Simultaneous transcription and translation in prokaryotes Green arrow = E. coli DNA Red arrow = mRNA combined with ribosomes

Eukaryotic RNA Differences RNA processing –Primary transcript produced in the nucleus –Processed before transported to the cytoplasm

A cap consisting of 7-methylguanosine is added to the 5’ end of the transcript

3’ poly (A) tail

Eukaryotic RNA RNA processing –5’ cap Protects RNA from degradation Required for binding to the ribosome during initiation of protein synthesis (translation) –3’ poly (A) tail Protects RNA from degradation by nucleases

Eukaryotic RNA RNA processing –Splicing Removes intervening sequences in RNA

Many eukaryotic genes contain internal sequences that do not encode amino acids – introns (light colored areas) Sequences that encode amino acids – exons (darker colored areas)

Splicing removes the introns and brings together the coding regions

Gene Splicing Consensus sequence at intron-exon junction snRNAs pair complementarily with the splice site Splicing enzymes can then cut-out introns

Gene Splicing Sometimes, different introns are spliced-out determining the function (type) of protein that is made

The Central Dogma (Francis Crick, 1958) (Transcription) (Translation) DNA  RNA  Protein (Gene) (Phenotype) An informational process between the genetic material (genotype) and the protein (phenotype

Proteins Proteins are just long polymers of amino acids –So, the basic unit of a protein is an amino acid –20 different amino acids

Proteins Amino acids in a protein are held together by peptide bonds –Facilitated by peptidyltransferase

Proteins A long string of amino acids is called a polypeptide A protein has an amino (the first amino acid in the chain) and a carboxyl (the last amino acid in a chain) ends

Aminoacyl site: new amino acid brought in Peptidyl site: peptidyltransferase attaches amino acid to chain Ribosome moves in this direction Translation (protein synthesis)

Animation of protein synthesis hill.com/sites/ /student_view0/c hapter12/animation_quiz_2.htmlhttp://highered.mcgraw- hill.com/sites/ /student_view0/c hapter12/animation_quiz_2.html

Cells have adapter molecules called tRNA with a three nucleotide sequence on one end (anticodon) that is complementary to a codon of the genetic code. There are different transfer RNAs (tRNAs) with anticodons that are complementary to the codons for each of the twenty amino acids. Each tRNA interacts with an enzyme (aminoacyl-tRNA synthetase) that specifically attaches the amino acid that corresponds to its anticodon. For example, the tRNA to the right with the anticodon AAG is complementary to the UUC codon in the genetic code (mRNA). That tRNA would carry the amino acid phenylalanine (see genetic code table) and only phenylalanine to the site of protein synthesis. When a tRNA has its specific amino acid attached it is said to be “charged.”

Proteins Protein can have a Primary structure Secondary structure Tertiary structure Quaternary structure

Primary structure The order of the amino acids The order is the primary determinant of protein function The primary structure is determined by the code on the DNA/RNA synthesized

Tryptophane Synthase A Protein 268 amino acids long Primary structure Amino end Carboxyl end

Alpha Helix Secondary structure Interaction of side groups, giving polypeptides a periodic structure Stabilized by hydrogen bonds

Alpha Helix

Beta Pleated Sheet

Tertiary structure The folding or bending of the polypeptide

Tertiary structure can be affected by environmental factors such as temperature

Enzymes are proteins: if the tertiary structure is changed (mutation or temperature) the enzyme cannot carry out its function

Bovine Insulin Protein Quaternary structure Two or more polypeptides combine to form a functional protein

Proteins The order of the amino acids (the primary structure) can affect the secondary, tertiary and quaternary structures –Possibly affecting the function of the protein

Hemoglobin Alpha chains each have 141 amino acids Beta chains each have 146 amino acids

Change in beta chain at amino acid 6 out of the 146 amino acids (change in codon from GAG to GUG)

Proteins The order of the amino acids in a polypeptide is like the order of words in a sentence

Proteins If you change one word you can change the meaning significantly –John only punched Jim in his eye.

Proteins If you change one word you can change the meaning significantly –John only punched Jim in his eye. –John only punched Jim in his dreams.

Proteins This is what happens in mutations –If the code changes (DNA), new amino acids can be put in the polypeptide, changing “the meaning” of the polypeptide

Genetic Code One fundamental question: How can DNA and RNA, each consisting of only four different nucleotides (bases), encode proteins consisting of 20 amino acids? –Solving the genetic code became the most important biological question of the late 1950s and early 1960s