through the exit tunnel of the ribosome’s large subunit Translation assembly comes apart

Initiation of Translation

Elongation

Termination

Polyribosomes  a string of ribosomes trailing along one mRNA to make many copies of a polypeptide very quickly