 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

 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

 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

 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

 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?

 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

 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….

 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

 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

 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”)

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

 RNA pol. moves along DNA and untwists it 10-20 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

 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

 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

 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

 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

 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

 Transcribed from template DNA strand in nucleus
 Used repeatedly
 About 80 nucleotides long, single stranded with Hbonds causing a 3D structure

 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

 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)

 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!

 3 stages of translation
 Initiation
 Elongation
 Termination






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




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
1.
2.
3.
3 steps to elongation
Codon recognition
Peptide bond formation
Translocation (moving along A, P, E sites)

 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

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